Sperm competition and female avoidance of polyspermy mediated by sperm-egg biochemistry

Nearly simultaneous fertilization of an egg by two or more sperm (polyspermy) is a lethal condition in most organisms. Sperm competing for an egg face opposing selective pressures. The race to fertilize favours rapid penetration of the egg’s outer protective layer; a close finish between two sperm leads to polyspermy and death. Under most conditions of sperm competition, selection favours maximal speed of penetration by sperm in spite of potentially significant mortality imposed on both sperm and eggs. Eggs, in response to sperm competition and polyspermy, are favoured to increase the difference in arrival times between competing sperm. I model this sperm–sperm–egg conflict to study the population genetic consequences of polyspermy. To separate sperm arrival times, selection typically favours polymorphism of egg characters that influence the rate of passage by sperm through the egg’s outer protective layer. In response to diverse eggs, the population of sperm characters may be favoured to diversify in a matching way or to stabilize at a point that maximizes average penetration speed. Divergence of reproductive characters by sexual selection is frequently cited as a potentially important factor in reproductive isolation and speciation. The biochemistry of fertilization characters provides a useful model system to study these processes.