Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 25, 2005
Publication Date: February 3, 2006
Citation: Freking, B.A., Leymaster, K.A. 2006. Evaluation of the ovine callipyge locus: IV. Genotypic effects on reproductive traits. Journal of Animal Science. 84:311-316. Interpretive Summary: The impact on reproduction traits of a single base mutation with large favorable effects on lean and fat tissue of sheep was investigated. Sheep with specific allelic forms of this gene, callipyge, have extreme beneficial values for traits associated with carcass leanness. However, sheep which exhibit the lean phenotype have also displayed undesirable aspects associated with tenderness and marbling of loin chops. No information was previously available concerning the impact of this mutation on reproduction. Productivity and longevity through 5 years of age were recorded on a total of 265 F2 ewes. Genotypes for the callipyge mutation were determined using a direct DNA test. There was in general a lack of evidence to conclude any biologically important influence of this mutation on reproduction or fitness. Maintaining a ewe flock fixed for the mutation to produce rams for a terminal sire mating system would present no unusual problems from a reproduction standpoint. Industry adaptation of the callipyge phenotype is limited only by the antagonism with tenderness of loin chops.
Technical Abstract: A resource population of F2 ewes derived from Dorset and Romanov grandparents was generated over five years to provide genotypic and phenotypic data to estimate effects of each callipyge genotype on component traits of reproduction. Effects on ovulation rate were estimated from data recorded by examination of ovaries from serially slaughtered ewe lambs (n = 174) born in 1994 and 1995. Productivity and longevity through 5 yr of age were recorded on a total of 265 F2 ewes produced in 1996 through 1998. Number of lambs born, birth weights, and weaning weights of lambs were calculated for each ewe. Weaning weight was recorded for each lamb and adjusted for age to 56 and 32 d for dam- and nursery-reared lambs, respectively. Longevity was determined as a binary trait based on the presence or absence of the ewe in the breeding flock at 5 yr of age. Ewes generated 937 records for breeding weight and 925 records for conception rate, which was determined on ewes exposed and still present at the start of each lambing season. A total of 717 records were analyzed for traits based on number of ewes lambing. Genotypes for the causative callipyge mutation were determined using a MALDI-TOF mass spectrometry assay. Each ewe was classified into one of four genotypes; N**MatN**Pat, N**MatC**Pat, C**MatN**Pat, or C**MatC**Pat, with N representing wild type and C representing the mutant allele. Mean ovulation rate of ewe lambs genotyped N**MatN**Pat was greater (P < 0.02) than that exhibited by N**MatC**Pat. However, the estimated 0.25 ovum difference in ewe lambs did not affect number born. Live weight of N**MatC**Pat ewes at breeding was 2.5 kg less than other genotypes (P < 0.01). Callipyge genotype class did not significantly affect conception rates, fecundity, maternal ability, or ewe longevity. Maintaining a ewe flock to produce homozygous mutant rams for use in terminal sire mating systems presents no unusual problems from a reproduction standpoint.