EFFECTS OF GAMETE TRAITS ON FERTILIZATION IN THE SEA AND THE EVOLUTION OF SEXUAL DIMORPHISM

THE evolution of egg and sperm(1,2), and more derived forms of sexual dimorphism, is thought to be driven by sperm competition and postzygot ic survival; males are limited by fertilizations, females by resources (3). Evidence of sperm competition comes from internal fertilizers, or cases where sperm are deposited on eggs(4), but in free-spawners, the ancestral mating strategy (refs 1,5 but see ref. 6), females are ofte n sperm limited(7,8). Laboratory experiments on sea urchins demonstrat e that intraspecific differences in gamete attributes, such as egg siz e: can influence rates of fertilization. Field experiments in which ga metes are released and recaptured demonstrate that the influence of ga mete traits on fertilization is not overwhelmed by sea: conditions, an d that variation in gamete traits can have important fitness consequen ces. These results suggest a new mechanism for the evolution of anisog amy and sexual dimorphism, in which sperm limitation is important, and natural selection for enhanced fertilization acts on females as well as males.