Horizontal transfer

From EvoWiki

Horizontal gene transfer is the movement of genetic material across species boundaries. In more extreme cases, genes can be moved across the boundaries of kingdoms. Several mechanisms have been proposed to explain these transfers; some of these have been verified.

• Conjugation Many bacterial species normal transfer genes among themselves via conjugation. Conjugation can also result in the transfer of genetic material between species. Conjugation has been demonstrated to enable transfer of bacterial genes into Archea and Eukaryotes.

• Transposable elements Transposable elements are pieces of DNA that encode enzymes that recognize sequences on the element itself and catalyze its movement to new locations in the genome. The process of Transposition is often inaccurate, and results in the inclusion of genes from near the element's initial location. As transposable elements may cross species boundaries, these events can result in transfer of genetic material.

• Viral Transfer Many viruses go through a replication cycle that involves integration into the host genome. As with transposable elements, errors in this process may result in the inclusion of host genes in the viral genome and ultimately, in transfer of these genes to a new host. Viruses frequently cross species boundaries.

Proximity between species may play a significant role in enabling these mechanisms, as many examples of horizontal gene transfer are between organisms that have a symbiotic or parasitic relationship. Internal symbionts, such as the Chloroplast and Mitochondria, have also resulted in wholesale genomic transfer to Eukaryotic organisms. The role of horizontal gene transfer in the vertebrate genome is somewhat controversial, as some genes of bacterial origin are present in a limited number of vertebrate lineages, indicating either repeated transfer within vertebrates, or the loss of a gene from a subset of lineages. Nevertheless, the completion of the human genome has provided evidence of over 100 genes of bacterial origin (see the bottom of this page for a brief discussion).

Horizontal gene transfer does not pose any difficulty for evolutionary theory, as it can be viewed as simply another source of genomic diversity that is subjected to selective pressure. Transferred genes can be viewed as simply a short cut to what gene duplication and mutation could produce. It does, however, cause difficulties in generating phylogenetic trees. In cases where complete genome sequence is unavailable, the relatedness of species is often assayed based on a limited number of genes. Should any of these genes have entered a genome by horizontal gene transfer, then it can suggest a different date of divergence between two species than the date that would be obtained by examining the majority of other genes in the genome. This is generally more of an issue in bacteria, where conjugation is a regular occurrence.