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This is 3 years old, but it says here that the cannabis plant gained its elevating function through horizontal gene transfer from a virus!

2021-07-10

This comes from sciencedaily.com and was relayed via a comment on the New York Times website to an article about illicit drugs in general. The comment was written by “TDHawkes” of Eugene, Oregon on July 9, 2021. The Science Daily article is a popularization of another article published in Genome Research and entered in PubMed, a repository of generally available scientific articles maintained by the federal government.

The Science Daily article states that a new, more complete transcription of the Cannabis sativa genome revealed that the genes for the production of the prized active ingredient, delta nine tetrahydrocannabinol (THC) are contained within a region that appears to be transposed from an ancient virus. This suggests that the cannabis plant did not evolve its ability to produce this ingredient on its own but received its powers through a process known as horizontal gene transfer (HGT).

What is Horizontal Gene Transfer?

HGT occurs when an organism from one species contributes genes to cells of another species, which are then incorporated into the genome. This genome is passed down to all the cells’ offspring as a permanent part of its DNA. This way to pick up DNA is widely understood between bacteria and from viruses to bacteria, but the ability of viruses to give genes to higher animals, including man, has only recently been elucidated.

When genes are transferred through HGT, a species can suddenly get entirely new functions, conferred by complete proteins or sets of proteins. A multicellular organism could theoretically gain a new organ, like an eye or a liver, which it never had before.

Transfer of genes from one bacterial species to another was first discovered in 1928. HGT from a virus to the causative agent of diphtheria was described in 1951, explaining how virulent bacteria can arise from non-virulent strains, sometimes within a single patient who has, at first, an infection with Corynebacterium diphtheriae without any symptoms, but later develops fever and sore throat.

The role of HGT in the development of resistance to antibiotics was discovered subsequently in the 1960’s. It wasn’t until the turn of the century that geneticists learned that large parts of the genomes of higher animals and man contains the remains of many viruses. The ability of human immunodeficiency virus (HIV) to incorporate itself into the human genome was discovered in the 1980’s and was thought to be an anomaly; later research revealed that this incorporation is extremely common.

The presence of HGT in higher animals and humans and its importance as a feature in evolution has not been recognized until the last twenty years. Recently, hundreds of different human genes have been proposed to be the result of HGT from viruses; this proposal is highly controversial.

Horizontal Gene Transfer and the Cannabis plant

Deciphering the genetic map of cannabis has been made very difficult by the presence of millions of duplicated stretches of DNA originating from viruses, resembling puzzle pieces all of the same color. The new study reported by the Science Daily article results from an advance which combines these duplicated stretches into larger pieces with, so to speak, different colors on their edges.

These advances have made it possible to determine that the two major products of the cannabis plant, THC and cannabidiol (CBD), are made by different enzymes, both of which come from a common ancestor gene probably derived from a virus. They also revealed a third product called cannabichromene (CBC). CBC has potent anti-inflammatory properties which have not been well-studied.

There is yet another cannabis product known as delta-eight tetrahydrocannabinol which has largely unknown properties, other than the ability to induce an unusual “high” which is not well-characterized but clearly differs from that produced by THC. This chemical is being actively studied, but it is not clear from whence it comes.

Evolution and Horizontal Gene Transfer

The discovery of HGT and its near-universal presence in all forms of life has caused consternation among classical evolutionary biologists. Once the “tree of life” was a neat analogy that explained evolution and allowed scientists to believe that there was a “common ancestor” that was shared by all species of organisms. Now the “tree of life” has been shown to be an inadequate metaphor; evolution appears to be more like a mosaic or a meshwork of many shared genes that underlie the profusion of different organisms.

Implications for Cannabis breeding

The discovery of separate genes that produce CBD and THC suggests that it should be possible to selectively breed cannabis strains that specialize in producing only one of the two chemicals. So far, breeding experiments have failed to satisfactorily separate the two.

In addition, the experiments show that the many different cannabinoids can probably be separated and may be useful for numerous different medical purposes. In particular, the chemical CBC may be a useful anti-inflammatory product that could be produced entirely separately from THC. This is important because medicine could find another anti-inflammatory very helpful, especially if it has different properties from the other known drugs used for this purpose– with troublesome side-effects that limit their effectiveness.

Caveats

Horizontal gene transfer really exists, but it has been proven only in viruses, bacteria, and unicellular animals like protozoa. Plants also appear to engage in this odd behavior. Higher animals may not have many genes that come from viruses, although those that cause disease (pathogenic viruses) often display the ability to incorporate themselves into the genomes of their victims.

The phenomenon of HGT may thus be unique to unicellular organisms and plants as a factor in evolution. However, higher animals originated as unicellular organisms billions of years ago, and large parts of their original genomes may have been assembled from the genomes of viruses. The telltale tracks of repetitive DNA sequences and other residual elements indicate that horizontal gene transfer was an important mechanism of evolution in the early days that is still used as a method for viruses to infect vertebrates, including humans, to this day.

photo courtesy of Brent Barnett via pixabay.com

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