Postzygotic reproductive isolation among Rhodohypoxis baurii varieties and Rhodohypoxis milloides (Hypoxidaceae)

Abstract

The biodiversity we see on earth is generated through the process of speciation. Understanding how species form is central to evolutionary biology, as species are the basic units of evolutionary biology. In order to define species, biologists use species concepts as frameworks to test hypotheses around species characterization. In this study, the Biological Species Concept was used because it emphasizes that reproductive isolation occurs through the accumulation of reproductive barriers that reduce gene flow between closely related taxa. Prezygotic reproductive barriers occur before fertilization, and include habitat and pollinator isolation. Postzygotic barriers occur after fertilization and mainly manifest in hybrids through intrinsic or as extrinsic hybrid inviabilities. Postzygotic barriers are detectable through hybrids, hybridization is an important process through which reproductive isolation can be studied. Hybridization is also important inevolutionary trajectories of species because it has the potential to lead to speciation. In this study, the aim was to test for the presence of postzygotic barriers among the three varieties of Rhodohypoxis baurii and R. milloides from two different localities. Data obtained from controlled greenhouse crosses showed successful interbreeding between the varieties, as well as between the varieties and R. milloides. Seeds germinated from these controlled crosses were used to test hybrid viability through germination success. The analysis of gene flow and population structure between the four taxa was done using amplified fragment polymorphisms (AFLPs). Flow cytometry was used to determine ploidy and found that R. baurii var. baurii and R. baurii var. confecta to be diploid, and R. baurii var. platypetala and R. milloides to be tetraploid. Ploidy was not found to affect hybrid or seed viability. Results also show that there are few prezygotic barriers between the three R. baurii varieties and the three R. baurii varieties and R. milloides. Both the Hebron Farm and Sunset Farm populations are highly admixed due to ongoing introgression. Postzygotic isolation between Rhodohypoxis baurii varieties and R. milloides does not manifest as seed inviability, but may be seen in later hybrid generations in the form

of extrinsic hybrid inviability, or as pollen inviability. The main findings imply that Rhodohypoxis baurii varieties cannot be elevated to specific level, and that hybridization and habitat isolation have defining roles in the evolutionary trajectory of the species.