A new Vrs1 allele identified in 2-row Spanish landraces

Abstract

Vrs1, the gene determining the type of spike in barley has been extensively studied. The wild dominant allele encodes a homeodomain-leucine zipper transcription factor whose activity results in a two-rowed spike, whereas the recessive allele produces a six-row phenotype. Phylogenetic analysis of barley cultivars identified two alleles in two-rowed types and at least four different alleles in six-rowed barleys. Previous results genotyping with MWG699, a marker closely linked to Vrs1, suggested different geographic origins for six-row alleles, among them the vrs1.a2 allele originated in the Western Mediterranean. Using this same marker, we showed that a large proportion of Spanish and Moroccan landraces (both 2- and 6-rowed) as well as Moroccan Hordeum spontaneum lines, all shared the same haplotype. We then analyzed the sequence of the Vrs1 gene in those lines. All (11) six-rowed barleys sequenced carried the vrs1.a2 allele, but we found different Vrs1 alleles among 2-rowed types: the material from Morocco, both wild (3) and cultivated (2), carried the Vrs1.b2 allele, which was absent from the Spanish 2-rowed landraces studied. The most common allele among these was Vrs1.b3, in 46 lines out of 53 evaluated. The other seven lines presented a new Vrs1 allele, Vrs1.b5. This new allele contains a ‘T’ insertion in exon 2, originally proposed as the causal mutation giving rise to the 6-row vrs1.a2 allele, but has an additional upstream deletion that results in the change of 15 amino acids and a potentially functional protein. These results add a new hypothesis to the origin of the 6-rowed vrs1.a2 allele, which could result from a mutation either at the Vrs1.b2 allele (classical hypothesis) or at the newly found Vrs1.b5 allele identified in Spanish landraces.