Adaptive climatic molecular evolution in wild barley at the Isa defense locus

Wild barley (Hordeum spontaneum) represents a significant genetic resource for crop improvement in barley (Hordeum vulgare) and for the study of the evolution and domestication of plant populations. The Isa gene from barley has a putative role in plant defense. This gene encodes a bifunctional alpha-amylase/subtilisin inhibitor that inhibits the bacterial serine protease subtilisin, fungal xylanase, and the plant's own alpha-amylase. The inhibition of plant alpha-amylases suggests this protein may also be important for grain quality from a human perspective. We identified 16 SNPs in the coding region of the Isa locus of 178 wild barley accessions from eight climatically divergent sites across Israel. The pattern of SNPs suggested a large number of recombination events within this gene, indicating that the low-outcrossing rate of wild barley is not a barrier to recombinant haplotypes becoming established in the population. Seven amino acid substitutions were present in the coding region. Genetic diversity for each population was calculated by using Nei's diversity index, and a Spearman rank correlation was carried out to test the association between gene diversity and 16 ecogeographical factors. Highly significant correlations were found between diversity at the Isa locus and key water variables, evaporation, rainfall, humidity, and latitude. The pattern of association suggests selective sweeps in the wetter climates, with resulting low diversity and weaker selection or diversifying selection in the dryer climates resulting in much higher diversity.