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.