Fitness consequences of hybridization between ecotypes of Avena barbata: hybrid breakdown, hybrid vigor, and transgressive segregation.

Hybridization is an important factor in the evolution of plants; however, many of the studies that have examined hybrid fitness have been concerned with the study of early generation hybrids. We examined the early- and late-generation fitness consequences of hybridization between two ecotypes of the selfing annual Avena barbata in a greenhouse environment as well as in two natural environments. Fitness of early generation (F2) hybrids reflects both the action of dominance effects (hybrid vigor) and recombination (hybrid breakdown) and was not significantly different from that of the midparent in any environment. Fitness of later generation (F6) recombinant inbred lines (RILS) derived from the cross reflect both the loss of early generation heterozygosity as well as disruption of any coadapted gene complexes present in the parents. In all environments, F6 RILs were on average significantly less fit than the (equally homozygous) midparent, indicating hybrid breakdown through the disruption of epistatic interactions. However, the inbred F6 were also less fit than the heterozygous F2, indicating that hybrid vigor also occurs in A. barbata, and counteracts hybrid breakdown in early generation hybrids. Also, although the F6 generation mean is lower than the midparent mean, there are individual genotypes within the F6 generation that are capable of outperforming the parental ecotypes in the greenhouse. Fewer hybrid genotypes are capable of outperforming the parental ecotypes in the field. Overall, these experiments demonstrate how a single hybridization event can result in a number of outcomes including hybrid vigor, hybrid breakdown, and transgressive segregation, which interact to determine long-term hybrid fitness.