|dc.description.abstract||Myostatin (MSTN), which is also known as growth and differentiation factor 8 (GDF8), has been reported to act as a negative regulator of myogenesis during skeletal muscle development. It has also been implicated in the regulation of adipocyte function and in controlling the structure and function of tendons. Variation in the myostatin gene (MSTN) has been associated with differences in muscularity in certain breeds of sheep.
Polymerase chain reaction - single strand conformational polymorphism (PCR-SSCP) analysis coupled with nucleotide sequencing was used to assess variation in MSTN in New Zealand (NZ) sheep. In this study, a total of 218 sheep from a range of breeds were screened for variation across an extended region spanning nucleotide c.-1199 in the promoter region to c.*1813 in the 3’UTR. The breeds investigated included dual-purpose breeds for meat and wool production (Coopdale, Coopworth, Corriedale, Perendale and NZ Romney sheep), meat breeds (Dorper, Dorset Down, Poll Dorset, Suffolk and Texel sheep), a wool breed (Merino) and some NZ cross-bred sheep.
A total of 28 nucleotide substitutions were identified in these breeds across the region investigated. Of these substitutions, one (c.101G>A) is located in a coding region and results in an amino acid substitution of glutamic acid (Glu) with glycine (Gly) at codon 34. The other nucleotide substitutions are located within non-coding regions. Ten substitutions at positions c.-959C>T, c.-784A>G, c.373+563A>G, c.373+607A>G, c.374-654G>A, c.374-54T>C, c.748-54T>C, c.*83A>G, c.*455A>G and c.*709C>A are reported for the first time in this study. These substitutions suggest ovine MSTN is highly variable and that a potential exists for such variation to affect MSTN activity, and thus to be used as a gene-marker to help with breeding selection for muscle yield.
Based on the genetic variation found in this study, a strategic PCR-SSCP typing approach was used to define the haplotypic diversity of MSTN. Eight haplotypes (designated H1-8) of MSTN spanning an extended region including the 5’UTR, coding regions, intronic regions and 3’UTR were characterised in the different breeds. Certain haplotypes (e.g. H2 and H3) were found with high frequencies whereas others (e.g. H8) were less common. Two nucleotide substitutions, c.101G>A and c.*83A>G specifically defined haplotypes H7 and H8 respectively in this study and have not been reported previously.
Of the various breeds studied, the NZ Romney is the most common breed in NZ. Thus the association of MSTN with meat production in NZ Romney is of utmost interest to the NZ sheep industry. Associations between the various MSTN haplotypes and variation in lamb growth and carcass traits in 1376 NZ Romney lambs from 19 sire-lines were investigated. The lambs were selected from both North Island and South Island farms over four consecutive years from 2006 to 2009.
Five extended MSTN haplotypes (H1, H2, H3, H5 and H7) were identified in these NZ Romney sheep while H4, H6 and H8 were not found. Of these five haplotypes, H1 carrying MSTN c.*1232A is reported for the first time in NZ Romney sheep.
Using a Restricted Maximum Likelihood (REML) mixed-model approach in both half-sib and pooled-data analyses, H1 was associated with an increase in birth weight, tailing weight and draft weight in lambs, and had a significant effect on loin yield, leg yield, total yield and proportion loin yield from lamb carcasses. Breeding for an increased number of lambs carrying H1 may therefore lead to an improvement in lamb growth and meat yield, and thus underpin increased productivity and improved economic returns in the NZ sheep industry.
Haplotype H2 in lambs was associated with an increase in draft weight and an earlier drafting age, together with an increase in loin, shoulder and total yield of lean meat. H3 was associated with an increase in growth rate to weaning, leg yield and total yield of lean meat. The presence of haplotype H5 in lambs was associated with a decrease in loin yield, shoulder yield and total yield of lean meat yield. The association of haplotype H7 with an increase in hot carcass weight and a decrease in leg yield was identified for the first time in this study. This suggests that H7 may have a possible effect on growth but not meat yield. In this context, it would seem advantageous to promote the selection of NZ Romney for haplotypes H2 and H3 and not H5 and H7.
Pleiotropic effects of MSTN c.*1232A were also investigated. A total of 79 NZ Romney lambs derived from a single sire-line that was heterozygous for c.*1232G>A were investigated. A factorial REML mixed-model was used in the half-sib data analyses. A significant association was found between MSTN c.*1232 genotype and gender with the MSTN c.*1232AG genotype being more common in ewe lambs. Thus, MSTN may also affect other physiological activities during embryonic development. This may offer an explanation for the abnormal gender ratio effect observed in this study. It is important for the livestock industry to balance the benefit of increased birth weight, lamb growth and carcass yield against potential pleiotropic effects caused by MSTN c.*1232A.
The work presented in this thesis suggests that the variation in ovine MSTN in NZ Romney is associated with differences in lamb growth and carcass traits, and thus meat production. It also suggests that using MSTN as a marker-assisted selection tool for improved carcass traits in NZ Romney is a future possibility.||en