Repeated Evolution of Asexuality Among Hybrids of A Recently Diverged Subspecies Pair in Boechera

Abstract

The impacts of hybridization on speciation and biodiversity can be influenced by the persistence and the reproductive mode of hybrids. Persistent sexual hybrids can facilitate gene flow between parent taxa, which can potentially lead to swamping, fusion or introgression, while ephemeral and asexual hybrids cannot. Across a range of taxa, hybrid asexuality has been found to be prevalent among taxon pairs with lower divergence than taxon pairs who display hybrid sterility and other classic reproductive isolation mechanisms, supporting the hypothesis that hybrid asexuality is an early post-zygotic barrier to gene flow. Understanding the evolution of hybrid asexuality, especially in terms of how often it evolves, is important to evaluate the extent of its influence as a reproductive barrier. In this dissertation, we ask if apomixis - a form of asexuality involving clonal reproduction by seed – has a single origin, or has evolved repeatedly and independently, among a population consisting of both sexual and apomict hybrids of a pair of recently diverged subspecies. We also ask if evidence of introgression, fusion or swamping, is present despite the high rates of asexuality among hybrids.We find strong evidence that asexuality likely evolves frequently, repeatedly, and independently, with apomicts being locally generated on fine scales (~1km). There is no evidence of swamping or fusion, and evidence of introgression is inconclusive; this is consistent with asexuality acting as a significant barrier to gene flow. However, ecological divergence could also explain retained parental differentiation, either alone or in conjunction with the barrier posed by asexuality.