Sperm precedence in the bulb mite, Rhizoglyphus robini: context-dependent variation

Abstract

Sperm precedence was studied in Rhizoglyphus robini under a range of mating situations in order to examine the generality of P2 (proportion of eggs fertilized by the second of two males to mate with the female) as a predictor of selective pressures associated with sperm competition. Paternity was estimated using the sterile male method. Number of sperm transferred by males during copulation was indirectly controlled by manipulation of male mating frequency: males that were isolated from females for 2 hr before experiments (short mating interval) transferred fewer sperm than those that were prevented from mating for 2 days (long mating interval). The second of the two female's mates had slightly, but significantly, higher probability of fertilising her eggs when the two copulations were less than 1 hr apart, both in experiments with two males that had short mating interval (P2 = 56.5%), and with two males that had long mating interval (P2 = 61.1%). However, the last male sperm precedence held no longer when the first male's sperm had a numerical advantage: males that had high sperm reserves due to a 2-day sexual abstinence fertilised more eggs, when they were the first to mate with females, than males that had 2 hr mating interval. Moreover, the effects of the interval between, and the number of, female's matings on sperm precedence were studied using long mating interval males. Compared to 1 hr mating interval, P2 decreased by 11% when copulations with two consecutive males were 6 hr apart. When females mated with three males, the third fertilised on average 39.3% of eggs, and P3 was significantly lower than P2 for double-matings. Thus, sperm precedence was found to vary depending on the mating context.