A new and endemic species of Drosera (Droseraceae) from Madagascar

1Botanische Staatssammlung München (SNSB-BSM), Menzinger Strasse 67, D-80638 Munich, Germany 2GeoBio-Center LMU (Center of Geobiology and Biodiversity Research, Ludwig-Maximilians-University), Munich, Germany 3Missouri Botanical Garden, Madagascar Research and Conservation Program, BP 3391, Antananarivo 101, Madagascar 473 000 Chambéry, France 5Instituto Nacional da Mata Atlântica (INMA), Av. José Ruschi, 4, Santa Teresa, Espírito Santo, Brazil *Corresponding author: fleischmann@bio.lmu.de REGULAR PAPER


INTRODUCTION
The genus Drosera L. (Droseraceae, non-core Caryophyllales) comprises approximately 250 species of herbaceous carnivorous plants. The diversity centers of the genus lie in the Southern Hemisphere in South-West and Northern Australia, the Cape Region of South Africa and central-eastern Brazil (Fleischmann et al. 2018a). So far, five Drosera species have been recorded from Madagascar, one of them, Drosera humbertii Exell & J.R.Laundon, endemic to the island (Exell & Laundon 1956;Keraudren-Aymonin 1982;Robinson et al. 2017). The other species, D. madagascariensis DC., D. natalensis Diels, D. burkeana Planch., and D. indica L., are widespread in tropical and subtropical Southern Africa, the first and the last extending into tropical East and WestAfrica, D. indica even further into (sub)tropical Asia (Hutchinson & Dalziel 1954;Laundon 1959;Robinson et al. 2017).
The Forêt de Vohibe (Vohibe Forest) is a low to midelevational (326-1008 m a.s.l.), moist evergreen tropical rainforest covering 3117 ha in northwestern Vatomandry District, Eastern Madagascar (Rakotoarivony & Rasoa viety 2007;Rabearivony et al. 2015). It hosts a rich fauna and flora (Rakotoarivony & Rasoaviety 2007;Rakotoarivelo et al. 2013), including at least 672 plant species with an aston-ishing endemism rate of 76% (Rakotoarivelo et al. 2013). The homonymous "Vohibe low-altitude rainforest fragment" mentioned by Andrianandrasana et al. (2013) west of the confluence of the Mangoro and Nosivolo rivers is a different area of lower elevation and ca. 80 km to the south of the present study site.
During expeditions for floristic inventories to Vohibe Forest in 2010, 2015 and 2016, an unknown species of Drosera was collected by N.H. Rakotoarivelo and co-workers, tentatively identified as "D. aff. indica" or "Drosera sp.", respectively. Photographs of these herbarium specimens in the MO database were immediately identified as distinctive new species by A. Fleischmann, A. Roccia, and P.M. Gonella (as well as independently also by Jan Schlauer and Thilo Krueger). A herbarium revision of Drosera species from Madagascar by A. Fleischmann revealed a further collection of this new taxon made at Mont Taolana (Sud Betsileo) in 1941 (P04582299!; erroneously cited as "D. humbertii" by Keraudren-Aymonin 1982). This species is described as new to science here.

Additional material examined
likely result from caterpillars of the pterophorid moth Buckleria Tutt, 1905 which are oligophages on the genus Drosera (Gielis 2014). The Malagasy endemic Buckleria madecassea Gibeaux, 1994 has been described from Ambatolampy, Antananarivo Province (Gibeaux 1994, host species and larval stages unknown), which is just 117 km distant from Vohibe Forest, thus this moth could probably also occur there, using D. arachnoides as a host plant there. Etymology -The specific epithet "arachnoides" (Greek "arachne" = spider, "-oides" = resembling) refers to the spider-like appearance of the plant, both in terms of its numerous long, linear-lanceolate leaves covered with a pubescent indumentum as well as its habit clinging to vertical rock. Conservation assessment -Vulnerable (VU criterion D1 and D2; IUCN 2012). The four populations at Tsitondroina waterfall are situated within the borders of the NAP de Vohibe (Vohibe Forest New Protected Area, part of the conservation unit Ankeniheny-Zahamena corridor, which is protected as a key biodiversity area MDG-96; CEPF 2014), but lie outside the strict conservation zone of the NAP (Rakotoarivelo 2015). The populations at sites 1 and 4 comprise fewer than 50 individuals, at site 2 several hundred plants of D. arachnoides were found, at site 3 fewer than 20 individuals. Therefore, the total known population size is fewer than 1000 individuals (IUCN criterion VU D1). Even if no direct threat is observed to influence the known populations to date, the species is restricted and currently known from only four sites, covering a very small EOO and AOO, hence it could be prone to human interference or stochastic events that would make it extinct or endangered/critically endangered in a short time (IUCN criterion VU D2). The single collection from Mont Taolana [Itaolana] dates from 1941, and no recent records are known from this area, hence the current population size or presence of the species in Fianarantsoa Province remains unknown. Given that both known collection localities lie more than 300 km apart in a rather poorly botanically explored region, it is likely that further occurrences of Drosera arachnoides might exist in Eastern Madagascar. Taxonomic remarks -Drosera arachnoides is a new and distinctive species, morphologically well distinguishable from all Drosera species known from Madagascar. Additionally, it is geographically and ecologically isolated from the morphologically closest species, D. humbertii. Its morphological diagnosability (and occupation of a different ecological adaptive zone) supports our hypothesis that this new species forms a separately evolving metapopulation that is not conspecific with any other named species of Drosera known to date. Affinity -Drosera arachnoides belongs to Drosera subgenus Drosera section Ptycnostigma Planch. (sensu Fleischmann et al. 2018aFleischmann et al. , 2018b. It is most similar to and apparently closely related to D. humbertii, a montane species from Northeastern Madagascar, where it is restricted to the summits of the Marojejy Massif and Upper Manantenina (Antsiranana Province; fig. 4), where it grows in open areas of ericoid vegetation and montane scrub among sclerophyllous (high-) montane forest at elevations of (1400-)1800-2137 m (Exell & Laundon 1956;Schlosser 2005;Robinson et al. 2017). While D. humbertii is a highland species found at montane to subalpine elevations, D. arachnoides is a lowland species that grows in small open areas of mid-elevation tropical evergreen forests.
Drosera arachnoides superficially resembles some Brazilian species of D. section Brasilianae Rivadavia, Gonella & A.Fleischm. (like D. humbertii also does), such as D. latifolia (Eichler) Gonella & Rivadavia, D. riparia Gonella & Rivadavia or D. villosa A.St.-Hil., with which it shares a similar habit, circinate leaf vernation and similar habitat on vertical wet walls and slopes (Gonella et al. 2014). However, D. arachnoides and D. humbertii do not possess the characteristic yellow-translucent short-stalked glands (TSG trichomes) on their leaves, which represent an apomorphy for D. sect. Brasilianae (Fleischmann et al. 2018b; however, these glands are lacking in members of the D. villosa complex, see Gonella et al. 2014 Gonella et al. 2014). Hence the morphological similarity of D. arachnoides and D. humbertii with their New World congeners is convergent and merely superficial, but not based on phylogenetic affinity. Morphology -The circinate leaf vernation of D. arachnoides and D. humbertii is rare among African Drosera, but a typical feature in the Brazilian D. sect. Brasilianae (Fleischmann et al. 2018b); hence the superficial similarity to some New World species. Among the African D. sect. Ptycnostigma, D. arachnoides and D. humbertii are the sole hemicryptophyte members with circinate leaf vernation, the remainder of species have geniculate-involute vernation (with the rare exception of one clone of the South African D. capensis L. that is only known from cultivation). Only some of the geophyte species of D. sect. Ptycnostigma from the Western Cape of South Africa share involute leaf vernation (i.e., the affinity of D. cistiflora L. and D. trinervia Spreng., with the notable exception of D. pauciflora Banks ex DC.). Circinate leaves among African Drosera are further found only in the unique species D. regia Stephens (D. subgenus Regiae; sister to all remaining species of Drosera) from the Western Cape of South Africa, as well as in the widespread tropical annual D. indica (D. subgenus Drosera sect. Arachnopus). Overall, circinate vernation is a homoplastic character connected to linear leaf shape in the genus Drosera, having evolved in several lineages but not mirroring phylogenetic affinity. Drosera humbertii has been described by Exell & Laundon (1956) as having adaxially glabrous petioles, but this is not the case in all specimens studied by A. Fleischmann (including type material), and this is also not evident from the in situ photographs of the species from Marojejy summit published in Robinson et al. (2017). In fact, D. humbertii shares an identical leaf indumentum of eglandular white hairs with D. arachnoides, but the hair cover on the petiole upper surface is sparser in D. humbertii, so that the petioles appear subglabrous. The pubescence of white eglandular hairs on the petioles of D. arachnoides also slightly varies depending on growing conditions (as is the case in many species of Drosera), with specimens growing in more shaded conditions ( fig. 2A & B and e.g., N.H. Rakotoarivelo, A. Razanatsima, L.J. Razafitsalama, L.R. Andriamiarisoa, L. Raboto, L. Dedesy & C. Zafindrafeno 358) displaying a less dense indumentum compared to specimens growing in more exposed, sunnier habitats ( fig. 2C-E and e.g., N.H. Rakotoarivelo,F. Rakotoarivony,C. Zafindrafeno & Y.J. Lezafy 788,Nivo H. Rakotoarivelo,F. Rakotoarivony,C. Zafindrafeno & Y.J. Lezafy 789). However, in all specimens the petioles are more pubescent than in the related D. humbertii.
Further, Exell & Laundon (1956) described the (immature) seeds of D. humbertii as "likely fusiform" (a seed shape which would link it to D. madagascariensis). However, in the scarce fruiting material among the specimens examined by A. Fleischmann the seeds were angulate-ellipsoidal, hence identical in shape to those of D. arachnoides.