Sperm precedence in Callosobruchus chinensis estimated using the sterile male technique

P₂, the proportion of offspring sired by the second male to mate, is an indicator of the outcome of postcopulatory sexual selection, which occurs through sperm competition and/or cryptic female choice. We determined the appropriate dose of gamma radiation for sterilization of adult males and, using the sterile male technique, measured P₂ in the adzuki bean beetle, Callosobruchus chinensis. Adult males of C. chinensis were almost completely sterilized when irradiated at 80 Gy. Thus, we obtained sterile males through irradiation at this dose. Neither the probability of female first mating nor the probability of female remating was affected by whether females were paired with normal or sterile males. The P₂ calculated from the hatching success of eggs laid by females that mated both with normal and sterile males did not differ between reciprocal mating sequences, indicating that the sterilization has no effect on sperm fertilizing ability. The P₂ was estimated at 0.25. This study shows that female remating in C. chinensis means the coexistence of sperm from two males and thus the occurrence of postcopulatory sexual selection within the female reproductive tract, resulting in first-male sperm precedence.