Evidence that an imprinted gene on the X chromosome increases ovulation rate in sheep

Ovulation rate records from 1311 female progeny of 50 Coopworth rams were u sed to study the inheritance of ovulation rate in a screened high prolifica cy sheep flock. Breeding values (BV) for ovulation rate for 33 sires used w ithin the screened flock and ovulation rate deviations for a further 17 sir es progeny tested in commercial flocks suggest that a major gene (Woodlands gene) for ovulation rate with a non-Mendelian inheritance pattern is segre gating in a family line. Rams assigned as carriers of the putative gene did not produce carrier sons (zero of three), and this coupled with the observ ation that daughters of carrier rams had ovulation rates of 0.39 (standard error of difference [SED] = 0.06) higher than contemporaries without a sign ificant increase in the variance of log ovulation rate strongly suggests th at the gene is on the X chromosome. The evidence suggests that the gene is also maternally imprinted because ovulation rate data indicate that it is e xpressed where females inherit a paternal allele but is silenced when inher ited on a maternal allele. Maternal granddaughters of carrier rams had mean ovulation rates that were only 0.02 (SED = 0.06) higher than noncarrier ew es from the same flock. Furthermore, carrier dams expressing the gene (pate rnal allele) had 24 sons, none of which had female offspring that expressed the gene, whereas carrier dams not expressing the gene (maternal allele) h ad 7 out of 17 sons that had female progeny expressing the gene. There is n o evidence of the infertility that occurs in homozygous ewes carrying the X -linked Inverdale gene. Collectively, these results suggest the existence o f a novel gene for prolificacy located on the X chromosome that is maternal ly imprinted. The Woodlands gene was only expressed upon paternal inheritan ce from carrier males that were the progeny of nonexpressing carrier dams. The gene was not expressed in ewes that received it from either carrier dam s (expressing or nonexpressing) or from carrier males that were the progeny of expressing carrier dams.