Genomic differences between Rambouillet sheep selected for high and low reproductive rate

Abstract Text:

The Rambouillet sheep selection program at Montana State University began in 1968 with the establishment of the high (HL) and low (LL) reproductive rate lines. Sheep within these lines were selected based on the index of “I” = total number of lambs born in a lifetime / (age of ewe – 1).  The lines have significantly differentiated phenotypically for, number of lambs born, number of lambs born per ewe exposed and per ewe lambing, and total kilograms of lamb weaned (P < 0.01) Furthermore, systemic progesterone concentrations during the luteal phase of the estrous cycle differ for HL and LL ewes (P < 0.05).  Previous research in these flocks has shown differences between the lines for lambing rate, litter size and ovulation rate. Objectives of the present study were to: 1) evaluate if there are genomic differences between lines; and, 2) identify quantitative trait loci associated with each line and candidate genes within these loci. A sample set of 50 and 46 HL and LL sheep, respectively, were genotyped using the Ovine 60K SNP chip. The data for the genotypes were analyzed using the Golden Helix commercial software package. Principal component analysis indicated distinct clusters for the two lines of sheep when the first two eigenvectors were plotted; demonstrating that these lines are, in fact, genetically different. Using an additive correlation association model and a Bonferroni correction there were 14 markers that differed  (P< 0.01). These markers are on chromosomes 1, 3, 9, and 24. The candidate genes that appear to differ include CHP2, ACOT11, NOS1AP and EGFR. Further analyses and additional samples from each line are needed to better map the differences between the lines. In conclusion it appears that long-term selection for reproductive, a trait known to have low heritability, can be successful in generating animals, at least in sheep that are phenotypically and genetically different.

Keywords: genetic selection, genomics, physiology, reproduction