Investigations into the GDF8 g+6723G-A polymorphism in New Zealand Texel sheep.

This work investigated effects of carrying 0, 1, or 2 copies of the A allele resulting from the g+6723G-A transition in growth differentiation factor gene (GDF8) in New Zealand Texel-cross sheep at different lamb ages and carcass weights. Two Texel-cross sires carrying 1 copy of the A allele were mated to approximately 200 ewes carrying 0, 1, or 2 copies of the A allele. A total of 187 progeny were generated and genotyped to determine whether they were carrying 0, 1, or 2 copies of the A allele. The progeny were assigned to 1 of 4 slaughter groups balanced for the 3 genotypes, sex, and sire. The 4 groups were slaughtered commercially when their average BW (across all progeny in the slaughter group) reached 33, 40, 43, and 48 kg, respectively. Measurements of BW, and carcass dimensions and yield were made on all animals using Viascan (a commercial 2-dimensional imaging system that estimates lean content of the carcass as a percentage of total carcass weight). Additional measurements were made on the fourth slaughter group, which was computed tomography scanned at each slaughter time point to obtain 4 serial measures of lean and fat as estimated from the computed tomography images. The A allele did not have an effect on any BW traits. The A allele was associated with increased muscle and decreased fat across the variety of measures of muscling and fat, explaining between 0.2 and 1.1 of a residual SD unit. Estimates for an additive effect were significant and were positive for muscle and negative for fat traits. No dominance effect estimates (positive or negative) were significant. There was no significant interaction between A allele number and carcass weight or slaughter group for any trait. This is the first systematic study of the effect of the A allele copy number over a range of carcass weights (13 to 20 kg) and ages and results suggest the size of the effect across these endpoints is proportionately the same. Testing for the A allele therefore offers breeders the potential to improve rates of genetic gain for lean-meat yield across most production systems.

Comparative mapping of sheep chromosome 2q.

Sheep chromosome 2q (OAR2q), which is homologous with human chromosome 2q (HSA2q), and cattle chromosome 2 (BTA2), is known to contain several loci contributing to carcass traits. However, the chromosomal rearrangements differentiating these chromosomes among the three species have not yet been determined and thus precise correspondences between the locations of sheep and human genes are not known. Twenty-six genes from HSA2q (2q21.1-->2q36) have been assigned to OAR2q by genetic linkage mapping to refine this area of the sheep genome. Seventy-six genes were initially selected from HSA2q. Sixty-eight percent of the PCR primer sets designed for these genes amplified successfully in sheep, and 34% amplified polymorphic products. Part of the proximal arm of OAR2q was found to be inverted compared with HSA2q. The breakpoint has been localised near the growth differentiation factor 8 gene (GDF8), spanning 380 kb between the positions of the hypothetical protein (FLJ20160) (HSA2:191008944-191075046) and glutaminase (GLS) (HSA2:191453847-191538510) (Build36.1).

An enhanced linkage map of the sheep genome comprising more than 1000 loci.

A medium-density linkage map of the ovine genome has been developed. Marker data for 550 new loci were generated and merged with the previous sheep linkage map. The new map comprises 1093 markers representing 1062 unique loci (941 anonymous loci, 121 genes) and spans 3500 cM (sex-averaged) for the autosomes and 132 cM (female) on the X chromosome. There is an average spacing of 3.4 cM between autosomal loci and 8.3 cM between highly polymorphic [polymorphic information content (PIC) > or = 0.7] autosomal loci. The largest gap between markers is 32.5 cM, and the number of gaps of > 20 cM between loci, or regions where loci are missing from chromosome ends, has been reduced from 40 in the previous map to 6. Five hundred and seventy-three of the loci can be ordered on a framework map with odds of > 1000 : 1. The sheep linkage map contains strong links to both the cattle and goat maps. Five hundred and seventy-two of the loci positioned on the sheep linkage map have also been mapped by linkage analysis in cattle, and 209 of the loci mapped on the sheep linkage map have also been placed on the goat linkage map. Inspection of ruminant linkage maps indicates that the genomic coverage by the current sheep linkage map is comparable to that of the available cattle maps. The sheep map provides a valuable resource to the international sheep, cattle, and goat gene mapping community.

Isolation and mapping of the first ruminant multidrug resistance genes.

The first ruminant multiple drug resistance gene (MDR1) has been cloned and sequenced from sheep. Sequence data revealed the sheep MDR1 gene to have high sequence and structural similarity to other characterized MDR proteins from humans and rodents. A restriction fragment length polymorphism was discovered using the EcoRI enzyme and used to map the MDR1 gene to sheep chromosome 4. Physical mapping using fluorescent in situ hybridisation confirmed this map placement and assigned the MDR1 locus in the region 4q15-q21. The ovine MDR2 gene was also cloned and found to map to the same region as MDR1.

The application of AFLP fingerprinting to construct a YAC contig containing ADH2 and MTP on sheep chromosome 6.

The low density of genetic markers on livestock maps limits progress in positional cloning projects. We demonstrate a strategy of combining comparative mapping with AFLP fingerprinting to develop physical maps in a defined region of the sheep genome. Sequence tagged sites for alcohol dehydrogenase 2 (ADH2) and microsomal triglyceride transfer protein (MTP) were developed and used to screen a sheep yeast artificial chromosome (YAC) library. Nine YACs were identified containing the microsatellite marker BM1329 and either ADH2 or MTP. Additional markers in the region were not available, and AFLP analysis was developed to identify sheep-specific bands within the YACs to determine their degree of overlap. Fourteen bands common to more than one YAC were analysed and provided the markers necessary to develop a YAC contig containing the three STS markers. One YAC (yac260B5) containing all three markers (ADH2, MTP, and BM1329) was mapped to sheep chromosome 6q1.6-->q1.8 by FISH analysis.

Genetic and physical mapping of the ovine cystic fibrosis gene.

Two yeast artificial chromosomes (YACs 48B6 and 88F7) that span the ovine cystic fibrosis transmembrane conductance regulator (CFTR) gene locus have been isolated. These YAC clones have been physically assigned to the interval 4q23-->q25 on sheep chromosome 4, using fluorescence in situ hybridization. A sequence polymorphism in exon 7 of the CFTR gene was identified in Merino sheep and present at a low frequency. This polymorphism segregated in several flocks. Linkage analysis confirmed the location of CFTR on sheep chromosome 4. The most likely location is within a 13.7-cM region close to markers MAF70 and TGLA116.