Begonia myanmarica (Begoniaceae), a new species from Myanmar, and molecular phylogenetics of Begonia sect. Monopteron.

BACKGROUND: A new species, Begonia myanmarica, was discovered from Myanmar and herein documented. Characterized by a single developed wing in the ovary/fruit, this species would be assigned to sect. Monopteron (sensu Doorenbos et al. in The sections of Begonia including descriptions, keys and species lists: studies in Begoniaceae VI. Wageningen Agricultural University, Wageningen, 1998) that is known by B. griffithiana and B. nepalensis from the Himalaya. To confirm its sectional assignment, we conducted morphological, phylogenetic and cytological studies.
RESULTS: Morphological observations indicated that B. myanmarica was distinguishable from the two known species of sect. Monopteron by the leaf shape and size, 1-locular ovary, parietal placentation and chromosome number. Molecular phylogenetic analysis using nrITS sequences showed that B. myanmarica was not allied with the clade of sect. Monopteron, though both were nested within sect. Platycentrum-sect. Sphenanthera clade.
CONCLUSIONS: Studies of morphology, molecular phylogenetics and cytology support the recognition of the new species, Begonia myanmarica, which is fully described and illustrated. Our results also indicate that B. myanmarica is not closely related to species previously assigned to sect. Monopteron, suggesting that the fruit morphology of a single developed wing in the ovary/fruit characterizing sect. Monopteron is homoplasious.

Background

Begonia L. (Begoniaceae), comprising more than 1800 species classified into 68 sections (Doorenbos et al. 1998; Hughes et al. 2015; Christenhusz and Byng 2016), is one of the largest genera of vascular plants. With more than 760 Begonia species in Asia, Doorenbos et al. (1998) recognized 18 sections [Alicida C.B. Clarke, Apterobegonia Warb., Baryandra A. DC., Bracteibegonia A. DC., Coleocentrum Irmscher, Diploclinium (Lindl.) A. DC., Haagea (Klotzsch) A. DC., Heeringia Irmscher, Lauchea (Klotzsch) A. DC., Monophyllon A. DC., Monopteron (A. DC.) Warb., Parvibegonia A. DC., Petermannia (Klotzsch) A. DC., Platycentrum (Klotzsch) A. DC., Putzeysia (Klotzsch) A. DC., Reichenheimia (Klotzsch) A. DC., Ridleyella Irmscher, and Sphenanthera (Hassk.) Warb.]. Thereafter, four additional Asian sections were proposed [Leprosae (T.C. Ku) Y.M. Shui, Monolobium T.C. Ku, Pleiothece T.C. Ku, and Symbegonia (Warb.) G. Forrest & Hollingsw.] (Ku 1999; Shui et al. 2002; Forrest and Hollingsworth 2003; Ku et al. 2007). These 22 Asian sections are highly unequal in species numbers: eight of the large sections (Petermannia, Platycentrum, Diploclinium, Reichenheimia, Coleocentrum, Parvibegonia, Sphenanthera, and Symbegonia) comprise 95% of Asian Begonia species and the rest 14 sections each with less than five species (Thomas 2010). Several molecular phylogenetic studies have demonstrated the paraphyly or polyphyly of these large sections, suggesting homoplasy of morphological characters used for current sectional delimitations (Tebbitt et al. 2006; Thomas et al. 2011; Chung et al. 2014). However, few studies have tested the monophyly of small Asian section of Begonia thus far [but see Rajbhandary (2010); Rubite (2010); Thomas (2010)].

Myanmar is botanically a most interesting country, but there have been no critical floristic surveys for nearly half a century. Thus far about 60 species of Begonia have been recorded from Myanmar (Hughes 2008; Tanaka and Hughes 2007; Tanaka and Hayami 2011; Peng et al. 2014b; Tanaka and Peng 2016). During the fieldwork in western Myanmar on 2 February 2012, the second author (YDK) collected an unknown Begonia with only one developed wing in ovary/fruit, which is the key character of Begonia sect. Monopteron sensu Doorenbos et al. (1998) first delimited by de Candolle (1864) as Mezierea sect. Monopteron. Presently, only two species, B. griffithiana Warb. and B. nepalensis Warb., are recognized in sect. Monopteron (de Candolle 1864; Doorenbos et al. 1998). Begonia nepalensis, the type species of sect. Monopteron, is native to Bhutan, Nepal and India (Fig. 5; Doorenbos et al. 1998; Hughes et al. 2015). Its chromosome number was reported to be 2n = 16 (Legro and Doorenbos 1971), with an uncertain chromosome count 2n = 28–42 by Sharma and Bhattacharyya (1961). Begonia griffithiana, occurring in Bhutan and India (Fig. 5), is characterized by lanceolate to oblong leaves with subcordate base. Chromosome number of B. griffithiana was documented as 2n = 22 (Doorenbos et al. 1998). Based on recent systematics and phylogenetics of Begonia, sect. Monopteron is nested within the Platycentrum-Sphenanthera clade (Rubite 2010; Thomas 2010; Rajbhandary et al. 2011; Leong 2017).

Fig. 5

Distribution map of Begonia griffithiana (circle), B. myanmarica (star), B. nepalensis (trangle). Distribution data of B. griffithiana and B. nepalensis is based on GPS data in the Begonia Resource Center (Hughes et al. 2015)

Although morphology of the 1-winged ovary/capsule of the undescribed Begonia should be assigned to sect. Monopteron, it differs from B. griffithiana and B. nepalensis significantly the leaf shape, leaf size and distribution. In this study, we described it as a new species. We also provide detailed morphological data and molecular phylogenetic analysis to elucidate the sectional assignment for this species.

Methods

Morphological observations

Rhizomes of Begonia myanmarica collected by YDK from Myanmar were cultivated in the experimental greenhouse of the Biodiversity Research Center, Academia Sinica, Taipei, Taiwan. Fully grown plants with flowers and fruits (Peng 23565, 23566) were used for morphological observation. The two species of sect. Monopteron, B. griffithiana (Peng 20851) and B. nepalensis (Peng 20854), cultivated in the greenhouse were also studied as a comparison.

Chromosome preparations

Root tips were obtained from cultivated materials from greenhouse of Academic Sinica. Somatic chromosome of the new species, B. myanmarica (Peng 23566), and two species of sect. Monopteron: B. griffithiana (Peng 20851) and B. nepalensis (Peng 20854), were examined using root tips following the methods by Peng et al. (2014a).

Phylogenetic analyses

DNA sequences of the nuclear ribosomal internal transcribed spacer (nrITS) were used to evaluate the phylogenetic relationship among new species and the two species of sect. Monopteron. DNA extraction, PCR amplification and DNA sequencing followed Chung et al. (2014). To test the monophyly of sect. Monopteron and sectional assignment of new species, nrITS of 96 species used in Chung et al. (2014) were adopted for phylogenetic analysis (see Appendix for details). Alignment was conducted using MUSCLE implemented in MEGA5.2 (Tamura et al. 2011) and verified in Mesquite v3.03 (Maddison and Maddison 2015). Phylogenetic relationships were constructed by Bayesian Inference (BI) method. The best nucleotide substitution models were determined by Modeltest v2.7 (Posada and Crandall 1998). For BI analysis, the consensus topology was based on Markov chains algorithm implemented in MRBAYES 3.0b4 (Huelsenbeck and Ronquist 2001). Four chains of Markov chain Monte Carlo (MCMC) simulation were carried out for 1,500,000 generations each with trees sampled per 500 generations. The first 500 trees of sampled trees were discarded before the node probability was calculated (posterior probability: PP).

Taxonomic treatment

Begonia myanmarica C.-I Peng & Y. D. Kim, sp. nov. (Figs. 1, 2)

Fig. 1

Begonia myanmarica C.-I Peng & Y. D. Kim. A Habit. B Leaf adaxial surface. C Stipule. D Male flower, face view, D′ Male flower, side view. E, E′, E″, E‴ Stamen. F Female flower, face view, F′ Female flower, side view. G Style and stigma. H, H′, H″, H‴ Cross section of ovary. I Capsule

Fig. 2

Begonia myanmarica C.-I Peng & Y. D. Kim. a Habit and habitat. b Cultivated plant at anthesis. c Leaf abaxial view. d Stipule. e Bract. f Male flower, face view. g Male flower, side view. h Female flower, face view. i Female flower, side view. j Cross section of ovaries. k Capsule

Type

MYANMAR. Sagaing Region, Alangdaw Kathapa National Park, 22°18′ 47.7″ N, 94°28′32.7″ E, alt. 438 m, mixed deciduous forest, along the stream. Living collection made by Seong-hyun Cho, Young-dong Kim, Yong-in Kim & Jeong-hun Lee MM-0611, 2 Feb 2012; type specimens (with flowers and fruits) pressed from plants cultivated in the experimental greenhouse, Academia Sinica, Taipei, Taiwan, 20 Mar 2016, Ching-I Peng 23566 (holotype: RAF; isotypes: HAST, KB).

Diagnosis

Begonia myanmarica is a unique species with an erect habit; large, ovate to broadly ovate leaves (ca. 20–40 cm long, 22–30 mm across); sole, much protruded wing in ovary/fruit; 1-locular ovary with parietal placentation and 2 placentae; and the somatic chromosomes are determined as 2n = 38.

Herbs monoecious, perennial. Rhizomes stout, 2–4 cm across, to 9 cm long; erect stem 50–90 cm tall, 1–2 cm thick, internodes 5–15 cm, glabrous. Stipules caducous, glabrous, triangular, apex aristate or apiculate, margin entire, 7–15 mm long, 6–15 mm wide. Leaves alternate, green with slightly paler veins; petiole 20–40 cm long, 2.2–3 cm across, glabrous; leaf blade fleshy, asymmetric, ovate to broadly ovate, 21–35 cm long, 15–28 cm wide, upper surface glabrous, underside slightly hairy on veins, base obliquely cordate, margin irregularly loosely serrulate or denticulate, apex acute or short acuminate; venation 7-or 8-palmate. Inflorescence mainly terminal but also axillary racemes of dichasial cymes, bisexual, protandrous; cyme 10–15 cm long, with 2 female flowers at apex and 7–9 male flowers at base, peduncle 2–5 cm long, glabrous; bracts deciduous, ovate to triangular, apex acuminate, margin entire, 0.7–1.5 cm long, 0.4–0.6 cm wide. Male flower: pedicel 2–2.8 cm long, glabrous; tepals 4, white to pinkish, outer 2 ovate or orbicular, 1.9–2.3 cm long, 1.5–2 cm wide, inner 2, broadly oblong, 1.8–2.3 cm long, 0.5–1.5 cm wide, glabrous; androecium actinomorphic, subglobose, ca. 0.7 cm across, stamens 60–80, yellow, clavate; filaments ca. 2 mm long, fused to a short central column; anthers 1–1.2 mm long, apex truncate. Female flower: pedicel 1.7–3.5 cm long, glabrous; ovary white, wings 3, manifestly unequal, 2 side wings almost undeveloped, abaxial wing much protruded, white to pale greenish or pinkish, 1-locular; placentation parietal, placentae 2, each bilamellate; tepals 5, white in the greenhouse (pinkish in the wild), unequal, elliptic to obovate, 1.5–2 cm long, 0.5–1.1 cm wide, apex obtuse; styles 2, ca. 5 mm long, 2- or 3-cleft, fused at base, stigmatic band wavy-twisted and spiralled. Capsule nodding, stalk 3.5–5.5 cm long, abaxial wing triangular to rectangular, 2.4–3.8 cm tall, 2.0–2.4 cm wide, lateral two wings barely developed, rounded, 0.2–0.4 cm tall, 1.8–2.2 cm wide. Seeds barrel-shaped, 0.25–0.3 mm long.

Distribution and habit

Known only from the type locality.

Etymology

The epithet refers to Myanmar (formerly Burma) where it was discovered.

Additional specimens examined

MYANMAR. Sagaing Region: Alang Daw Kathapa National Park, 22°18′47.7″ N, 94°28′32.7″ E, 438 m, 2 Feb 2012, Peng 23565 (HAST); 22°18′49.5″ N, 94°28′30.2″ E, 434 m, 2 Feb 2012, MM-0556 (KB); 22°18′47.7″ N, 94°28′32.7″ E, 438 m, 2 Feb 2012, MM-0611 (KB, HHU); 22°18′44.2″ N, 94°28′28.7″ E, 380 m, 2 Feb 2012, MM-0616 (KB); 22°19′25″ N, 94°29′37.7″ E, 380 m, 5 Feb 2012, MM-0848 (KB, RAF).

Chromosome cytology

Somatic chromosome at metaphase of B. myanmarica were shown to be 2n = 38 in this study (Fig. 4c).

Fig. 4

Somatic chromosomes at metaphase of Begonia. a B. griffithiana (2n = 16, Peng 20851). b B. nepalensis (2n = 16, Peng 20854). c B. myanmarica (2n = 38, Peng 23566). Scale bar 5 µm

Discussion

Begonia myanmarica has only one developed wing in ovary/fruit (Figs. 2k, 3c, d), the key character of sect. Monopteron in Begonia (Doorenbos et al. 1998). The new species, however, deviates from sect. Monopteron with axillary placentation and two locules in ovary (Fig. 3e, f) in having 1-locular ovary and parietal placentation (Fig. 2j). Additionally, B. myanmarica has ovate to broadly ovate leaves and large leaves (ca. 20–40 cm long, 22–30 mm across) (Fig. 2b, c), whereas leaves of B. griffithiana and B. nepalensis are lanceolate to oblong and no longer than 20 × 10 cm (Fig. 3a, b). Cytologically, somatic chromosome of B. myanmarica is determined to be 2n = 38 (Fig. 4c), while chromosomes of B. griffithiana and B. nepalensis are both 2n = 16 (Fig. 4a, b) in our study. Geographically, B. myanmarica is endemic to Myanmar while B. griffithiana and B. nepalensis are distributed in India, Nepal and Bhutan (Fig. 5). We concluded that B. myanmarica is sharply distinct from B. griffithiana and B. nepalensis.

Fig. 3

Overview of leaves, fruit and cross section of ovary in Begonia griffithiana and B. nepalensis. a, b Leaves. c, d Fruit. e, f Cross section of ovary. Begonia griffithiana (a, c, e). Begonia nepalensis (b, d, f)

In our molecular phylogenetic study, B. griffithiana and B. nepalensis form a strongly supported clade (posterior probability, PP = 1) nested within the clade dominated by sect. Platycentrum-sect. Sphenanthera clade [Fig. 6; Clade PLA-SPH in Chung et al. (2014)], congruent with the studies of Rubite (2010), Thomas (2010), Rajbhandary et al. (2011), and Leong (2017). Two sampled individuals of B. myanmarica also fall within the Clade PLA-SPH but not clustered within sect. Monopteron clade (Fig. 6), suggesting that the fruit morphology of a single developed wing in the ovary/fruit is homoplasious. Morphologically, the key characters for sect. Platycentrum-sect. Sphenanthera clade are evergreen, rhizomatous and two locules in ovary (Leong 2017). Begonia myanmarica is evergreen with stout rhizome, but having 1-locular ovary. Compared with other species in sect. Platycentrum-sect. Sphenanthera, B. myanmarica is unique with a single developed wing and having 1-locular ovary not known in any other taxa in this clade. Further studies with increasing sampling of Myanmar Begonia are needed to place B. myanmarica in its proper infrageneric position.

Fig. 6

Phylogenetic tree of section Platycentrum-section Sphenanthera in Begonia generated from Bayesian analysis of nrITS sequence data. Numbers on the branches indicate posterior probability of Bayesian inference analysis. The inset is simplified phylogenetic tree based on the nrITS dataset and sectional classification of Chung et al. (2014)

Conclusion

Studies of morphology, molecular phylogenetics and cytology support the recognition of the new species, Begonia myanmarica, which is fully described and illustrated. Our results also indicate that B. myanmarica is not closely related to species previously assigned to sect. Monopteron, suggesting that the fruit morphology of a single developed wing in the ovary/fruit characterizing sect. Monopteron is homoplasious.