A POSSIBLE EXPLANATION FOR CRYPTIC FEMALE CHOICE IN THE YELLOW DUNG FLY, SCATHOPHAGA-STERCORARIA (L.)

Cryptic or post-copulatory, female choice could markedly affect the ou tcome of sperm competition, i.e. a female could differentially manipul ate ejaculates within her own body, affecting the fertilization succes ses of her mating partners. Female yellow dung flies, Scathophaga (Sca tophaga) stercoraria, have three spermathecae, the sperm-storage organ s, and can to some extent store the sperm of different males in differ ent places. I show that a female's body size, as well as those of her mates, influences the process of sperm storage. Furthermore, females l ay eggs of different genotypes under different environmental condition s. Females use both cues correlated with single locus variation (at th e locus for the enzyme phosphoglucomutase, PGM) and quantitative trait variation (in body size and development time) when using sperm to fer tilize their eggs. It is proposed that this allows a female to match t he genotypes of her offspring to the conditions in which the larvae mu st grow, thus increasing their subsequent fitness. I describe an exper iment where larvae of different PGM genotypes were raised in different environments and the most successful genotype was different in differ ent environments. The complexity of the female reproductive system may therefore have evolved because the best father for a female's offspri ng, from the female's viewpoint, is different under different environm ental conditions. The effect interacts with the established male-deter mined effects to influence the outcome of sperm competition.