Alien introgressions represent a rich source of genes for crop improvement

Our major crop species represent the relatively few species that were selected by our ancestors from a multitude of extant species, and the resulting narrow germplasm forms the basis of modern monoculture in many areas of the world. Crop monoculture over vast areas inevitably leads to boom and bust cycles alternating between susceptible and resistance genotypes, disease epidemics, and loss of productivity after the appearance of new virulent races of pathogens. Breeders and geneticists have increasingly sought new sources of resistance in diverse germplasm, often involving distant wild relatives. Often, such alien introgressions are accompanied by linkage drag where recombination is suppressed in the target gene region, and standard recombination-based approaches cannot be used in the molecular dissection of the target genes. In PNAS, Cao et al. (1), however, circumvent the problems and report the isolation of a resistance gene from an alien introgression in wheat using a combination of cytology and gene expression analysis. As a rule, polyploids containing more than two sets of chromosomes have highly buffered genotypes, are more tolerant, and are more likely to benefit from alien introgression than diploids. Common bread wheat (Triticum aestivum L.; 2n = 6x = 42, genomes AABBDD) is hexaploid, consisting of A, B, and D progenitor genomes. Most of the highest yielding wheat cultivars carry portions of alien chromosomal introgression from related weedy species. The process of alien introgression is illustrated in Fig. 1. The alien introgression T1BL·1RS, for …