Sperm-female coevolution: mechanisms, models and theoretical implications.
Recent work has demonstrated that the shapes of sperm and of the female reproductive tract evolve in concert. This pattern is attributable to a process known as female "sperm choice," with female reproductive tract characteristics producing a bias in patterns of paternity when females mate with multiple males, such as biasing fertilization in favor of longer sperm. Research proposed here will investigate (1) physiological mechanisms of female sperm choice, (2) possible benefits to females of having the ability to "discriminate" among sperm variants, (3) costs and benefits to males of variation in traits that contribute to fertilization success, and (4) implications of producing few, large sperm for sex difference theory.
Disparity between the sexes in the size of their sex cells is believed to be the cornerstone of all sex differences (e.g., body size, aggressiveness, ornamentation). However, contrary to popular belief, males of all species do not produce many tiny sperm. In fact, sperm cells are the most diverse cell type: in nearly all animal groups, they evolve in size, shape, and structure so rapidly that even closely related species can usually be discriminated by their sperm. Because having the correct species-specific sperm structure is pivotal for successful reproduction, sperm divergence may function as an "engine of speciation." Understanding the causes of sperm evolution is thus important to our understanding of the biological basis of sex differences and of the process of speciation.
