Sperm displacement in the yellow dung fly, Scatophaga stercoraria: An investigation of male and female processes

Despite the ubiquity with which patterns of sperm utilization have been stu died, the mechanisms underlying fertilization in insects are far from clear . One well-studied system is the: yellow dung fly, in which the last male's ejaculate is thought to displace rival sperm from the female's sperm store s. Here we follow the movement of the copulating male's ejaculate through t he female's reproductive tract using males labeled with different radioisot opes. We find that males ejaculate into the bursa copulatrix and that male- 1 sperm are displaced from the spermathecae during copulation. The increase in male-2 ejaculate in the spermathecae matches the pattern of male-2 fert ilization gain, indicating that only spermathecal sperm are utilized at fer tilization. Previously we have analyzed this system with a direct model of sperm displacement in which the male displaces rival sperm from the spermat hecae. The data, and morphology of the female, clearly preclude such a mech anism. Here we contrast this model with a new indirect model, in which the female facilitates displacement by exchange of sperm from the bursa copulat rix to the spermathecae. The two models give equivalent fits to the observe d sperm utilization patterns because the rate of sperm transfer into the bu rsa copulatrix greatly exceeds the rate of sperm exchange with the spermath ecae so that the concentration of the first male's sperm in the bursa remai ns considerably lower than that of the second male. These analyses provide a quantitative attempt to incorporate female processes into the analysis of sperm utilization patterns in insects.