of considerable debate in the Agrobacterium transformation community. Recent analyses of Agrobacterium T-DNA inserts into Arabidopsis provide data to address the question. The Salk Institute maintains a population of 32,500 Arabidopsis thaliana lines with Agrobacterium-mediated T-DNA inserts in and around the 25,500 genes in the genome of this species. An analysis shows the loci of insertion to be reasonably evenly distributed along the entire genome. Some of the inserts are between genes and others interrupt genes (Ecker, 2003).

In a related study comparing T-DNA with transposons inserted into the Arabidopsis genome, for transposons a slight preference was found for insertion into coding sequences of genes. As with the study of Ecker (2003), Pan and colleagues (2003) found that both T-DNA and transposon insertions are reasonably evenly distributed throughout the genome.

These data indicate that there is no strong recombinational hotspot or strong preferential insert site for T-DNA. Similarly, there is no evidence to suggest the CaMV 35s promoter in GE plants is any more unstable than the CaMV 35s promoter in ordinary plants infected with CaMV.

Recombination Hotspots and Genetic Instability

The range of loci in the Arabidopsis genome into which T-DNA has been inserted indicates that the insertion location is random or near random (Ecker, 2003). If insertional hotspots do exist, they clearly are not exclusive, but at most they are mildly preferred points of recombinational insertion. Genetic instability at singular loci in itself is not hazardous unless the unstable nature necessarily gives rise to hazardous outcomes. However, genetic instability, when it occurs, affects only one cell or plant at a time, and the result is typically a loss of activity of the relevant gene. An example is the case where a herbicide tolerance gene is inserted into an unstable locus and then grown in a field of the herbicide-tolerant plants. When an instability incident occurs in this crop, the herbicide tolerance is lost—the plants experiencing this instability event revert to being herbicide susceptible. When the field is then sprayed with the herbicide, only those few individual plants suffering the instability succumb to the spray.

Because the timing of such destabilizing events is rare and does not strike an entire population simultaneously, it is unlikely that the loss of a small number of crop plants would be noticed compared with the loss of weeds or with nonherbicide-tolerant volunteer crop plants. If genetic instability were more common or synchronized, such that it were noticeable, the breeder would not be able to obtain certification for the new variety due to failure to meet the genetic stability requirement.

Genetic instability is observed from time to time, both in conventional breeding and in genetic engineering. Robertson (1978) described a conventional corn line with a high degree of genetic instability due to an endogenous transposon. As



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