gene Flow: Implications for Crop Diversity and Wild Relatives

and they do not pose a concern, yet crop-to-crop gene flowoften is a concern. The possible dispersal range of reproductivepropagules (i.e. pollen) is dependent on many different climatic(including wind, humidity, temperature, etc.) and biologicalfactors (height of plant, size of propagule, natural outcrossingrates, etc.), but human dispersal can also broaden this range.Second, in order for gene flow to occur, there must be anoverlap in phenology (flowering and fertility times) betweenthe transgenic crop and recipient population. Flowering timesmay be affected by ecological and/or biological factors insome circumstances, leading to partial or total reproductiveisolation among neighbouring populations. Third, any matingbetween a transgenic crop and a landrace or wild relative mustproduce fertile and viable offspring. Reproductive barriers tointrogression are strong, especially where ploidy number (genomiccopy number) differs between domesticated and wild croprelatives (Jenczewski et al. 2003). This may only occur in limitedscenarios. Plants that normally are only self-compatible, i.e. havethe capacity to only mate with itself, also represent a type ofreproductive barrier. Fourth, the offspring of the new transgenichybridplant must also be viable and fertile to some extent, anda lack of survivors means that any potential gene flow wouldcease at this point. Yet even a low level of fertility can lead tofully viable populations in subsequent generations, as wouldbe the case with backcrossing (mating ‘back’ or again, with theparent population) into the wild progenitor populations.When these four conditions are met, transgene flow is likely.In some of these cases, offspring will have reduced fitness, orproduce sterile (unviable) seeds. In other cases they will haveimproved vigour (Singh et al. 1995; Hauser et al. 1998) andfitness, yet the advantages may reduce or reverse over time.Thus, there must be a minimum level of fertility in order forthe recipient population to maintain the transgene(s) and surviveto the next generation. Lastly, it should be mentioned that the5