223. The epididymis secretes proteins involved in sperm competition: evidence from the echidna

Abstract

Although there is general recognition that the mammalian epididymis is unique among the vertebrates and is essential for male fertility, there is relatively little understanding of its role in the process of reproduction. It has been suggested that it plays an important role in the competition between males to achieve paternity (sperm competition) by accumulating and storing sperm.1 We now provide evidence that specific protein synthesized and secreted by the epididymis of the echidna plays a major role in sperm competition by binding spermatozoa into bundles of about a 100 sperm and consequently greatly enhancing their rate of motility, particularly in viscous medium. The epididymis of the echidna is structurally differentiated into a large caput epididymidis that is structurally similar to the ‘initial segment’ of other mammals, and a small cauda epididymidis. Using radiolabelling and 2-D electrophoresis we have shown that the caput epididymidis synthesises and secretes a similar pattern of proteins along most of its length where sperm are not associated with one another. The sperm form into bundles as they enter the cauda epididymidis where the pattern of protein secretion changes considerably, being characterised by a new band of about 80 kDa in reduced gels. Electron microscopy of the bundles shows electron dense material binding the sperm together in the bundles. The bundles persist for more than an hour after sperm are released into physiological media. (1)Jones, RC (1998). Evolution of the vertebrate epididymis. J. Reprod. Fertil. Suppl. 53, 163–182.