The Role of Weed-Crop Hybridization in the
Evolution of a Noxious Introduced Weed

* Presented at Southern California Botanists Annual Meeting on 22 October 1994 at California State University Fullerton *

Paul Arriola
Department of Botany and Plant Sciences
University of California, Riverside

The role of crop to weed hybridization in the continued evolution of weed plants has often been overlooked. It has been suggested that traits bred into crop plants would be detrimental to weedy plants that possess them; however, this may not always be the case. I created an experimental system in order to address the question of whether a weed-crop hybrid could persist in the field. I measured the rate of spontaneous weed-crop hybridization between a wind-pollinated crop plant, Sorghum bicolor, and related weedy plant, Sorghum halepense, and compared fitness characters of the hybrids with those of their respective parents. An isozyme marker was used to identify hybrid plants through progeny testing so that the rate and incidence of hybridization could be measured. Fitness correlates of hybrid plants from forced crosses grown under field conditions were measured. Incidence and rate of hybridization varied with target distance from the crop, location of the field, and year. Preliminary measurements of hybrid progeny from the first year of the study yielded no significant differences between the hybrids and their maternal weed parents. The results imply that natural spontaneous weed-crop hybridization in Sorghum occurs at a measurable rate. Also F1 plants do not appear to suffer any significant reduction in measurable fitness correlates when compared to the maternal parent. Weed-crop mating can introduce variation into populations upon which selection can act, perhaps leading to better adpated and more competitive weeds. In the case of hybridization involving genetically engineered crops, the liklihood for weed enhancement may even increase. The appearance of enhanced weeds in natural populations could lead to a host of problems, not the least of which may be replacement of endangered and other native plant species.