Sexual selection by female immunity against paternal antigens can fix loss of function alleles

Humans lack the Neu5Gc molecule because of the loss of function of the gene CMAH. Neu5Gc is a sailic acid molecule that is involved in cellular recognition during infection, development, and immune regulation. In humans, the CMAH gene was switched off about 3 million years ago and quickly driven to fixation. The authors of this paper test whether pathogen-driven fixation would be the sole cause of the prevalence of this pseudogenization. They are interested in an alternative hypothesis of sexually-selected change because of reproductive compatibility. In order to test this, they use transgenic and wild type mice to test the immune response to Neu5Gc in CMAH (-/-) females, and by testing if human anti-Neu5Gc antibodies and would attack chimpanzee sperm in vitro. Both tests produced positive results, indicating that selection due to female immunity against foreign antigens can fix loss-of-function alleles from moderate initial frequencies. Interestingly, Neandertals also have a loss-of-function CMAH gene, supporting the idea that they could easily hybridize with Homo sapiens.