Characterizing the factors contributing to intraspecific pollen-mediated gene flow between cropped and volunteer spring wheat (Triticum aestivum L.)

Transgenic crops have been widely adopted throughout many parts of the world. Their popularity is not without concern regarding the ecological and agricultural quandaries associated with their cultivation. Several traits have recently been incorporated into hexaploid wheat (Triticum aestivum L.) using recombinant DNA technology and therefore, the resulting transgenic lines may require segregation from non-transgenic wheats to satisfy international markets. However, segregation may be complicated by the underlying biological characteristics of wheat plants that collectively allow for the transfer of genes between cropped and volunteer spring wheat populations. To this end, the objectives of this thesis were: i) to identify a hybridization window in spring wheat crops that would facilitate temporal isolation, thus minimizing flowering synchrony and pollen-mediated gene flow (PMGF); ii) to determine whether increasing crop density of spring wheat crops which are taller than volunteer genotypes could reduce flowering synchrony and PMGF between the two populations and; iii) to ascertain whether a relationship exists between flowering synchrony and PMGF such that flowering synchrony could be used to predict PMGF. Synchrony of flowering and PMGF were always greatest when seedling emergence timing of volunteers coincided with the wheat crop. Flowering synchrony ranged between 0% and 86% depending on emergence timing, and a distinct hybridization window 125 GDD in length was indentified in which seedling emergence of volunteer wheat plants would result in PMGF regardless of volunteer density. Volunteers emerging after the wheat crop (which cannot be selectively controlled) contributed substantially to PMGF and although plant height appeared to have some merit in reducing flowering synchrony, it had no influence on PMGF. Increasing crop density, on the other hand, resulted in an exponential decline in PMGF in all site-years. However, further reductions above the recommended planting rate of 250 plants m -2 were not realized. Overall, this research suggests that the potential for PMGF between volunteer wheat plants and a wheat crop is real and perhaps absolute. Nevertheless, results presented in this thesis lend support for the possibility of coexistence between transgenic and non-transgenic wheat within an appropriate framework that includes the establishment of realistic thresholds for admixture.