gene Flow: Implications for Crop Diversity and Wild Relatives

that introgression rates of genes from one population to anothercan be quite rapid even when the fitness advantage is small(Barton & Dracup 2000), or when there is a high frequency oftransgressive hybrids (Reiseberg & Wendel 1993). A modellingstudy conducted by Haygood et al. (2003) demonstrated thatcrop alleles can be rapidly fixed in a recipient populationwhen the migration frequency exceeds the selection threshold,even when they have a negative impact on fitness. Their studyexpands on how demographic swamping (reduced fitness in thehybrid’s offspring populations) can facilitate genetic assimilationjust where high rates of gene flow occur from agriculturalpopulations. In this situation, gene flow that reduces fitnesswill become stable in the population when the migration rateof the alleles exceeds the level of selection, leading to reducedpopulation size and perhaps local extinction. Further, extinctionthrough hybridisation is a valid concern not only when itinvolves transgenic plants, but in any situation of non-nativebiological or genetic invasions where hybridisation may increasea plant’s invasiveness (Ellstrand & Schierenbeck 2000).3.1 Types of transgene flows and theirimplicationsWith the now decade-long history of GMO commercialisation,the world has already witnessed a number of cases oftransgene flow, from crop to wild relatives, crop to landraceand crop to crop. Within each type of transgene flow, a hostof environmental, agronomic, cultural, and intellectual propertyconcerns emerge in conjunction with the biological andevolutionary considerations of gene flow. While research hasmade some progress, there is still much to be learned.11