Tuesday, May 08, 2007

Egg protection   posted by Razib @ 5/08/2007 03:19:00 PM

Evidence of Amino Acid Diversity - Enhancing Selection within Humans and among Primates at the Candidate Sperm-Receptor Gene PKDREJ:
Sperm-egg interaction is a crucial step in fertilization, yet the identity of most interacting sperm-egg proteins that mediate this process remains elusive. Rapid evolution of some fertilization proteins has been observed in a number of species, including evidence of positive selection in the evolution of components of the mammalian egg coat. The rapid evolution of the egg-coat proteins could strongly select for changes on the sperm receptor, to maintain the interaction. Here, we present evidence that positive selection has driven the evolution of PKDREJ, a candidate sperm receptor of mammalian egg-coat proteins. We sequenced PKDREJ from a panel of 14 primates, including humans, and conducted a comparative maximum-likelihood analysis of nucleotide changes and found evidence of positive selection. An additional panel of 48 humans was surveyed for nucleotide polymorphisms at the PKDREJ locus. The regions predicted to have been subject to adaptive evolution among primates show several amino acid polymorphisms within humans. The distribution of polymorphisms suggests that balancing selection may maintain diverse PKDREJ alleles in some populations. It remains unknown whether there are functional differences associated with these diverse alleles, but their existence could have consequences for human fertility.

To my understanding this sort of barrier between sperm & egg is a variant of prezygotic isolation, a gene flow barrier which crystallizes speciation. If I read their last sentence correctly it seems that the researchers are implying that polymorphism in regards to this character might be a source of reduced fertility between some individuals because of the gametic incompatibilities.

Note: Sperm related genes are always a good target of adaptive evolution because of their proximal relationship to fitness. For example, Pervasive adaptive evolution in primate seminal proteins.

Related: Sperm competition.