Puzzles of mammalian fertilization - and beyond

A complexity imposed on the events of conception during the evolution of eutherian mammals is reflected not only in the placenta but also in a series of gamete-related novelties that involve the design of sperm and eggs, their interactions leading to fertilization, epididymal function, and even the scrotal state. While their functional significance has been difficult to determine, I suggest on the basis of present evidence that the genesis of these novel features relates ultimately to changes in the egg associated with nidation, that they imply a new fertilization strategy, and that most are causally linked - a major first 'domino' being the formidable zona pellucida. The eutherian zona pellucida stands out by virtue of an elastic resilience and thickness which allow it to stretch around the expanding blastocyst. However, this quality of the zona together with its relative protease-insensitivity creates a physical challenge which appears to have determined the design of the sperm head, its behaviour at fertilization, and indirectly even the pattern of sperm maturation in the epididymis. In turn, that pattern appears to have evoked the development of the sperm storage function of the cauda region, with both sperm capacitation and the scrotal state as ultimate legacies of this. Coincidentally, the vulnerability of the small yolkless egg to polyspermy appears to have selected for a unique parsimonious pattern of sperm transport in the Fallopian tube, and possibly for the role of the cumulus oophorus as a sperm sequestering device. These ideas are supported generally by evidence from marsupials, as well as by the deviant patterns seen in some insectivorous mammals.