Horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT)[1][2][3] is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction).[4] HGT is an important factor in the evolution of many organisms.[5][6] HGT is influencing scientific understanding of higher-order evolution while more significantly shifting perspectives on bacterial evolution.[7]
"HGT" redirects here. For other uses, see HGT (disambiguation).
Horizontal gene transfer is the primary mechanism for the spread of antibiotic resistance in bacteria,[8][5][9][10] and plays an important role in the evolution of bacteria that can degrade novel compounds such as human-created pesticides[11] and in the evolution, maintenance, and transmission of virulence.[12] It often involves temperate bacteriophages and plasmids.[13][14][15] Genes responsible for antibiotic resistance in one species of bacteria can be transferred to another species of bacteria through various mechanisms of HGT such as transformation, transduction and conjugation, subsequently arming the antibiotic resistant genes' recipient against antibiotics. The rapid spread of antibiotic resistance genes in this manner is becoming a challenge to manage in the field of medicine. Ecological factors may also play a role in the HGT of antibiotic resistant genes.[16]
Horizontal gene transfer is recognized as a pervasive evolutionary process that distributes genes between divergent prokaryotic lineages[17] and can also involve eukaryotes.[18][19] It is postulated that HGT promotes the maintenance of a universal life biochemistry and, subsequently, the universality of the genetic code.[20]
History[edit]
Griffith's experiment, reported in 1928 by Frederick Griffith,[21] was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation.[22][23] Griffith's findings were followed by research in the late 1930s and early 1940s that isolated DNA as the material that communicated this genetic information.
Horizontal genetic transfer was then described in Seattle in 1951, in a paper demonstrating that the transfer of a viral gene into Corynebacterium diphtheriae created a virulent strain from a non-virulent strain,[24] simultaneously revealing the mechanism of diphtheria (that patients could be infected with the bacteria but not have any symptoms, and then suddenly convert later or never),[25] and giving the first example for the relevance of the lysogenic cycle.[26] Inter-bacterial gene transfer was first described in Japan in a 1959 publication that demonstrated the transfer of antibiotic resistance between different species of bacteria.[27][28] In the mid-1980s, Syvanen[29] postulated that biologically significant lateral gene transfer has existed since the beginning of life on Earth and has been involved in shaping all of evolutionary history.
As Jian, Rivera and Lake (1999) put it: "Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes"[30] (see also Lake and Rivera, 2007).[31] The phenomenon appears to have had some significance for unicellular eukaryotes as well. As Bapteste et al. (2005) observe, "additional evidence suggests that gene transfer might also be an important evolutionary mechanism in protist evolution."[32]
Grafting of one plant to another can transfer chloroplasts (organelles in plant cells that conduct photosynthesis), mitochondrial DNA, and the entire cell nucleus containing the genome to potentially make a new species.[33] Some Lepidoptera (e.g. monarch butterflies and silkworms) have been genetically modified by horizontal gene transfer from the wasp bracovirus.[34] Bites from insects in the family Reduviidae (assassin bugs) can, via a parasite, infect humans with the trypanosomal Chagas disease, which can insert its DNA into the human genome.[35] It has been suggested that lateral gene transfer to humans from bacteria may play a role in cancer.[36]
Aaron Richardson and Jeffrey D. Palmer state: "Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryotes. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remain unclear."[37]
Due to the increasing amount of evidence suggesting the importance of these phenomena for evolution (see below) molecular biologists such as Peter Gogarten have described horizontal gene transfer as "A New Paradigm for Biology".[38]
Compounds found to promote horizontal gene transfer[edit]
Through research into the growing issue of antibiotic resistance[123] certain compounds have been observed to promote horizontal gene transfer.[124][125][126][127] Antibiotics given to bacteria at non-lethal levels have been known to be a cause of antibiotic resistance[127] but emerging research is now showing that certain non-antibiotic pharmaceuticals (ibuprofen, naproxen, gemfibrozil, diclofenac, propranolol, etc.) also have a role in promoting antibiotic resistance through their ability to promote horizontal gene transfer (HGT) of genes responsible for antibiotic resistance. The transfer of antibiotic resistance genes (ARGs) through conjugation is significantly accelerated when donor cells with plasmids and recipient cells are introduced to each other in the presence of one of the pharmaceuticals.[124] Non-antibiotic pharmaceuticals were also found to cause some responses in bacteria similar to those responses to antibiotics, such as increasing expression of the genes lexA, umuC, umuD and soxR involved in the bacteria's SOS response as well as other genes also expressed during exposure to antibiotics.[124] These findings are from 2021 and due to the widespread use of non-antibiotic pharmaceuticals, more research needs to be done in order to further understanding on the issue.[124]
Alongside non-antibiotic pharmaceuticals, other compounds relevant to antibiotic resistance have been tested such as malachite green, ethylbenzene, styrene, 2,4-dichloroaniline, trioxymethylene, o-xylene solutions, p-nitrophenol (PNP), p-aminophenol (PAP), and phenol (PhOH).[125][126] It is a global concern that ARGs have been found in wastewater treatment plants[125] Textile wastewater has been found to contain 3- to 13-fold higher abundance of mobile genetic elements than other samples of wastewater.[125] The cause of this is the organic compounds used for textile dying (o-xylene, ethylbenzene, trioxymethylene, styrene, 2,4-dichloroaniline, and malachite green)[125] raising the frequency of conjugative transfer when bacteria and plasmid (with donor) are introduced in the presence of these molecules.[125] When textile wastewater combines with wastewater from domestic sewage, the ARGs present in wastewater are transferred at a higher rate due to the addition of textile dyeing compounds increasing the occurrence of HGT.
Other organic pollutants commonly found in wastewater have been the subject of similar experiments.[126] A 2021 study used similar methods of using plasmid in a donor and mixing that with a receptor in the presence of compound in order to test horizontal gene transfer of antibiotic resistance genes but this time in the presence of phenolic compounds.[126] Phenolic compounds are commonly found in wastewater and have been found to change functions and structures of the microbial communities during the wastewater treatment process.[126] Additionally, HGT increases in frequency in the presence of the compounds p-nitrophenol (PNP), p-aminophenol (PAP), and phenol. These compounds result in a 2- to 9-fold increase in HGT (p-nitrophenol being on the lower side of 2-fold increases and p-aminophenol and phenol having a maximum increase of 9-fold).[126] This increase in HGT is on average less than the compounds ibuprofen, naproxen, gemfibrozil, diclofenac, propranolol, o-xylene, ethylbenzene, trioxymethylene, styrene, 2,4-dichloroaniline, and malachite green[124][125] but their increases is still significant.[126] The study that came to this conclusion is similar to the study on horizontal gene transfer and non-antibiotic pharmaceuticals in that it was done in 2021 and leaves room for more research, specifically in the focus of the study which is activated sludge.[126]
Heavy metals have also been found to promote conjugative transfer of antibiotic resistance genes.[127] The paper that led to the discovery of this was done in 2017 during the emerging field of horizontal gene transfer assisting compound research.[127] Metals assist in the spread of antibiotic resistance through both co-resistance as well as cross-resistance mechanisms.[127] In quantities relevant to the environment, Cu(II), Ag(I), Cr(VI), and Zn(II) promote HGT from donor and receptor strains of E. coli.[127] The presence of these metals triggered SOS response from bacterial cells and made the cells more permeable. These are the mechanisms that make even low levels of heavy metal pollution in the environment impact HGT and therefore the spread of ARGs.