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In this study, actinin-3 ( ACTN3 ) gene expression was investigated in relation to the feed efficiency phenotype in  Bos indicus  -  Bos taurus  crossbred steers. A measure of relative feed efficiency based on residual feed intake relative to predictions from the NRC beef cattle model was analyzed by the use of a mixed linear model that included sire and family nested within sire as fixed effects and age, animal type, sex, condition, and breed as random effects for 173 F 2  Nellore-Angus steers. Based on these residual intake observations, individuals were ranked from most efficient to least efficient. Skeletal muscle samples were analyzed from 54 steers in three groups of 18 (high efficiency, low efficiency, and a statistically average group).  ACTN3 , which encodes a muscle-specific structural protein, was previously identified as a candidate gene from a microarray analysis of RNA extracted from muscle samples obtained from a subset of steers from each of these three efficiency groups. The expression of  ACTN3  was evaluated by quantitative reverse transcriptase PCR analysis. The expression of  ACTN3  in skeletal muscle was 1.6-fold greater in the inefficient steer group than in the efficient group ( p  = 0.007). In addition to expression measurements, blocks of SNP haplotypes were assessed for breed or parent of origin effects. A maternal effect was observed for  ACTN3  inheritance, indicating that a maternal  B. indicus  block conferred improved residual feed efficiency relative to the  B. taurus  copy ( p  = 0.03). A SNP haplotype analysis was also conducted for m-calpain ( CAPN2 ) and fibronectin 1 ( FN1 ), and a significant breed effect was observed for both genes, with  B. indicus  and  B. taurus  alleles each conferring favorable efficiency when inherited maternally ( p  = 0.03 and  p  = 0.04). Because the  ACTN3  structural protein is specific to fast-twitch (type II) muscle fibers and not present in slow-twitch muscle fibers (type I), muscle samples used for expression analysis were also assayed for fiber type ratio (type II/type I). Inefficient animals had a fast fiber type ratio 1.8-fold greater than the efficient animals ( p  = 0.027). Because these fiber-types exhibit different metabolic profiles, we hypothesize that animals with a greater proportion of fast-twitch muscle fibers are also less feed efficient.

Skeletal Muscle Expression of Actinin-3 (ACTN3) in Relation to Feed Efficiency Phenotype of F2Bos indicus - Bos taurus Steers