Scathophaga

Abstract: Both female choice and male-male competition may take place during reproduction in many species. Female choice tends to be less obvious than male-male competition and consequently has received less attention from re- searchers. The opportunity for cryptic female choice arises after multiple insemination. Through postcopulatory pro- cesses, a female could alter the pattern of paternity among her offspring so that it does not directly reflect the different contributions of sperm made by her mates. To be able to determine if a female alters the relative sperm contributions of her mates, the behaviors and influences of the males must therefore be first taken into account. The interest of each male is to father all the offspring, and the interest of each female is to maximize paternal quality. Female yellow dung flies have complex internal reproductive tracts that may give them considerable control over the fertilization success of stored sperm from different males. In laboratory trials to date, the last male to mate has usually been most successful. In the present study, cryptic choice occurred in Scathophaga stercoraria and the pattern of choice was consistent with previously reported results. The fertilization success of a female's second mate (P2) was substantially larger if a female was kept at constant temperature and if the second male was genetically similar to her at the phosphoglucomutase (Pgm) locus. Females from the field normally have three spermathecae, but some have four. Lines were bred to have either three or four spermathecae. Flies from the different lines were crossed to generate females with similar genetic backgrounds that had either three or four spermathecae. P2 was significantly lower for high-quality females, that is, those that laid larger-than-average-clutches, with four spermathecae than for low-quality females with four spermathecae; female quality had no influence on P2 for females with three spermathecae. The results suggest that only large females may benefit from increased spermathecae number by being able to act against male interests. Females may only have three spermathecae, even though genetic variation for more is present, because selection for more spermathecae is weak.

Abstract: Cryptic, or post-copulatory, female choice could markedly affect the outcome of sperm competition, i.e. a female could differentially manipulate ejaculates within her own body, affecting the fertilization successes of her mating partners. Female yellow dung flies, Scathophaga (Scatophaga) stercoraria, have three spermathecae, the sperm-storage organs, and can to some extent store the sperm of different males in different places. I show that a female's body size, as well as those of her mates, influences the process of sperm storage. Furthermore, females lay eggs of different genotypes under different environmental conditions. Females use both cues correlated with single locus variation (at the locus for the enzyme phosphoglucomutase, PGM) and quantitative trait variation (in body size and development time) when using sperm to fertilize their eggs. It is proposed that this allows a female to match the genotypes of her offspring to the conditions in which the larvae must grow, thus increasing their subsequent fitness. I describe an experiment where larvae of different PGM genotypes were raised in different environments and the most successful genotype was different in different environments. The complexity of the female reproductive sysrem may therefore have evolved because rhe best father for a female's offspring, from the female's viewpoint, is different under different environmental conditions. The effect interacts with the established male-determined effects to influence the outcome of sperm competition.

Abstract: Seasonal variation in mean hind tibia length and mean testes length is investigated in the yellow dung fly, Scathophaga stercoraria (L.). There is a cycle in mean hind tibia length and mean testes length over a season. The body size curve peaks later than the testes length curve, showing that there is no fixed relationship between the two variables. The causes of variation in testes size and its influence on copula duration are experimentally examined. Increasing the number of Drosophila eaten per day leads to increased mature testes length. Males with larger testes copulate for longer than males with smaller ones, and smaller males copulate for longer than do larger males. While testes shrivel with successive copulas, copula duration remains constant. The more females a male is prepared to copulate with in a day (up to five), the longer he copulates with each. The shrinkage of the testes of males collected throughout a day suggests that males copulate with an average of 4 females per day. The costs of sperm production are thus shown to have a significant influence on the copula duration.