We have been warned that T-HRCs can lead to the evolution of "super-weeds" that will inherit the earth (Kling, 1996). The rapid commercial release of such crops has often been without broad-based scientific scrutiny with the most competent experts being involved, and there is little published physiological, biochemical, ecological, genetic, and agronomic data to scrutinize. This leads to a certain degree of skepticism among scientists, which contributes to the public questioning the needs, utility, risks, and values associated with the use of T-HRCs. The severe pressures by detracting groups on policy makers make it politically incorrect to pursue public sector research in this area, which prejudices the ability to perform experiments to obtain accurate information about the risks. These pressures also prevent generating crops needing resistance to herbicides where the agrochemical or seed industry perceives little profit.
Herbicide-resistant rice and wheat can be of great benefit only if used with care to prevent or mitigate gene transfer to related feral and wild rices and related weeds of wheat such as Aegilops cylindrica. Whereas the wild and feral rices are widely distributed, A. cylindrica has a relatively narrow distribution, although it can be quite pernicious. No solution in agriculture has been forever; farmers have always had to deal with evolution. More sophistication will be needed than is presently being used with transgenic wheat and rice to mitigate introgression delaying such evolution.
Discussions of T-HRCs have rarely dealt with the risks from a weed biology perspective. The main risk stated by the detractors is claimed to be that of the T-HRCs becoming "volunteer" weeds (in following crops) or their introgressing traits into a wild relative, rendering it weedier: the su-perweeds of the mass media. An attempt at such an assessment based on weed science was made using a defined set of uniform criteria in a decision tree format (Gressel and Rotteveel, 2000). Decision trees, by requiring discrete answers to sequential, stepped questions, lower the bias in arriving at conclusions vis-à-vis the risks deriving from a given hazard.
Conversely, there are wild species that are unlikely ever to become weeds unless they evolve a large number of weedy traits (Keeler et al., 1996). Unfortunately, too many risk studies do not differentiate between weedy relatives of crops and wild relatives (e.g., Sindel, 1997). Risk assessment must be performed on a local or regional basis, as the risks from the same T-HRC will vary greatly from one agricultural ecosystem to another. A second assessment should be done (but has not been done in the past) about the effects of introgression from transgenic crops on weed flora of countries that import bulk unprocessed commodities such as wheat and oilseed rape (Gressel, 1997b).
The risks of introgression have been assessed on a case-by-case basis by regulatory authorities. The Canadians delineated criteria before even having to evaluate plants with novel traits, whether or not transgenic (Anonymous, 1994a), and then specifically evaluated oilseed rape in the context of these criteria (Anonymous, 1994b). In a series of documents they further evaluated oilseed rapes resistant to imidazolinone (Anonymous, 1995a), gly-phosate (Anonymous, 1995b), and glufosinate (Anonymous, 1996). The decision process was based on their perception of the risks to the regional agricultural ecosystems in western Canada, without considering other regions that may be importing the crops. Internationally, the OECD (Organization for Economic Cooperation and Development) and UNIDO (United Nations Industrial Development Organization) are developing a series of consensus documents on the biology of various crops (with regard also to related weeds) so that there is a common starting point to evaluate each cropping situation (Anonymous, 1997). Most of the stated hazards of interspecific introgression from T-HRCs are based on unpredictive laboratory experiments, which prove that introgressions could occur. Thus, they show that the hazard exists but give little indication of risk: how quickly such transfers will occur in the field or how fit recipients will be to cope with competition in a multfactorial situation. The time factor is not inconsequential; if resistance introgresses to produce resistant populations more slowly than natural mutational evolution of resistance, what is the significance of introgression?
A. Vertical, Horizontal, and Diagonal Gene Transfer
Two types of gene transfer are widely discussed: (1) vertical, within a species, and (2) horizontal, transfer among unrelated species, usually by asexual means. Biology is not as clear-cut; there can be some sexual transfer between plant species in the same genus and closely related genera. These are typically included in discussions of introgression as horizontal gene transfer. Horizontal gene transfer via plasmids is common in prokaryotic organisms. Because extrapolations are often made from these rare cases of gene transfer among closely related species to "prove" that all horizontal transfers are possible, I suggest terming these special interspecific cases in plants as "diagonal" gene transfer, denoting the gray area where they exist.
Vertical and diagonal transfer possibilities are obvious to any biologists, but horizontal transfers, in eukaryotes, with their potentially disastrous implications for agriculture are not. The possibilities of true horizontal transfers are extrapolated from the intergeneric and interfamilial plasmid-medi-ated transfer of traits among microorganisms, which have allowed transfer of antibiotic resistance (analogous to herbicide resistance) among unrelated pathogens. The claim continues that because plasmids are often used as vectors in the genetic engineering of crops, interfamilial transfers will become commonplace or at least inevitable. This claim does not stand up to epidemiological experience with organisms such as Agrobacterium tumefa-ciens and A. rhizogenes. The plasmids used for laboratory gene transfers from these Agrobacterium spp. naturally infect a broad range of dicots, using the plasmid as part of the infection process.
If such interfamilial transfers were to occur via Agrobacterium, they would have been seen over the past 50 years with naturally occurring her bicide resistances. There are no known cases where such genes have transferred interfamiliarly from any crop to weed via Agrobacterium, despite the great selective advantages that such weeds would have and the ubiquity of Agrobacterium in the environment. The more than 10 million hectares of herbicide-resistant weeds that have appeared in the past 30 years can all be traced to evolution resulting from mutant selection, not to plasmid-mediated horizontal gene transfer. In addition, an extensive survey of the GenBank database found few Agrobacterium DNA sequence pieces in any of the plant genes, which would have been expected in the millions of years of cohabitation. This matching task took hundreds of hours of computer time (Rubin and Levy, 1999). Horizontal gene transfer need not be discussed further; diagonal gene transfer is a hazard for which the risks must be estimated in particular cases.
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