The evolutionary significance of variation in sperm-egg interactions

In sexual reproduction, interactions between the gamete cells, be they sperm and egg or different mating types, initiate the next generation, so an insight of evolutionary processes relies on understanding factors that influence these gametic interactions. This chapter focuses on the present state of the field of fertilization and the nature of sperm-egg interactions (SEIs), with particular emphasis on evolutionary mechanisms and processes that have driven this important aspect of the life history of sexual organisms. The goal of the chapter is to illustrate the variation in fertilization systems at the level of SEIs and common/disparate SEI processes and to provide evidence for the evolutionary significance of SEIs. A detailed understanding of the molecular makeup of sperm will be necessary to fully assess the role sperm play in SEIs. They contain other essential elements necessary for production of a viable diploid organism carried into the maternal cytoplasm during fertilization. During fertilization and early embryonic development, sperm and egg interact in a variety of ways. The chapter primarily focuses on the initial stages of sperm-egg interactions near/at their surfaces and ending at the stage of the first mitotic division of the embryo. The SEIs discussed encompass those interactions that take place between the gametes both at their surfaces, at or near these surfaces, and subsequently, once sperm are inside the egg at the terrestrial invertebrates.