Identification of a quantitative trait locus for wool strength in Romney sheep

Abstract

Intermediate filament (IF) keratins and associated matrix proteins were selected as candidate genes for the identification of a genetic marker of wool strength. These proteins are the principal structural components of the wool fibre and a number of studies have reported considerable heterogeneity at the protein level. The genes encoding particular wool proteins were specifically amplified using polymerase chain reaction (PCR) methods and nucleotide sequence differences were characterised.

Fifteen nucleotide substitutions, some of which were non-synonymous, were identified in each of the genes encoding the B2A and B2C proteins of the B2 high-sulphur protein family. A length polymorphism was also identified in the B2A gene and was characterised as the insertion or deletion of a 30-nucleotide sequence. The inserted/deleted sequence translated to a decapeptide repeat which varies in frequency between the four B2 high-sulphur proteins. Regions which encode highly conserved amino acids of IF keratins were PCR-amplified from one Type I and two Type II wool IF keratin genes. Two silent nucleotide substitutions were characterised in Exon 1 of the gene for the KRT1.2 protein, a Type I IF keratin. However, there were no findings of polymorphism which would alter the primary structure of the IF keratin protein.

Paternal half-sib pedigrees of the collaborative WRONZ/ AgResearch Wool Strength Selective Breeding Programme in Romney sheep were used for linkage analyses. Tight linkage between the B2A, B2C and KRT1.2 genes was demonstrated by the lack of any recombination between these loci in the genotyped half-sib progeny. Evidence from the literature indicated that these gene loci were located on ovine Chromosome 11q.

The paternal half-sib progeny of 13 sires, which were heterozygous for at least one polymorphic marker in either the B2A, B2C or KRT1.2 genes, were genotyped and classified into two groups according to which allele was inherited from the sire. The data from all sire groups were pooled and a residual maximum likelihood (REML) analysis was conducted on the variate of hogget staple strength (N/ktex). The fixed effects were all interactions of the terms birth year, sex and selection line, and the random effect was genotype-within-sire.

There was a significant (P < 0.01) genotype-within-sire effect for staple strength (N/ktex). This suggests that there is a quantitative trait locus (QTL) for wool staple that is linked to the B2A/B2C/KRT1.2 loci on ovine Chromosome 11q. The differences of mean staple strength (N/ktex; adjusted for sex effects) between each genotype classification within sire were tested for significance. Two sires showed a significant genotype effect (P < 0.02). This result is valuable for an improved understanding of the biological basis of wool strength and the future development of genetic markers to identify genetically superior sheep.