Environmental Safety of Genetically Engineered Crops

Factors Influencing the Genetic Diversity of Plant Species and the Potential Impact of Transgene Movement

61 Factors Influencing the Genetic Diversity of Plant Species and the Potential Impact of Transgene Movement James F. Hancock Biodiversity in native and agroecosystems is a function of species composition (number of species, relative abundance) and the genetic makeup of individual species. Biodiversity is negatively affected if there are reductions in species abundance or levels of genetic diversity within species. The potential impact of a GE crop on biodiversity will depend on the invasiveness of the crop itself and changes in the competitive ability of any relatives that receive a transgene through hybridization. Herein, I will discuss the major evolutionary forces that assort genetic variability in native species and then use this information to describe how transgene movement might affect levels of genetic diversity in native and agroecosystems. Patterns of genetic variability in natural populations are regulated by three major forces of evolution: (1) migration, (2) selection, and (3) genetic drift. Migration or gene flow is the movement of genes within and between populations via pollen or seeds. Selection is a directed change in a population’s gene frequency due to differential survival and reproduction, while genetic drift is a nondirectional change in gene frequency due to random events. Selection is often believed to be the major force shaping the diversity patterns of native populations, but in many instances migration and genetic drift play just as significant or even a more significant role. In this chapter I will discuss the impacts of these various forces on levels of genetic variability in natural populations, and then will describe how conventionally bred and genetically engineered (GE) crops have affected genetic variability in compatible relatives. Migration Migration between Populations Gene flow between populations can have a major impact on levels of genetic variability, if the populations carry different alleles or differ in allele frequencies. The amount of migration 62| James F. Hancock between populations is regulated by the type of pollen vector, the breeding system of a species, its mode of seed dispersal, and the distance between populations. The most extensive information available on the factors regulating gene movement can be extracted from the recommended isolation distances for the production of pure seed lines of crops (Levin and Kerster 1974; Ellstrand and Hoffman 1990). Most wind-­ pollinated species require distances of less than 200 m, while most insect-­ pollinated crops require separations of more than 500 m. The average for 35 insect-­ pollinated species is 657 m, while that for wind-­ pollinated species is 244 m. Self-­ fertilized species require isolation distances below 200 m, while most outcrossing species require much greater distances. The average isolation distances are 385 m for 14 predominantly selfing species, 493 m for 16 mixed-­ pollination (both selfed and outcrossed) species, and 846 m for 21 outcrossed species. This is not to say that long-­ distance dispersal does not occur even in the crops with the shortest isolation distances. While most pollen travels only a few hundred meters, the tails of the distribution are often very lengthy and of unspecified distance. This allows for low frequencies of gene movement to distant individuals in both insect-­and wind-­ pollinated species . Long-­ distance gene dispersal has been documented at distances of one kilometer or more (Klinger et al. 1991; Watrud et al. 2004; Fénart et al. 2007). The mode of dispersal of seeds can also greatly influence how far gene flow occurs and influences patterns of genetic variability (Levin and Kerster 1974; Harper 1977; Cain et al. 2000). Gravity-­ dispersed seeds usually travel only a few meters, and most seeds are clumped about the mother. Seeds with explosive or plumose seeds travel a little bit further 1–­ 10 m with a more even distribution. Animal-­ dispersed seeds are often clumped about the mother because of gravity, but can have by far the longest tails. Mammals can have territories of dozens of miles, and bird migration distances can span thousands of miles. Human beings have played a particularly important role in plant evolution by carrying both native-­ collected and crop seeds long distances, planting them next to compatible native populations and providing disrupted sites for population expansion (Anderson 1948; Hancock 2004). Migration within Populations The distance that pollen and seeds move not only affects variation patterns between populations , but also within populations. Limited gene flow results in a highly substructured population where only adjacent plants have a high likelihood of mating. Several terms have been developed to describe the substructuring of plant populations due to limited gene flow. A deme...