Chapter 4 - Genetic variation in ovulation rate in sheep

Abstract

Ovulation rate sets the upper limit to litter size, and accounts for an overwhelming proportion of the genetic variation in litter size both among and within breeds of sheep. Because most of the variation in litter size attributable to embryo mortality is non-genetic this masks genetic variation in litter size due to ovulation rate, with the result that heritability of litter size is lower than the heritability of ovulation rate. Where within-breed selection is the only available option for genetic improvement of prolificacy it is shown that the annual rate of genetic improvement in litter size can be doubled when ovulation rate is used as the selection criterion. The demonstration that a single gene with a large effect on ovulation rate (about 1.5 ova) is segregating in Booroola Merino sheep makes it possible to substantially alter the reproductive rate of a target population by introducing this gene. Backcrossing with selection for the gene would allow its incorporation while recovering the genetic background of the target population. Recent evidence suggests that genes with effects of similar magnitude are segregating in Icelandic sheep, in the Cambridge breed and in Javanese sheep. Another approach to breed transformation is to use genetic material from prolific breeds like the Finn and Romanov, for which the evidence suggests that high ovulation rate is not due to a single major gene. The utility of these breeds may be enhanced by selection for further increases in ovulation rate. Results are presented for a Finn line in which selection has increased ovulation rate by 1.5 ova. Use of this line rather than unselected Finn sheep allows a reduction of about 50% in the proportion of Finn genes required for a given increase in the ovulation rate of a target population.