Item

Molecular genetics underlying ovine growth and carcass traits in New Zealand breeds: A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Ekegbu, Ugonna Jane
Date
2019
Type
Thesis
Fields of Research
ANZSRC::0604 Genetics , ANZSRC::060412 Quantitative Genetics (incl. Disease and Trait Mapping Genetics) , ANZSRC::070201 Animal Breeding
Abstract
As a major source of protein in many nations, the global demand for meat is rising. The multi-billion dollar New Zealand (NZ) export sheep meat industry is always searching for new ways to increase profitability, improve productivity and increase efficiency. Sheep growth and carcass traits are therefore considered to be of economic importance; with greater emphasis being placed on the use of genetic tools in breeding for leaner, faster-growing animals that yield more meat with good taste characteristics. This research examined four sheep genes: insulin-like growth factor 1 receptor (IGF1R), growth hormone (GH), POU class 1 homeobox 1 (POU1F1), and PROP paired-like homeobox 1 (PROP1). The biological function of the protein products of these genes, and their direct or indirect effect on traits associated with meat production, suggested they were good candidates for further study. This research investigated whether nucleotide sequence variation was present in these genes, and whether associations exist between variation in the genes and variation in selected sheep growth and carcass traits. To achieve this goal, firstly, ten different NZ sheep breeds were screened to ascertain whether nucleotide sequence variation was present in selected regions of the genes. This involved using a combination of polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP) analysis, coupled with nucleotide sequencing. Next, with NZ Romney sheep that had well-recorded pedigree and growth and carcass traits, general linear mixed-effects models (GLMMs) were then used to ascertain whether or not any nucleotide sequence variation detected was associated with the growth and carcass traits. The IGF1 receptor mediates key signalling processes that are crucial to cell growth, protein synthesis, proliferation, survival and metabolism. This study examined three fragments of ovine IGF1R, one of which spanned from exon 9 to intron 10. Five variants (A9 to E9) defined by ten nucleotide substitutions, including a deletion (c.2201+114_118delCGCAG), were detected in the exon 9 to intron 10 region. Lambs possessing the A9 variant grew faster by 10.8 g/day (P = 0.001), had 9% less fat thickness (P = 0.038), weighed 3.2% more at slaughter (P < 0.001), and had 1.2% increase in total lean meat yield (P = 0.002). In contrast, the E9 variant was associated with slower-growing lambs (P = 0.001), increased fat depth (P = 0.022), and decreased muscularity (P = 0.009). Growth hormone is a key mediator of growth and other metabolic processes. Variation in GH has been reported in several species where it showed association with production traits. In this study, a fragment of ovine GH spanning the exon 2, intron 2 and exon 3 regions was found to have 11 nucleotide substitutions that were arranged in seven variants (named A3 to G3). Four of the substitutions were found in the coding region of GH, namely c.51C>T (p.Leu17Leu), c.59C>T (p.Pro20Leu), c.79G>A (p.Ala27Thr), and c.103G>C (p.Gly35Arg). The B3 variant was associated with an increase in average daily weight gain by 17.4 g/day (P < 0.001), lower fat depth at the 12th rib by 0.48 mm (P = 0.015), and increased total lean meat yield by 1.2% (P = 0.007). In contrast, the F3 variant was associated with decreased average daily weight gain (P < 0.001), lower weight at slaughter (P < 0.001), and decreased total lean meat yield (P < 0.001). Lambs possessing the A3B3 genotype grew the fastest (274.59 ± 10.56 g/day; P < 0.001) and had the highest lean meat yield (53.47 ± 0.19%, P < 0.001). POU1F1 positively regulates the functions of three key hormones: GH, thyroid-stimulating hormone, and prolactin. Three fragments were explored in this study, but only the exon 1 fragment revealed any sequence variation with two variants, A and B. Associations with growth and carcass traits were not tested. PROP1 controls prenatal muscle development and differentiation. Two fragments spanning the exon 1 and exon 2 regions of ovine PROP1 were examined. Three variant sequences were revealed in each region: exon 1 - A1, B1 and C1, and exon 2 - A2, B2 and C2. The more abundant A1 and A2 variants were associated with an increase in growth rate (P < 0.001) and higher total lean meat yield (P = 0.001). Variant B1 was associated with a decrease in birth weight by 0.14 kg (P = 0.025) and an increase in fat thickness by 0.45 mm (P = 0.013), while variant C2 was associated with lower growth rate of 22.3 g/day (P = < 0.001) and decreased total lean meat yield by 1% (P = 0.025). The sequence variations found in this study could potentially be used in marker-assisted selection (MAS) for improving growth and carcass traits in sheep.
Source DOI
Rights
Creative Commons Rights
Attribution-NonCommercial-NoDerivatives 4.0 International
Access Rights