Author : Prins, T.W.; Zadoks, J.C.

Summary : Abstract: If Horizontal Gene Transfer (HGT) in plants were a regularly occurring phenomenon, it would profoundly affect risk assessment of Genetically Modified Organisms (GMOs) since it would be an additional way of spreading a transgene. Therefore, a careful and critical analysis was made of a large number of references. Most supposed HGT cases were based on DNA sequence homologies of similar genes in different, sexually incompatible organisms. Detailed analysis indicated that sequence homology is a weak argument indeed. Even strong homology is not a sufficient argument for HGT, when the necessary sequence of biochemical steps for HGT is not indicated. Usually, the chain of events needed to explain HGT is not indicated. An alternative explanation for strong homology is derived from the evolution theory. Without exception, supposed cases of HGT are 'gene swaps', so that organisms do not gain new traits. Distinction should be made between generative HGT (HGT by which the unrelated DNA is incorporated into the germ-line of the recipient plant, followed by sexual reproduction) and vegetative HGT (where this is not the case). HGT should be considered an illegitimate way of explaining DNA sequence homologies between sexually incompatible species. HGT may have occurred in evolutionary history, but there is no solid evidence, ultimo 1992, that HGT is a recent and regular phenomenon. Risk analysis for gentically modified plants may ignore HGT.