Life-history consequences of divergent selection on egg size in Drosophilamelanogaster

Life histories are generally assumed to evolve via antagonistic pleiotropy (negative genetic correlations) among traits, and trade-offs between life-h istory traits are typically studied using either phenotypic manipulations o r selection experiments. We investigated the trade-off between egg size and fecundity in Drosophila melanogaster by examining both the phenotypic and genetic relationships between these traits after artificial selection for l arge and small eggs, relative to female body size. Egg size responded stron gly to selection in both directions, increasing in the large-egg selected l ines and decreasing in the small-egg selected lines. Phenotypic correlation s between egg size and fecundity in the large-egg selected lines were negat ive, but no relationship between these traits occurred in either the contro l or small-egg selected lines. There was no negative genetic correlation be tween egg size and fecundity. Total reproductive allocation decreased in th e small-egg selected lines but did not increase in the large-egg lines. Our results have three implications. First, our selection procedure may have f orced females selected for large eggs into a physiological trade-off not re flected in a negative genetic correlation between these traits. Second, the lack of a negative genetic correlation between egg size and number suggest s that the phenotypic trade-off frequently observed between egg size and nu mber in other organisms may not evolve over the short term via a direct gen etic trade-off whereby increases in egg size are automatically accompanied by decreased fecundity. Finally, total reproductive allocation may not evol ve independently of egg size as commonly assumed.