Deepening the Wheat Gene Pool

Summary For many crop species, genetic uniformity is a result of modern breeding and farming practices. Common wheat (Triticum aestivum L.) is an exception, having been a genetically narrow species throughout its entire existence. This paper discusses the evolutionary bottlenecks through which today's wheat germplasm has descended, and the ways in which the wheat gene pool is being enriched with genes from other species. The worldwide gene pool of common wheat is descended from a very small number of spontaneous interspecific hybrids, which originated as a result of two natural amphiploidization events. In the more recent event, plant(s) of emmer wheat, which were being cultivated at the time by early Neolithic fanners, were fertilized by a weedy diploid goatgrass, Aegilops tauschii, producing primitive common wheat. Because of the rarity of this event, today's common wheat has extremely low levels of polymorphism at enzyme, storage protein, and DNA marker loci, compared with its parent species, especially Ae. tauschii. In fact, the bulk of evolutionary evidence suggests that common wheat began its existence as a highly monomorphic species and that its genetic variation was reduced further by domestication. Despite common wheat's narrow genetic base, human-guided evolution has produced a profusion of distinct land races over a period of five or more millenia, and modern breeding has maintained steady genetic improvement throughout the current century. To protect these gains, humans have resorted to interspecific crossing to improve wheat's pest resistance. But why should wheat's progenitors not be regarded as sources of useful genetic variation for all economic traits? Humans would have been fortunate indeed if the rare amphiploid(s) that gave rise to common wheat carried the ideal allele at every locus. This paper provides a critical analysis of methodologies for bypassing wheat's genetic bottleneck and deepening its primary gene pool. It emphasizes the two general approaches to expanding the wheat gene pool: amphiploidization and direct backcrossing. Advantages and disadvantages of both methodologies are provided, drawing on examples of gene transfer from the various subspecies of T. monococcum, T. urartu, T. turgidum, T. timopheevii, ssp. monococcum, Ae. speltoides, and Ae. tauschii.