A test for Y-linked additive and epistatic effects on surviving bacterial infections in Drosophila melanogaster.

Y- and W-chromosomes offer a theoretically powerful way for sexual dimorphism to evolve. Consistent with this possibility, Drosophila melanogaster Y-chromosomes can influence gene regulation throughout the genome; particularly immune-related genes. In order for Y-linked regulatory variation (YRV) to contribute to adaptive evolution it must be comprised of additive genetic variance, such that variable Ys induce consistent phenotypic effects within the local gene pool. We assessed the potential for Y-chromosomes to adaptively shape gram-negative and gram-positive bacterial defence by introgressing Ys across multiple genetic haplotypes from the same population. We found no Y-linked additive effects on immune phenotypes, suggesting a restricted role for the Y to facilitate dimorphic evolution. We did find, however, a large magnitude Y by background interaction that induced rank order reversals of Y-effects across the backgrounds (i.e. sign epistasis). Thus, Y-chromosome effects appeared consistent within backgrounds, but highly variable among backgrounds. This large sign epistatic effect could constrain monomorphic selection in both sexes, considering that autosomal alleles under selection must spend half of their time in a male background where relative fitness values are altered. If the pattern described here is consistent for other traits or within other XY (or ZW) systems, then YRV may represent a universal constraint to autosomal trait evolution.

The role of gene flow asymmetry along an environmental gradient in constraining local adaptation and range expansion.

Theoretically, asymmetric gene flow along an environmental gradient can limit species range expansion by keeping peripheral populations from locally adapting. However, few empirical studies have examined this potentially fundamental evolutionary mechanism. We address this possibility in the cricket Allonemobius socius, which exist along a season-length gradient where the probability of producing a single generation per year (univoltinism) increases with latitude. As the probability of univoltinism increases northwards, populations are expected to hedge their bets by producing a greater proportion of diapause eggs when exposed to a mild diapause cue. However, gene flow from southern populations may disrupt local adaptation in the north by reducing the proportion of diapause eggs (expected to be 100% in pure univoltine environments). This may limit range expansion along the northern periphery where A. socius compete with A. fasciatus, a sister species that exhibits an invariant diapause-only egg-laying strategy. To assess the potential for range limitation, we examined diapause incidence (the proportion of diapause eggs produced under diapause conditions), gene flow symmetry and population structure across nine A. socius populations. We found that gene flow was asymmetric and biased northwards towards the periphery. Furthermore, peripheral populations that inhabited pure univoltine environments produced numerous nondiapause eggs (a southern, bivoltine diapause phenotype), which we assume to be a suboptimal phenotype. These patterns suggest that asymmetric gene flow along the gradient constrains adaptation in peripheral populations, potentially constraining species range expansion.

The evolutionary genetics of sexual size dimorphism in the cricket Allonemobius socius.

In recent years, investigations into the evolution of sexual size dimorphism have moved from a simple single trait, single sex perspective, to the more robust view of multivariate selection acting on both males and females. However, more accurate predictions regarding selection response may be possible if some knowledge of the underlying sex-specific genetic architecture exists. In the striped ground cricket, Allonemobius socius, females are the larger sex. Furthermore, body size appears to be closely associated with fitness in both males and females. Here, we investigate the role that genetic architecture may play in affecting this pattern. Employing a quantitative genetic approach, we estimated the sex-specific selection gradients and the (co)variance matrix for body size and wing morphology (that is, either a long-winged flight-capable phenotype or a short-winged flightless phenotype) to predict phenotypic change in the next generation. We found that the sexes differed significantly in their selection gradients as well as several of their genetic parameters. Our predictions of next-generation change indicated that the within-sex genetic correlations, as well as the between-sex genetic correlations, should play a significant role in sexually dimorphic evolution in this system. Specifically, the female size response was increased by approximately 178% when the between-sex genetic correlations were considered. Thus, our predictions reinforce the notion that genetic architecture can produce counterintuitive responses to selection, and suggest that even a complete knowledge of the selection pressures acting on a trait may misrepresent the trajectory of trait evolution.