THE GENETICS OF PRENATAL SURVIVAL OF PIGS AND RABBITS - A REVIEW

Current knowledge on the genetic variability of prenatal survival (PS) in pigs and rabbits is reviewed. There is a large amount of variation between lines or breeds and these differences are not always negatively correlated with ovulation rate (OR); a line with a high OR can also have a high level of PS (e.g. the Meishan pig). Crossbreeding studies show that the maternal genotype is much more important in the control of line differences in PS than the the embryo/fetus, particularly the former, enhances PS, demonstrating the importance of non-additive genetic variation in the control of this trait. Only few estimates of the within breed genetic parameters of PS are available in the literature. Heritability seems to be low, with estimates ranging from 0 to 0.23. PS is negatively correlated with OR and positively correlated with number of embryos/fetuses (NE) or litter size (LS), but estimates of genetic correlations differ widely between studies. Selection for OR generally leads to an increase of NE at mid-gestation, but not at birth. Selection on a linear index combining OR and PS has not proved to be more efficient than selection on LS. New methods, such as unilateral hysterectomy/ovariectomy, which increases the emphasis on fetal survival and aims to measure uterine capacity, are currently under study and seem to be promising.