Begonia wuzhishanensis (sect. Diploclinium, Begoniaceae), a new species from Hainan Island, China

Background Hainan is the largest island of the Indo-Burma Biodiversity Hotspot and has the best preserved and most extensive tropical forests in China. A recent study on distribution of endangered species in China identifies southern Hainan as one of eight hotspots for plant conservation in the country. In continuation of our studies of Asian Begonia, we report the discovery of an attractive undescribed species, B. wuzhishanensis C.-I Peng, X.H. Jin & S.M. Ku, from Hainan Island. Results Living plant of the new species, Begonia wuzhishanensis, was collected in 2009 and cultivated in the experimental greenhouse for morphological and cytological studies. It flowered consecutively in 2012 and 2013 in the experimental greenhouse, Academia Sinica. It was assigned to the large, heterogeneous sect. Diploclinium. The chromosome number of this new species was determined to be 2n = 26. Conclusions A careful study of literature, herbarium specimens and living plants, both in the wild and in cultivation, support the recognition of the new species Begonia wuzhishanensis, which is described in this paper. Begonia wuzhishanensis is currently known only from Fanyang, Wuzhishan Mountain in the center of the island. A line drawing, color plate, and a distribution map are provided to aid in identification. Electronic supplementary material The online version of this article (doi:10.1186/1999-3110-55-24) contains supplementary material, which is available to authorized users.


Background
Hainan is the largest island of the Indo-Burma Biodiversity Hotspot and has the best preserved and most extensive tropical forests in China (Deng et al. 2008;Zang and Ding 2009). The island harbors the Hainan Island Monsoon Rain Forest Ecoregion, one of the 26 terrestrial habitats of China internationally recognized by the World Wildlife Fund for its global ecological importance (Francisco-Ortega et al. 2010). A recent study on distribution of endangered species in China identifies southern Hainan as one of eight hotspots for plant conservation in the country (Zhang and Ma 2008 (Gu et al. 2007). In continuation of our studies of Asian Begonia (e.g., Chung et al., 2014;Hughes et al. 2011;Nakamura et al. 2013;Peng et al. 2012Peng et al. , 2013Peng et al. , 2014Rubite et al.: Begonia chingipengii [sect. Baryandra, Begoniaceae], a new species from Luzon Island, Philippines, submitted), we report the discovery of an additional undescribed species, B. wuzhishanensis, endemic to the island.

Chromosome preparations
Somatic chromosomes of the new species, Begonia wuzhishanensis (Ku & Jin 2093) were examined using root tips. The methods of pretreatment, fixation and staining for chromosome observations followed Peng et al. (2012). Classification of the chromosome complements based on centromere position at mitotic metaphase follows Levan et al. (1964). Voucher specimens have been deposited in Herbarium, Biodiversity Research Center, Academia Sinica, Taipei (HAST).

Chromosome cytology
Somatic chromosomes at metaphase of B. wuzhishanensis (Ku & Jin 2093, HAST) were determined to be 2n = 26 (Figure 3). The 26 chromosomes gradually varied from ca. 1.0 to 2.0 μm long in length. A pair of the longest chromosomes has centromeres at submedian; several chromosomes at median, however, those of smaller chromosomes could not be determined. Satellites were not observed.

Ecology and distribution
Wuzhishan (1,876 m), located in the center of Hainan, is the highest mountain on the island. Begonia wuzhishanensis is a rare species found at lower part (ca. 200-500 m altitude) of Wuzhishan ( Figure 5). It occurs on wet mossy rocks by streams in tropical forest.

Etymology
The specific epithet is derived from the type locality, Wuzhishan ('Five-fingered Mountain', literally) of Hainan Island, China.

Phenology
Flowering from August to December; fruiting from October to December.

Cultivation
Begonia wuzhishanensis can be grown from seeds. Alternatively, they can be propagated by the elongate tubers or leaf cuttings with ease ( Figure 2O).

Notes
Begonia wuzhishanensis has prominent elongate tubers that are rarely encountered in the genus. This feature resembles those observed in B. lithophila C.Y. Wu (sect. Reichenheimia; Li 2006) from Yunnan, China and B. woodii Merr. (sect. Baryandra) from Palawan, the Philippines ( Figure 6). The perennating tubers of B. woodii have been erroneously interpreted as short rhizomes in the past (Merrill 1925;Hughes et al. 2010).
Placentation of B. wuzhishanensis is noteworthy: its ovary is 3-locular with 2 placentas each growing from middle of the septa ( Figures 1J, 2M, 7B). Such placentation was classified as 'septal' in African Begonia ( Figure 7C; Reitsma 1984). Septal placentation was also observed in part of the ovary of some species in the Asian sect. Coelocentrum (e.g.  Figure nine). With 3-locular ovaries, bifid placentas and perennating tubers, etc. but without axile placentation in B. wuzhishanenis ( Figure 7B), we have tentatively placed it in the polymorphic and 'dust-bin' sect. Diploclinium . Further molecular phylogenetic studies to clarify its placement are underway.
Begonia wuzhishanensis somewhat resembles B. obsolescens Irmsch. and B. fimbristipula Hance. Comparison of salient features of the three species is provided in Table 1.

Conclusions
A careful study of literature, herbarium specimens and living plants, both in the wild and in cultivation, support the recognition of the new species Begonia wuzhishanensis. It is currently known only from Fanyang, Wuzhishan Mountain in the center of Hainan island, China. Begonia wuzhishanensis is unique in having elongate tubers and septal placentation. It is tentatively assigned to sect. Diploclinium. Further studies to clarify its phylogenetic position are underway.