THE COADAPTATION OF PARENTAL AND OFFSPRING CHARACTERS

Parents often have important influences on their offspring's traits an d/or fitness (i.e., maternal or paternal effects). When offspring fitn ess is determined by the joint influences of offspring and parental tr aits, selection may favor particular combinations that generate high o ffspring fitness. We show that this epistasis for fitness between the parental and offspring genotypes can result in the evolution of their joint distribution, generating genetic correlations between the parent al and offspring characters. This phenomenon can be viewed as a coadap tive process in which offspring genotypes evolve to function with the parentally provided environment and, in turn, the genes for this envir onment become associated with specific offspring genes adapted to it. To illustrate this point, we present two scenarios in which selection on offspring alone alters the correlation between a maternal and an of fspring character. We use a quantitative genetic maternal effect model combined with a simple quadratic model of fitness to examine changes in the linkage disequilibrium between the maternal and offspring genot ypes. In the first scenario, stabilizing selection on a maternally aff ected offspring character results in a genetic correlation that is opp osite in sign to the maternal effect. In the second scenario, directio nal selection on an offspring trait that shows a nonadditive maternal effect can result in selection for positive covariances between the tr aits. This form of selection also results in increased genetic variati on in maternal and offspring characters, and may, in the extreme case, promote host-race formation or speciation. This model provides a poss ible evolutionary explanation for the ubiquity of large genetic correl ations between maternal and offspring traits, and suggests that this p attern of coinheritance may reflect functional relationships between t hese characters (i.e., functional integration).