Abstract
Domestication of barley and other cereals was accompanied by an increase in seed size which has been ascribed to human selection, large seeds being preferred by early farmers or favoured by cultivation practices such as deep sowing. An alternative suggestion is that the increase in seed size was an indirect consequence of selection for plants with more vigorous growth. To begin to address the latter hypothesis we studied the diversity of HvWAK1, a wall-associated kinase gene involved in root proliferation, in 220 wild barley accessions and 200 domesticated landraces. A 3655-bp sequence comprising the gene and upstream region contained 69 single nucleotide polymorphisms (SNPs), one indel and four short tandem repeats. A network of 50 haplotypes revealed a complex evolutionary relationship, but with landraces largely restricted to two parts of the topology. SNPs in the HvWAK1 coding region resulted in nonsynonymous substitutions at nine positions in the translation product, but none of these changes were predicted to have a significant effect on the protein structure. In contrast, the region upstream of the coding sequence contained five SNPs that were invariant in the domesticated population, fixation of these SNPs decreasing the likelihood that the upstream of a pair of TATA boxes and transcription start sites would be used to promote transcription of HvWAK1. The sequence diversity therefore suggests that the cis-regulatory region of HvWAK1 might have been subject to selection during barley domestication. The extent of root proliferation has been linked with traits such as above-ground biomass, so selection for particular cis-regulatory variants of HvWAK1 would be consistent with the hypothesis that seed size increases during domestication were the indirect consequence of selection for plants with increased growth vigour.