Sperm competition and female avoidance of polyspermy mediated by sperm-eggbiochemistry

Nearly simultaneous fertilization of an egg by two or more sperm (polysperm y) is a lethal condition in most organisms. Sperm competing for an egg face opposing selective pressures. The race to fertilize favours rapid penetrat ion of the egg's outer protective layer; a close finish between two sperm l eads to polyspermy and death. Under most conditions of sperm competition, s election favours maximal speed of penetration by sperm in spite of potentia lly significant mortality imposed on both sperm and eggs. Eggs, in response to sperm competition and polyspermy, are favoured to increase the differen ce in arrival times between competing sperm. I model this sperm-sperm-egg c onflict to study the population genetic consequences of polyspermy. To sepa rate sperm arrival times, selection typically favours polymorphism of egg c haracters that influence the rate of passage by sperm through the egg's out er protective layer. In response to diverse eggs, the population of sperm c haracters may be favoured to diversify in a matching way or to stabilize at a point that maximizes average penetration speed. Divergence of reproducti ve characters by sexual selection is frequently cited as a potentially impo rtant factor in reproductive isolation and speciation. The biochemistry of fertilization characters provides a useful model system to study these proc esses.