Speciation and hybridization among Houstonia (Rubiaceae) species: the influence of polyploidy on reticulate evolution.
Chloroplast and nuclear DNA sequence variation among populations and species was used to examine the phylogenetic history and hybridization of the North American Houstonia lineage. The ancestral species in the lineage do not show evidence of hybridization; however, the more recently derived species in eastern North America contain a wide degree of morphological and genetic variation both within and among species. Furthermore, there is a clear association between hybridization and polyploidy in the Houstonia lineage, with all potential hybrids occurring among species that contain polyploid populations. This suggests that polyploidy may break down species barriers and allow hybridization among lineages. These results indirectly support speciation models that involve genetic incompatibilities among species arising from gene silencing or genomic reorganization.