Bt Brinjal The scope and adequacy of the GEAC environmental risk assessment

16 Bt Brinjal: The GEAC environmental risk assessmentInserted TransgeneFinding 8. The description of the transgene in Bt brinjal EE-1 is inadequate to support risk assessment. It isnot clearly known that there is only one transgene incorporated into the brinjal genome, that the inserted transgeneexpresses the intended gene product or that the transgene does not interrupt a functional plant gene.(A) Number of inserts. The Dossier indicates that one transgene insert remains in Bt brinjal EE-1. Based on the evidenceprovided, the conclusion is not fully justified. There are several sources of uncertainty that weaken the conclusion, and three ofthese sources of uncertainty may affect risk. Two may increase risk and one may increase or decrease risk.• Uncertainty arises because the size of the digoxigenin (DIG)-labelled Bt probe is not specified. If it is too large, it may notdetect independently inserted fragments;if it is too small, it may not detectBox 1independently inserted fragments notBackcrossing and Transgene Characterizationassociated with the probe.Backcrossing is one way to create crop varieties that are similar to previous elite• Uncertainty also arises because probes varieties with a few new traits added. It is also a useful way to clean up “dirty”transformation events that generate transgenic plants.for other parts of the plasmid, such asAt the time Bt brinjal event EE-1 was created, all know methods for creatingthe nptII gene and several others, are not transgenic events created “dirty” transgenic events, and EE-1 is no exception. Aused. If fragments are inserted in such “dirty” event is one in which there are multiple transgene insertions, often withinversions of the transgene, interspersion of plant DNA, artificially createda way that a new open reading frame isintrons, and transgene fragments. All of the “dirty” elements of the event couldcreated that is associated with a plant create serious risks to the environment.promoter, novel gene products may beThe multiple insertions typically end up on different plant chromosomes or somedistance apart on the same chromosome. Recurrent backcrossing can help toexpressed at high rates.clean up these dirty events.• The third source of uncertainty isOnce a transgenic plant cell is created, it must be regrown into a whole plant.Backcrossing involves crossing the newly created transgenic plant with an eliteassociated with the breeding program.variety or line that the breeder wants the transgenic plant to resemble. RecurrentAlthough the file makes it clear that the backcrossing involves multiple generations of crossing offspring back to the sametransformed plant is backcrossed to elite elite variety or line as diagrammed in the figure below.brinjal germplasm, it is not clear howRecurrent Backcrossing to Elite Line(B 1is the first backcross generation, etc.)many backcross generations were done.B 1B 2B 3B 4Each backcross generation reduces theEliteLineprobability that an additional integrationevent is retained into the final brinjalX X X Xproduct. In other words, the moreTransformedPlantbackcross generations, the less likely anTransgeneundetected integration event is retained inthe final brinjal product (Box 1). Because Proportion ofthe number of backcross generations is Transformed Plant (½) 1 (½) 2 (½) 3Remaining in Offspringnot specified for each of the Bt brinjalproducts, it is uncertain how well the For brinjal’s 12 linkage groups (Doganlar et al. 2002), about 10 backcrossinserted transgene has been characterised. generations are necessary to eliminate all but the chromosome with the transgenewith 99% probability. Ten BC generations would also eliminate all but 0.1% (on(B) Sequence of the inserted transgene. average) of the genetic material associated with the transgene. If only 5 backcrossUnder the assumption that there is only generations were used, the probability that all other chromosomes have beeneliminated is only 62%, and about 3% of the chromosome associated with theone transgene inserted into Bt brinjal EE-1transgene remains. If only 5 backcross generations were used, a significantas asserted by the file, the sequence ofamount of germplasm that potentially contains unintended transgenes remains inthis one transgene was not reported. The the final variety. This creates unnecessary risks to the environment that couldhave been easily reduced with a bit more breeding effort. Hence the number ofsequence of the inserted transgene is typicallybackcross generations is an essential element to characterizing a transgene.required by regulatory authorities around the