Two new species of Begonia sect. Coelocentrum, B. guixiensis and B. longa, from Sino-Vietnamese limestone karsts

In our recent molecular phylogenetic study of Asian Begonia, two undescribed species, B. guixiensis sp. ined. (S. Guangxi, China) and B. longa sp. ined. (Vietnam), were sampled and placed within the strongly supported clade composed of Begonia sect. Coelocentrum and other co-distributed rhizomatous species in the Sino-Vietnamese limestone karsts. While Begonia sect. Coelocentrum has been recircumscribed based on the phylogenetic relationships, B. guixiensis sp. ined. and B. longa sp. ined. remain illegitimate names. In continuation of our studies in Asian Begonia, these two new species are described and illustrated. Begonia guixiensis resembles B. cylindrica in the peltate, subcoriaceous leaves, differing by the shape of ovary/fruit and the type of placentation. In aspect, B. longa bears a superficial resemblance to B. brevipedunculata in leaf shape in particular, differing by many other features such as the long internodes, shorter petioles and smaller leaves, longer peduncles and 3-locular ovary. The chromosome number of both new species is determined as 2n = 30. A careful study of the literature, herbarium specimens and living plants, both in the wild and in cultivation in the experimental greenhouse, support the recognition of the two new species, which are described and illustrated herein.

Our recent molecular phylogenetic analysis  demonstrates that Begonia sect. Coelocentrum is not monophyletic but instead dominates in a strongly supported clade that otherwise also includes B. Despite the disparity in placentation (vs. axile) and fruit types (cylindric and berry-like in sect. Leprosae), these six species are all rhizomatous and distributed exclusively in the Sino-Vietnamese limestone karsts. Given the strongly supported phylogenetic relationship, the presence of additional placentation and fruit types in the clade composed of species mainly with parietal placentation and dry capsule further attests the labile nature of ovary and fruit types for the infrageneric classification of Begonia highlighted by previous works (e.g., Tebbitt et al. 2006;Thomas et al. 2011). Because of the strongly supported phylogenetic relationship and apparent cohesiveness in terms of their perennation mode, geographic distribution, and ecological preference, Chung et al. (2014) expands the concept of Begonia sect. Coelocentrum to include the abovementioned species. Meanwhile, in the phylogeny of Chung et al. (2014), two undescribed species, B. guixiensis sp. ined. and B. longa sp. ined., were sampled and grouped within the recircumscribed Begonia sect. Coelocentrum with strong support ; Figure 1). Detailed morphological description and cytological examination of the two new species are provided below.

Chromosome preparations
Somatic chromosomes of the two new species were examined using root tips. The methods followed Peng et al. (2014a). The classification of the chromosome complements based on the centromere position at mitotic metaphase described in Levan et al. (1964) was adopted. Voucher specimens (Begonia guixiensis: Peng et al. 20310;B. longa: Peng et al. 20076) were deposited in the Herbarium of the Biodiversity Research Center, Academia Sinica (HAST).
Additional specimens examined

Chromosome cytology
Somatic chromosomes at metaphase of Begonia guixiensis were determined to be 2n = 30 ( Figure 6A). The thirty chromosomes, ranging from ca. 1.1 to 2.1 μm long, showed a gradual change in chromosome length. Centromeres of most chromosomes are median or submedian. Satellite chromosomes were not observed.

Notes
Begonia guixiensis resembles B. cylindrica D.R. Liang and X.X. Chen (Figure 7) in the peltate, subcoriaceous leaves, differing by the shape of ovaries/fruits and placentation. Compared to the wingless and long-cylindric berrylike   fruits of B. cylindrica, B. guixiensis possesses ovaries that are trigonous-ellipsoidal with three distinct wings. Its capsule-like fruits dry up and do not dehisce when mature. While the placentation of B. cylindrica is axile throughout the ovary, that of B. guixiensis is parietal on upper part of the ovary but the placental branches gradually fused side by side below and assuming a 3-locular appearance. Serial cross-sections to show the gradual changes are depicted in Figure 8.
Both Begonia guixiensis and B. cylindrica have clustered stomata and hypodermis (Figures D, E, G, H). These characters were also seen in B. leprosa : figure nine-H, I), the most closely related species in the phylogeny (Figure 1), and are known in a number of limestone Begonia species from the Philippines and China Rubite et al. 2014).

Ecology and distribution
Begonia longa is currently known only from Quan Ba District, Ha Giang Province, northern Vietnam ( Figure 4).
It grows on limestone rock face, in shaded and moist broadleaf forest. The plants were locally frequent, often associated with Amorphophallus sp., Asplenium sp., Colocasia giganta, Hemiboea sp., Hoya sp., Nephrolepis cordifolia and Pilea sp.

Etymology
The specific epithet refers to the long creeping rhizome in this species.
Additional specimens examined

Chromosome cytology
Somatic chromosomes at metaphase of Begonia longa were determined to be 2n = 30 ( Figure 6B). The thirty chromosomes gradually varied from ca. 1.2 to 1.8 μm long in length. Most chromosomes have centromeres at median, submedian and subterminal positions. Satellites were not observed.
The Asian section Coelocentrum consistently shows the chromosome number of 2n = 30, with the exception of some probable autotriploid individuals with 2n = 45 in B. longgangensis . Chromosome numbers of the two new species, B. guixiensis (see above) and B. longa, studied here are in agreement with previous reports for species of Begonia in this section.

Notes
In aspect, B. longa resembles B. brevipedunculata Y.M. Shui (Shui 2006) in the leaf shape, differing by many features such as the elongate internodes, shorter petioles, smaller leaves, longer peduncles and its 3-locular ovary. Detailed comparison of salient features of the two species is provided in Table 1.