Timing and Extent of Embryonic Mortality in Pigs Sheep and Goats: Genetic Variability

Abstract

Embryonic mortality rate in pigs, sheep and goats ranges around 30%. It occurs mainly during the first month of pregnancy and mostly before implantation. In pigs, a second less important stage observed about Day 50 of pregnancy is resulting from overcrowding of the uterine horns. In these three species, chromosomal abnormalities such as centric fusions or reciprocal translocations cause the formation of embryos with unbalanced karyotypes which do not survive until parturition. This may considerably increase the basal loss. Several examples of major genes involved in embryonic mortality are reviewed though few cases are conclusive. The between breed and crossbred variability in embryo loss is reviewed. In pigs, large between breed variability independant of the ovulation rate is observed as well as significant individual and maternal heterotic effects. In sheep, this variability is less marked and almost no heterosis is observed. The within breed variability does not include an additive genetic component in the pig but is mainly related to the variability of the ovulation rate due both to genetic correlations and to maternal uterine effects, though the mechanism is not well understood. The success of selection experiments for prolificacy in sheep seems to be due to a lower correlation between ovulation rate and embryonic mortality than exists in the pig. The existence of pig and sheep breeds with low rates of embryonic mortality and the selection of highly prolific lines within these species provide ideal material for studies on embryonic mortality.