Horizontal gene transfer (HGT) or lateral gene transfer (LGT) refers to transfer of genes between organisms other than through sexual or asexual reproduction (referred to as vertical gene transfer VGT). It is quite prevalent in prokaryotes and unicellular organisms but not as common in multicellular eukaryotes. As with mutation, HGT is mostly neutral and seldom negative but occasionally provides an advantage—for example, antibiotics resistance in bacteria. And in this regard, in the simpler organisms, it has played an important role in evolution. Straight infection of organisms with just the pathogenic DNA (e.g., crown gall disease) is usually not referred to as HRT if only introgression of pathogenic sequences is involved—transduction of genes from a previous host would have had to occur, and this has also been documented.
Under the endosymbiotic theory, subcellular organelles chloroplasts and mitochondria probably originated as bacterial endosymbionts of a progenitor to the eukaryotic cell. Analysis of DNA sequences suggests that horizontal gene transfer has also occurred within eukaryotes from the chloroplast and mitochondrial genomes to the nuclear genome, but, again, this is a very limited notion of HGT. The massive accumulation of sequencing data shows extensive genetic similarity among genomes of diverse organisms that are only remotely related. For example, parts of the nucleic acid sequence of a common bacterium present in our guts, E. coli, have been found in the DNA of organisms such as oilseed rape, amphibians, birds, grasses and mammals—including humans. Such findings put in doubt the value of assigning genes to a particular species and the validity of using terms such as “species-specific DNA.” And HGT is a confounding factor in inferring phylogenetic trees based on the sequence of single genes.