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The zona pellucida is an elevated glycoproteineous envelope that surrounds the egg and is involved in the initial stages of sperm- egg interaction ( Wassarman 1999). Positive Darwinian selection, sexual conflict, fertilization, likelihood ratio test, speciation IntroductionĬomponents of the mammalian egg zona pellucida are subjected to positive Darwinian selection, indicating an adaptive value to diversify the primary amino acid sequence ( Swanson et al.

This is a more refined test of positive selection than the previous LRTs which only identified if there was a class of sites with a d N/ d S ratio >1 but did not test if that ratio was significantly greater than one.

A new LRT is implemented to determine if the d N/ d S ratio is significantly greater than one. Taken together, these represent the major candidates involved in mammalian fertilization, indicating positive selection is pervasive amongst mammalian reproductive proteins. Several of these proteins are sperm proteins that have been implicated in binding the mammalian egg coat zona pellucida glycoproteins, which were shown previously to be subjected to positive selection. In this study, we demonstrate that the evolution of seven genes involved in mammalian fertilization is promoted by positive Darwinian selection by using likelihood ratio tests (LRTs). Mammalian fertilization exhibits species specificity, and the proteins mediating sperm-egg interactions evolve rapidly between species.