title: Monotremes provide a key to understanding the evolutionary significance of epididymal sperm maturation.
abstract: It has been widely accepted that mammalian spermatozoa are infertile when  they leave the testes and require a period of maturation in both the  epididymis and the female reproductive tract before acquiring the ability to  fertilize an oocyte. However, the necessity for such a complex process of  posttesticular sperm maturation appears to be unique to mammals because it is  well established that these processes do not directly influence the  fertilizing ability of the spermatozoa of birds, reptiles, and other lower  vertebrates. Because of their key evolutionary position and form of  reproduction, we contend that monotremes (platypus and echidna) provide a  unique model for resolving why these processes are necessary. In the present  review, we examine evidence that the epididymal maturation of monotreme  spermatozoa is far less complex than in other mammals. However, a unique  feature of the monotreme epididymis lies in its ability to promote the  formation of elaborate sperm bundles that serve to greatly enhance the cells'  motility. It is suggested that this intriguing cooperative strategy used by  monotreme sperm represents an early form of epididymal maturation that appears  to have been elaborated upon during the evolution of higher mammals, possibly  as an adaptation for sperm competition.
authors: School of Environmental and Life Sciences, Discipline of Biological Sciences, University of Newcastle, University Drive, Callaghan, NSW 2308, Australia. brett.nixon@newcastle.edu.au
authors: Brett
authors: B
authors: Nixon
authors: Heath W
authors: HW
authors: Ecroyd
authors: Jean-Louis
authors: JL
authors: Dacheux
authors: Russell C
authors: RC
authors: Jones
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004822:Epididymis / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D008297:Male; D010987:Platypus / Q000378:metabolism / Q000502:physiology*; D013079:Sperm Maturation; D013094:Spermatozoa / Q000254:growth & development* / Q000378:metabolism; D004442:Tachyglossidae / Q000378:metabolism / Q000502:physiology*