An ovine quantitative trait locus affecting fibre opacity in wool

• Genotypes from a sheep disease study were used in a genome scan for wool QTL.
• There was a significant QTL for yield on OAR1 in one of three sire families.
• We detected very strong evidence for a QTL for fibre opacity on OAR20.
• We inferred that the opacity variation was due to medullation and not pigmentation.

This experiment was originally structured as a genome-wide screen for quantitative trait loci (QTL) affecting resistance or susceptibility to facial eczema disease (FE), a liver mycotoxicosis. Two Finnish Landrace (F) rams were crossed to three Texel (T) ewes to produce three cross-breed (designated F × T) rams. The F × T rams were then outcrossed to 210–230 Coopworth ewes each to generate three half-sib families, and 200 progeny per sire family were used in the FE QTL experiment. In a FE-phenotyping trial, the 600 five-month old lambs were orally challenged with a fixed dose rate of sporidesmin (FE mycotoxin) to determine their disease status. About 217 informative microsatellite markers, which were evenly spaced throughout the 26 ovine autosomes, were analysed in the FE study. Genotyping for the FE experiment was conducted in two stages: a primary and a secondary screen. In the initial primary screen, 46 most FE-resistant and 46 most FE-susceptible progeny of each sire family were selectively genotyped with the markers and analysed. Resulting chromosomes carrying suggestive and significant FE QTL were then followed up in a secondary screen when all progeny were genotyped for final analysis; there were seven chromosomes that underwent secondary screening, viz. OAR1, OAR2, OAR6, OAR13, OAR18, OAR19 and OAR20. During the FE-phenotyping trial period, mid-side wool samples were collected from the unshorn lambs for fibre measurement; the wool traits measured included yield, staple length (StapLen), crimp frequency, fibre curvature (FCurv), fibre opacity (FOpac), fibre diameter mean (FDMean) and fibre diameter standard deviation (FDsd). In a linkage analysis of wool traits, using the Haley–Knott regression method with the available “FE-generated” genotypes, a highly significant QTL for fibre opacity was detected on OAR20.