I came across an interesting article today at Nature News. It discusses a report on transgenic rice, engineered to over-express 5-enolpyruvoylshikimate-3-phosphate (EPSP) synthase which confers resistance to the herbicide glyphosate (Roundup). This transgenic rice line was then out-crossed with a weedy rice relative and the F2 generations were evaluated for fitness, in the absence of glyphosate. The F2s demonstrated clear competitive advantages over wild-type rice (eg. 48-128% increased seed production per plant).
These results are disconcerting if you consider that this report would suggest if transgenic crops were to outcross with weedy relatives it could result in escape of the trans-gene in to wild populations. Compounding this problem of trans-gene escape, is the fact that the transgene confers a competitive advantage even in the absence of herbicide application. However, the problem with this inference is that rice does not normally out-cross, and only appears to do so under forced, laboratory conditions, as were performed in this report.
This is a very interesting article and anyone involved in plant biotech should read it. Just keep in mind the caveats that this model system does not normally out-cross. Nevertheless it is important to keep these considerations in mind when discussing GMOs in the environment.