Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2017
Publication Date: 3/6/2018
Publication URL: http://handle.nal.usda.gov/10113/5934969
Citation: Freking, B.A., King, D.A., Shackelford, S.D., Wheeler, T.L., Smith, T.P.L. 2018. Effects and interactions of myostatin and callipyge mutations: I. Growth and carcass traits. Journal of Animal Science. 96:454–461. https://doi.org/10.1093/jas/skx055.
Interpretive Summary: Improving carcass leanness of U.S. lamb is important to the sheep industry to allow it to remain competitive in our domestic and global market. Genotypic effects and potential interactions of two genetic variants, myostatin and callipyge, both influencing muscle growth and carcass composition but through different mechanisms were estimated. This study provides the comparative information to assess four combinations of myostatin and callipyge genotypes for growth and carcass traits testing for the presence of interaction effects that could be exploited to produce leaner lamb carcasses in terminal crossbreeding systems. A single copy of the myostatin mutation common in Texel sheep had little to no effect on growth rate and modest impacts on carcass fat relative to a single copy of the callipyge mutation. An interaction above and beyond the independent effects of the two genes was detected for a subjective measure of leg conformation. Some additional interactions between the two loci were more modest in effect but were detected for growth and carcass traits measurements with the direction of those effects varying by trait and sex. Both basic and applied knowledge resulted from the outcomes of this research to allow producers to make informed decisions concerning genetic selection for improved carcass leanness.
Technical Abstract: Objectives were to document effects of the Texel myostatin mutation (MSTN) on growth and carcass traits and also test whether or not interactions with the callipyge mutation (CLPG) could be detected. Twelve rams heterozygous at both loci on the two different chromosomes were mated to 215 terminal-sire type composite crossbred ewes genotyped as non-carriers for both loci. A total of 365 lambs were born, 362 of those were genotyped and 236 lambs contributed carcass data to estimate effects and interactions among the four genotype combinations produced. The four genotype combinations were defined as follows: ++/++ for wildtype at both loci; ++/C+ for wild-type at MSTN and heterozygous at CLPG; M+/++ for heterozygous at MSTN and wild-type at CLPG; and M+/C+ for heterozygous at both loci. The two independently segregating sire-derived alleles represent different breed-of-origin contrasts at each locus (Texel vs. composite origin for MSTN and Dorset vs. Texel origin for CLPG). Birth weight was recorded on all lambs, and subsequent body weights were adjusted to 56 (weaning), 70, and 140 d of age. Within sire-sex-genotype subgroups, naturally reared lambs were assigned to one of eight slaughter groups accounting for variation in birth date. Lambs were serially slaughtered at weekly intervals, 30 lambs per group, from roughly 26 to 33 wk of age. In addition to standard carcass traits, subjective leg scores were assigned and widths of carcasses were measured at the widest points of the shoulder and rump. Differences in birth weight were detected (P < 0.01) for the combination of the two loci and birth type, with single-born differences among genotypes exceeding differences among twin born progeny. Those interaction differences among genotypes were not as important at weaning (P = 0.36). Impact on growth rate differences among the genotypes during the post-weaning period were variable and dependent on sex of the lamb (P < 0.01). A synergistic interaction between MSTN and CLPG was observed for leg muscling scores (P < 0.05) but no other measures of carcass shape were affected. One copy of MSTN had a more modest impact on fat deposition and muscle conformation than did CLPG and did not interact (all values P > 0.20). Although some non-additive interactions that vary by trait and sex were detected, in general the data are consistent with the two mutations acting on muscle growth through independent pathways.