Structural variation and evolution of a defense-gene cluster in natural populations of Aegilops tauschii.

Genetic mapping and sequencing of plant genomes have been useful for investigating eukaryotic chromosome structural organization. In many cases, analyses have been limited in the number of representatives sampled from specific groups. The degree of intraspecific genome diversity remains in question. The possibility exists that a single model genome may have limited utility for identifying genes in related members of the species or genus. Crop improvement programs have particular interests in disease resistance genes that are harbored by wild relatives of modern cultivated crops. These genes are evolutionarily dynamic and under selective pressure by a broad range of pathogenic organisms. Using resistance gene analogs as models for gene evolution, intraspecific genome comparisons were made among populations of wild diploid wheat (Aegilops tauschii). We observed that deletion haplotypes are occurring frequently and independently in the genome. Haplotypes are geographically correlated and maintenance of gene complements in localized populations indicates selective advantage. Furthermore, deletion haplotypes are not detrimental to plant health, since genes without adaptive value in alternate environments are eliminated from the genome. Deletion haplotypes appear to be a common form of allelic variation in plants, and we address the consequences on genome restructuring and gene evolution.