Genetic variation within and between subpopulations of the Australian Merino breed

Abstract

Genetic variation within and between Australian Merino subpopulations was estimated from a large breeding nucleus in which up to 8500 progeny from over 300 sires were recorded at eight sites across Australia. Subpopulations were defined as genetic groups using the Westell–Quaas model in which base animals with unknown pedigree were allocated to groups based on their flock of origin if there were sufficient ‘expressions’ for the flock, or to one of four broad sheep-type groups otherwise (Ultra/Superfine, Fine/Fine-medium, Medium/Strong, or unknown). Linear models including genetic groups and additive genetic breeding values as random effects were used to estimate variance components for 12 traits: yearling greasy and clean fleece weight (ygfw and ycfw), yearling mean and coefficient of variation of fibre diameter (yfd and ydcv), yearling staple length and staple strength (ysl and yss), yearling fibre curvature (ycuv), yearling body wrinkle (ybdwr), post-weaning weight (pwt), muscle (pemd) and fat depth (pfat), and post-weaning worm egg count (pwec). For the majority of traits, the genetic group variance ranged from approximately equal to two times larger than the additive genetic (within group) variance. The exceptions were pfat and ydcv where the genetic group to additive variance ratios were 0.58 and 0.22, respectively, and pwec and yss where there was no variation between genetic groups. Genetic group correlations between traits were generally the same sign as corresponding additive genetic correlations, but were stronger in magnitude (either more positive or more negative). These large differences between genetic groups have long been exploited by Merino ram breeders, to the extent that the animals in the present study represent a significantly admixed population of the founding groups. The relativities observed between genetic group and additive genetic variance components in this study can be used to refine the models used to estimate breeding values for the Australian Merino industry.

Genetic parameters for wool traits, live weight, and ultrasound carcass traits in Merino sheep1

“…The estimates of heritability for the adult wool traits (available in Supplementary Table S2; see the online version of the article at http://journalofanimalscience.org) were generally consistent with the yearling traits shown here. These estimates of heritability for the wool production and wool quality traits are generally consistent with those reviewed by Safari et al (2005) and more recent reports from large Merino data sets (Asadi Fozi et al, 2005;Safari et al, 2007a;Swan et al, 2008Swan et al, , 2016Brown et al, 2010Brown et al, , 2013. The few reports of estimates for heritability of scoured wool yellowness color (Y-Z) in Merino sheep range from 0.25 for yearlings and 0.29 for adults (Smith and Purvis, 2009) to 0.42 ± 0.14 (James et al, 1990) and 0.45 ± 0.08 (Hebart and Brien, 2009), with estimates from Coopworth and Romney sheep being lower (0.13 ± 0.06; Bigham et al, 1983).…”

“…The heritability estimates for yGFW (0.57 ± 0.05) and yearling CFW (yCFW; 0.52 ± 0.05) were high. There was also considerable genetic group variation for these traits, and the results are generally consistent with those of Swan et al (2016). The estimates of heritability for the adult wool traits (available in Supplementary Table S2; see the online version of the article at http://journalofanimalscience.org) were generally consistent with the yearling traits shown here.…”

“…These genetic correlations for yearling fleece weight and yFD with live weights were generally higher than those reviewed by Safari et al (2005) and those reported by Safari et al (2007b) in research flocks, although they were generally close to the estimates reported from industry data . Our estimates for these wool traits are lower than those reported by Swan et al (2016) for pwWT using similar data. The other wool traits had lower genetic correlations with the live weights, which is generally consistent with the limited number of traits and magnitude of the SE of the estimates in these other reports.…”

“…Initially, mixed linear sire models were developed to identify those fixed effects influencing the wool traits and the weight and ultrasound traits separately. The fixed effects included site (8 levels), year of birth (5 levels), sex (2 levels), sheep type (3 levels: ultra/super fine, fine fine/medium, and medium/strong to account for sires being from different types of Merino; Swan et al, 2016), type of birth and rearing (6 levels: 11, 21, 22, 31, 32, or 33 for the number of lambs born/reared, respectively), and dam age (7 levels: 2 to greater than or equal to 7 yr of age). For the live weight and ultrasound traits, management group nested within site was also fitted as well as age at observation as a linear covariate.…”

“…All models included the random effects of animal and genetic group. The genetic group effect represents the proportion of genes from various Merino bloodlines of each animal defined from its pedigree using the method initially described by Quaas (1988) and modified by Swan et al (2016). For each trait, the genetic groups fitted were derived from the pedigrees of animals with phenotypic records for that trait and ranged in number from 130 to 134 across the traits.…”