GENETIC-VARIATION OF PRENATAL SURVIVAL IN RELATION TO OVULATION RATE IN SHEEP - A REVIEW

Varying rates of partial fertilization (2-40%) were found in literatur e. These data gave evidence that the 'all-or-none' hypothesis on ferti lization is not correct, at least in a number of sheep breeds and expe rimental conditions. Furthermore, there are indications for variabilit y in prenatal mortality at different stages of pregnancy, as a functio n of ovulation rate (OR). Consequently, higher partial fertilization a nd foetal mortality rates interfere with the determination of prenatal survival (PS), based on the difference between OR and the number of l iving embryos at varying times before or at lambing, as was done in ma ny studies on that subject. Indications for variability in PS within b reeds were mainly an effect of differentiation between lines, dependin g on the selection criteria followed, e.g., growth rate vs, multiple o vulations. In some cases, this effect was associated with dam-embryo i nteractions or line-age interactions. Data on differences in PS betwee n breeds and crosses were based, in most cases, on differences between twin and multiple ovulating ewes, or in ewes carrying multiple foetus es after egg transfer. Among twin ovulating ewes, all born to Romney d ams, uterine efficiency (UE) was highest in Finnish Landrace (FL) shed ewes. The contribution of FL breeding to superiority in UE was suppor ted in other FL crossbreeds and in crossbred genotypes with FL genes f rom their ancestry. Crossbreeding effects of the Booroola Merino on ut erine capacity (UC) or on UE were variable, depending on the breeds us ed, on the presence of the Booroola Fecundity gene and on OR. A clear distinction should be made between UE (the marginal increase in LS as OR increases) and UC (the maximum number of foetuses a dam is able to support at birth) when comparing Finnish Landrace and Booroola Merino with other prolific breeds. There were indications of a higher genetic variability in PS, independent from the genetic variability in OR, in the Romanov breed than in other breeds and crosses. in conclusion, wh ile genetic variation in litter size (LS) within some breeds can be st atistically predicted on the basis of OR, between-breed variation in L S has consider the genetic variation in PS. (C) 1998 Elsevier Science B.V. All rights reserved.