Characterisation of genes associated with sheep growth and carcass traits

Abstract

Animal growth and carcass composition are of commercial importance for sheep meat production. Growth and fat deposition are affected by both environmental factors and genetic factors. Genetic selection to produce leaner, fast growing lambs has a long-term benefit for the sheep industry. Recent evidence supports the contention that lipolysis is important in the regulation of animal feed conversion efficiency and energy utilisation. In humans, genes encoding the β₃-Adrenergic Receptor (ADRB3), Hormone-Sensitive Lipase (HSL) and Uncoupling Protein 1 (UCP1) have been found to be associated with variation in growth-rate and body composition. Given these associations in humans, the aims of this thesis were to characterise variation in the ovine genes ADRB3, HSL and UCP1, and assess whether there is an association between variation in these genes and variation in sheep growth and carcass traits. Using a Polymerase Chain Reaction-Single Strand Conformational Polymorphism (PCR-SSCP) method, variation in the three genes was investigated. Next, General Linear Mixed-effects Models (GLMMs) were used to investigate associations between variation in these genes and variation in various growth and carcass traits. The following was found: The intron and the 3’untranslated region (3’UTR) of ovine ADRB3 were screened for genetic variation in 808 NZ Suffolk sheep and 140 NZ Merino sheep. In the NZ Merino sheep, six 3’UTR variants (named a-f) were detected. Sequencing revealed three SNPs (g.*233A>C, g.*271C>G, g.*357A>T) and a single nucleotide deletion (g.*257delG) in the 3’UTR of the gene. Sixteen ovine ADRB3 intron-3’UTR haplotypes were deduced (named A-a, A-b, A-c, B-c, C-e, D-d, E-e, F-e, G-f, H-f, I-a, J-f, K-f, L-e, M-f). For the 808 Suffolk sheep investigated, in single variant model, the presence of intron variant C was associated with higher weaning-weight (P = 0.021) while the presence of A was associated with lower weaning-weight (P < 0.001). The presence of C was found to be associated with higher post-weaning growth-rate (P = 0.002) and higher Fat Depth above the eye Muscle (FDM) (P= 0.033). The presence of B was found to be associated with lower post-weaning growth-rate (P = 0.002). The presence of 3’UTR variant b was associated with lower weaning-weight (P = 0.001). The presence of a was associated with higher post-weaning growth-rate (P = 0.021) and c was associated with lower post-weaning growth-rate (P = 0.005). The presence of haplotype A-b was associated with a lower weaning-weight (P = 0.001). The presence of C-a was found to be associated with higher post-weaning growth-rate (P = 0.008), while the presence of B-c was associated with lower post-weaning growth-rate (P = 0.005). Variation within three regions (exon 3-4, exon 5-6 and exon 9) of ovine HSL and its association with post-weaning growth (n = 538) and carcass traits (n = 262) was investigated in NZ Suffolk sheep. Four intron 5 variants (designated A-D) and two exon 9 variants (designated a and b) of ovine HSL were detected. In the single variant models, the presence of intron 5 variant A in a lamb's genotype was associated with lower Eye Muscle Depth (EMD) (P = 0.036) and Eye Muscle Width (EMW) (P = 0.018), whereas the presence of C was associated with higher EMD (P < 0.001), EMW (P < 0.001) and FDM (P = 0.017). The association of C with higher EMD (P = 0.002) and EMW (P = 0.002) persisted in the multi-variant model. No association between HSL intron 5 variants and post-weaning growth, or between HSL exon 9 variants, post-weaning growth or carcass traits, were found. Variation within three regions (the promoter region, intron 2 and exon 5) of ovine UCP1 and the association of this variation with variation in growth and carcass traits was investigated in 587 NZ Suffolk sheep and 236 NZ Romney sheep. Three promoter variants (designated A-C) and two intron 2 variants (designated a and b) of ovine UCP1 were detected. In the Suffolk sheep studied, promoter variant C was found to be associated with higher FDM in single variant model (P =0.033). In the Romney sheep studied, the presence of variant B in a lamb's genotype was associated with lower subcutaneous carcass fat grade (VGR) (P = 0.005), whereas two copies of C was associated with higher VGR (P < 0.001), and a lower hind leg yield (P = 0.032). The results suggest these genes could be used as gene-markers in sheep breeding to select for desirable growth and carcass quality characteristics.