Isotrema sanyaense, a new species of Aristolochiaceae from Hainan, China

Abstract Isotrema sanyaense R.T.Li, X.X.Zhu & Z.W.Wang, sp. nov., a new species from Hainan island, China, is described and illustrated here. It is morphologically most similar to I. ledongense (Han Xu, Y.D.Li & H.J.Yang) X.X.Zhu, S.Liao & J.S.Ma and I. jianfenglingense (Han Xu, Y.D.Li & H.Q.Chen) X.X.Zhu, S.Liao & J.S.Ma in the shape of leaf, flower, and the yellow to brown villous indumentum of the pedicel, ovary and calyx. However, I. sanyaense can be easily distinguished from the latter two species by its 1–5-flowered cymes, in hanging clusters of 1 to numerous branches, upper calyx tube obviously longer than basal calyx tube, calyx limb discoid, yellow inside, with purple-red stripes and spots, about 13–18 mm in diameter, glabrous, and a throat dark red without spots, 4–6 mm wide.


Introduction
Isotrema Raf. (Aristolochiaceae), previously treated as a subgenus of Aristolochia L., was recently reinstated as an independent genus based on molecular and morphological evidence . It differs from other genera of Aristolochiaceae by a combination of characters: calyx strongly curved, gynostemium 3-lobed, anthers paired on the outer surface of each gynostemium segment, and capsule dehiscing basipetally (Do et al. 2015a. Currently, Isotrema comprises 99 species and one subspecies, mainly distributed in East and South Asia, with some species scattered in North and Central America (Zhou et al. 2019. China accommodates 59 species and one subspecies, of which 47 species and one subspecies are endemic to the country (Hwang et al. 2003, Zhou et al. 2019.
During our field investigations to South Hainan Province, China, in 2017 and 2018, an unknown species of Aristolochiaceae was discovered. The horseshoe-shaped calyx tube, 3-lobed calyx limb and gynostemium, anthers adnate in pairs opposite the gynostemium lobes, and capsule dehiscing basipetally indicate it to be a member of Isotrema. After comparing with other species of the genus, we confirmed that the unidentified species from Hainan island represents a new taxon, so here, we describe and illustrate it.
Phenology. Flowering specimens have been collected in October and in fruiting specimens in May.
Etymology. The specific epithet is derived from the type locality, Sanya City, in Hainan island, China. The Chinese name is given as "三亚关木通".
Distribution and habitat. Isotrema sanyaense is currently known from Haitangwan Town, Haitang District, Sanya City, Hainan Province, China. It grows in lowland dry forests dominated by families including Euphorbiaceae, Fagaceae, Lauraceae, Myrtaceae, Arecaceae and Rubiaceae at elevations between 332-400 m.
Conservation status. Isotrema sanyaense is only known from two populations in Sanya City, Hainan island, China, with fewer than 30 individuals seen at each site. Therefore, the new species is assigned a preliminary status of Vulnerable (VU D2) according to IUCN Red List Criteria (IUCN 2012), indicating a population with a very restricted area of occupancy (typically less than 20 km 2 ) or number of locations (typically five or fewer).
Additional specimens examined (

Discussion
Morphologically, Isotrema sanyaense resembles I. ledongense and I. jianfenglingense in having similar leaf blade shape (lanceolate or elliptic-lanceolate, entire, base shallowly cordate) and the yellow to brown villous indumentum of the pedicel, ovary and calyx, but I. sanyaense and I. ledongense are significantly different in the inflorescence (cymes 1-5-flowered, in hanging clusters of 1 to numerous branches vs. solitary), the pedicel (nearly equal in length to flower vs. significantly shorter than flower), the calyx tube (upper tube obviously longer than basal tube vs. upper tube almost equal to basal tube), the calyx limb (about 13-18 mm in diameter, with purple-red stripes and spots vs. 5-7 mm in diameter, only with unobvious light red spots), and the throat (dark red without spots vs. dark red with light yellow spots). Isotrema sanyaense can also be easily distinguished from I. jianfenglingense by the inflorescence (cymes 1-5-flowered, in hanging clusters of 1 to numerous branches vs. solitary), the calyx (white with purple-red stripes vs. light red to brown without stripes), the calyx limb (discoid, yellow, with purple-red stripes and spots, lobes without papillae and pubescent vs. trumpetshaped, pink, without stripes and spots, lobes densely papillae and pubescent), and the throat (dark red without spots, significantly smaller than the limb width vs. pink with red-brown spots, approximately to the limb width) (summarized in Table 1). Considering the discovery of this new species from Hainan island, along with the species previously described in China, Myanmar, Peninsular Malaysia, Thailand and Vietnam in recent years (González and Poncy 1999, Phuphathanaphong 2006, Hansen and Phuphathanaphong 2009, Liu and Deng 2009, Xu et al. 2011, Yao 2012, Huang et al. 2013, Wu et al. 2013, Do et al. 2014, 2015a, 2015b, 2015c, 2015d, 2017, 2019, Huong et al. 2014, Lu and Wang 2014, Ohi-Toma et al. 2014, Ravikumar et al. 2014, Zhu et al. 2015, 2017a, 2017b, Gong et al. 2018, Zhou et al. 2019, we predict that more and more new species of Isotrema will be found after extensive investigations and studies. Key to Isotrema sanyaense and closely related species (including all species of Aristolochia and Isotrema in Hainan island, China)