Selection in favor of the Ped fast haplotype occurs between mid-gestation and birth

The preimplantation embryo development (Ped) gene that encodes the class Ib major histocompatibility complex protein Qa-2 influences the rate of embryonic cleavage during the preimplantation stages of development. Embryos from strains of mice that lack the Fed gene cleave slowly, while embryos that have a functional Fed gene cleave more rapidly. This effect is observed both in vivo and in vitro with the Fed fast haplotype showing dominance over the Fed slow haplotype. The Fed gene is associated with pleiotropic effects on reproduction. Certain strains of mice lacking the Fed gene (Ped slow) have smaller litters and the pups weigh less at birth and at weaning. Previously our laboratory reported that in litters derived from Fed fast/ slow F1 mice backcrossed to the slow/slow parent, there were significantly more Fed fast pups than the 50% expected, at two months of age. This implies that there is selection in favor of the Fed fast haplotype at some point during development. The present study was designed to determine at what point during development selection occurs. Using a polymerase chain reaction assay, we determined that selection does not occur by days post coitus 14.5. However, our results show that there are significantly more Fed fast pups than Fed slow pups remaining in backcross litters just after birth, indicating that selection in favor of the Fed fast haplotype occurs between day 14.5 and birth.