Copula in yellow dung flies (Scathophaga stercoraria): investigating spermcompetition models by histological observation

While sperm competition has been extensively studied, the mechanisms involv ed are typically not well understood. Nevertheless, awareness of sperm comp etition mechanisms is currently recognised as being of fundamental importan ce for an understanding of many behavioural strategies. In the yellow dung fly, a model system for studies of sperm competition, second male sperm pre cedence appears to result from a combination of sperm displacement and sper m mixing. Displacement was until recently thought to be directly from the f emale's sperm stores, the spermathecae (i.e. males were thought to ejaculat e directly into these stores), and under male control. However, recent work indicates displacement is indirect (i.e. males do not ejaculate directly i nto the sperm stores) and that it is female-aided. although the evidence wa s not based on direct observation. Here, we used histological techniques to directly determine interactions during copula and sperm transfer. Our resu lts are consistent with inference and clearly show that males ejaculate int o the bursa copulatrix. Our data are also consistent with active female inv olvement in sperm displacement, which is indirect, and indicate the aedeagu s may remove some spermatozoa from the bursa at the end of copula. In addit ion, evidence suggests females aid sperm transport to and from the spermath ecae, possibly by muscular movement of a spermathecal invagination. (C) 200 0 Elsevier Science Ltd. All rights reserved.