The evolutionary genetics of an adaptive maternal effect: Egg size plasticity in a seed beetle

In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases , maternal effects may evolve as mechanisms for transgenerational phenotypi c plasticity whereby, in response to a predictive environmental cue, a moth er can change the type of eggs that she makes or can program a developmenta l switch in her offspring, which produces offspring prepared for the enviro nmental conditions predicted by the cue. One potentially common mechanism b y which females manipulate the phenotype of their progeny is egg size plast icity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a. seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid an seeds of two host plants (Acacia greggii and Cercidium flor idum; h(2) ranged between 0.217 and 0.908), and that the heritability of eg g size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation acro ss host plants (r(G) > 0.6). However, the cross-environment genetic correla tion is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size pl asticity is thus capable of evolving in response to natural selection.