Introduction

Ontogeny concerns the study of living things' development from the basal cell's emergence to their full maturity (Fontquer 1985). The study of floral ontogeny is fundamental to clarify several events, such as the order of organ initiation, heterochrony, the establishment of symmetry, and organ proliferation or reduction (Tucker 1987a, b, 1989, 1992a, 1997, 1999). The study of floral ontogeny with a morphological approach covers from the initiation of the primordia (organogenesis stages), passing through elongations and differentiation of forms, differentiation and specializations of cells (intermediate and final stages of development), until the moment of anthesis, elucidating several aspects of the evolution of flowers in important clades of angiosperms. Extensive investigations on floral development within the Fabaceae family have been conducted by notable researchers, including Tucker, Teixeira, Sinjushin, and Prenner, among others. (Leite et al. 2015; Mansano et al. 2002. Pedersoli et al. 2010; Pedersoli and Teixeira 2016; Teixeira et al. 2009; Sinjushin 2018, 2021, 2023; Tucker 1984, 1989, 1992a,b, 1997, 1999; Tucker and Douglas 1994). Many of these works have been significant in terms of their relevance to the systematics of the group.

The study of floral development is an excellent approach for understanding complex floral architectures, which often have ecological impacts and determine the evolutionary success of plants. In this context, it is crucial to contextualize the taxonomic position of Mimosa to understand the relevance of the wide variation in the construction of the floral architecture in this genus (Barneby 1991). The genus Mimosa L. belongs to the family Fabaceae, the third largest angiosperm family, subfamily Caesalpinioideae, clade mimosoid (Lewis et al. 2005; LPWG 2017). According to the new circumscription that emerged with the subdivision of the family into six subfamilies: Cercidoideae, Detarioideae, Duparquetioideae, Dialioideae, Caesalpinioideae, and Papilionoideae (LPWG 2017), Mimosoideae, formerly recognized as a subfamily, became a distinct clade now placed within the subfamily Caesalpinioideae and referred to as the mimosoid clade (LPWG 2017). The genus Mimosa stands out not only for being one of the largest genera of Fabaceae, with more than 500 species, distributed mainly in the Neotropical region (Simon et al. 2011), but also for the characteristics that deviate from the floral trait pattern of the family, especially concerning the number of parts that make up the perianth (Simon et al. 2011), making it an excellent study model for ontogenetic analysis. It is important to note that the legume family is known to display pentamerous flowers, comprising 21 floral organs (five sepals, five petals, five antesepalous stamens, five antepetalous stamens, and one central carpel) (Bruneau et al. 2014). The floral characters have crucial importance in the infrafamilial classification in Fabaceae.

Thus, Mimosa (mimosoid clade) stands out not only for being the type genus of the former subfamily Mimosoideae (Lewis et al. 2005; LPWG 2017), but also for its wide inter- and intraspecific variation in floral characters (Barneby 1991). In Fabaceae, although the flowers display a wide variety of shapes, sizes, and pollination syndromes (Arroyo 1981), there is a well-defined pattern in the number of the organs (pentamerous flowers) (Bruneau et al. 2014; Lewis et al. 2005; LPWG 2017). Besides, floral organs’ positional arrangement (median sagittal plane) is usually stable in each subfamily (Tucker 2003). In this context, considering the taxonomic position of Mimosa, the flowers in this genus are peculiar mainly because of the great lability in merism. Most floral whorls are composed of three, four, five, or more rarely, two or six organs, besides to have some flowers without carpel (andromonoecious species) the uncommon characteristic among most species of the family and even among close genera, such as members of the Piptadenia group, which have pentamerous flowers (Barneby 1991; Simon et al. 2011). Thus, the study of the development of floral organs can greatly contribute to uncovering atypical structures (Paulino et al. 2013; Pedersoli et al. 2010, 2019; Tucker 2003).

Studies of floral ontogeny in species of the mimosoid clade have significantly contributed to broadening our understanding of the processes that lead to important variations in floral architecture. One example is the reduction of floral organs, resulting from carpel abortion, that leads to dioecy in species of Parkia R. Br. and Stryphnodendron Mart. (Pedersoli et al. 2023), which is considered an atypical floral construction in the family (Tucker 2003). Flowers fail to produce male or female gametophytes via functional or structural mechanisms, the latter resulting from the absence of carpels or stamens (Dellaporta and Calderón-Urrea 1993; Matsunaga and Kawano 2001). Mitchell and Diggle (2005) point out two ontogenetic pathways that result in diclinous flowers: (i) abortion in the course of organ development or (ii) absence of the reproductive whorl from inception. The first case happens when the androecium and gynoecium are initiated, but only one reaches maturity and full functionality. In the second case, the flowers are diclinous from the beginning of their development; the meristem initiates the formation either of carpels or stamens only, without any trace of the other whorl. Cases like this require investigation to elucidate the floral ontogeny of the group.

On the other hand, the reduction or proliferation of organs located in the perianth whorls, especially the petals, have been described and analyzed from an ontogenetic point of view, most commonly in species of Dialioideae (Falcão et al. 2020; Zimmerman et al. 2013), Detariodeae (Bruneau et al. 2014; Kochanovski et al. 2018; Pedersoli et al. 2010; Tucker 2000), Papilionoideae (Leite et al. 2015; Paulino et al. 2013), and other Caesalpinioideae species (Prenner 2004; Tucker 1988, 1990, 1992a). Regarding the mimosoid clade (with polysymmetric flowers, in contrast to monosymmetric flowers in previously mentioned clades), little is known about the ontogenetic pathways that lead to the reduction or proliferation of organs, especially of the perianth organs. Ontogenetic studies have focused on the perianth merism instability in some species of this clade (Paulino et al. 2017; Prenner 2011), but the developmental pathways leading to fixed or labile meristic changes, as occurs in Mimosa, are underexplored. Merism or merosity is the number of floral parts per whorl in flower (with whorled phyllotaxis), usually based on the parts of the perianth (Ronse de Craene 2016, 2022).

In Fabaceae, some cases of extreme reductions of floral organs have been reported, such as Dialium L., Apuleia Mart., and Dicorynia Benth. (Dialioideae), (Falcão et al. 2020; Zimmerman et al. 2013), or subtle reductions of floral organs in Amhertsia Muro, Hymenaea L., and Copaifera L. (Detarioideae), in which two sepals of a pentamerous calyx are fused, leading to a tetramerous pattern in anthesis (Kochanovski et al. 2018; Pedersoli et al. 2010; Tucker 2000). Further, there is the case of species of basal lineages of Papilionoideae, in which the corolla undergoes extreme reduction to one petal or complete absence of petals (Leite et al. 2015; Moço and Pinheiro 1999; Paulino et al. 2013; Tucker 1990). Therefore, it is evident that in a speciose family like Fabaceae each group needs to be evaluated individually.

Thus, the present work aims to elucidate the floral development of four species of the genus Mimosa, focusing on the merism, which deviates from the pentamerous pattern of the family. For this, the following questions were investigated: (i) Which ontogenetic pathways give rise to the peculiar morphological configuration that deviates from the pentamerous pattern of the family? (ii) Does the reduction in the number of perianth parts affect the subsequent development pattern of the whorls? (iii) Which ontogenetic pathway leads to haplostemony in M. pudica? (iv) Which ontogenetic pathway results in the formation of the staminate flowers in andromonoecious species? (v) In a broader context, what are the reasons contributing to the variation in merism?

Materials and methods

Study species and collection area

Four species were selected to cover the variations in merism in Mimosa: M. caesalpiniifolia Benth. (Fig. S1a, b); M. pudica var. hispida Brenan and M. pudica var. tetrandra (Humb. & Bonpl. ex Willd.) DC. (Fig. S1c-e); M. bimucronata (DC.) Kuntze (Fig. S1f, g); and M. candollei R. Grether (Fig. S1h, i) (Table 1).

Table 1 Characteristics of the selected species with variations in merism, stamen configuration and sexual systems
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Flowers and floral buds at different stages of development were collected. To measure the size of each floral organ, ten flowers were dissected, and their parts were measured with the Leica S APO stereomicroscope coupled to a DFC320 camera. The measurements were made using the LAS V3.8 program. The mean and standard deviation were calculated. Plant height, habitat, flower and leaf color, texture and odor, and the date and location of the collection site were recorded in the field. The voucher specimens were deposited in the herbarium of the Instituto de Pesquisas Jardim Botânico do Rio de Janeiro. Individuals were identified with the help of specific literature and confirmed by the taxonomist and specialist in the group Lucas Sá Barreto Jordão.

Surface analysis (SEM)

Floral buds at various stages of development were collected, fixed in FAA 70 (formalin–acetic acid–alcohol; Johansen 1940), gradually dehydrated in an ethanol series and stored in 70% ethanol. Then they were dissected, critical-point-dried in a Bal-Tec CPD 030 Bal-Tec (AG, Liechtenstein—JBRJ-RJ) apparatus, mounted on metal supports, placed on Carbon-coated adhesive tape and then covered with Gold in an Emitech K550X (Ashford, UK—JBRJ-RJ) metallizer. Observations and illustrations were made on a JEOL—JSM-6490LV (CBPF-RJ) and Tescan-VEGA 3 LMU (NUMPEX, UFRJ-RJ) scanning electron microscope at 15, 20, or 30 kv.

Illustrations and terminologies

The images obtained using the different techniques were processed using Adobe Photoshop CS5 software. The terminology adopted followed Tucker (1987a, b, 2003) for the description of floral development stages, and Claßen-Bockhoff and Bull-Hereñu (2013) and Beentje (2010) for the description of inflorescences.

Results

Mimosa caesalpiniifolia (trimerous flowers, diplostemonous)

Organography

The flowers are arranged in cylindrical spike-like inflorescences (Fig. S1b). Each flower is inserted in the axil of an abaxial bract; there are no bracteoles. The flowers are white and predominantly trimerous, diplostemonous, with actinomorphic symmetry (Fig. S1b, 1a-c). They are usually perfect (Fig. 1b, c), but few staminate flowers may occur (Fig. 1d). The calyx is white, glabrous, gamosepalous, campanulate, and irregularly lobed; the sepals are ca. 0.7 ± 0.1 mm long (Fig. 1b); the calyx is trimerous, rarely composed of two or four sepals. The corolla is campanulate, ca. 2.2 ± 0.1 mm long, formed by three petals (Fig. 1b, c), occasionally two or four petals, white colored, with slightly greenish lobes, and conspicuous veins. Most inflorescences only produce trimerous flowers. The androecium is composed of six (varying from four to nine) free stamens (Fig. 1c) and is the most conspicuous whorl; each stamen is formed by a long (ca. 6.2 ± 0.2 mm) linear filament and a small, orbicular, dithecous, dorsifixed anther with longitudinal dehiscence and yellow color (Fig. 1b, c). The gynoecium is monocarpellate, with the carpel, 7.1 ± 0.3 mm long, formed by a short stipe, 0.3 ± 0.1 mm long, a glabrous ovary, ca. 0.7 ± 0.1 mm long (Fig. 1c) enclosing ca. five ovules, a long, slender style, ca. 5.9 ± 0.3 mm long, and a terminal, poriform stigma (Fig. 1c).

Fig. 1
figure 1

Floral diagrams with floral formulae and drawing of Mimosa flowers. a-d M. caesalpinifolia. a Floral diagram. b Overview of the flower. c Flower in lateral view, showing the trimerous corolla, the androecium with six stamens, and the central carpel. d Detail of the aborted carpel in the staminate flower. e–g M. pudica. E. Floral diagram. f Overview of the flower. g Flower in lateral view, showing the tetramerous corolla, the androecium with four stamens, and the central carpel. h–k M. bimucronata. h Floral diagram. i Overview of the flower. j Flower in lateral view, showing the tetramerous corolla, the androecium with eight stamens, and the central carpel. k Detail of the aborted carpel in the staminate flower. l-o M. candollei. l Floral diagram. m Overview of the flower. n Flower in lateral view, showing the pentamerous corolla, the androecium with ten stamens, and the central carpel. o Detail of the aborted carpel in the staminate flower. The floral diagrams illustrate the predominant merism in each species. Bracts and sepals are represented in green; petals in white; anthers in yellow; and central carpel in olive green. The floral diagrams were built in the "Floral diagram generator" (Kebert 2015). The floral formulae follow the format proposed by Prenner et al. 2010. (Drawings: Maria Alice de Rezende)

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Organogeny

The inflorescences are formed as an open reproductive system. The inflorescence is characterized by an indeterminate reproductive meristem with spike-like subunits (Fig. 2a) of axillary inflorescences at different stages of development. The floral meristems are initiated in the axil of an abaxial bract (Fig. 2a–c), varying from rounded (Fig. 2b) to elliptical (Fig. 2c) in shape, measuring 50–70 µm wide, and are quite congested in the inflorescence (Fig. 2a). The bract elongates early in development, covering and protecting the floral meristem. There is no formation of bracteoles. The development of floral organs in the meristems of the same inflorescence subunit is usually synchronous (Fig. 2a). The emergence of floral organs in vertical succession is mixed acropetal, initiating in the following sequence: sepals, petals, antesepalous stamens, carpel, and antepetalous stamens.

Fig. 2
figure 2

Floral organogenesis in Mimosa caesalpiniifolia. a Cylindrical spike-like inflorescence subunit. Note the synchronous development of floral buds. b, c Floral meristem in rounded (b) and elliptical (c) shape. d, e Simultaneous sepal initiation. Note the arising of three (d) and four (e) sepal primordia. f Formation of an early ring-like calyx. Note the fusion (arrowhead). g Simultaneous initiation of elliptical petal primordia. Sepals removed. h Simultaneous initiation of antesepalous stamens. i Initiation of the central carpel primordium. j Simultaneous initiation of antepetalous stamen primordia and onset of carpel elongation. Sepals and petals removed. k All organs are initiated. Note the carpel cleft formation at this stage (asterisk). Symbols: a = antepetalous stamen; A = antesepalous stamen; br = bract; c = carpel; fm = floral meristem; p = petal; s = sepal. Scale bars: a = 1 mm; b–k = 20 µm

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During the initiation of the sepals, the floral meristem assumes a triangular shape. The sepals arise simultaneously on the floral meristem as individualized primordia (Fig. 2d). The median sepal is usually adaxial, and there may be a slight shift to the right or left of the median sagittal plane (Fig. 2d, e). The union of the three (Fig. 2d), rarely two or four sepal primordia occurs precociously (Fig. 2e), and the whorl assumes a ring shape very early, even before the initiation of petal primordia (Fig. 2f).

The petal primordia appear with an elliptical shape, in a simultaneous order, and alternate with the sepals (Fig. 2g). In trimerous flowers, the median petal is usually in the abaxial position (Fig. 2g), and there may be a slight displacement to the right or left of the median sagittal plane. Predominantly three petals are initiated in the floral buds (Fig. 2g–j), but on rare occasions, floral meristems initiate four petals (Fig. 3d) and, even more rarely, two petals.

Fig. 3
figure 3

Intermediate and final stages of floral development in Mimosa caesalpiniifolia. a-d Sepals elongation. a Fusion of the sepals forming irregular lobes without completely enclosing the floral bud. b Corolla elongates and begins to surpass the calyx in length. c Detail of the stoma on the petal adaxial surface. d Corolla surpasses the calyx. Note the closure of the floral bud in valvate aestivation, becoming the protective whorl (b-e). f Detail of the apical papillae of the petals that intertwine and help the closing of the corolla (arrowheads). g Floral bud in lateral view, showing the basal union of petals. h, i Androecium elongation. Sepals and petals removed. Note the antesepalous stamens elongation and the carpel cleft (asterisks). j Staminate floral bud, showing the aborted carpel. k Folded filaments in pre-anthesis. l Pollen grains in a polyad. m–o Carpel development. m Detail of the glabrous ovary. n Poriform stigma with one fitted polyad (arrow). o Dissected ovary with visible ovules. Symbols: a = antepetalous stamen; A = antesepalous stamen; br = bract; c = carpel; cl = aborted carpel; ov = ovule; ovr = ovary; p = petal; s = sepal, st = stipe; stl = style. Scale bars: a, b, f, h, i = 50 µm; c = 20 µm; d, e, j, m, o = 100 µm; g, k = 200 µm; l = 2 µm; n = 10 µm

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The stamens arise in two whorls: an outermost antesepalous and an innermost antepetalous whorl. The antesepalous stamens initiate simultaneously, alternating with the petals (Fig. 2h). The antepetalous stamens initiate alternately to the antesepalous ones in simultaneous order and after the carpel initiation (Fig. 2j). Therefore, the antepetalous stamens constitute the last whorl to arise in the floral meristem. Three stamen primordia are commonly formed in both whorls, always alternating with the primordia of the outermost whorl. In flower buds with variation in the perianth merism, especially in the petalous whorl, it was possible to observe a wide variation in the number of initiated stamen primordia. However, the androecium most often remains with six stamens, especially in buds with a tetramerous corolla, with the formation of three antesepalous stamens, one abaxial and two lateral, without the initiation of the fourth adaxial primordium and, three antepetalous stamens, two abaxial and one adaxial, the latter elongating faster than the other primordia of the same whorl due to the space left by the absence of the fourth adaxial antesepalous stamen primordium (Fig. 3h).

In perfect and staminate flowers, the carpel initiates early as a single protuberance in the center of the floral meristem after the antesepalous stamens (Fig. 2i). Early in the development, the carpel becomes protuberant, with a circular shape (Fig. 2j). In perfect flowers, the development of the plicate carpel gives way to the emergence of the carpel cleft, facing the adaxial side, after initiating the antepetalous stamens (Fig. 2k). The position of the carpel cleft varies among floral buds: the cleft can be initiated opposite to the adaxial antesepalous stamens or shifted to the right or left (Figs. 2k, 3h, i). In staminate flowers, an early abortion of the carpel happens in some cases when the carpel cleft initiates, but the carpel primordium has its development arrested. Yet, the other organs continue the elongation and differentiation process (Fig. 3j).

Neither in trimerous flowers nor flowers with a variation of merism that deviate of the pentamerous pattern did we find the initiation of additional primordia of sepals, petals, antesepalous or antepetalous stamens, with subsequent abortion. The decreased number of organs of the calyx, corolla, and androecium is caused by an absence of organs from inception.

Intermediate and final stages of development

Fusion, elongation, and specializations of the organs characterize the intermediate and late stages of development. The sepals that assumed a ring shape at the beginning of development start to elongate, culminating in a campanulate gamosepalous calyx with irregular lobes that does not enable a perfect closure of the floral bud (Fig. 3a–c). During development, the calyx does not exceed the height of the corolla and the other whorls, so it does not entirely enclose the flower bud throughout development (Fig. 3a–c).

The petals elongate and unite, enclosing the bud in valvate aestivation (Fig. 3a, b, d, e). In this stage, the petals present papillae in their apical region (Fig. 3b, f) that intertwine and help the petals enclose the floral bud. The petals then become the whorl that protects the innermost reproductive organs. At the end of development, the calyx reaches ca. half of the total length of the corolla (Fig. 3g). Stomata are present on the adaxial surface of the petals (Fig. 3b, c).

The antepetalous stamens lag behind antesepalous stamens in the initiation of anther elongation and differentiation (Fig. 3h–j); at the end of elongation, both whorls occupy the same circumference. The filaments remain folded until anthesis, when they extend and expose the anthers, surpassing the perianth in length (Fig. 3k). The differentiated anthers shelter pollen grains, which are aggregated in polyads formed by eight pollen grains each (Fig. 3l).

Carpel elongation in perfect flowers is precocious, taking place before the elongation of the stamens (Fig. 2k, 3h, i); later, the carpel cleft closes, and the differentiation of the style and the stigma occurs (Fig. 3m–o). At the end of development, the carpel differentiates into a proximal ovary housing ovules exhibiting marginal placentation (Fig. 3o), with a cylindrical style (Fig. 3m), and a terminal poriform stigma, in which one polyad fits (Fig. 3n). The elongation of the carpel primordium is suppressed in staminate flowers and the carpel remains rudimentary, very small and without style, stigma, or ovules (Fig. 3j).

Mimosa pudica var. hispida and var. tetrandra (tetramerous, haplostemonous flowers)

Organography

The species presents the flowers grouped in globose spike-like inflorescences (Fig. S1c-e). The flowers, inserted in the axil of an abaxial bract and without bracteoles, have a pink color and are generally tetramerous, haplostemonous, perfect, with actinomorphic symmetry (Fig. S1c-e, 1e-g). The calyx is white, glabrous, gamosepalous, campanulate, diminutive, and irregularly lobed; the sepals are ca. 0.3 ± 0.1 mm long (Fig. 1f); the calyx is usually tetramerous (Fig. 1e), rarely composed of five sepals. The corolla is tubular, white at the base with pink lobes, pilose on the abaxial side, ca. 2 ± 0.12 mm long, usually tetramerous (Fig. 1e–g), rarely with five petals. The androecium comprises four free stamens and is the most conspicuous whorl; each stamen is formed by a linear, pink, cylindrical, long filament, ca. 5.1 ± 0.3 mm long, and a diminutive, orbicular, dithecous, dorsifixed anther, with longitudinal dehiscence and of yellowish color (Fig. 1f, g, S1c–e). The gynoecium is monocarpellate, formed by a short stipe, ca. 0.4 ± 0.1 mm long, a glabrous ovary, ca. 0.4 ± 0.1 mm long, usually enclosing four ovules, a glabrous, long, cylindrical, and thin style attached to the ovary in a slightly subterminal position, ca. 5.4 ± 0.4 mm long, and a terminal poriform stigma (Fig. 1g).

Organogeny

The inflorescences are an open reproductive system; the meristem is in the apical portion. The indeterminate inflorescence meristem forms axillary globose spike-like inflorescence subunits; each subunit develops in the axil of a bract (Fig. 4a). Glandular trichomes form at the base of the inflorescence subunits (Fig. 4b, c). Each floral meristem is generally rounded, 55 to 75 µm in diameter, inserted in the axil of the abaxial bract (Fig. 4d), which elongates rapidly and protects the floral meristem. There is no formation of bracteoles. The development of floral meristems in the inflorescence is synchronous (Fig. 4b). The initiation of floral organ primordia in vertical succession is acropetal, in the following sequence: sepals, petals, stamens, and carpel.

Fig. 4
figure 4

Floral organogenesis in Mimosa pudica. a Apex of an indeterminate inflorescence, with developing inflorescence subunits. b Spike-like inflorescence subunit. Note the synchronous development of floral meristems. c Trichomes at the base of the inflorescence subunits. d Rounded floral meristem in the axil of the abaxial bract. e–g Simultaneous initiation of sepals (e); of petals (f) and of antesepalous stamens (g). h Carpel initiation as a central protuberance. e-i Observe that the sepal primordia are precociously fused, forming a calyx with irregular lobes. i Note that no initiation of antepetalous stamen primordia. Symbols: A = antesepalous stamen; br = bract; c = carpel; fm = floral meristem; is = inflorescence subunit; p = petal; s = petal. Scale bars: a = 500 µm; b = 200 µm; c, e, f, i = 50 µm; d, g, h = 20 µm

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The sepal primordia unite very precociously, forming a ring-like calyx in the early development stage, making it difficult to determine the order of initiation in this whorl (Fig. 4e, f). Because of this, it was not possible to define whether the sepal primordia initiating in the sagittal plane were median or lateral.

Petal primordia emerge simultaneously after the formation of the ring-like calyx (Fig. 4f). In most buds, four petal primordia appear (Fig. 4f–i); more rarely, five primordia may appear (Fig. 5e). In tetramerous buds, the abaxial and adaxial petal primordia are in a median position in the sagittal plane. Two other primordia are in a lateral position. Currently, the floral bud assumes a tetragonal shape (Fig. 4f).

Fig. 5
figure 5

Intermediate and final stages of floral development in Mimosa pudica. a-b Elongation of the ring-like calyx with irregular lobes formation. c Onset of petals elongation with a pronounced inclination towards the center of the floral meristem. d, e Corolla in valvate aestivation with four (d) and five petals (e). f Detail of the papillae at the apex of a petal (arrowheads). g Corolla elongation, showing the trichomes on the apical portion. Note the height proportion between the calyx and corolla. h Trichomes and papillae on the apical portion of the corolla. i-l Androecium elongation. Sepals and petals partially removed. i Anther differentiation and carpel cleft formation (asterisk). j, k Elongation of two first congested stamens. k The elongation of the second pair of stamens. l Pre-anthesis folded filaments. m Ovary and basal portion of style. n Poriform stigma, with one fitted polyad (arrow). Symbols: A = antesepalous stamen; an = anther; br = bract; c = carpel; co = corolla; f = filament; k = calyx; ovr = ovary; p = petal; stg = stigma; stl = style. Scale bars: a, b, e, f, i = 50 µm; c = 25 µm; d, g, j = 100 µm; h = 20 µm; k-m = 200 µm; n = 10 µm

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The stamens appear in an antesepalous single whorl. After the initiation of petal primordia, the four stamen primordia appear in simultaneous order and alternate to the petals (Fig. 4G). There is no initiation of antepetalous stamen primordia; haplostemony results from organ absence from inception.

The carpel appears as a protuberance in the center of the floral meristem after stamen initiation, being the last organ to initiate (Fig. 4h). No buds with abortion or absence of carpels were observed. The cleft in the plicate carpel arises after the petals enclose the floral bud (Fig. 5i).

The decrease in the number of organs from the pentamerous to the tetramerous pattern occurs by an absence of organs from inception, i.e., there was no initiation of additional primordia of sepals, petals, or stamens with subsequent abortion.

Intermediate and final stages of development

The sepal primordia, already organized in a concentric ring, start their elongation, delimiting the floral meristem, which at this stage is rounded and prominently convex (Fig. 4e). The ring-like calyx elongates rapidly, forming irregular lobes without completely enclosing the floral bud at any stage (Fig. 4f–i, 5a–e). At the end of the elongation, a gamosepalous, campanulate calyx is formed, with acute and diminutive lobes (Fig. 5g). There are structures similar to glandular trichomes in the apical portions of sepals.

The petals begin their elongation early, with a pronounced inclination of their primordia towards the center of the floral meristem, exerting pressure on the floral meristem in the region between the primordia of antesepalous stamens (Fig. 5c). The petals then continue their elongation, unite and enclose the floral bud in valvate aestivation throughout the development of the floral bud (Fig. 5d, e). The same whorls Conspicuous papillae are formed mainly on the adaxial tip of the petals (Fig. 5f). The intertwining of these papillae helps to keep the floral bud tightly closed (Fig. 5g). At the end of development, various tector trichomes and stomata are formed on the abaxial side of the petals, distributed mainly in the apical region (Fig. 5g, h).

The androecium is formed only by the whorl of the antesepalous stamens. There are four stamens alternate with the petals that start to elongate after the elongation of the petals (Fig. 5i). Despite the common origin (same whorl) of the four stamen primordia, the floral buds show a very congested architecture, and two stamen primordia elongate and differentiate first, presenting their anthers tightly joined (Fig. 5j), and then the second pair elongates (Fig. 5k). At the end of development, the four stamens occupy the same circumference. The filaments are folded at pre-anthesis (Fig. 5l); at anthesis, the filaments extend and surpass the corolla, exposing the anthers (Fig. 1f, g). The differentiated anthers shelter pollen grains, which are aggregated in tetrahedral tetrads.

Only perfect flowers were found in the observed samples. In these flowers, the elongation and differentiation of the carpel lag behind those of the stamens, especially in relation the two first stamens that start to elongate (Fig. 5j, k). The carpel differentiates into a proximal ovary, a cylindrical style (Fig. 5m), and a terminal poriform stigma, in which one polyad fits (Fig. 5n).

Mimosa bimucronata (tetramerous, diplostemonous flowers)

Organography

The flowers are arranged in globose spike-like inflorescences. Each flower is inserted in the axil of an abaxial bract, without bracteoles. The flowers are white, predominantly tetramerous, diplostemonous, glabrous, and actinomorphic (Fig. S1f, g, 1 h-j). They are usually perfect, with few cases of staminate flowers (Fig. 1k). The calyx is gamosepalous, campanulate, ca. 0.7 ± 0.2 mm long, with irregular lobes and distally arranged trichomes (Fig. 1i); the calyx is in general tetramerous (Fig. 1h) and may present, in rare cases, five or six sepals. The corolla is tubular, ca. 2.2 ± 0.1 mm long, with erect lobes and papillae in the distal portion (Fig. 1i, j); like the calyx, the corolla is formed, in general, by four petals (Fig. 1h), rarely five or six petals. The androecium is the most conspicuous whorl, and it is formed by eight free stamens with white, linear, long filaments, ca. 7.2 ± 0.6 mm long, and orbicular, dorsifixed, bithecous anthers with longitudinal dehiscence (Fig. 1i, j). The gynoecium is monocarpellate, formed by a short stipe, ca. 0.2 ± 0.2 mm long, the ovary, ca. 1 mm long, covered by glandular trichomes and enclosing ca. eight ovules, a long and slender style, ca. 6.8 ± 0.1 mm long, and a terminal, poriform stigma (Fig. 1j).

Organogeny

The inflorescences are formed as an open reproductive system, i.e., indeterminate, with subunits of axillary inflorescences at different stages of development (Fig. 6a); each subunit is a globose spike-like inflorescence and develops in the axil of a bract of the first order (Fig. 6b). The floral meristem is rounded, 70–80 µm in diameter and inserted in the axil of an abaxial bract (Fig. 6c). The bract shows a pronounced elongation at the beginning of development, culminating in a robust, laterally expanded structure with apical projections and stomata located on the abaxial surface (Fig. 6b). There is no formation of bracteoles. The development of floral meristems in the inflorescence is synchronous (Fig. 6b), and the initiation of floral organ primordia in vertical succession is mixed acropetal, in the following sequence: sepals, petals, antesepalous stamens + carpel and antepetalous stamens.

Fig. 6
figure 6

Floral organogenesis in Mimosa bimucronata. a Apex of an indeterminate inflorescence, with developing inflorescence subunits. b Spike-like inflorescence subunit. Note the synchronous development of floral meristems. c Rounded floral meristem. d-e Reversed unidirectional order of sepal primordia initiation. f Simultaneous initiation of petal primordia. g Antesepalous stamen primordia initiation and inception of central carpel. h Antepetalous stamen primordia initiation. Note the carpel cleft formation (asterisk). Symbols: a = antepetalous stamen; A = antesepalous stamen; br = bract; c = carpel; fm = floral meristem; im = inflorescence meristem; is = inflorescence subunit; p = petal; s = sepal. Scale bars: a = 200 µm; b, g = 100 µm; c, h = 50 µm; d, e = 25 µm; f = 20 µm

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Sepal primordia are initiated in reversed unidirectional order and are four in number, rarely five or six. The first primordium starts in the adaxial position, followed by the lateral ones. Finally, the abaxial primordium appears (Fig. 6d). In tetramerous floral buds, two primordia are initiated in the median position in the sagittal plane, one adaxial and one abaxial, and two primordia are lateral (Fig. 6d, e). After all sepal primordia emerge, they unite, assuming the ring shape early in development (Fig. 6e).

Petal initiation begins simultaneously after the emergence and subsequent union of sepals (Fig. 6f). Four petal primordia (Fig. 6d, e), rarely five (Fig. 7f) or six (Fig. 7h), appear alternate to the sepals. In tetramerous floral buds, there are no median petals in the sagittal plane (Fig. 6f-h).

Fig. 7
figure 7

Intermediate and final stages of floral development in Mimosa bimucronata. a, b Campanulate calyx elongation. c Detail of a stoma on the adaxial surface of the calyx. d Detail of the sepal, showing trichomes on the apical edge. e–h Petals in valvate aestivation. Note the floral bud with four (e), five (f) and six sepals (h). g Detail of some stomata (circles) and spherical substances (arrows) on the adaxial surface of the petals. i Corolla elongation. Note the height proportion between calyx and corolla. j Detail of the papillae at the petal apex (arrowheads). k Beginning of the stamens elongation and carpel cleft formation (asterisk). l Staminate floral bud, showing the aborted carpel. m Filaments folded in pre-anthesis. n Ovary and basal portion of style. Note the trichomes on the ovary. o Dissected ovary, showing ovules arranged in two rows. p Polyad. q Trichomes with a glandular aspect on the ovary external epidermis (region referring to the dotted square of figure o). Symbols: a = antepatalous stamen; A = antesepalous stamen; an = anther; c = carpel; cl = aborted carpel; co = corolla; f = filament; k = calyx; ov = ovule; ovr = ovary; p = petal; s = sepal; stl = style; t = trichome. Scale bars: a, d, j = 50 µm; b, g, h, k = 100 µm; c = 20 µm; e, f, n–o = 200 µm; i, l–m = 500 µm, p = 5 µm; q = 10 µm

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After petal initiation, the floral meristem assumes a tetragonal shape (Fig. 6f). The stamens are initiated in two distinct whorls: first, the outermost antesepalous stamens (Fig. 6g), and then the innermost antepetalous stamens (Fig. 6h). In the antesepalous stamens, two primordia are in the abaxial position and two in the adaxial position. In contrast, in the antepetalous stamens, two primordia are lateral, and two are median, one abaxial and the other one adaxial (Fig. 6g, h).

The carpel arises as an individualized primordium in the center of the floral meristem, concurrent with the antesepalous stamens and before the arising of the antepetalous stamen primordia (Fig. 6g). The carpel cleft appears after the initiation of all floral organ primordia in an adaxial position and may be displaced to the right or left in some cases (Fig. 6h).

The shift from the pentamerous to the tetramerous pattern shows a reduction of organs from inception. No initiation of additional primordia of sepals, petals, antesepalous and antepetalous stamens followed by abortion was observed. Abortion of the carpel is the ontogenetic pathway that results in the few staminate flowers of the species (Fig. 7m).

Intermediate and final stages of development

The sepal primordia that merged, assuming a ring shape shortly after the initiation of the primordia (Fig. 6e), continue their elongation, forming a campanulate gamosepalous calyx, usually with four lobes (Fig. 7a, b). The calyx initially elongates and encloses the other floral organs still in development, but soon the corolla exceeds the calyx in elongation (Fig. 7e–i). In the intermediate and final stages of development, stomata are observed on the abaxial surface (Fig. 7c) and tector trichomes and trichomes with a glandular aspect on the apical edges of the sepals (Fig. 7b–d).

The petals elongate and unite, enclosing the floral bud in valvate aestivation. Petal elongation lags behind sepal elongation; however, at the end of development, the corolla is longer than the calyx, assuming a protective function for the inner floral organs (Fig. 7e–i). Conspicuous papillae form on the adaxial surface, mainly on the apical portion of the petals (Fig. 7j), intertwine and contribute to tightly enclosing the flower bud. At the end of development, numerous stomata are formed on the abaxial surface of the petals, distributed mainly in the apical portion. Spherical-shaped secretions are observed near the stomata (Fig. 7g).

The antesepalous stamens begin developing before the antepetalous ones, with the differentiation of the anthers and subsequent elongation of the filaments (Fig. 7l–m). After the formation of the carpel cleft, the antesepalous anthers differentiate (Fig. 7l), and the filaments subsequently elongate (Fig. 7m). Then, the anthers of the antepetalous stamens differentiate, and the filaments elongate, culminating in a single circumference of stamens. In pre-anthesis, the filaments are folded and perfectly fitted within the floral bud (Fig. 7m). With the floral opening, the filaments extend at anthesis, surpassing the corolla in length and exposing the anthers (Fig. 1i). The differentiated anthers shelter pollen grains, which are aggregated in polyads formed by eight pollen grains each (Fig. 7p).

In perfect flowers, the carpel's elongation occurs after the carpel cleft's closure; the long and slender style and the filaments of the stamens remain bent in the bud, extending in anthesis. At the end of development, the carpel is formed by a small stipe, the ovary covered by several trichomes with a tector and glandular aspect (Fig. 7q), sheltering ovules with marginal placentation (Fig. 7o), a long, cylindrical style (Fig. 7n, o), and a terminal stigma with a tiny orifice. In staminate flowers, the carpel primordium is aborted after the carpel cleft formation, and the primordium ceases its development, remaining as a rudimentary carpel (Fig. 7l).

Mimosa candollei (pentamerous, diplostemonous flowers)

Organography

The flowers of this species are arranged in globose spike-like inflorescences. Each flower is inserted in the axil of an abaxial bract. The flowers are pink, predominantly pentamerous, diplostemonous, with actinomorphic symmetry (Fig. S1i, 1 l, m). They are usually perfect (Fig. 1m, n), and rarely staminate (Fig. 1o). The calyx is gamosepalous and campanulate, ca. 0.5 ± 0.0 mm long, with short lobes (Fig. 1m), usually formed by five sepals. The corolla is glabrous, tubular, ca. 2 ± 0.1 mm long, with erect lobes, predominantly composed of five petals (Fig. ln), in rare cases, four or six petals. The petals are pink in the apical portion. The androecium, the most conspicuous whorl of the flower, is formed by ten free stamens, composed of a long, cylindrical, linear filament, ca. 6.1 ± 0.3 mm long, pink in color, and an orbicular, yellow, dorsifixed, bithecous anther with longitudinal dehiscence (Fig. 1m, n). The gynoecium is monocarpellate, formed by a short stipe, ca. 0.4 ± 0.1 mm long, the ovary, ca. 1.4 ± 0.1 mm long, a long, cylindrical, and glabrous style, ca. 3.4 ± 0.2 mm long, and a terminal, poriform stigma (Fig. 1n).

Organogeny

The inflorescences are presented as an open reproductive system. The apex is indeterminate, formed by an inflorescence meristem. The apical reproductive meristem forms subunits of axillary spike-like inflorescences, which develop in the axil of a bract (Fig. 8a, b). Each floral meristem is found in the axil of an abaxial bract, which elongates, protecting it (Fig. 8b). There is no formation of bracteoles. The development of the floral meristems in the inflorescence is synchronous (Fig. 8a, b), and the initiation of floral organ primordia in vertical succession is mixed acropetal in the following sequence: sepals, petals, antesepalous stamens, carpel, and antepetalous stamens.

Fig. 8
figure 8

Floral organogenesis in Mimosa candollei. a Apex of an indeterminate inflorescence, with inflorescence subunits. b Developing spike-like inflorescence subunit. Note the synchronous development of floral meristems. c Development of sepals, forming the precocious ring-like calyx. d Simultaneous initiation of petal primordia. Simultaneous initiation of antesepalous stamen primordia. f Carpel formation as a central protuberance. g Antepetalous stamen primordia and carpel cleft formation (asterisk). Sepals and petals partially removed. Symbols: a = antepetalous stamen; A = antesepalous stamen; br = bract; c = carpel; im = inflorescence meristem; is = inflorescence subunit; p = petal; s = sepal. Scale bars: a = 200 µm; b = 100 µm; c–g = 50 µm

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The sepal primordia unite and form a ring-like calyx very early, which makes it difficult to determine the order of initiation in this whorl (Fig. 8c) and, consequently, the position of the primordia in the sagittal plane at the time of initiation.

Petal primordia emerge in simultaneous order after the formation of the sepal ring (Fig. 8d). Predominantly, five primordia are initiated; at this stage, the buds assume a pentagonal shape. In pentamerous buds, the abaxial petal is the median in the sagittal plane (Fig. 8d, e).

The stamens arise in two whorls. The five antesepalous stamen primordia initiate in simultaneous order, after the initiation of the petals, opposite to the sepal primordia (Fig. 8e). The antepetalous stamen primordia appear after the formation of the carpel, interspersed with the antesepalous stamen primordia. The antepetalous stamens are the last organs to form on the floral meristem (Fig. 8g).

The carpel appears as a protuberance in the center of the floral meristem after initiation of the antesepalous stamen primordia and before the appearance of the antepetalous stamen whorl (Fig. 8f). The cleft in the plicate carpel forms adaxially after the formation of the antepetalous stamens (Fig. 8g).

Intermediate and final stages of development

The sepal primordia, already organized in a concentric ring, continue their elongation, delimiting the floral meristem, which at this stage is prominently convex (Fig. 8c). With elongation, irregular lobes are formed; however, the calyx does not entirely enclose the floral bud at any stage of development (Fig. 8d–f, 9a–c). The petal, antesepalous stamen, and carpel primordia begin partially exposed, covered only by the bracts and partly by the sepals (Fig. 8d–g). A glabrous, gamosepalous, campanulate calyx is formed at the end of elongation with small, slightly rounded lobes (Fig. 9c).

Fig. 9
figure 9

Intermediate and final stages of floral development in Mimosa candollei. a-c Calyx and corolla elongation. a Floral bud in frontal view. Note the irregular calyx lobes. b, c Valvate aestivation of petals. d Detail of the papillae (arrowheads) at the apex of three contiguous petals. e Differentiation of the antesepalous anthers. f Stamens elongation. Note one antepetalous elongating more than others from the same whorl almost matching in height to the antesepalous ones. Note the carpel cleft (asterisk). g Anthers differentiated and stigma (arrow) and style differentiation. h Style bends towards the adaxial region. Poriform stigma differentiation (arrow). i Staminate floral bud, showing the aborted carpel. j Folded filament in pre-anthesis. k Detail of tetrads. l Ovary, showing the trichomes on the surface. m Detail of a trichome with a glandular aspect. n Dissected ovary with visible ovules. Symbols: a = antepatalous stamen; A = antesepalous stamen; an = anther; c = carpel; cl = aborted carpel; f = filament; k = calyx; ov = ovule; ovr = ovary; p = petal; stl = style. Scale bars: as: a, e = 50 µm; b, d, f, g, i = 100 µm; c, j, l, n = 200 µm; h = 250 µm; k = 5 µm; m = 10 µm

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The petals elongate towards the center of the floral meristem. Their margins touch each other, enclosing the floral bud in valvate aestivation, protecting the organs of the innermost whorls still in development (Fig. 9b, c). The petals are united in the basal portion. Their lobes are free, but conspicuous papillae formed in the apical portion of the lobes, especially in the adaxial surface, strongly intertwine and contribute to enclosing the floral bud (Fig. 9d). At the end of development, the corolla presents at least three times the size of the calyx in length and closes the floral bud, becoming the protective whorl. Stomata can also be observed in the apical portion of the petal lobes, formed during elongation (Fig. 9d).

Anther differentiation and subsequent filament elongation occur first in the antesepalous stamens than in antepetalous stamens and soon after the beginning of carpel elongation in perfect flowers (Fig. 9e, f). The antepetalous stamens are the last organs to differentiate and elongate (Fig. 9f). In some flower buds, one or two antepetalous stamens elongate more than the other organs of the same whorl and come close in size to the antesepalous stamens; in these cases, the antesepalous stamens appear to be supernumerary in the intermediate stages (Fig. 9f), but at the end of development the ten stamens of the two whorls all occupy the same circumference (Fig. 9g, h). The filaments of all stamens are folded and perfectly joined until anthesis (Fig. 9j), when they extend and expose the anther, surpassing the perianth (Fig. 1m, n). The differentiated anthers shelter pollen grains, which are aggregated in tetrahedral tetrads (Fig. 9k).

In perfect flowers, the central carpel begins its elongation before the stamens (Fig. 9e), as the carpel cleft closes and the terminal style and the stigma differentiate (Fig. 9g, h). At the beginning of the elongation, the style bends towards the adaxial region (Fig. 9h) and remains bent until anthesis (Fig. 9n). At the end of development, the carpel is formed by a small stipe, the ovary covered by several epidermal appendages with a glandular aspect arranged in two rows close to the suture region (Fig. 9l, m) usually enclosing 24 ovules in two rows, with marginal placentation (Fig. 9n), a long and cylindrical style and a terminal poriform stigma (Fig. 9n). In staminate flowers, the carpel primordium forms a cleft. Right after the formation of the carpel cleft, the development is interrupted, and the organ is aborted, culminating in a rudimentary carpel (Fig. 9i).

Discussion

Mimosa is an interesting genus in terms of floral development, particularly due to the marked interspecific and intraspecific merism variation, ranging from dimery to hexamery, in addition to the coexistence of staminate and perfect flowers in the same plant (Barneby 1991), characterizing andromonoecy (Richards 1997). Therefore, elucidating the ontogenetic processes that lead to these patterns may be relevant to understanding floral evolution in the genus and especially shed light on merism instability in the mimosoid clade, one of the most striking features of this clade. The mimosoids is a clade of Caesalpinioideae with a tropical distribution (Koenen et al. 2020), that due to a set of peculiar floral characteristics, such as radial symmetry, valvate aestivation of the petals, presence of polyandry and polycarpelly (Koenen et al. 2020; LPWG 2017; Paulino et al. 2014; Tucker 2003), used to be ranked as a subfamily (LPWG 2013, 2017). Some of the characteristics that deserve mention are (i) the number of floral parts per whorl, (ii) the degree of fusion in the whorls (Koenen et al. 2020; Pedersoli et al. 2023), and (iii) the presence of merism instability, already reported for some taxa such as Mimosa, Senegalia, Acacia, Inga, and Parkia (Barneby 1991; Paulino et al. 2017; Prenner 2011; Anderson Javier Alvarado Reyes, personal communication; Renan Siqueira Moraes, personal observation). The merism instability is one of the most intriguing and little explored traits regarding floral development.

Ontogenetic pathways leading to merism variation and expression of the andromonoecious sexual system in Mimosa

The analysis of floral development revealed two distinct ontogenetic pathways leading to the vastly diverse floral morphology in Mimosa, culminating in an atypical floral construction concerning the pattern of Fabaceae flowers (LPWG 2017). The perianth, recorded as trimerous, tetramerous, pentamerous, and more rarely hexamerous, has the pentamerous condition considered as plesiomorphic, based on comparisons with the outgroup, the Piptadenia group, all of which have five organs per perianth whorl (Barneby 1991; Simon et al. 2011). The floral development of Mimosa species analyzed in this study, including the trimerous, tetramerous, and pentamerous conditions, revealed that the wide merism variation in this genus results from the absence of organs from inception. Intraspecific variation is also a result of this ontogenetic pathway. In dimerous, trimerous, or tetramerous flowers, the remaining organ primordia expected in the calyx, corolla, and androecium whorls are not initiated. Considering the pentamerous pattern as a plesiomorphic condition (Sinjushin 2021) and the high lability of merism in this genus, further studies focusing on floral vascularization may broaden our understanding of floral evolution in this group.

The shape and size of the petal and stamen primordia at the time of their initiation, especially in the predominantly trimerous species, M. caesalpiniifolia, merit attention. These primordia initiate in an elliptical shape, expanded laterally, occupying a greater extension of the floral meristem at their emergence. According to Ronse de Craene (2016), larger primordia in the perianth reduce the available space in the floral meristem and can reduce the number of organs. Interestingly, M. caesalpiniifolia, whose primordia exhibit more clearly the elliptical shape, shows both more pronounced reduction, predominantly trimerous, and greater instability in the number of organs per whorl, ranging from dimery to hexamery. Furthermore, in this species, the number of organs initiated in the outermost whorls does not act as strongly on standardizing the number of organs initiated in the subsequent inner whorls. Therefore, we hypothesize that reducing the number of primordia initiated in the meristem makes room for an oscillation in primordium shapes and sizes, which may lead to fluctuation in the number of organs initiated.

The absence of carpels in staminate flowers observed in M. caesalpiniifolia, M. bimucronata, and M. candollei is due to the ontogenetic pathway of organ abortion, with the formation of a carpel rudiment in the center of the flower. The monocarpellate gynoecium is a relatively conserved feature in Fabaceae, in addition to the rare polycarpellate flowers (Acacia celastrifolia Benth.—Prenner 2011; Inga bella M. Sousa, I. congesta T.D. Penn., I. hispida Schott ex Benth., I. grandis T.D. Penn. and I. feuillei DC. – Paulino et al. 2014), we also have rare cases of carpel abortion in Mimosa and other taxa of the mimosoid clade: Neptunia pubescens Benth. (Tucker 1988), Parkia multijuga Benth., Stryphnodendron adstringens (Mart.) Coville (Pedersoli and Teixeira 2016), Mimosa caesalpiniifolia, M. bimucronata, M. candollei – (present study); and Acacia berlandieri Benth. (Goméz-Acevedo et al. 2007), and more rarely in members of other subfamilies, such as Apuleia leiocarpa (Vogel) J.F. Macbr.—Dialioideae (Falcão et al. 2020), and Cordyla pinnata (A. rich.) Milne-Redh. (Papilionoideae), a species with actinomorphic flowers (Sinjushin 2018). It is noteworthy that supernumerary gynoecia, generally, linked with polyandrous androecia in Mimosoids seems to be associated with an expansion in meristem size (i.e., ring meristem formation), enabling the emergence of new organs (Paulino et al. 2014; 2017; Ronse de Craene 2016; Tucker 2003).

Floral development elucidates the modification from diplostemonous to haplostemonous flowers in Mimosa

Haplostemonous flowers in Mimosa are predicted to have evolved from a diplostemonous androecium, the predominant pattern in the Piptadenia group (outgroup) and the whole family. Therefore, haplostemony is considered the apomorphic state (Barneby 1991). The diplostemonous condition predominates in Mimosa, with changes to haplostemonous flowers hypothetically occurring at least six times (Simon et al. 2011). The haplostemonous condition observed in M. pudica (present work) results from the absence of antepetalous stamens since inception. It is worth mentioning that unlike what was observed for diplostemonous species, in M. pudica, the petal primordia bend towards the center of the floral meristem very early, compressing the region where the antepetalous stamen primordia are supposed to emerge. Therefore, the haplostemony in this species could result from the mechanical pressure exerted by the petals on the floral meristem at the beginning of their elongation. The developing floral meristems are subjected to pressures caused by organs of the adjacent whorl or external to the floral bud, and these pressures often leave marks. Imprinted shapes are the shapes caused by pressure from contiguous organs. Such pressures can change the autonomous shape of an organ that would develop without physical influence from adjacent organs (Endress 2008). Similar patterns can be observed in Astragalus (Fabaceae) species, in which the pressure of the bracts influences the sequence of sepal initiation, with the first sepals appearing preferentially where pressure is lower (Naghiloo et al. 2012). Increased pressure throws the regularity of flowers into disarray and can even lead to disrupted organ development (Ronse de Craene 2018).

It is interesting to note that haplostemonous flowers are quite common in the Mimosa genus, while diplostemonous flowers exhibit greater variability in the androecium (Barneby 1991; Ramírez-Domenech and Tucker 1989; Wyatt and Lipow 2021). The androecium, being the most variable whorl in flowers, tends to show more variability when the number of stamens increases. Our comparative study of species, such as M. caesalpiniifolia, M. bimucronata, M. candollei (which exhibit diplostemonous flowers), and M. pudica (with haplostemonous flowers) shows that the trimerous-diplostemonous M. caesalpiniifolia (lower merism) showed greater merism lability, while the tetramerous-haplostemonous M. pudica showed a lower merism lability and was the only species where carpel abortion was not observed (see Table 2). However, Barneby (1991) reported the presence of staminate flowers in M. pudica, indicating that while rare, the condition is not entirely absent compared to other species.

Table 2 Comparative survey of floral development characters in species of the mimosoid clade (Caesalpinioideae subfamily)
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Merism lability in the mimosoid clade: inflorescence structure, floral symmetry, and meristem size

Merism or merosity is one of the most exciting aspects of floral architecture as it is intricately linked to the floral bauplan, symmetry, and phyllotaxis (Ronse de Craene 2022). Angiosperms commonly have fixed numbers: trimerous flowers in monocots and pentamerous or tetramerous flowers in eudicots (Pentapetalae) (Ronse de Craene 2016, 2022). However, because it is rare, merism variation has attracted the attention of researchers. It has been widely reported in the literature in different families of flowering plants, such as Polemoniaceae (Ellstrand and Mitchell 1988), Araliaceae (Nuraliev et al. 2010, 2014), Anonnaceae (Xu & Ronse de Craene 2010), Burseraceae (Daly et al. 2011), Melastomataceae (Wanntorp et al. 2011), Menispermaceae (Meng et al. 2012), Moraceae (Leite et al. 2018), Lythraceae (Sinjushin and Ploshinskaya 2020) and, in some cases, Fabaceae (present study, Falcão et al. 2020; Mansano et al. 2002; Paulino et al. 2013, 2017; Pedersoli and Teixeira 2016; Sinjushin 2023; Sinjushin and Karasyova 2017; Tucker 1991). Besides the variations among species of the same group, intraspecific instability and even variation in flowers of a single inflorescence are notorious. This has been, therefore, a recurrent subject studied from different perspectives (Leite et al. 2018; Ronse de Craene 2016; Ronse de Craene and Smets 1994; Specht and Bartlett 2009). In this context, the mimosoid clade is a morphologically inclusive group encompassing most species with actinomorphic flowers of Fabaceae (LPWG 2017) but with species showing significant variations in merism in flowers of the same inflorescence (see Table 2). Thus, in the present study, using the variation in Mimosa as a model, we will compare and discuss the possible causes that lead to the high instability in the mimosoid clade.

It is important to stress that species of the mimosoid clade mostly present flowers arranged in capitula, spikes, glomerules, and less frequently in umbels (see Table 2) (LPWG 2017; Queiroz 2009). These types of inflorescences usually have large numbers of small, sessile, radial flowers, and the flowers are consequently very congested, a condition that may be associated with high merism instability in each inflorescence due to ontogenetic restriction, limiting the development of the flowers, as previously reported in Hydrangea paniculata Siebold and Sedum maximum Hoffm. (Sinjushin 2023). In Urticaceae species, the reduction from pentamery to tetramery, especially in staminate flowers (Pedersoli et al. 2022), may be related to compact cymose inflorescences (Dong 2016). In more extreme cases, spatial restriction associated with inflorescence architecture and variation in the timing of initiation of floral apices (heterochrony) in pseudoracemes of Pongamia pinnata (L.) Pierre and Clitoria fairchildiana R.A. Howard (Papilionoideae) lead to limited growth of the central short-shoot flower (Tucker 1987a, b). Such reports reinforce the hypothesis that physico-dynamic forces (Ronse de Craene 2018) exerted by inflorescence architecture drive the lability of the floral merism in some species.

The plane of floral symmetry in Mimosa and the mimosoid clade, in general, is one of the most notable features of these plants, as all its members share it (LPWG 2017; Tucker 2003). Floral symmetry emerged in different lineages independently, and it is postulated that zygomorphic symmetry was derived from actinomorphic symmetry (Endress 2001; Spencer and Kim 2018). Zygomorphic flowers, associated with higher specificity in pollination events, are considered to have a more stable merism than generalist actinomorphic flowers (Endress 2006; Ronse de Craene 2016; Sinjushin and Karasyova 2017). As previously mentioned, actinomorphy is a striking floral characteristic in flowers of the mimosoid clade, which indicates that this condition may be strongly associated with the high instability in merism (Sinjushin 2023), which is reported more frequently in taxa of this clade compared to other taxa of Fabaceae (present work; Barneby 1991; Paulino et al. 2017; Pedersoli and Teixeira 2016; Prenner 2004, 2011; Tucker 1988; Anderson Javier Alvarado Reyes, personal communication; Renan Siqueira Moraes, personal observation).

In addition to pressures mediated by pollination processes, genetic factors controlling floral morphology must be considered. The genes that establish floral symmetry have been extensively studied in model species of Antirrhinum spp. (Corley et al. 2005; Galego and Almeida 2002; Luo et al. 1996; Preston and Hileman 2009). Genes such as CYCLOIDEA (CYC) and DICHOTOMA (DICH), RADIAL (RAD) determine the identity of the adaxial petal, which is responsible for establishing zygomorphic symmetry, while DIVARICATA (DIV) determines the identity of the abaxial petal (Corley et al. 2005; Galego and Almeida 2002; Luo et al. 1996; Spencer and Kim 2018). In mutants of Antirrhinum majus L., the lack of expression of genes responsible for the identity of the adaxial petal results in ventralization, i.e., all petals have a ventral identity, with change from zygomorphic symmetry to an exceptionally actinomorphic symmetry (Almeida et al. 1997; Luo et al. 1996). Given this finding, Sinjushin and Karasyova (2017) suggest that the adaxial domain of the floral meristem needs to be better defined in actinomorphic flowers. However, further studies related to the expression of CYC or similar genes are needed to assess whether these genes are involved in determining merism stability in zygomorphic flowers. Such a finding would provide strong evidence supporting the hypothesis that merism instability is related to the actinomorphic condition in flowers of the mimosoid clade.

As previously mentioned, the genus Mimosa is an excellent model for studies of ontogenetic processes that lead to variation in the floral merism variation, as it exhibits remarkable fixed and labile merism variation, in particular, of the perianth, with species ranging from predominantly trimerous to predominantly hexamerous (Barneby 1991; Simon et al. 2011). In this sense, the size of the floral meristem is a factor that deserves attention because, according to Ronse de Craene (2016), this feature associated with the size of floral organs can influence the merism and the arrangement of the whorls. Relatively larger meristems likely have room to originate more organs, while relatively smaller meristems may cause a reduction in the number of the initiated organs (Ronse de Craene 2016; Sinjushin 2021). Therefore, the size of the floral meristem associated with the arrangement of inflorescences in Mimosa, and even in the mimosoid clade, may be determinant to explain the high interspecific and intraspecific variation.

It is noteworthy that species with higher variation in meristem size, such as those of Mimosa (50—80 µm; present work; Ramírez-Domenech 1989; Ramírez-Domenech and Tucker 1989), Calliandra angustifolia Spruce ex Benth. (50—70 µm; Prenner 2004) and the species N. pubescens (50—80 µm; Tucker 1988), which produce spike-like inflorescences, also show high merism instability: meristems measuring 50 µm would be able to produce a lower number of organs, while meristems measuring 70 µm enabled the development of more organs. In Gleditisia spp., a basal genus of Caesalpinioideae, the terminal floral meristem in inflorescences had a larger diameter (150–160 µm) than the lateral floral meristems (95–150 µm), and the terminal flowers were commonly pentamerous, while the lateral flowers varied from pentamerous to trimerous (Tucker 1991). In species with supernumerary floral organs, such as Inga species, the subsequent expansion of the meristem results in polyandry and polycarpelly, and the initial size of the floral meristem is relatively large (75—195 µm), likely favoring the greater number of perianth organs (Paulino et al. 2014; 2017). Genes that act in the regulation of meristem size may be vital to understanding merism evolution (Specht and Barlett 2009) since genes such as ULTRAPETALA associated with the CLAVATA pathway are responsible for regulating floral meristem size (Carles et al. 2004; Ronse de Craene 2016; Specht and Barlett 2009). The ULT1 mutants enable the increase in the number of floral organs occasioned by the increase in floral meristem size (Carles et al. 2004; Spetch and Barlett 2009). Thus, the increase in the number of organs in different whorls is not yet fully understood, but meristem size variation seems to be a plausible explanation and deserves more attention (Ronse de Craene 2016; Sinjunshin 2023). We stress, therefore, that additional studies focusing on meristem size are needed to formulate well-founded hypotheses.

In the Xylia clade (Koenen et al. 2020), the first to diverge within mimosoid, no instability in the perianth has been reported (Barros et al. 2017; Ramírez-Domenech and Tucker 1990), but the abortion of the antepetalous stamens in Pentaclethra macroloba (Willd.) Kuntze led to a reduction in the merism of the androecium (Barros et al. 2017), which may suggest two events: (i) that the high merism lability in the perianth initiates in clades that diverged later, such as the Dychrostachys clade (Koenen et al. 2020); and (ii) that the merism of the androecium varies from at an early stage in the evolution of Mimosoids. Therefore, a phylogenetic reconstruction study is needed to better understand the lability of distinct whorls in the mimosoid clade. Furthermore, the Parkia clade (Koenen et al. 2020) and the Stryphnodendron clade (Koenen et al. 2020) have shown moderate lability compared to the other clades. On the other hand, the Mimosa clade (Koenen et al. 2020) and the Inga clade (Koenen et al. 2020) exhibit greater instability, showing high lability in all floral whorls (Barneby 1991; Paulino et al. 2017).

Thus, there is notable evidence that advancing studies exploring the merism instability in reproductive structures, specifically focusing on floral ontogeny and control of gene interactions in the mimosoid clade, which has great ecological relevance, can enhance our knowledge about the high lability in flowers of the same inflorescence observed in this clade.

Potential ontogenetic synapomorphies for the mimosoid clade

Papillae on the adaxial and apical surfaces of the petals are essential for efficient flower bud closure, as the petals surpass the developing sepals and enclose the flower bud, becoming the main protective organs. Besides the presence in Mimosa species (present work; Ramırez-Domenech and Tucker 1990), apical papillae have been documented in Acacia Mill. (Gómez-Acevedo et al. 2007; Prenner 2011), Inga Mill. (Paulino et al. 2017), C. angustifolia (Prenner 2004), N. pubescens (Tucker 1988), P. multijuga, Stryphnodendron astringens (Pedersoli and Teixeira 2016), and several other taxa of the mimosoid clade (Ramırez-Domenech and Tucker 1990; Pedersoli et al. 2023) (see Table 2), indicating that the mechanism of corolla closure by papillae or long unicellular trichomes is a potential developmental trait shared by species of the mimosoid clade.

The position of the median sepal and petals in the sagittal plane, commonly classified as the adaxial sepal and abaxial petal, opposite to the pattern reported for other species of Fabaceae (Tucker 2003), is not an entirely homogeneous feature in the group. Many species follow this pattern, such as Adenanthera microsperma Teijsm. & Binn. (Domenech-Ramirez and Tucker 1990), Parkia platycephala Benth. (Personal observation), P. multijuga, Stryphnodendron astringens (Pedersoli and Teixeira 2016) and Mimosa candollei (5-merous species; present work). However, with the advancement of floral morphology and development studies, many exceptions have been reported, such as Inga congesta, I. grandis (Paulino et al. 2017), and Senegalia polyphylla (DC.) Britton (Anderson Javier Alvarado Reyes, personal communication) (see Table 2). In addition, other caesalpinioids with polysymmetric flowers, such as Ceratonia siliqua L. and Erythrophleum suaveolens (Guill. & Perr.) Brenan are also unusual in having the median sepal on the adaxial side (Tucker 1992b). Therefore, as previously suggested, this is not a potential synapomorphy for the group (Tucker 2003). Furthermore, ontogenetic data have shown that, due to the reduction or even instability in merism, the position of the median sepal and petal in the sagittal plane can be very variable.

Conclusion

We conclude that the variation in perianth and androecium merism in Mimosa results from the ontogenetic pathway of organ absence since inception, while the presence of staminate flowers, without developed carpels in some floral morphotypes of Mimosa, results from the ontogenetic pathway of carpel abortion. In a broader perspective, the merism lability in the mimosoid clade, one of the most intriguing features of eudicot flowers, is further associated with a set of shared morphological characteristics, such as the production of tight inflorescences of the capitulum, spike, glomerule, and umbel types with rather congested, floral meristem size variations, and actinomorphic flowers.

We also highlight that the genus Mimosa, whose merism variations and other alterations related to the organization of floral architecture can be readily recognized, is an excellent model for ontogenetic studies to understand the diversity and evolution of floral morphology. Studying this genus can be especially important because the floral uniformity seen in most Fabaceae species, which share the organization of floral organs in five sepals, five petals, ten stamens, and one carpel, totaling 21 organs (Tucker 2003), does not apply to species of the mimosoid clade.

Our data and literature showed that understanding ontogenetic processes is crucial to broaden our understanding of the floral evolution in Mimosa and shedding light on the unstable condition of merism in the mimosoid clade. The merism variation in Mimosa seems to be primarily associated with space availability, nonetheless the intriguing floral variation and merism lability in the mimosoid clade requires further ontogenetic studies. Although a few taxa of the mimosoid clade have been studied it is necessary to increase the sampling examined to better understand the floral evolution of the group.