discussed earlier, unstable pigmentation in GE petunias provides an example of a systemic instability in particular GE plants (Meyer et al., 1992).
Spontaneous mutations change DNA also, affecting everything from single base changes, known as “point mutations,” to entire genomes. Deletions of entire segments from a genome are predictably severe and deleterious, but point mutations or even more substantial mutations, such as those affecting large tracts of DNA in chromosomes, can be beneficial and adaptive.
Because of the ubiquitous nature of spontaneous mutations, all plant varieties—whether conventional or GE—exhibit a small degree of genetic instability and generate mutants with some degree of frequency. If the frequency of spontaneous genetic instability is low, the typical loss-of-function mutants will probably not be noticed; if it is high, the variety cannot be commercialized.
Natural types of recombination can also result in the same effects. Transposons, even Agrobacterium insertions, all interrupt any DNA sequence where they insert, regardless of whether they insert into homologous or nonhomologous DNA. Depending on the function of the interrupted DNA, there may or may not be phenotypic consequences from the insertion.
Agrobacterium naturally inserts DNA into the host plant-cell genome, typically in a genetically stable manner. The substitution of desirable DNA by genetic engineering for the phyto-oncogenic DNA of the wild strains does not affect the mechanics of transfer. The genes responsible for Agrobacterium genetic transformation and recombination in the plant are physically and functionally separate from the T-DNA actually transferred and integrated. The mechanics of Agrobacterium appear the same whether the event is staged through a natural infection process by a crown gall-producing wild-type strain or by the same strain that has been disarmed by the removal of the gall-producing genes and subsequently replaced with known, desirable DNA.
Predicting the likelihood of unintended hazards from compositional changes associated with genetic modifications does not fit a simple dichotomy comparing genetic engineering with non-genetic engineering breeding. This is because there are many mechanisms shared in common by both GE and non-GE methods, and also because there are techniques that slightly overlap each other. Furthermore, within the scope of genetic engineering (rDNA) technology, several mechanisms for genetically transforming plants are available as options to scientists, such as Agrobacterium-mediated gene transfer and the use of the particle gun (McHughen, 2000). These two examples of genetic engineering are as different mechanistically as the conventional methods of narrow crossing and wide crossing (discussed in Chapter 2). Consequently, it is unlikely that all methods of