Speciation and gene flow in Central American Begonia L. (Begoniaceae)

Abstract

Begonia L. is one of the largest plant genera, comprising over 1500 species. Weak species cohesion, and the rapid evolution of reproductive barriers in allopatry, are two processes that have been postulated to explain the generation of such hyper-diversity of taxa within a single genus of plants. The aim of this thesis is to investigate whether these factors are likely to have been important contributors to the diversity of species found in Central American Begonia. Species cohesion was analysed in the widespread Central American species Begonia heracleifolia and B. nelumbiifolia. Interpopulation seed flow was estimated with seven plastid microsatellites. Breeding system estimates and measures of genetic differentiation at nine nuclear microsatellites were used to infer levels of interpopulation pollen flow. Controlled crosses were employed to assess the strength of reproductive barriers both between populations within species, and between species differing in ecology. The potential for gene flow between species in the wild was assessed in natural hybrid zones using molecular markers. Finally a quantitative trait locus (QTL) approach was employed to investigate the genetic basis of reproductive traits that differ between species. No plastid polymorphisms were found in B. nelumbiifolia, suggesting it has been through a recent population bottleneck. In contrast, B. heracleifolia possessed many plastid haplotypes that were strongly differentiated between populations (G’ST = 0.829). Nuclear microsatellites showed high genetic differentiation within species, and both species were self-compatible and self-fertilize at a moderate rate (B. heracleifolia F’ST = 0.506, FIS = 0.249; B. nelumbiifolia F’ST = 0.439, FIS = 0.380). F1s between ecologically similar B. heracleifolia and B. sericoneura were partly fertile (2-5% seed set), and F1s and early generation backcrosses were found in a hybrid swarm. F1s between B. heracleifolia and the ecologically contrasting B. nelumbiifolia were pollen sterile, and 3 hybrid swarms showed no evidence of hybrids beyond the F1 generation. Seven QTL were found for reproductive traits, including: sex ratio, pollen sterility and stamen number. The population biology of Begonia, with limited seed and pollen dispersal, small population sizes and frequent self-fertilization predisposes them to genetic isolation, increasing the chances that reproductive barriers evolve. These characteristics may underlie the large number of endemics in Begonia.