Potential for Gene Flow from Cultivated Wheat to Weedy Relatives in the Great Plains of North America

Abstract

Common or bread wheat (Triticum aestivum), introduced to North America, has become a stable and often dominant component of agricultural systems in the semi-arid western portion of the Great Plains and Intermountain regions of North America. To date, transgenic (GMO) wheats have not been deployed in this region. Concern over potential loss of export markets has been the primary factor in restricting, to date, use of GMO wheat. An additional factor has been concern over possible outcrossing between cultivated wheat and weedy relatives, with potential establishment of transgenes in naturalized populations. Throughout the Great Plains, common wheat grows in close proximity to introduced, and often permanently established, populations of the related species Triticum cylindricum (=Aegilops cylindrica, jointed goat-grass), Secale cereale (rye) and Elytrigia intermedia (=Agropyron intermedium, intermediate wheat grass). While hybrids can be produced between wheat and both rye and intermediate wheat grass, and both have been used as sources of genes for wheat improvement, naturally occurring hybrids are rare, always sterile, and unlikely to serve as bridges for gene flow. Jointed goat-grass, however, is more closely related to wheat, and natural hybrids are quite common. Common wheat is an allohexaploid containing the A, B and D genomes. Jointed goat-grass is an allotetraploid, and carries the C and D genomes. Thus, in F1 hybrids between the two species, there are seven common chromosomes, which will pair at meiosis and allow for limited fertility as a female parent. Male (pollen) fertility in such hybrids is, however, low or non-existent. Still, seed may be formed if pollen arrives from either wheat or jointed goat-grass populations. Thus, potential bridges for gene flow exist. A survey of more than 60 naturalized populations of jointed goat-grass has been conducted using genes encoding the wheat seed storage proteins known as high-molecular-weight glutenins and gliadins. To date, no evidence for transfer and permanent establishment of these genes in goat-grass populations has been uncovered. A survey during the 2003 season did reveal the presence of natural hybrids in 10% of observed populations; however, no seed was recovered from these hybrid plants. Jointed goat-grass is naturalized over a wide expanse of agricultural land in western North America. Even if successful, seed set by natural F1 hybrids is a rare event, the potential for gene flow from wheat remains a possibility, and will require continuous monitoring to adequately assess the risk of such events.