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Plant Systematics and Evolution

, Volume 304, Issue 5, pp 665–677 | Cite as

Seed micromorphology of Orchidaceae in the Gulf of Guinea (West Tropical Africa)

  • Roberto Gamarra
  • Emma Ortúñez
  • Pablo Galan Cela
  • Ángela Merencio
Original Article
  • 159 Downloads

Abstract

Qualitative micromorphological characters of seeds were analysed using Scanning Electron Microscopy for 95 species of 34 genera, belonging to the subfamilies Vanilloideae, Orchidoideae and Epidendroideae (Orchidaceae) from the Gulf of Guinea countries. Three genera (Dinklageella, Platylepis and Rhipidoglossum) and 69 species were studied for the first time. Vanilla shows a pattern of seed coat distinct to the rest of the studied taxa. Orchidoideae and Epidendroideae show variation in several features: seed shape, orientation of testa cells, morphology of anticlinal walls, cell corners, shape and ornamentation of the periclinal walls and presence of waxes. Two main patterns have been observed: one with parallel medial cells along the longitudinal axis, thin and straight anticlinal walls, with no raised and no thickened cell corners, wide periclinal walls and lack of waxes, for the terrestrial orchids of the subfamily Orchidoideae, the tribes Tropidieae, Nervilieae, Collabieae, Cymbidieae and the subtribe Malaxidinae (Epidendroideae); the other one showed twisted medial cells, thickened anticlinal zone with prominent ridges, raised and thickened cell corners, narrow-to-not visible periclinal walls and presence of waxes, in epiphytic orchids of Vandeae, Podochileae and Dendrobiinae (Epidendroideae). In the epiphytic genera Ansellia and Graphorkis (Cymbidieae) and in Liparis epiphytica (Malaxideae), thickened anticlinal walls and presence of waxes also occurred but parallel medial cells along the longitudinal axis and wide periclinal walls have been observed. This study confirms the diagnostic value of the qualitative features of the seed coat in Orchidaceae and suggests a probable relation with adaptations to habitat for seed dispersal.

Keywords

Anticlinal walls Periclinal walls Qualitative characters Seed coat SEM Waxes 

Introduction

Orchidaceae is one of the largest families of vascular plants in tropical Africa, of about 1500 species (Cribb and Govaerts 2005). In the Gulf of Guinea countries (Nigeria, Cameroon, Equatorial Guinea, Sao Tomé and Principe, Gabon and Congo), 569 species and 60 genera have been recognised (Govaerts et al. 2017), which following the taxonomical scheme of Chase et al. (2015) are placed into 13 tribes: Vanilleae (subfamily Vanilloideae), Cranichideae and Orchideae (Orchidoideae), Neottieae, Tropidieae, Triphoreae, Gastrodieae, Nervilieae, Malaxideae, Cymbidieae, Collabieae, Podochileae and Vandeae (Epidendroideae).

Along this region, there is a wide diversity of vegetation types, including one of the largest remaining continuous moist tropical forest, and other communities as grasslands or afro-alpine vegetation (White 1983). Two main orchid habits are recognised in this area: terrestrial (including climbing and mycoheterotrophic) and epiphytic (sometimes lithophytic), with 227 and 342 species, respectively (Govaerts et al. 2017).

Orchid seeds are extremely small and numerous. They comprise a testa for protection and a simple embryo (Molvray and Chase 1999). Previous studies on seed coat in Orchidaceae have demonstrated their diagnostic value (Clifford and Smith 1969; Barthlott 1976; Barthlott and Ziegler 1981; Ziegler 1981; Tohda 1985; Chase and Pippen 1988; Molvray and Kores 1995; Cameron and Chase 1998; Arditti and Ghani 2000). Nowadays, the knowledge about seeds in Orchidaceae was summarised by Barthlott et al. (2014).

Studies on seed micromorphology in Africa are scarce (Kurzweil 1993). In tropical areas of America and Asia, the analysis of seed morphology has demonstrated the importance of seed coat, related to the taxonomy and ecology of some orchids (Barthlott and Ziegler 1980; Ziegler 1981; Chase and Pippen 1988, 1990; Swamy et al. 2004, 2007; Tsutsumi et al. 2007; Verma et al. 2012; Chaudhary et al. 2014; Zhang et al. 2015).

During our research, we have analysed the testa cells in orchids of the northern hemisphere using Scanning Electron Microscopy (SEM) (Gamarra et al. 2007, 2008, 2010, 2012, 2015; Galán Cela et al. 2014), demonstrating the importance of the qualitative characters. Nowadays, within the revision of the family Orchidaceae for the project “Flora de Guinea Ecuatorial”, we have extended our research on seed micromorphology to the taxa which occur in West Tropical Africa.

The aim of this study was to analyse the qualitative micromorphological features of the seed coat in taxa from the Gulf of Guinea countries (Nigeria, Cameroon, Equatorial Guinea, Sao Tomé and Principe, Gabon and Congo), belonging to the subfamilies Vanilloideae, Orchidoideae and Epidendroideae, and provide new insights to identify diagnostic characters that could contribute to understand the ecology of the orchids. Qualitative data of seed coat of the genera Dinklageella, Platylepis and Rhipidoglossum, and of 69 species of different tribes were carried out for the first time.

Materials and methods

We have analysed seeds of 95 species belonging to 34 genera of the subfamilies Vanilloideae, Orchidoideae and Epidendroideae (Table 1), according to the taxonomical scheme of Chase et al. (2015).
Table 1

Taxonomic groups and genera according to Chase et al. (2015)

Subfamily

Tribe

Subtribe

Genus

Plant habit

Species examined/recognised in the study area

Vanilloideae

Vanilleae

 

Vanilla

Ts

1/13

Orchidoideae

Cranichideae

Cranichidinae

Cheirostylis

T

2/2

   

Platylepis

T

1/2

   

Zeuxine

T

2/4

  

Manniellinae

Manniella

T

1/2

 

Orchideae

Brownleeinae

Disperis

T

2/11

  

Orchidinae

Brachycorythis

T

5/10

   

Cynorkis

T

2/3

   

Habenaria

T

13/65

   

Holothrix

T

1/3

   

Platycoryne

T

3/6

Epidendroideae

Tropidieae

 

Corymborkis

T

1/2

 

Nervilieae

Nerviliinae

Nervilia

T

2/6

  

Epipogiinae

Epipogium

T

1/1

 

Malaxideae

Dendrobiinae

Bulbophyllum

E

5/60

   

Genyorchis

E

1/8

  

Malaxidinae

Liparis

T/E

5/17

   

Malaxis

T

2/3

   

Orestias

T

1/4

 

Cymbidieae

Eulophiinae

Eulophia

T

6/31

   

Oeceoclades

T

1/3

   

Ansellia

E

1/1

   

Graphorkis

E

1/1

 

Collabieae

 

Calanthe

T

1/1

 

Podochileae

 

Stolzia

E

1a/4

 

Vandeae

Polystachyinae

Polystachya

E(L)

23/76

  

Angraecinae

Ancistrorhynchus

E

1/13

   

Angraecopsis

E

1/9

   

Bolusiella

E

2/5

   

Calyptrochilum

E

2/2

   

Diaphananthe

E

2/25

   

Dinklageella

E

1/3

   

Rhipidoglossum

E

1/16

   

Solenangis

E

1/3

E epiphyte, L lithophyte, T terrestrial, Ts terrestrial scandent

aAnalysed material from species outside the study area

Seeds were removed from mature capsules of two sources: field collections in Equatorial Guinea and herbarium specimens (K, MA, WAG, acronyms according to Thiers 2017) from the study area (Nigeria, Cameroon, Equatorial Guinea, Sao Tomé and Principe, Gabon and Congo). When it was not possible, seeds of specimens from other countries of tropical Africa were provided; only in the genus Stolzia (Epidendroideae), the studied material belongs to a species outside of this area. A list of voucher specimens and localities is given in Appendix, with scientific names according to Govaerts et al. (2017).

The samples were mounted on SEM stubs and coated with gold in a sputter-coater (SEM Coating System, Bio-Rad SC 502). More than 15 seeds of each sample were examined with a Philips XL30, with a filament voltage of 20 kV. Qualitative data such as seed shape, morphology and orientation of testa cells, and several features (morphology, ornamentation) of the anticlinal and periclinal walls were analysed, and selected images were recorded.

The terminology and methods were adopted from Arditti et al. (1979) and Barthlott et al. (2014) for seed characters, and Barthlott et al. (1998) for epicuticular waxes.

Results

Seeds of the studied orchids show diversity of qualitative micromorphological characters.

Ovoid seeds with thickened, hard and warty seed coat (Fig. 1a) and not visible cell borders (Fig. 1b) were found only in Vanilla imperialis (Vanilloideae).
Fig. 1

Vanilla imperialis: a seed morphology; b detail of the seed coat. Scale bars: a 300 µm; b 100 µm

Main differences were found in the subfamilies Orchidoideae and Epidendroideae, regarding seed shape, orientation of testa cells, morphology of anticlinal walls (longitudinal and transversal), cell corners, shape and ornamentation of the periclinal walls and presence of waxes (Table 2).
Table 2

Main micromorphological characters in the studied genera of the subfamilies Orchidoideae and Epidendroideae

Subfamily

Tribe

Subtribe

Genus

Seed shape

Orient. testa cells

Long. anticl. walls

Transv. anticl. walls

Cell corners

Pericl. walls

Ornament. pericl. walls

Waxes

Orchidoideae

Cranichideae

Cranichidinae

Cheirostylis

Platylepis

Zeuxine

Filiform

Parallel

Thin

Straight

With intercellular gaps

Wide

Smooth

Absent

Manniellinae

Manniella

Filiform

Parallel

Thin

Straight

With intercellular gaps

Wide

Smooth

Absent

Orchideae

Brownleeinae

Disperis

Fusiform

Parallel

Thin

Straight

No raised, no thickened

Wide

Smooth

Absent

Orchidinae

Brachycorythis

Cynorkis

Habenaria

Holothrix

Platycoryne

Fusiform to filiform, sometimes clavate or globose in Brachycorythis and Habenaria

Parallel

Thin

Straight

No raised, no thickened

Wide

Smooth, sometimes reticulated in Holothrix

Absent

Epidendroideae

Tropidieae

 

Corymborkis

Filiform

Parallel

Thin

Straight

No raised, no thickened

Wide

Smooth

Absent

Nervilieae

Nerviliinae

Nervilia

Fusiform

Parallel

Thin

Straight

With intercellular gaps

Wide

Smooth

Absent

Epipogiinae

Epipogium

Globose

Parallel

Thin

Straight

No raised, no thickened

Wide

Smooth

Absent

Malaxideae

Dendrobiinae

Bulbophyllum

Genyorchis

Fusiform

Slightly twisted

Thickened with prominent ridges in adhesion zone

Curved

Raised and thickened

Narrow or not visible

Smooth

Present

Malaxidinae

Liparis

Malaxis

Orestias

Fusiform, globose in L. epiphytica

Parallel

Thin (thickened in L. epiphytica)

Straight

No raised, no thickened (except L. epiphytica)

Wide

Smooth, reticulated in L. epiphytica

Absent (except in L. epiphytica)

Cymbidieae

Eulophiinae

Eulophia

Oeceoclades

Fusiform

Parallel

Thin

Straight

No raised, no thickened

Wide

Thin and transversal ridges

Absent

Ansellia

Graphorkis

Fusiform Pyriform

Parallel

Thickened

Straight

Raised and thickened

Wide

Thick and longitudinal ridges

Present (wax caps)

Collabieae

 

Calanthe

Filiform

Parallel

Thin

Straight

No raised, no thickened

Wide

Reticulated

Absent

Podochileae

 

Stolzia

Fusiform

Slightly twisted

Thickened with prominent ridges in adhesion zone

Curved

Raised and thickened

Narrow or not visible

Smooth

Present

Vandeae

Polystachyinae

Polystachya

Fusiform

Strongly twisted

Thickened with prominent ridges in adhesion zone

Curved

Raised and thickened

Narrow or not visible

Smooth

Present

Angraecinae

Ancistrorhynchus

Angraecopsis

Bolusiella

Calyptrochilum

Diaphananthe

Dinklageella

Rhipidoglossum

Solenangis

Fusiform to filiform

Slightly to strongly twisted

Thickened with prominent ridges in adhesion zone

Curved

Raised and thickened

Narrow or not visible

Smooth

Present

Seed shape was mostly filiform (Fig. 2a) to fusiform (Fig. 2b) in the studied taxa. Clavate to globose seeds (Fig. 2c) were also observed in Brachycorythis and Habenaria (Orchideae, Orchidoideae), Epipogium (Nervilieae, Epidendroideae) and Liparis epiphytica (Malaxideae, Epidendroideae). In the genus Graphorkis (Cymbidieae, Epidendroideae) seeds were pyriform in shape (Fig. 2d).
Fig. 2

Seed shape. Dinklageella liberica: a filiform seed. Polystachya fusiformis: b fusiform seed. Liparis epiphytica: c globose seed. Graphorkis lurida: d pyriform seed. Scale bars: a 300 µm; b, c 50 µm; d 100 µm

Medial cells of the seed coat were quadrangular to rectangular or elongated (Fig. 3a‒c). Parallel testa cells along the longitudinal axis (Fig. 3d) were observed in the tribes Cranichideae and Orchideae (subfamily Orchidoideae), in Tropidieae, Nervilieae, Malaxideae (Malaxidinae), Cymbidieae and in Calanthe sylvatica (Collabieae) within the subfamily Epidendroideae. Twisted testa cells along the longitudinal axis were common in subtribe Dendrobiinae (Malaxideae), Podochileae and Vandeae, varying in the degree of torsion from slightly (Fig. 3e) in Dendrobiinae, Podochileae, and in the genera Ancistrorhynchus, Dinklageella and Bolusiella (Vandeae), to strongly (Fig. 3f) in the rest of the tribe Vandeae.
Fig. 3

Morphology and orientation of testa cells. Platylepis glutinosa: a quadrangular cells with intercellular gaps (arrow) at the cell corners. Nervilia bicalcarata: b rectangular cells with intercellular gaps (arrow) at the cell corners. Eulophia cucullata: c elongated cells. Eulophia bouliawongo: d parallel testa cells. Bulbophyllum falcatum: e slightly twisted testa cells. Diaphananthe bidens: f strongly twisted testa cells. Scale bars: a 30 µm; b 20 µm; c, e 50 µm; d 300 µm; f 100 µm

Testa extensions were observed in several taxa of the tribe Vandeae, forming capitate ends (Fig. 4a) in Polystachya, Ancistrorhynchus and Bolusiella, and elaborate hooks in Dinklageella and Solenangis (Fig. 4b).
Fig. 4

Testa extensions. Ancistrorhynchus clandestinus: a capitate ends. Dinklageella liberica: b elaborate hooks at the apical pole. Scale bars: a 20 µm; b 50 µm

Thin and straight longitudinal anticlinal walls, with no raised and no thickened cell corners (Fig. 5a), were observed in the tribes Cranichideae and Orchideae (Orchidoideae), Tropidieae and Nervilieae, mostly of the subtribe Malaxidinae (Malaxideae), in the genera Eulophia and Oeceoclades (Cymbidieae) and Calanthe (Collabieae) within the subfamily Epidendroideae.
Fig. 5

Morphology of anticlinal walls. Corymborkis corymbis: a thin and straight. Rhipidoglossum densiflorum: b thickened with raised cell ends. Polystachya polychaete: c wide and thickened with prominent ridges (arrows). Polystachya concreta: d curved transversal anticlinal walls. Scale bars: a 50 µm; b 5 µm; c, d 10 µm

Thickened longitudinal anticlinal walls, with raised and thickened cell ends (Fig. 5b), were observed in the tribes Podochileae, Vandeae and in the subtribe Dendrobiinae (Malaxideae), covering more surface than periclinal walls in the whole seed coat and with prominent ridges in both sides of the adhesion zone (Fig. 5c). In the genera Ansellia and Graphorkis (Eulophiinae, Cymbidieae) and in L. epiphytica (Malaxidinae, Malaxideae), anticlinal walls were thickened but lack of prominent ridges, not covering more surface than periclinal walls (Fig. 2c‒d).

Transversal anticlinal walls were straight, but curved (Fig. 5d) in the tribes Podochileae, Vandeae and in the subtribe Dendrobiinae (Malaxideae).

Intercellular gaps at the cell corners were mostly absent, except in the studied taxa of the tribe Cranichideae (Fig. 3a) and the subtribe Nerviliinae (Fig. 3b).

Wide and clearly visible periclinal walls were observed in the subfamily Orchidoideae, and in the studied taxa of the tribes Tropidieae, Nervilieae, Cymbidieae, the subtribe Malaxidinae and in the genus Calanthe (Collabieae) within Epidendroideae. These walls were usually smooth (Figs. 3b, 5b), sometimes provided of ornamentation with thin (< 1 µm) and transversal ridges (Fig. 6a) in Eulophia and Oeceoclades, thick (> 1 µm) and longitudinal ridges (Fig. 6b) in Ansellia and Graphorkis, or reticulated (Fig. 6c) in L. epiphytica, Calanthe and Holothrix. Narrow-to-not-visible periclinal walls (Fig. 6d) were observed in the subtribe Dendrobiinae (Malaxideae) and the tribes Podochileae and Vandeae.
Fig. 6

Morphology and ornamentation of the periclinal walls. Eulophia cucullata: a wide periclinal walls with thin and transversal ridges. Graphorkis lurida: b wide periclinal walls with thick and longitudinal ridges. Liparis epiphytica: c wide periclinal walls with reticulated ornamentation. Calyptrochilum emarginatum: d not visible periclinal walls. Scale bars: a, b, d 50 µm; c 30 µm

Waxes appeared only in the subfamily Epidendroideae, in all the studied taxa of the tribes Podochileae and Vandeae, in the subtribe Dendrobiinae (Fig. 7a), in the genera Ansellia and Graphorkis and in L. epiphytica. Wax caps covering the cell corners (Fig. 7b) were observed in the genera Ansellia and Graphorkis (Eulophiinae, Cymbidieae).
Fig. 7

Waxes. Genyorchis apetala: a waxes over all the testa cells. Ansellia africana: b detail of the prominent wax caps (arrows) at the cell corners. Scale bars: a, b 20 µm

Discussion

Based on qualitative features, our micromorphological study reveals different patterns in the seed coat of the analysed taxa.

Vanilla is the only genus with ovoid seeds, provided with hard and warty seed coat. Our results are congruent with those of Ziegler (1981) and Cameron and Chase (1998).

The studied taxa of the subfamily Orchidoideae share a set of micromorphological features with those of the tribes Tropidieae, Nervilieae and Collabieae, the subtribe Malaxidinae (except L. epiphytica) and the genera Eulophia and Oeceoclades (Cymbidieae) of the subfamily Epidendroideae: parallel medial cells along the longitudinal axis, thin anticlinal walls, straight transversal anticlinal walls, no raised and no thickened cell corners, wide periclinal walls and lack of waxes. All these taxa are terrestrial, and their common features are consistent with previous results in terrestrial orchids of other geographic areas (Arditti et al. 1979, 1980; Tohda 1983, 1985; Kurzweil 1993; Ortúñez et al. 2006; Gamarra et al. 2007, 2012, 2015; Barthlott et al. 2014; Galán Cela et al. 2014; Riverón-Giró et al. 2017).

Other qualitative characters show morphological variations among and within tribes, such as seed shape and the ornamentation of the periclinal walls. Seed morphology shows the greatest variability in Orchideae, a characteristic previously observed in temperate regions (Gamarra et al. 2007, 2012, 2015). Our results about the sculpturing of the periclinal walls in Cymbidieae, Collabieae and Orchideae are congruent with Barthlott et al. (2014). The patterns of ornamentation of the periclinal walls were confirmed as a good diagnostic value regarding to the taxonomy of different genera in Orchideae by Gamarra et al. (2007, 2008, 2010, 2012, 2015) and Galán Cela et al. (2014), suggesting the importance of future studies on seed coat in African orchids.

The tribe Cranichideae (Orchidoideae) and the genus Nervilia (tribe Nervilieae, Epidendroideae) share the presence of intercellular gaps at the cell corners. This feature was observed in Cranichideae (Tohda 1985), in the tribes Diurideae (Molvray and Kores 1995) and Chloraeeae (Chemisquy et al. 2009) within Orchidoideae, and in Bromheadia finlaysoniana (Barthlott et al. 2014) within Epidendroideae. These results suggest that the intercellular gaps at the cell corners occur only in terrestrial orchids, however, Barthlott et al. (2014: 179, Fig. 99C-D sub B. biflorum) mentioned this character in the epiphyte Bulbophyllum bifarium Hook.f. (Dendrobiinae, Malaxideae, Epidendroideae). This is not congruent with our results (Fig. 3e) either with the images published by Swamy et al. (2004) and Shimizu et al. (2012) in this genus. The presence of intercellular gaps has been related to facilitate entry of the hyphae of fungi to provide nutrients to the embryo (Molvray and Chase 1999).

The seed coat of the studied taxa of the subtribe Dendrobiinae (Malaxideae) and the tribes Podochileae and Vandeae (Epidendroideae) share a set of features: twisted medial cells along the longitudinal axis, thickened anticlinal walls with prominent ridges in adhesion zones, curved transversal anticlinal walls, raised and thickened cell corners, narrow-to-not-visible periclinal walls and presence of waxes. These taxa are epiphytes and our results are congruent with previous studies. Twisted medial cells in epiphytic orchids were revealed by Vij et al. (1992) and confirmed by Barthlott et al. (2014) in representatives of the tribes Podochileae and Vandeae. Thickened anticlinal walls with prominent ridges in the adhesion zones and narrow-to-not-visible periclinal walls are also congruent with Swamy et al. (2004), Barthlott et al. (2014) and Ramudu and Khasim (2015). According to Ziegler (1981), the presence of raised anticlinal walls could be related to the slowing down of the seeds to better attach to the substrate. Ongoing studies about the aerodynamics of the seeds must be provided to demonstrate the relationships among these features and the dispersal adaptation to epiphytic habit.

The presence of waxes in epiphytic taxa is congruent with Chase and Pippen (1990) and Barthlott et al. (1998). The verrucosities mentioned by Ziegler (1981), Chase and Pippen (1988) and Molvray and Chase (1999) on seed coat could be matched with waxy deposits observed by us in several genera of Epidendroideae (Fig. 5b‒d). The presence of waxes could be related to adaptations to epiphytism. Waxes act as hydrophobic surfaces and probably contribute to the protection of embryo in the first stages. Future research about the presence of waxes and their chemical composition in epiphytic orchids along the tropical regions and taxonomic groups must be provided.

Seed coat of the epiphytic taxa of the genera Ansellia and Graphorkis (Eulophiinae, Cymbidieae), and L. epiphytica (Malaxidinae, Malaxideae) share a set of qualitative features with the studied taxa of the subtribe Dendrobiinae (Malaxideae) and the tribes Podochileae and Vandeae (Epidendroideae), such as thickened anticlinal walls and the presence of waxes, but differ in the occurrence of parallel medial cells along the longitudinal axis and wide periclinal walls, both features shared with the terrestrial taxa mentioned above. The subtribes Eulophiinae and Malaxidinae are represented by terrestrial and epiphytic orchids. According to the habit, seed coats of representatives of both subtribes showed differences in the micromorphological features (Tsutsumi et al. 2007, Barthlott et al. 2014). Similar results were obtained by Zhang et al. (2015) in the genera Paphiopedilum and Cypripedium (Cypripedioideae).

On the other hand, several genera of the tribe Vandeae share the presence of testa extensions, forming capitate ends or elaborate hooks. Testa extensions were also observed in this tribe by Ziegler (1981) and Chase and Pippen (1988). In accordance with these authors, the presence of the extensions would provide a better attachment to the bark of the trees and also contribute to the slowing of the fall of the seed. Future studies linking these features with seed dispersal strategies in epiphytic orchids should be carried out.

Our results suggest a probable relation with ecological adaptations required for seed dispersal, which would explain the presence of common and different features among the taxa. The main different characters observed among the terrestrial and epiphytic orchids are the orientation of medial cells along the longitudinal axis, the thickness of the anticlinal walls, the width of the periclinal walls and the presence of waxes.

New insights into tropical Orchidaceae could be useful to advance in the knowledge of the orchid seeds and the understanding of their dispersal mechanisms.

Notes

Acknowledgements

This study was supported by the Ministry of Economy and Competitiveness of Spain through the research project CGL2012-32934. The authors express their gratitude to the curators and collection managers of the herbaria K, MA and WAG. We are much indebted to Yolanda León, the technical staff at the SEM laboratory at the Royal Botanic Gardens, Madrid. We thank Francisco Cabezas, Maximiliano Fero and Mauricio Velayos who participated in Equatorial Guinea’s botanical expeditions. We appreciate Dr Mauricio Velayos and Dr Javier Fuertes for their constructive comments.

Compliance with ethical standards

Conflict of interest

All authors declare that they have no conflict of interest.

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Departamento de BiologíaUniversidad Autónoma de MadridMadridSpain
  2. 2.Departamento de Sistemas y Recursos NaturalesUniversidad PolitécnicaMadridSpain

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