Gene flow from tree plantations and implications
for transgenic risk assessment
Stephen P. DiFazio, Gancho T. Slavov, Jaroslaw Burczyk, Stefano Leonardi and Steven H. Strauss
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Gene flow is a major determinant of impacts of forest tree plantations on surrounding populations and ecosystems. Realistic predictions of gene flow are therefore essential for scientifically credible assessments of the impacts of transgenic plantations. The choice of methods for measuring gene flow will be dictated by the organism and environment under study, the availability of tools and resources, and the desired scope of inference.
Methods include direct tracking of propagule movement, parentage analysis, and analysis of genetic diversity in seeds, seedlings, and pollen. Gene flow estimates can be integrated with ecological and demographic data in spatiallyexplicit simulation models to allow projections of transgene dispersal over large areas and long time frames. Such models allow exploration of a large number of scenarios of plantation cultivation, and help to identify the key parameters controlling gene flow. We describe a simulation study of gene flow from hybrid poplar plantations that illustrates some of the key issues in estimating and modeling transgene flow from plantations. For example, the results suggested that accurate estimates of long-distance gene flow are considerably more important to prediction of transgene dispersal than are estimates of local (<1 km) dispersal patterns. |
A major issue for plant genetic engineering is the extent to which transgenes will escape from cultivation and cause negative impacts in wild ecosystems [1-3]. Gene flow to wild relatives occurs for nearly all crops in some places where they are grown [4]. However, it is of particular concern for forest trees because they are virtually undomesticated [5], they have the potential for spatially extensive gene flow [6,7], and they can have large effects on ecosystem processes and biological diversity when they are the dominant life form [8].
The ecological impacts of transgenic trees will primarily depend on the traits conferred by the transgene and the environment in which the trees are grown [9,10]. Risk assessment therefore requires detailed consideration of the specific ecological consequences of individual transgenes in different settings. However, gene flow is a prerequisite for most ecological impacts outside of plantations, so baseline estimates of introgression will apply to most environmental risk assessments for transgenic trees [11,12].
Many commercially-grown trees are cultivated in close proximity to interfertile wild relatives, and many exotic plantation species have naturalized feral populations with which they can interbreed [8,13]. However, because of their inherent ecological novelty, exotic tree introductions can have large effects on ecosystems in the absence of transgenes [14]. Therefore, cultivation of exotic transgenic trees raises qualitatively different issues than cultivation of native transgenic trees. Also, because exotic trees are readily identified in wild settings, gene movement is simpler to monitor than for native species—where genetic techniques that distinguish morphologically similar progeny are required. In this chapter we will outline approaches to measuring transgene introgression, emphasizing plantations of native species. We will then discuss a risk assessment for transgenic trees, focusing on a simulated planting of transgenic hybrid poplars in the United States.
Measuring gene flow
Gene flow is a complex process that can be broken down into the following components:
• Dispersal from plantations,
• Dispersal in wild populations,
• Landscape dynamics and availability of establishment sites,
• Establishment and competitiveness of conventional plantation progeny, and
• Effects of transgenes on fertility, establishment, growth, and mortality.
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