Inbreeding changes the shape of the genetic covariance matrix in Drosophila melanogaster

The pattern of genetic covariation among traits (the G matrix) plays a cent ral role in determining the pattern of evolutionary change from both natura l selection and random genetic drift. Here we measure the effect of genetic drift on the shape of the G matrix using a large data set on the inheritan ce of wing characteristics in Drosophila mellanogaster. Fifty-two inbred li nes with a total of 4680 parent-offspring families were generated by one ge neration of brother-sister mating and compared to an outbred control popula tion of 1945 families. In keeping with the theoretical expectation for a co rrelated set of additively determined traits, the average G matrix of the i nbred lines remained proportional to the outbred control G matrix with a pr oportionality constant approximately equal to (1 - F), where F is the inbre eding coefficient. Further, the pattern of covariance among the means of th e inbred lines induced by inbreeding was also proportional to the within-li ne G matrix of the control population with a constant very close to the exp ectation of 2F. Although the average G of the inbred lines did not show cha nge in overall structure relative to the outbred controls, separate analysi s revealed a great deal of variation among inbred lines around this expecta tion, including changes in the sign of genetic correlations. Since any give n line can be quite different from the outbred control, it is likely that i n nature unreplicated drift will lead to changes in the G matrix. Thus, the shape of G is malleable under genetic drift, and the evolutionary response of any particular population is likely to depend on the specifics of its e volutionary history.