Abstract
BackgroundBelgian Blue cattle are famous for their exceptional muscular development or “double-muscling”. This defining feature emerged following the fixation of a loss-of-function variant in the myostatin gene in the eighties. Since then, sustained selection has further increased muscle mass of Belgian Blue animals to a comparable extent. In the present paper, we study the genetic determinants of this second wave of muscle growth.ResultsA scan for selective sweeps did not reveal the recent fixation of another allele with major effect on muscularity. However, a genome-wide association study identified two genome-wide significant and three suggestive quantitative trait loci (QTL) affecting specific muscle groups and jointly explaining 8-21% of the heritability. The top two QTL are caused by presumably recent mutations on unique haplotypes that have rapidly risen in frequency in the population. While one appears on its way to fixation, the ascent of the other is compromised as the likely underlying MRC2 mutation causes crooked tail syndrome in homozygotes. Genomic prediction models indicate that the residual additive variance is largely polygenic.ConclusionsContrary to complex traits in humans which have a near-exclusive polygenic architecture, muscle mass in beef cattle (as other production traits under directional selection), appears to be controlled by (i) a handful of recent mutations with large effect that rapidly sweep through the population, and (ii) a large number of presumably older variants with very small effects that rise slowly in the population (polygenic adaptation).Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-796) contains supplementary material, which is available to authorized users.
Highlights
Belgian Blue cattle are famous for their exceptional muscular development or “double-muscling”
That homozygosity for the p.D273RfsX13 MSTN variant only accounts for part of the spectacular muscular hypertrophy of modern Belgian Blue Cattle (BBC) is obvious from the comparison of their carcass scores with that of present-day Blancs bleus mixtes (BBM) animals that are homozygous for the p.D273RfsX13 MSTN mutation (Additional file 1: Figure S1)
To address the first question we aimed at identifying genomic regions characterized by (i) reduced genetic variability in “modern” BBC, and (ii) high genetic differentiation (FST-based measure; see M&M) with “old” BBC, BBM, and Holstein-Friesian (HF) animals
Summary
Belgian Blue cattle are famous for their exceptional muscular development or “double-muscling”. A small population (~3,400) of dualpurpose animals corresponding to this ancestral “Race de Moyenne et Haute Belgique” still exists ( referred to as “Blancs Bleus Mixtes” or BBM), intense selection for increased muscle mass, driven by premiums paid for doublemuscled carcasses and enabled by the systematization of. It is generally not appreciated, that (i) the heritability of muscular development remained as high as 30-45% in BBC after fixation of the p.D273RfsX13 MSTN allele, and (ii) that this residual genetic variation has been exploited to further increase muscle mass of BBC. Did it involve the rapid fixation of one or more mutations with effects of a magnitude similar to the p.D273RfsX13 MSTN mutation? Are Quantitative Trait Loci (QTL) with large effects on muscle mass segregating in present-day BBC? Or is the residual heritability for muscle mass in present-day BBC largely polygenic or “quasi-infinitesimal”?
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