title: Selection in action: dissecting the molecular underpinnings of the increasing muscle mass of Belgian Blue Cattle.
abstract: BACKGROUND
  Belgian 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.
  
  
  RESULTS
  A 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.
  
  
  CONCLUSIONS
  Contrary 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).
authors.affiliation: Unit of Animal Genomics, GIGA-R & Faculty of Veterinary Medicine, University of Liège - B34 (+1), Avenue de l'Hôpital 1, Liège B-4000, Belgium. tom.druet@ulg.ac.be.
authors.forename: Tom
authors.initials: T
authors.lastname: Druet
authors.forename: Naima
authors.initials: N
authors.lastname: Ahariz
authors.forename: Nadine
authors.initials: N
authors.lastname: Cambisano
authors.forename: Nico
authors.initials: N
authors.lastname: Tamma
authors.forename: Charles
authors.initials: C
authors.lastname: Michaux
authors.forename: Wouter
authors.initials: W
authors.lastname: Coppieters
authors.forename: Carole
authors.initials: C
authors.lastname: Charlier
authors.forename: Michel
authors.initials: M
authors.lastname: Georges
mesh_terms: D000818:Animals; D002417:Cattle; D019143:Evolution, Molecular; D006239:Haplotypes / Q000235:genetics; D006720:Homozygote; D009132:Muscles / Q000033:anatomy & histology*; D009154:Mutation; D009929:Organ Size / Q000235:genetics; D040641:Quantitative Trait Loci / Q000235:genetics; D012641:Selection, Genetic