The quantitative genetics of two life history trade-offs in the yellow dung fly in abundant and limited food environments

The trade-offs between body size and development time and between egg size and egg number ( clutch size) are central to life history theory, but evidence for them, particularly in terms of genetic correlations, is equivocal. For the yellow dung fly Scathophaga stercoraria ( Diptera: Scathophagidae), we investigated variation in phenotypic and genetic variances and covariances, i.e. heritabilities and genetic correlations, of these life history traits ( plus diapause) in benign and stressful larval field or adult laboratory food environments. We found both trade-offs to be weak, as evidenced by low phenotypic and genetic correlations, but stronger in the food limited environments. Broad sense heritabilities were generally significant for all traits considered, whereas the narrow sense heritabilities for egg and clutch size were nil. With regard to the question of how environmental stress affects heritabilities, we found a whole range of responses within one single species depending on the traits considered. All three possible patterns occurred, i.e. increased h(2) due to increased V-G or decreased V-P, decreased h(2) due to increased V-P, and no change in h(2) due to increased V-G and V-P. These can be explained by the particular ecological circumstances yellow dung flies face in their natural environment. Nevertheless, the majority of patterns was consistent with the idea that stressful conditions amplify phenotypic differences between genotypes. Such variable responses of traits even within one organism underscores the complexity of this issue and may well explain the multiple patterns found in various organisms.