gene Flow: Implications for Crop Diversity and Wild Relatives

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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
dispersal of seeds themselves can also be an agent of gene flow.The movement of seeds can occur in a range of ways, mostlyby human activities, such as transportation (Figure 1), or bywind or wild animals.Figure 1. A maize plant growing on the side of the highwayoutside Guadalajara, Mexico. This plant presumably arrived asa seed fallen from a transport truck. (Photo: D. Quist, 2002)6
Page 1 and 2: 11Vertical (Trans)geneFlow: Implica
Page 3 and 4: Vertical (Trans)gene Flow: Implicat
Page 5 and 6: Chapter 6.Practical Considerations
Page 8 and 9: Chapter 1IntroductionThe purpose of
Page 10 and 11: Chapter 2Overview of Vertical Gene
Page 14 and 15: 2.3 In what species or kinds of cro
Page 16 and 17: Identifying the most important comp
Page 18 and 19: that introgression rates of genes f
Page 20 and 21: of transgenic field trials, in fede
Page 22 and 23: A number of important transgene flo
Page 24 and 25: particularly in the USA - with vari
Page 26: of our monitoring efforts be reason
Page 29 and 30: introduction or interaction with ot
Page 31 and 32: 6.2 Context-specific considerations
Page 34 and 35: ReferencesAlvarez Morales, A. (2002
Page 36 and 37: Hauser, T. P., R. B. Jorgensen, et
Page 38 and 39: Ortiz-Garcia, S., E. Ezcurra, et al
Page 40: Titles in the TWN Biotechnology & B

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