Evolution of species-specific major seminal fluid proteins in placental mammals by gene death and positive selection

The seminal fluid is a complex substance composed of a variety of secreted proteins and has been shown to play an important role in the fertilisation process in mammals and also in Drosophila. Several genes under positive selection have been documented in some rodents and primates. Our study documents this phenomenon in several other mammalian taxa. We study the evolution of genes that encode for 20 proteins that are quantitatively predominant in the seminal fluid of at least one out of seven domestic animal species. We analyse the amino acid composition of these proteins for positive selection and for the presence of pseudogenes. Genes that disappeared through pseudogenisation include KLK2 in cattle, horse and mice. Traces of positive selection are found in seven genes. The identified amino acids are located in regions exposed to the protein surface, suggesting a role in the interaction of gametes, with possible impact on the process of speciation. Moreover, we found no evidence that the predominance of proteins in seminal fluid and their mode of evolution are correlated, and the uncoupled patterns of change suggest that this result is not due solely to lack of statistical power.