gene Flow: Implications for Crop Diversity and Wild Relatives

Identifying the most important components to survival is notstraightforward, and must be considered within the ecologyof transgenic hybrids. Variation in fitness is also likely acrosshybrid generations. With such little knowledge on the behaviourof transgenes in unintended and new genomic and ecologicalbackgrounds, prediction of real-world effects is particularlychallenging.One principal concern of transgene flow is the loss of potentiallyuseful crop genetic diversity in the recipient population(whether other crops, landraces or wild relatives). Outbreedingdepression (the reduction of fitness from hybridisation) can leadto a decrease in allelic diversity by extinction of members of adiverse gene pool that are less adapted to survive because of theparticular introgressed transgenic trait. This is loss of diversitythrough negative selection. On the other hand, when transgenehybrids have an increased fitness, and can survive into the nextgeneration, genetic assimilation (loss of unique genetic identitythrough continual hybridisation and backcrossing) will have ahomogenising effect on the recipient population, also leading toa less diverse gene pool. Thus, both instances can have negativeeffects on genetic diversity. The magnitude of these selectiveforces within the new genomic and ecological backgroundof the recipient population will largely determine the rate ofevolutionary change in the recipient population (Gepts & Papa2003).So how can we predict the outcomes of transgene flows ona recipient population? Population matrix models have beensuggested as useful ways to estimate this risk (Parker & Kareiva1996; Bullock 1999). However, the magnitude and evidence ofeffects is idiosyncratic, and may take years to develop (Ellstrand& Hoffman 1990). Few direct studies have been conducted tomeasure the fitness effects of transgenes in wild populations(Linder 1998; Linder et al. 1998; Snow et al. 2001; Spencer &9