Genotype-by-environment interaction and the fitness of plant hybrids in the wild

Natural hybrid zones between related species illustrate processes that cont ribute to genetic differentiation and species formation. A common viewpoint is that hybrids are essentially unlit, but they exist in a stable tension zone where selection against them is balanced by gene flow between the pare nt species. An alternative idea is that selection depends on the environmen t, for example, by favoring opposite traits in the two parental habitats or favoring hybrids within a bounded region. To determine whether selection o f hybrids is environment dependent, we crossed plants of naturally hybridiz ing Ipomopsis aggregata and I. tenuituba in the Colorado Rocky Mountains an d reciprocally planted the seed offspring into a suite of natural environme nts across the hybrid zone. All types of crosses produced similar numbers a nd weights of seeds. However, survival of the offspring after 5 years diffe red markedly among cross types. On average, the F, hybrids had survival and growth rates as high as the average for their parents. But hybrid survival depended strongly on the direction of a cross, that is, on which species s erved as the maternal parent. This fitness difference between reciprocal hy brids appeared only in the parental environments, suggesting cytonuclear ge ne interactions that are environment specific. These results indicate that complex genotype-by-environment interactions can contribute to the evolutio nary out come of hybridization.