Genetic factors controlling wool shedding in a composite Easycare sheep flock

Abstract

Historically, sheep have been selectively bred for desirable traits including wool characteristics. However, recent moves towards extensive farming and reduced farm labour have seen a renewed interest in Easycare breeds. The aim of this study was to quantify the underlying genetic architecture of wool shedding in an Easycare flock. Wool shedding scores were collected from 565 pedigreed commercial Easycare sheep from 2002 to 2010. The wool scoring system was based on a 10-point (0-9) scale, with score 0 for animals retaining full fleece and 9 for those completely shedding. DNA was sampled from 200 animals of which 48 with extreme phenotypes were genotyped using a 50-k SNP chip. Three genetic analyses were performed: heritability analysis, complex segregation analysis to test for a major gene hypothesis and a genome-wide association study to map regions in the genome affecting the trait. Phenotypes were treated as a continuous or binary variable and categories. High estimates of heritability (0.80 when treated as a continuous, 0.65-0.75 as binary and 0.75 as categories) for shedding were obtained from linear mixed model analyses. Complex segregation analysis gave similar estimates (0.80 ± 0.06) to those above with additional evidence for a major gene with dominance effects. Mixed model association analyses identified four significant (P < 0.05) SNPs. Further analyses of these four SNPs in all 200 animals revealed that one of the SNPs displayed dominance effects similar to those obtained from the complex segregation analyses. In summary, we found strong genetic control for wool shedding, demonstrated the possibility of a single putative dominant gene controlling this trait and identified four SNPs that may be in partial linkage disequilibrium with gene(s) controlling shedding.

Estimating heritability of wool shedding in a cross‐bred ewe population

“…No such backcrossing was included in the mating design of this flock, perhaps allowing extra variability in WS. The estimates of heritability of WS obtained by Matika et al (2013) in Easycare sheep were much higher than those obtained in the current study, ranging from 0.65 when treating WS as a binary expressed trait to 0.83 when WS was treated as a continuously expressed trait. Repeatability of WS was also moderate with mean 0.338 (SD 0.028).…”

“…Through the use of link functions (logit, probit), generalized linear mixed models are often used in the analysis of this trait (Pollott ; Matika et al . ). These methods, often known as ‘threshold models’, can be used under classical or Bayesian inference; Allain et al .…”

“…Pollott () and Matika et al . () reported that wool shedding was moderately to highly heritable, and therefore clearly under genetic control. Still, the extent of wool shedding for an animal can vary over time, which may reflect other physiological and environmental factors impacting its expression.…”

“…Easycare, Romane) with less than 100000 animals registered and only limited engagement of producers in record keeping and performance testing (Ricordeau et al 1992). Pollott (2011) and Matika et al (2013) reported that wool shedding was moderately to highly heritable, and therefore clearly under genetic control. Still, the extent of wool shedding for an animal can vary over time, which may reflect other physiological and environmental factors impacting its expression.…”

“…Because of the discrete nature and skewness of WS, normality cannot be assumed. Through the use of link functions (logit, probit), generalized linear mixed models are often used in the analysis of this trait (Pollott 2011;Matika et al 2013). These methods, often known as 'threshold models', can be used under classical or Bayesian inference; Allain et al (2011), for instance, adopted a Bayesian approach to evaluate WS.…”