Adaptive evolution at immune system genes and deep pregnancy implantation in primates.

A major evolutionary change in the lineage ancestral to humans, chimpanzee and gorilla (HCG) has been the embedding of the embryo into maternal tissue. Thus, the first layer of cells (trophoblast) to differentiate after fertilization must adapt to invade the uterus. Such event would likely leave signatures of positive selection at genes with roles in embryo implantation. Here, 163 pregnancy implantation genes are tested for evidence of adaptive diversification in the ancestral lineage to HCG. Two immune system genes, HLA-E and KIR2DL4 showed evidence of positive selection. Some of the positive selected sites involve amino acid substitution with predicted damaging effects on protein function, thus highlighting the possibility of antagonistic pleiotropic effects. Selection at a gene coding for a receptor expressed in uterine cells (KIR) that interacts with trophoblast human leukocyte antigen (HLA) genes suggests a main role for immunological adaptations in embryo deep invasion of the maternal endometrium.