In biological landscapes, spatial genetic structure of species may occur as the result of historical processes. Such partitioning can influence patterns of genetic variation both within and between populations of the same species. In this study, we used 2 chloroplast markers to measure genetic variation within and among five populations of the lady of the night orchid, Brassavola nodosa, in the Guanacaste province of northwestern Costa Rica. We addressed the following questions. Are there patterns of genetic structure within and among populations of Brassavola nodosa? If so, what do they imply about levels of gene flow: are populations in close proximity to one another fairly isolated, or do they exhibit panmixia? We tested this hypothesis by sequencing the trnL-rpL32F region and trnG intron of the chloroplast genome. A haplotype network was used to compare genetic variation and phylogenetic structure with spatial distribution of populations.
Results/Conclusions
A haplotype network constructed from chloroplast DNA sequences revealed 8 haplotypes across 186 individuals collected from 5 locales. Within each geographic region (north/south), gene flow was found to occur between populations, with considerably less structure in the north than in the south. This distribution of the maternal lineage in these orchids indicates that seed dispersal is widespread. Notably, there was a distinct phylogeographic split of 12 mutational events between the two northern and two southern populations (about 60 km apart). The fifth, centrally located population contained haplotypes found in both the northern and southern regions. This major genetic split between north and south has been documented in other plant species in the Guanacaste region, suggesting that a common geographic influence may result in concordant phylogeographic structure.