FEMALE GENOTYPE AFFECTS MALE SUCCESS IN SPERM COMPETITION

The central question addressed by most studies of sperm competition is : 'what determines which male's sperm are used at fertilization?' Empi rical and theoretical studies that address this question have traditio nally focused on adaptations which enhance male fertilization success while treating the female as a receptacle in which sperm competition i s played out. Here we provide evidence which suggests that female geno type strongly influences the outcome of sperm competition. When the sp erm of two males are in competition the proportion of offspring father ed by the second male to mate (P-2) was found to be highly repeatable only if the male pair were mated to three different, but genetically s imilar females (full-sisters to each other; unrelated to either of the males). In contrast? if a male pair were mated to three females that were unrelated then P-2 was either non-repeatable or marginally repeat able. We also show that male success in sperm competition is determine d, to a large extent, by gamete and/or male-female compatibility. This conclusion is derived from the observation that P-2 was repeatable am ong full-sisters mated to different, yet genetically similar male pair ings, whilst P-2 was non-repeatable among full-sisters mated to differ ent, genetically distinct male pairings.