Fitness Consequences of Hybridization Between Ecotypes of Avena Barbata: Hybrid Breakdown, Hybrid Vigor, and Transgressive Segregation

Abstract

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.

Genetic admixture and heterosis may enhance the invasiveness of common ragweed

“…The latter prediction is in line with the large variation in performance of native populations in our study as well as previous studies showing high differentiation and significant isolation by distance among native populations of A. artemisiifolia (Genton et al., 2005). On the other hand, admixed genotypes can be expected to exhibit outbreeding depression due to genetic incompatibilities arising from hybridization between differentiated populations, or through the loss of local adaptation following the introduction of unfavorable alleles or the breakup of adapted gene complexes (in particular in the F2 and later generations after recombination) (Johansen‐Morris & Latta, 2006). Although in our experiment, there was some evidence for outbreeding depression, the overall pattern in our study indicates that the benefits outweighed potential negative effects of admixture in the majority of crosses.…”

Epistasis in natural populations of a predominantly selfing plant

“…A situation can be even more complicated with both mechanisms simultaneously present and operating at the similar spatial scale p2 m at which major seed dispersal occurs (Volis et al, 2010). In contrast, some gene flow through sexual recombination seems to be important in a predominant selfer to prevent accumulation of mildly deleterious mutations (Fritsche and Kaltz, 2000;Lenormand and Otto, 2000;Johansen-Morris and Latta, 2006) and fixation of beneficial alleles in different genetic neighborhoods (Hill and Robertson, 1966;Felsenstein, 1974). In predominant selfers, occasional sexual recombination creates new combinations of alleles among and within loci, of which only a small fraction will persist over generations because of higher fitness than in existing combinations.…”

“…There are three possible routes for divergent epistasis to evolve. The structure of genetically divergent neighborhoods can be a result of divergent selection resulting from spatial heterogeneity, for example, soil or microclimatic differences (Allard et al, 1972;Latta et al, 2004;Johansen-Morris and Latta, 2006;Ramakrishnan et al, 2006). Such structure can be also caused by genetic drift under limited seed and pollen dispersal (Williams, 1994;Siol et al, 2008), and strongly resemble the micro-environmental selection effect when seed dispersal and environmental heterogeneity occur at similar scale (Volis et al, 2004a).…”

Linkage Maps of thedwarfand Normal Lake Whitefish (Coregonus clupeaformis) Species Complex and Their Hybrids Reveal the Genetic Architecture of Population Divergence

“…For example, emerging empirical studies on the fitness consequences of genotypes in inbred and outbred crosses are also finding that the fitness of early generation hybrids may reflect both the action of dominance effects (hybrid vigor) and recombination (hybrid breakdown) within the same family (e.g., Pélabon et al 2005;Johansen-Morris and Latta 2006;Ferreira and Amos 2007). These experiments demonstrate how a single hybridization event within one family can result in a number of outcomes, including hybrid vigor, hybrid breakdown, and transgressive segregation, which ultimately interact to determine long-term hybrid fitness.…”