gene Flow: Implications for Crop Diversity and Wild Relatives

Ellstrand et al. 1999). Thus, gene flow per se is not the mainconcern, but rather the types of genes, and the level of geneticheterogeneity or homogeneity (genetic diversity) that is spreadthrough gene flow and its effect on recipient populations, arethe relevant issues. Whether flow of new genes or gene variantsresults in a change in fitness, i.e. the ability of the organismto survive and produce viable offspring (either positively ornegatively), has been a central focus in population biology. Itshould be clearly pointed out that considerations of gene flowdiscussed here are not unique to transgenic crop varieties,but are relevant for all commercial crops. The introduction ofwholly new genomic identities into recipient populations fromcommercial traits should be equally scrutinised, but is outsidethe scope of this paper.If commercial crops have been exchanging genes with relatedspecies for some time, why are transgenic varieties of particularconcern? With transgenics, completely novel traits are passed onthat could dramatically affect the fitness of individuals receivingthe given gene in a population. Thus, the commercialisationof transgenic plants has sparked widespread interest in thepotential evolutionary significance of transgene flow. The centralquestion is how transgene introgression may impact fitness inthe new transgenic hybrids, and consequently, the significanceof maintaining important crop genetic diversity for future cropbreeding.2.2 Under what conditions does gene flowoccur?Hybridisation and subsequent gene flow depend on a numberof biological and ecological conditions. First, the sexuallycompatible plants need to be growing within a sufficient pollenor seed dispersal range of the transgenic crop. In many cases,there is no overlap between crops and wild/landrace relatives,4