The larger the catalog of sequenced genomes, the greater the chance for seeing horizontal gene transfer (HGT) in action. Although reports of HGT between prokaryotes and eukaryotes have appeared sporadically, it seems that incontrovertible evidence is only beginning to appear now. Indeed, the genomes of nematodes reveal that genes have been acquired from bacteria a number of times. A recent, one-page review of this subject discusses examples of this phenomenon.
Evidence for inter-domain HGT seems to be on its firmest footing when it comes to nematodes. Given the intimate contact between bacteria, fungi, and many soil-dwelling worms (many of which are bacteriovores), this should not be surprising. Nor is it unexpected that the genes that have been transferred encode enzymes that can break down otherwise indigestible sugars, such as glycosides and cellulose, that are constituents of plant food that the worms ingest. Little is known as yet about how the foreign genes “adapt” to their new genomic context, although it is known, at least in some cases, that they are indeed expressed. I can predict with confidence that this subject will be fertile territory for many years to come
Given that HGT provides endless opportunities for evolution, the question begs: Why does it not take place more often between prokaryotes and eukaryotes? Aren’t prokaryotes and their viruses a nearly endless repository of potentially useful genes? This grandmother of Talmudic Questions brings up the stark fact that organisms must be highly selective in what foreign genes they allow into their genome, lest they lose their identity and potentially their survival skills. But even without getting into the thicket of mechanisms designed to stabilize genomes and keep them from going astray, I still wonder why HGT between prokaryotes and eukaryotes (in either direction) appears to be something that may happen with some frequency but that is only rarely triumphant.