Mechanisms of Sperm–Egg Interactions: What Ascidian Fertilization Research Has Taught Us

Abstract

Fertilization is an essential process in terrestrial organisms for creating a new organism with genetic diversity. Before gamete fusion, several steps are required to achieve successful fertili-zation. Animal spermatozoa are first activated and attracted to the eggs by egg-derived chemoat-tractants. During the sperm passage of the egg’s extracellular matrix or upon the sperm binding to the proteinaceous egg coat, the sperm undergoes an acrosome reaction, an exocytosis of acrosome. In hermaphrodites such as ascidians, the self/nonself recognition process occurs when the sperm binds to the egg coat. The activated or acrosome-reacted spermatozoa penetrate through the pro-teinaceous egg coat. The extracellular ubiquitin–proteasome system, the astacin-like metalloprote-ases, and the trypsin-like proteases play key roles in this process in ascidians. In the present review, we summarize our current understanding and perspectives on gamete recognition and egg coat lysins in ascidians and consider the general mechanisms of fertilization in animals and plants.