Data from: Genetic differentiation and species cohesion in two widespread Central American Begonia species

Begonia is one of the ten largest plant genera, with over 1500 species.
This high species richness may in part be explained by weak species
cohesion, which has allowed speciation by divergence in allopatry. In this
study, we investigate species cohesion in the widespread Central American
Begonia heracleifolia and Begonia nelumbiifolia, by genotyping populations
at microsatellite loci. We then test for post-zygotic reproductive
barriers using experimental crosses, and assess whether sterility barriers
are related to intraspecific changes in genome size, indicating major
genome restructuring between isolated populations. Strong population
substructure was found for B. heracleifolia (FST=0.364, F′ST=0.506) and B.
nelumbiifolia (FST=0.277, F′ST=0.439), and Bayesian admixture analysis
supports the division of most populations into discrete genetic clusters.
Moderate levels of inferred selfing (B. heracleifolia s=0.40, B.
nelumbiifolia s=0.62) and dispersal limitation are likely to have
contributed to significant genetic differentiation (B. heracleifolia
Jost’s D=0.274; B. nelumbiifolia D=0.294). Interpopulation crosses
involving a divergent B. heracleifolia population with a genome size ~10%
larger than the species mean had a ~20% reduction in pollen viability
compared with other outcrosses, supporting reproductive isolation being
polymorphic within the species. The population genetic data suggest that
Begonia populations are only weakly connected by gene flow, allowing
reproductive barriers to accumulate between the most isolated populations.
This supports allopatric divergence in situ being the precursor of
speciation in Begonia, and may also be a common speciation mechanism in
other tropical herbaceous plant groups.