Multifaceted, Cross-Generational Costs of Hybridization in Sibling Drosophila species

Abstract

Maladaptive hybridization, as determined by the pattern and intensity of selection against hybrid individuals, is an important factor contributing to the evolution of prezygotic reproductive isolation. To identify the consequences of hybridization between Drosophila pseudoobscura and D. persimilis, we estimated multiple fitness components for F1 hybrids and backcross progeny and used these to compare the relative fitness of parental species and their hybrids across two generations. We document many sources of intrinsic (developmental) and extrinsic (ecological) selection that dramatically increase the fitness costs of hybridization beyond the well-documented F1 male sterility in this model system. Our results indicate that the cost of hybridization accrues over multiple generations and reinforcement in this system is driven by selection against hybridization above and beyond the cost of hybrid male sterility; we estimate a fitness loss of >95% relative to the parental species across two generations of hybridization. Our findings demonstrate the importance of estimating hybridization costs using multiple fitness measures from multiple generations in an ecologically relevant context; so doing can reveal intense postzygotic selection against hybridization and thus, an enhanced role for reinforcement in the evolution of populations and diversification of species.

Aspects, Including Pitfalls, of Temporal Sampling of Flying Insects, with Special Reference to Aphids

“…As is well known in biology, hybrids may have hybrid vigour and may sometimes be fertile, but often they have fitness costs and intermediate behaviours, which may be suboptimal in terms of survival, such that they are selected against. For example, in hybridization experiments with Drosophila pseudoobscura Frolova and D. persimilis Dobzhansky & Epling, Myers et al [135] showed that “ the cost of hybridization accrues over multiple generations and reinforcement in this system is driven by selection against hybridization above and beyond the cost of hybrid male sterility; we estimate a fitness loss of >95% relative to the parental species across two generations of hybridization .” Similarly, in sympatric hybridization of Hawaiian Drosophila, D. heteroneura Perkins and D. silvestris Basden, hybrids of both sexes were fertile but hybridization was not extensive (1.1% at three sites, i.e., 6 of 528 flies of both species surveyed; [136]). As well as hybridization per se, various levels of genetic introgression may occur, leading, as with the Sitobion aphids earlier discussed, to asymmetry in the insects resulting from males and females of the different parent species: “….…”