Detection of Quantitative Trait Loci for Meat Quality and Carcass Composition Traits in Blackface Sheep

Quantitative trait loci (QTL) were identified for traits related to carcass and meat quality in Scottish Blackface sheep. The population studied comprised a double backcross between lines of sheep previously divergently selected for carcass lean content (LEAN and FAT lines), comprising nine half-sib families. Carcass composition (600 lambs) was assessed non-destructively using computerised tomography (CT) scanning and comprehensive meat quality measurements (initial and final pH of meat, flavour, colour and carcass weight) were taken on 300 male lambs. Lambs and their sires were genotyped across candidate regions on chromosomes 1, 2, 3, 5, 14, 18, 20 and 21. QTL analyses were performed using regression interval mapping techniques. In total, eight genome-wise significant and 12 chromosome-wise and suggestive QTL were detected in seven out of eight chromosomal regions. The genome-wise significant QTL (chromosomes 1, 2, 3 and 5) affected flavour, meat colour, muscle density, bone density and live weight-related traits. The most significant QTL affected meat flavour and was segregating in several families on chromosome 1. This study provides information on new QTL affecting meat quality and carcass composition traits in sheep. Verification of these results is now required in independent sheep populations. Introduction Selection objectives in sheep breeding are changing as the nature of the pressures upon the sheep industry change. Sheep breeders particularly need to address product quality traits, as these will determine consumer acceptance of lamb, thus the long-term market prospects. Critical traits determining product quality are carcass composition and meat quality. A number of selection experiments have documented rapid changes in composition that are possible by selecting on live animal estimates of carcass weight. New measurement technologies, such as computerised tomography (CT), offer the potential for more accurate measurement of carcass traits in the live animal and consequently improved genetic gains. On the other hand, meat quality traits pose particular problems for improvement, as measurement is generally restricted to the slaughtered animal. Therefore, for meat quality quantitative trait loci (QTL) will be particularly important. The development of genetic markers and their application to farm animals has progressed rapidly, opening new prospects for identifying chromosomal regions defining QTL. There is less activity in QTL identification in sheep populations compared to other livestock species, and surprisingly few QTL have been published for traits of direct relevance to meat production, apart from studies of individual major genes such as the callipyge locus (Freking et al., 2002). This suggests there may be QTL effects for meat traits still to be found in sheep. The aim of this study is to identify QTL for carcass composition and meat quality traits. This will provide a basis for targeting genomic regions to verify QTL in independent sheep populations. Material and Methods Animals The population studied comprised lambs derived from LEAN (L) and FAT (F) lines of Blackface sheep, previously divergently selected for predicted carcass composition (Bishop, 1993). A double backcross design created 9 half-sib families for QTL detection, ranging from 23 to 141 individuals per family. Phenotypic measurements taken on cross-sectional scans at the ischium (ISC), the 5 lumbar vertebrae (LV5) and the 8 thoracic vertebrae (TV8) were obtained by computerised tomography (CT) on 600 5-month old lambs, from these 9 families. From each scan image the areas and image densities were obtained for the fat, muscle and bone components of the carcass. Additionally, meat quality measurements were made on 300 8-month old male lambs that had previously been CT scanned. Traits including initial and final pH of the meat, colour, flavour and carcass weight traits.