Homoploid hybrid speciation in Louisiana Iris

Background/Question/Methods

Natural hybridization is common in plants and has played an important role in the evolution of plant biodiversity. Although hybridization commonly results in the production of hybrids that are less fit than the parental species, some hybrid lineages may be fit in a novel habitat such that the hybrid lineage can diverge from the progenitor species. Homoploid hybrid speciation involves the evolution of reproductive isolation between such a hybrid lineage and the originally hybridizing taxa. Iris nelsonii is endemic to a deeply shaded cypress swamp system in southern Louisiana and a potential indicator species for the status of this vital ecosystem. When originally described, I. nelsonii was hypothesized to be a homoploid hybrid species derived from hybridization between three widespread species of Louisiana Iris (Iridaceae): Iris brevicaulis, I. fulva, and I. hexagona. In order to investigate the hypothesized hybrid origin of I. nelsonii, we first conducted a survey of genetic variation at gene-based markers in the three widespread species and I. nelsonii. Second, in order to understand ecological divergence between I. nelsonii and its progenitors, we conducted a common garden experiment to assay the response of the four species to different water and light-levels that characterize their divergent habitats. 

Results/Conclusions

Results from the molecular work reveal that I. nelsonii shares genetic variation with all three purported progenitor species. Furthermore, in the common garden experiment, I. nelsonii responded to light and water availability differently than its progenitors. Specifically, of all four species, I. nelsonii was least affected by heavy shading. These results support the hypothesis that I. nelsonii is indeed a homoploid hybrid species and that this species is ecologically divergent from the progenitor species in its ability to inhabit deeply shaded cypress swamps in southern Louisiana. Such information on the population genetics and ecology of I. nelsonii is vital to conservation efforts focused on this endemic species.