Begonia elachista Moonlight & Tebbitt sp. nov., an enigmatic new species and a new section of Begonia (Begoniaceae) from Peru

The world’s smallest Begonia, Begonia elachista Moonlight & Tebbitt sp. nov., is described and illustrated from a limestone outcrop in the Amazonian lowlands of Pasco Region, Peru. It is placed within the newly described, monotypic Begonia sect. Microtuberosa Moonlight & Tebbitt sect. nov. and the phylogenetic affinities of the section are examined. Begonia elachista sp. nov. is considered Critically Endangered under the International Union for the Conservation of Nature (IUCN) criteria.


Introduction
Begonia L. is a megadiverse, pantropically distributed genus with 1821 currently accepted species (Hughes et al. 2015). The majority of species are understory herbs and shrubs with many prized in the horticulture industry. L'Héritier de Brutelle described the fi rst Begonia species from Peru, B. octopetala L'Hér., in 1788 (L'Héritier de Brutelle 1788). Since this date, the number of Begonia species known from the country has increased rapidly. Smith & Schubert (1941) covered 34 species in their treatment of the Begoniaceae of Peru, although it did not cover all species known from the country at the time (e.g. B. albomaculata C.DC., described from Peru in 1906(de Candolle 1906). Recent fi eld and herbarium work continues to yield new species (e.g. Tebbitt 2011Tebbitt , 2015Tebbitt , 2016 and new records for Peru (e.g. Tebbitt et al. 2015). Hughes et al. (2015) now list more than 75 species classifi ed in 15 sections from Peru and at least ten species remain undescribed (unpubl. data). The Begonia fl ora of Peru is now the third largest in the Americas after Brazil (242 species) and Colombia (101 species). This study describes a highly unusual new species (B. elachista Moonlight & Tebbitt sp. nov.) from a limestone outcrop in lowland Pasco, Peru.
European Journal of Taxonomy 281: 1-13 (2017) 2 The phylogenetic relationships within Neotropical Begonia were fi rst investigated in depth by Moonlight et al. (2015), who used three cpDNA markers and identifi ed two clades of American Begonia within paraphyletic African Begonia: Neotropical Clade 1 (NC1) and Neotropical Clade 2 (NC2). This study presents a phylogeny of NC2 with increased sampling. We demonstrate that B. elachista sp. nov. is distantly related to all other tuberous western South American species and sections of Begonia; is resolved outside all closely related sections; and, given the species' unusual morphology, we describe a new section to encompass it (B. sect. Microtuberosa Moonlight & Tebbitt sect. nov.).

Phylogenetics
The dataset consisted of data from three non-coding plastid DNA regions (ndhA intron, nhdF-rpl32 spacer and rpl32-trnL spacer) and 68 species of Begonia (see Appendix). Species were chosen to be representative of all major groups within NC2 with B. sect. Augustia (Klotzsch) A.DC. chosen as an outgroup. A particular emphasis was placed upon the inclusion of other tuberous, western South American species and sections of Begonia. Ninety two sequences were newly generated for this analysis following the methods described in Moonlight et al. (2015).
Sequences were aligned manually in BioEdit v.7.2.5 (Hall 1999). Bayesian phylogenetic reconstruction was carried out in MrBayes v.3.2.1. (Huelsenbeck & Ronquist 2001). Models of molecular evolution were determined with jModelTest 2.1.7 (Darriba et al. 2012) on a maximum-likelihood topology with the Bayesian information criterion, resulting in the selection of the GTR+I+Γ model. Two searches each comprising two Markov chain Monte Carlo chains were run for 2.5 × 10 7 generations and sampled every 2500 generations with the burn-in determined as 6.25 × 10 6 generations following analysis of time series plots in Tracer v.1.6 (Rambaut & Drummond 2013) to ensure adequate sample size.

Taxonomic descriptions
The descriptions of B. sect. Microtuberosa sect. nov. and B. elachista sp. nov. presented herein are derived from herbarium material, material grown at the Royal Botanic Garden Edinburgh, and fi eld observations made by the authors during an expedition to Peru in 2016. Stable links to specimens held at E are included as hyperlinks and images of all cited specimens are available from Hughes et al. (2015). Comparisons to other sections of Begonia were made by reference to Doorenbos et al. (1998) and through reference to living material grown at the Royal Botanic Garden Edinburgh and Glasgow Botanic Gardens.

Phylogenetics
The topology of the 50% majority rule consensus tree ( Fig. 1) is entirely consistent with that of Moonlight et al. (2015). We resolve B elachista sp. nov. within a clade of exclusively eastern South American species. The monophyly of B. elachista sp. nov. is well-supported (p = 1.00) and it is resolved as sister to three species of B. sect. Gaerdtia (Klotzsch) A.DC., which will form the reinstated B. sect. Pereira Brade (Moonlight et al. in prep.), and B. sect. Trachelocarpus (Müll.Berol.) A.DC. as reciprocally monophyletic groups. This placement is moderately well-supported (p = 0.88). We resolve all other tuberous western South American species within a distantly-related clade containing all sampled members of B. sect.
Trachelocarpus. The majority of both fl oral and vegetative characters are, however, markedly different among the three sections.
Begonia sect. Microtuberosa sect. nov. is readily identifi ed as the only Neotropical section of Begonia with male fl owers with four or fewer stamens, and the combination of ovaries with two or three locules and entire placentas, and a tuberous habit.

Etymology
The name 'Microtuberosa' emphasises the diminutive and tuberous habit of the type species.

Distribution
On a limestone outcrop in lowland Amazonian Peru to the east of the Chemillén Cordillera at an altitude of 430 m. Diagnosis Begonia elachista sp. nov. is a highly distinct species with an unusual combination of features that is easily recognized as the only Peruvian species of Begonia that reaches maturity at fewer than 5 cm in height. It is also unique within Peru in having ovate leaves smaller than 3 × 3 cm and a combination of entire placentae and a tuberous habit.

Distribution and habitat
Begonia elachista sp. nov. is known only from the type locality in the Peruvian region of Pasco (Oxapampa Province) and has been collected on calcareous rocks by the entrance to a cave within primary lowland Amazonian forest, at an altitude of 430 m. It was observed growing on rocks free from other vascular plants in association with various bryophyte species in the almost continual shade of the surrounding forest.

Conservation status
The known range of B. elachista sp. nov. consists of the area immediately around a single cave mouth and nearby limestone outcrops in sector Paujil of Parque Nacional Yanachaga-Chemillén (AOO < 1 km 2 ). There are no other known limestone outcrops in sector Paujil of Parque Nacional Yanachaga-Chemillén or the surrounding areas thus its range is likely limited to this site. Around 5,000 plants are found at this locality and the population appeared stable during fi eldwork in February 2016. Although the area is currently protected and tourists are prohibited from visiting the site, the national park authorities are considering building a tourist trail to the cave. The resulting increased footfall would put considerable pressure on the species' habitat and inevitably decrease the population size. Accordingly, we classify B. elachista sp. nov. as Critically Endangered: CR B2 ab(iii) (IUCN 2001).

Discussion
The tuberous habit and western South American range of B. elachista sp. nov. suggest a close relationship with other tuberous Andean species of Begonia but this is not supported by our analysis. The majority of tuberous Andean Begonia species are currently classifi ed in B. sect. Eupetalum, which is distinguished by its geophytic tubers or fl eshy rhizomes and relatively large fl owers (Doorenbos et al. 1998;Tebbitt 2015). That B. sect. Microtuberosa sect. nov. is nested within a group of eastern Brazilian species suggests it represents an independent dispersal across the Amazon basin, in addition to those identifi ed in Moonlight et al. (2015). The majority of lowland Amazonia represents unsuitable habitat for most Begonia species but the genus appears to have dispersed across the area multiple times. The discovery of a further dispersal event highlights the abundance of 'rare' long-distance dispersal events within Begonia.
Begonia elachista sp. nov. is one of a number of recently described species of Peruvian Begonia. The number of species known from this country has risen from the 34 covered in the Flora of Peru (Smith & Schubert 1941) to more than 75 today (Hughes et al. 2015). This continued rapid rate of species discovery suggests many more species may remain undiscovered within the country, particularly in limestone areas.
A number of species are contenders for the world's smallest Begonia, and many have been designated epithets emphasising their diminutive statures. The fi rst of these was B. minor Jacq. in 1787, although in this case the name only implied it was slightly smaller than the 20 or so species previously described, and it still grows to more than 1 m. Perhaps the most delicate known Begonia species is the Malaysian B. sibthorpioides Ridl., whose leaves and fl owers are both smaller than those of B. elachista sp. nov.; however, the stem of B. sibthorpioides trails to 10 cm. Malagasy Begonia also include a number of tiny species, including B. leandrii Humb., B. nana L'Hér. and B. perpusilla A.DC., while the smallest mainland African species is B. wilksii Sosef, but all exceed the 3 cm total height of B. elachista sp. nov. in either their infl orescence height or petiole length. The recently described Vietnamese B. minuscula Aver. and Sumatran B. lilliputana M.Hughes are both small, but the rhizomes of the former reach 6 cm while the leaf length of the latter equals the entire height of B. elachista sp. nov. Begonia sleumeri L.B.Sm. & B.G.Schub. is particularly notable in being a small tuberous species from Andean South America. This Argentinian species has smaller leaf blades of a similar size (1-3.1 × 1.4 -4.6 cm) to B. elachista sp. nov. but has larger tubers (0.5 -2 cm in diam.), sometimes has taller stems (up to 4.5 cm tall), often has longer petioles (1.