volved in sperm–egg interaction. The classic example of a rapidly evolving gamete recognition protein is the abalone sperm protein lysin, perhaps the most rapidly diverging protein yet discovered (Lee and Vacquier, 1992). Lysin acts to dissolve the egg vitelline envelope, a process that demonstrates species specificity. In mammals it has been demonstrated that sperm and egg molecules are among the most diverse found within the genome, with a minimum of 10 reproductive genes showing evidence of adaptive evolution (Swanson et al., 2003b). One such gene is the mammalian egg coat protein ZP3. The region within this protein undergoing adaptive evolution corresponds to experimentally determined binding sites (Swanson et al., 2001a), suggesting that the rapid evolution relates to fertilization.
The second class of reproductive proteins exhibiting rapid evolutionary change are not directly involved in surface recognition of the gametes. These include components of seminal fluid (Kingan et al., 2003), pheromones and protamines. In Drosophila seminal fluid, an estimated 10% of the genes show the signatures of adaptive evolution (Swanson et al., 2003a). Many of these genes, called accessory gland proteins (ACPs) act to manipulate female reproductive behavior, thus increasing male fitness (Wolfner, 1997). In primates, seminogellin II (SEM2), a major component of seminal fluid, shows rapid adaptive evolution. This protein is involved in copulatory plug formation in rodents, and in primates its rate of evolution shows a correlation with mating system (Dorus et al., 2004).
The recent comparison of predicted genes in chicken genome with the human genome supports the pattern of divergence found in reproductive proteins across taxa (International Chicken Genome Sequencing Consortium, 2004). Genes implicated in reproduction appear less conserved between chicken and human than genes involved in typical “housekeeping” functions. For example, among genes classified into 10 different tissue specificities, those expressed in the testis showed the most divergence: 65% sequence conservation compared with the mean of 75% across all genes. Many of these genes, such as ZP3, have orthologues in birds and would be important candidates for targets of natural selection. Other potential target genes in birds include seminal fluid proteins. The prediction of natural selection on such genes in other species can be inferred from reproductive observations. Adkins-Reagan (1999) documented a viscous mucoprotein produced by Japanese quail (Corturnix japonica) thought to increase the probability of fertilization when a hard-shelled egg is in the uterus. The origin of such viscosity must have a basis in protein evolution, although the target loci have not yet been identified.
Cryptic female choice, sperm competition, and sexual conflict are three nonexclusive hypotheses for the forces driving the rapid evolution of these proteins (Swanson and Vacquier, 2002). Cryptic female choice of