Bidirectional selection for body weight on standing genetic variation in a chicken model

Experimental populations of model organisms provide valuable opportunities to unravel the genomic impact of selection in a controlled system. The Virginia body weight chicken lines represent a unique resource to investigate signatures of selection in a system where long-term, single-trait, bidirectional selection has been carried out for more than 60 generations. Using pooled genome resequencing of paired generations of these lines, we reveal the within and between-line genomic signatures of selection. At 55 generations of divergent selection, 14.2% of the genome showed extreme differentiation between the selected lines were contained within 395 genomic regions. The lines often displayed a duality of the sweep signatures: an extended region of homozygosity in one line, in contrast to mosaic pattern of heterozygosity in the other line. These haplotype mosaics consist of short, distinct haploblocks of variable between-line divergence. Formed during what probably was a complex history of bottlenecks, inbreeding, and introgressions, these mosaics represent the standing genetic variation available at the onset of selection in the founder population. Selection on standing genetic variation can thus result in different signatures depending on the intensity and direction of selection.