Tuesday, February 14, 2012
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.