Thilachium madagascariense (Capparaceae), a new species from eastern Madagascar with a key to the species of the genus

Thilachium madagascariense Fici, a small tree characterised by 1-foliolate leaves with articulate petioles, leaf blades elliptic or narrowly obovate with base attenuate and apex acuminate, flowers in groups of 2 – 3 at the top of lateral twigs and a high number of stamens, is described and illustrated from forest habitats of eastern Madagascar. The new species is similar to T. laurifolium Baker, a species endemic to central and eastern Madagascar, differing in the longer leaves with acuminate apex, flowers conferted at the top of lateral twigs, shorter pedicels, higher number of stamens and shorter gynophore. Its affinities are discussed and an updated key is provided for all the species of the genus Thilachium.


Introduction
The genus Thilachium Lour. (Capparaceae Juss.) comprises 15 species, seven of which endemic to Madagascar, six widespread in eastern Africa from Somalia southwards to Natal, one species, T. africanum Lour., recorded from both Madagascar and eastern Africa and one species, T. panduriforme Juss., from Madagascar and Mauritius (Elffers et al. 1964;Hadj Moustapha Haddade 1965;Harvey et al. 1995). The genus includes small trees, shrubs and a single herbaceous species (T. roseomaculatum Y. B.Harv. & Vollesen), with simple or 1 -3-foliolate leaves and in some cases with tuberous roots, occurring in dry lowland and mountain forests, riverine formations, deciduous woodlands, xerophilous bushlands and thickets, secondary scrubs, dunes and grasslands, from sea level up to 1600 m a.s.l. With regard to the generic name, the spelling Thylacium was adopted in Flora Zambesiaca (Wild 1960), Thylachium in Flora of Tropical East Africa (Elffers et al. 1964) and in Flore de Madagascar et des Comores (Hadj Moustapha Haddade 1965). Later the original spelling Thilachium was conserved (McVaugh 1968), and adopted in Flora of Somalia by Thulin (1993), and by Harvey et al. (1995) in describing a new species from Kenya. Historically Thilachium was included by Pax & Hoffman (1936) with the genera Maerua Forssk. and Courbonia Brongn.
in the tribe Maerueae Baill. belonging to the subfamily Capparoideae Burnett. Later DeWolf (1962) treated Courbonia as a synonym of Maerua and underlined that Thilachium "is a somewhat anomalous genus" showing affinities also with the genus Ritchiea R.Br. More recently Kers (2003) did not retain the subdivision of Capparoideae in tribes proposed by Pax & Hoffman (1936), stating that Thilachium is related, in addition to Maerua and Ritchiea, to other Old World woody genera, i.e. Boscia Lam., Bachmannia Pax and Cadaba Forssk. The same author (Kers 2003) distinguished Thilachium from the allied genera mainly by the sepals connate, rupturing transversally at the midline, the conical calyptra tardily falling off and the absence of petals. With regard to the phylogeny of Capparaceae, Hall et al. (2002), based on chloroplast sequence data, identifi ed within subfamily Capparoideae a mostly African Old World clade including Thilachium, Maerua, Ritchiea, Boscia and Cadaba.
Since the treatment of family Capparaceae provided by Hadj Moustapha Haddade (1965) no recent studies are available for Madagascar, which constitutes a centre of speciation of the genus Thilachium. During investigations on herbarium collections, material from the Lam & Meeuse expedition to Madagascar in 1938, originally identified as Capparis sp., turned out to belong to the genus Thilachium. Based on various differential features from the species known from Madagascar and eastern Africa, this specimen is referred to as a new species, distinguished by its leaf morphology and by the inflorescence and flower characters. The new species is here described and illustrated, its affinities are discussed and a key to the species of the genus is provided.

Materials and Methods
Herbarium investigations were carried out on historical and recent collections of Capparaceae from the Old World kept at the Nationaal Herbarium Nederland, Leiden University branch (L). To my knowledge the new species here described has been collected only once, in December 1938, by Lam & Meeuse in eastern Madagascar (Lam & Meeuse 5840).
The description and illustration are based on herbarium material. The species concept follows the one adopted by Elffers et al. (1964) andHadj Moustapha Haddade (1965). The terminology of the vegetative and reproductive structures, as well as the main diagnostic characters within the genus, are based on the same treatments. The herbarium acronyms follow Thiers (continuously updated), while authors and plant names are based on IPNI (2020). The examination of type specimens of other species of Thilachium was carried out through electronic images available at JSTOR Global Plants (n.d. ). The online collections available at P, K and MO and the Catalogue of the Vascular Plants of Madagascar (n.d.) were also consulted. The conservation status was provisionally assessed according to IUCN Red List Categories and Criteria (IUCN 2012).

Funding
Open access funding provided by Università degli Studi di Palermo within the CRUI-CARE Agreement.
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