Sperm maturation: Lessons from our distant relatives, the monotremes (#5)
Two of the major unresolved questions relating to sperm production in mammals are why spermatozoa are infertile when they leave the testes and require a period of maturation in the specific milieu provided by the epididymides, and why ejaculated spermatozoa cannot immediately fertilise an ovum until they undergo capacitation within the female reproductive tract. Understanding these processes is of strategic importance in the design of post-testicular methods of fertility regulation, the development of assisted conception systems for the preservation of endangered species and elucidation of the causes of human male infertility. Due to their key evolutionary position and form of reproduction, monotremes (echidna and platypus) provide a unique model for resolving how and why these processes are necessary. We have therefore sought to characterise the molecular mechanisms that govern the epididymal maturation and capacitation of monotreme spermatozoa.Our studies have demonstrated that monotreme sperm maturation is far less complex than in other mammals. However, the monotreme epididymis is distinctive in forming spermatozoa into bundles that display greatly enhanced motility compared with individual spermatozoa. Sperm bundle formation is mediated by interaction with epididymal proteins, and the bundles persist for 2-3 h during incubation in vitro, before dispersing spermatozoa that have the capacity to engage in oocyte interactions. It is suggested that the unique co-operative strategy employed by monotreme sperm represents an early form of epididymal maturation, and that their subsequent dispersal is also an early form of capacitation. It appears that these processes have since been elaborated upon during the evolution of higher mammals, possibly as an adaptation for sperm competition, that is, the competition between males to achieve paternity.