SPERM PRECEDENCE IN THE BULB MITE, RHIZOGLYPHUS-ROBINI - CONTEXT-DEPENDENT VARIATION

Sperm precedence was studied in Rhizoglyphus robini under a range of m ating situations in order to examine the generality of P-2 (proportion of eggs fertilized by the second of two males to mate with the female ) as a predictor of selective pressures associated with sperm competit ion. 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 fr om females for 2 hr before experiments (short mating interval) transfe rred fewer sperm than those that were prevented from mating for 2 days (long mating interval). The second of the two female's mates had slig htly, but significantly, higher probability of fertilising her eggs wh en the two copulations were less than 1 hr apart, both in experiments with two males that had short mating interval (P-2 = 56.5%), and with two males that had long mating interval (P-2 = 61.1%). However, the la st male sperm precedence held no longer when the first male's sperm ha d 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. Moreov er, the effects of the interval between, and the number of, female's m atings on sperm precedence were studied using long mating interval mal es. Compared to 1 hr mating interval, P-2 decreased by 11% when copula tions with two consecutive males were 6 hr apart. When females mated w ith three males, the third fertilised on average 39.3% of eggs, and P- 3 was significantly lower than P-2 for double-matings. Thus, sperm pre cedence was found to vary depending on the mating context.