The effect of GDF9 on litter size in Australian sheep

Growth differentiation factor 9 (GDF9) is a known autosomal gene which regulates ovulation rate in mammals. In sheep, numerous polymorphisms have been reported in coding regions of GDF9 with a significant impact on ovulation rate and hence litter size. To study the effect of GDF9 on litter size in Australian sheep breeds, an association analysis was performed between 1,600,633 imputed sequence single nucleotide polymorphisms (SNPs) on OAR5 and litter size phenotypes in 8,850 Merino and 7,613 maternal sheep breed ewes (predominantly Border Leicester, Coopworth, Corriedale and composite maternal lines) respectively. Results showed a significant association between litter size and SNPs in the GDF9 region in maternal breeds. After filtering for high linkage disequilibrium, a highly significant SNP (p_value = 9.09E-09) was found in an intron of the GDF9 gene at OAR5:41841588, which accounted for a 0.22 increase in litter size and explained 4.75% of the total genetic variance. This SNP and the surrounding SNPs in the region of GDF9 were not significantly associated with litter size in Merinos. Information on this SNP genotype could be useful for obtaining a more accurate estimate of genetic merit for reproduction traits in some breeds of sheep.