The project was designed to study the maternal contribution to high and low rates of prenatal survival. Lines of mice will be used that were produced from previous genetic selection experiments and represent extremes in the proportion of fertilized ova that develop into viable young. Allophenic (quadriparental) mice will be produced by fusing two eight-cell embryos, one from each line. Fused embryos will be cultured in vitro to the blastocyst stage and then transferred to pseudopregnant recipients for continued development to term. Reproductive performance of allphenic mice, which contain the complete genetic complement of each line, will be compared with that of females from the two selected lines and hybrids produced through crossbreeding. Comparisons to be made include ovulation rate, implantation rate, preimplantation survival, postimplantation survival, litter size, uterine luminal protein profiles, and growth rate. Sublines with biochemical markers in uterine tissue will be produced from the high and low embryo survival lines. Allophenic mice produced from these sublines will provide females with uterine tissue in which the cellular contribution of each line can be determined. Attempts to exceed selection plateaus for high prenatal survival will be made by crossing and combining two high embryo survival lines. The results from this research will be useful in determining the physiological basis for genetic difference in embryo survival. Such information may lead to methods of preventing or controlling embryonic mortality.