Genetic factors associated with mating system cause a partial reproductive barrier between two parapatric species of Leavenworthia (Brassicaceae)

Reproductive barriers play a major role in the origin and maintenance of biodiversity by restricting gene flow between species. Although both pre- and postzygotic barriers often isolate species, prezygotic barriers are thought to contribute more to reproductive isolation. We investigated possible reproductive barriers between Leavenworthia alabamica and L. crassa, parapatric species with high morphological and ecological similarity and the ability to hybridize. Using greenhouse and field experiments, we tested for habitat isolation and genetic incompatibilities. From controlled crosses, we identified unilateral incompatibility (a partial prezygotic barrier associated with the self-incompatibility system), but no evidence of other genetic incompatibilities. We found a small reduction in pollen viability of F1 hybrids and early germination of F1, F2, and BC hybrids relative to L. alabamica and L. crassa in a common garden experiment, but the effect on fitness was not tested. Field studies of hybrid pollen viability and germination are needed to determine if they contribute to reproductive isolation. In a reciprocal transplant, we found no evidence of habitat isolation or reduced hybrid survival (from seedling to adult stage) or reproduction. These data suggest unilateral incompatibility partially reproductively isolates L. alabamica and L. crassa, but no other reproductive barriers could be detected