Cannabis Genome Map Reveals How Repetitive, Virus-Like DNA Shaped THC and CBD Production
A detailed genetic map of Cannabis sativa has clarified how the plant’s major cannabinoid-producing genes are arranged—and offered clues to the evolutionary history of THC-, CBD- and CBC-producing varieties.
The study, published in Genome Research in 2019, combined long-read DNA sequencing with a genetic map created from a cross between the drug-type cultivar Purple Kush and the hemp variety Finola. The work was led by researchers from the University of Toronto, the University of British Columbia and the Icahn School of Medicine at Mount Sinai.
Earlier cannabis genome projects produced fragmented sequences that made it difficult to determine where important genes sat on the plant’s chromosomes. The new research placed much of the genome into ten chromosome-scale structures, making it easier to examine the regions associated with cannabinoid production.
The researchers identified the genes encoding THCA synthase and CBDA synthase—enzymes that help produce tetrahydrocannabinolic acid and cannabidiolic acid. When heated, those acidic compounds are converted into THC and CBD, respectively. Both genes were located in large, highly repetitive regions of chromosome 6.
Those regions contain abundant retrotransposons: mobile, virus-like DNA sequences that can copy themselves and insert copies elsewhere in a genome. This finding is more precise than saying that ordinary infectious viruses directly created the THC or CBD genes. The study supports a model in which an ancestral cannabinoid synthase gene was duplicated, while retrotransposon-rich DNA contributed to the rearrangement and diversification of the surrounding genomic regions over millions of years.
The THCAS and CBDAS sequences are closely related, consistent with their having descended from a common ancestral gene. Over time, gene duplication and mutation appear to have helped produce enzymes with different chemical outputs. The drug-type and hemp-type forms also occupy substantially different, repeat-rich genomic regions with limited recombination, which may help explain why their chemical profiles can remain distinct.
Humans later reinforced those differences through selective breeding. Cannabis cultivated for drug production was selected for elevated THC, while hemp was bred for traits such as fiber production and low THC. The genetic map provides researchers with a way to track those traits more precisely and could make it easier to breed plants with particular cannabinoid profiles.
The study also identified and experimentally characterized a gene for cannabichromenic acid synthase, or CBCAS. CBC is one of several minor cannabinoids produced by cannabis, but the discovery of its gene does not establish that CBC is responsible for the plant’s principal psychoactive effects. THC remains the cannabinoid most directly associated with cannabis intoxication, while the pharmacological effects of other cannabinoids are still being investigated.
One important result was the clearer separation of cannabinoid-production genes. The researchers found that the genes involved in making the precursor molecule for THC and CBD are generally not all inherited as a single block. They also identified a gene involved in precursor formation that is closely linked to a marker associated with total cannabinoid content. Such information could eventually help breeders select plants for cannabinoid concentration, composition and other agricultural traits.
The map was made possible in part by Pacific Biosciences long-read sequencing, which can span repetitive DNA more effectively than many older short-read methods. Repetitive sequences account for a large share of the cannabis genome, creating a difficult assembly problem. The resulting resource builds on the team’s 2011 draft cannabis genome, which provided an early sequence but could not reliably place many fragments on chromosomes.
The researchers made the resulting assembly available through public genome resources, including the NCBI Cannabis sativa genome record. Rather than explaining cannabis psychoactivity as the direct product of viral infection, the findings show how ancient mobile DNA, gene duplication and subsequent human selection may have helped shape the plant’s unusually diverse chemistry. The map gives scientists a stronger foundation for studying cannabinoid biology and developing cultivars with more predictable traits.
