Begonia wui-senioris (sect. Platycentrum, Begoniaceae), a new species from Myanmar

The flora of Myanmar is under-collected compared with all other tropical Asian countries. An unknown Begonia was grown from seeds collected from a limestone hill in Central Myanmar, and compared with potentially allied species. The unknown Begonia is rhizomatous, has peltate leaves, 2-locular ovaries, and is evergreen. It is clearly assignable to sect. Platycentrum. Only two other species of Begonia, B. josephii and B. subperfoliata, in Myanmar have peltate leaves, but they are deciduous tuberous plants with 3-locular ovaries and belong to sect. Diploclinium. Thorough studies of literature and herbarium materials support the recognition of a new species, Begonia wui-senioris, which is fully described and illustrated. Begonia wui-senioris has the lowest chromosome number (2n = 14) for the genus.


Background
The most recent checklist for the Begonia of Myanmar (Hughes, 2008) reports 57 species as native. Almost concurrently, a new species was reported, Begonia hayamiana (sect. Sphenanthera) (Tanaka and Hughes 2007), bringing to the total to 58. In 2009, Hong Wang, the 2nd author of this article collected seeds of a Begonia from Central Myanmar and sent them to the senior author, Peng, who germinated them and raised them to maturity in the experimental greenhouse of Academia Sinica, where they were carefully examined. Upon consulting herbarium materials from major herbaria, both domestic and abroad, the plants were found not to belong to any species known from Myanmar. However, a herbarium specimen in E, collected from the region where our seeds were gathered, corresponds to our living material. That specimen was annotated in 1992 as "Begonia lacei C. Y. Wu, sp. nov. ined.," by the late Prof. Zhengyi Wu, renowned botanist of the Chinese Academy of Sciences. This unpublished taxon was excluded from the current checklist for Myanmar (Hughes 2008).

Materials
Living plants of Begonia wui-senioris were raised from seeds collected from ca. 200 km northeast of Mandalay, Myanmar and grown in an experimental greenhouse at Academia Sinica. The type collection (Peng 22199) was gathered from the greenhouse and deposited at HAST herbarium.

Cryo scanning electron microscopy
Fresh leaves of Begonia wui-senioris were dissected and attached to a stub. The samples were frozen with liquid nitrogen slush, then transferred to a sample preparation chamber at −160°C and etched for 15 min at −85°C. After etching, the temperature reached −130°C for sample fracturing and coating. After coating, the samples were transferred to the SEM chamber and observed at −160°C with a cryo scanning electron microscope (FEI Quanta 200 SEM/Quorum Cryo System PP2000TR FEI).  Herbs, monoecious, evergreen, rhizomatous. Rhizomes 1 cm thick, to 10 cm long, internodes ca. 0.5 cm long. Stipules eventually deciduous, ovate-triangular, boatshaped, ca. 1.2 cm long, 0.9 cm wide, apex attenuate.

Habitat
Lithophytic. Disturbed tropical rain forest; on limestone hills, on shaded, humid, rock surfaces near caves by waterfalls (based on Peng 22199).

Phenology
Flowering April to August; fruiting June to October.

Etymology
The specific epithet commemorates the late Prof. Zhengyi Wu, a renowned Chinese botanist.

Chromosome cytology
Somatic chromosomes at metaphase of Begonia wuisenioris were determined to be 2n = 14 (Figure 5), which is the first confirmed report of the lowest chromosome number known for the genus. Somatic metaphase chromosomes of most species of Begonia are nearly always small (< 2 μm long), and karyotypes are mostly impossible  to be determined. Begonia wui-senioris, however, has longer chromosomes and its karyotype is fully described here. The 14 chromosomes gradually vary in length from ca. 2.2 to 4.2 μm long. Among them, ten 11,12), two ( Figure 5B; Nos. 9, 10) and two (Figure 5B: Nos. 13,14) chromosomes have the centromere in the median (m), submedian (sm) and subterminal (st) positions, respectively. Most chromosomes have secondary constrictions (SC) or satellites (sat): SCs are located at interstitial regions of the long arms in four m-( Figure 5B: Nos. 1, 2, 7, 8) and two sm-chromosomes ( Figure 5B: Nos. 9, 10); at interstitial region of the short arms in two m-chromosomes ( Figure 5B: Nos. 5, 6); satellites were observed in the distal regions of the short arms in two st-chromosomes ( Figure 5B: Nos. 13, 14). The karyotypic formula of B. wui-senioris is therefore 2n = 14 = 10m 6SC + 2sm 2SC + 2st 2sat .
Begonia section Platycentrum in Asia comprises about 110 species (Shui et al. 2002). To our knowledge, chromosome numbers of about 30 taxa have been reported for species in this section (e.g. Ye et al. 2004;Peng et al. 2006;Li et al. 2005). The karyotypes show wide variation in number, especially in Taiwanese Begonia (Oginuma and Peng 2002). Among the species belonging to sect. Platycentrum, B. wui-senioris is unique in having longer chromosomes and the lowest chromosome number (2n = 14) for the genus. Nakata et al. (2003) documented a probable chromosome count of 2n = ca. 14 for a sterile, not positively identified, plant from Yunnan, China.

Notes
Begonia wui-senioris, B. josephii and B. subperfoliata are the only three peltate-leaved species occurring naturally in Myanmar. The latter two, however, are deciduous, tuberous species with a single leaf and a 3-locular ovary and are classified within sect. Diploclinium.