Reproductive barriers between the cedar glade endemics Leavenworthia alabamica and L. crassa
Koelling, Vanessa Anne
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Speciation, the process by which new species form, is an important evolutionary process that generates biodiversity. The evolution of barriers to reproduction between populations is considered an integral part of the speciation process. Possible barriers can include genetic incompatibilities, habitat isolation, and mating system isolation. In the present work, I describe studies aimed at elucidating the presence of these barriers between Leavenworthia alabamica and L. crassa, two species in the family Brassicaceae that have been the subject of studies of the evolution of self-fertilization from self-incompatibility, but not previously studied with respect to speciation. To study reproductive barriers in this system, I performed the following: 1) crosses in the greenhouse between L. alabamica and L. crassa to test for genetic incompatibilities at the level of seeds produced, 2) common garden analyses of hybrid versus parent species floral morphology to test for genetic incompatibilities manifest at the phenotypic level, 3) a reciprocal transplant using both parent species and hybrid individuals to test for habitat isolation and genetic incompatibilities in the field, and 4) an analysis of genetic structure, diversity, and mating system of populations to test for mating system isolation. From the crosses, I found no evidence for genetic incompatibilities, other than the phenomenon known as unilateral incompatibility, which limits mating between self-incompatible and self-compatible individuals. In addition, hybrids produced in crosses had floral phenotypes within the range of the parent species, although hybrids tended to be intermediate for most traits measured. Hybrids did, however, germinate more quickly than the parent species, which could impact their fitness in the field. The results of the reciprocal transplant yielded no evidence of habitat isolation or of reduced hybrid fitness in the field. L. alabamica and L. crassa had similarly high levels of genetic structure and diversity, and genetic diversity was lower in self-compatible populations than self-incompatible. I also found that genetic structure was higher between self-incompatible compared to self-compatible populations than between comparisons of self-incompatible populations, indicating that mating system isolation may be present in this system. Thus, unilateral incompatibility and mating system isolation may limit mating between L. alabamica and L. crassa.