Jointed goatgrass (Aegilops cylindrica Host - CD genomes), a wild tetraploid wheat, was introduced into North America as a contaminant of wheat seed brought from Eastern Europe in the late 1800's and early 1900's. Since its introduction, it has become a serious crop weed in fields where hexaploid bread wheat is grown (Triticum aestivum L. - ABD genomes). Hybridization is common among members of the wheat complex, having played an important role in the evolution of this allopolyploid group. Wheat and jointed goatgrass share only the D genome. Therefore, hybrids are considered to be sterile, a fact supported by the identification of a gametocidal sterility system in jointed goatgrass (Endo, 1988). However, large numbers of partially female fertile hybrids are being found in the jointed goatgrass infested wheat fields of the Pacific Northwest. Studies of experimental material and hybrids collected in Oregon now suggest that neither hybridization nor seed production is a rare event. Preliminary genetic analyses using SDS-PAGE of the high molecular weight glutenin seed proteins, microsatellite (SSR) markers, and genomic in-situ hybridization suggest the development of a crop-weed complex via introgressive hybridization. To date, evaluation of hybrid material supports the possibility of a two-way introgression with both species capable of serving as the female parent. Wheat field populations appear to be a diverse mix of F1 and backcross (BC) hybrid generations. Work with experimental wheat x jointed goatgrass BC hybrids, with either species as the recurrent male parent, shows a rapid move to fully fertile, self-pollinating hybrids. The wheat-jointed goatgrass weed complex offers a model system for studying introgressive hybridization within the agricultural ecosystem. It also offers a timely opportunity to study gene flow risk in advance of the release of transgenic wheat.

Key words: crop-weed complex, introgression, jointed goatgrass, transgenic wheat