The cannabis plant has been exploited by humanity primarily for textiles and for intoxication, with hemp strains used for fabrics and marijuana strains for altering the mind. The changes to the genome that led to drug-producing plants is a mystery of cannabis evolution, but one that has now been solved, thanks to an article published in Genome Biology.

In the article, a team of Canadian researchers describe the first cannabis genome sequence, which they assembled using reads from a number of high-throughput sequencing strategies. Using DNA from the potent Purple Kush marijuana strain, obtained from a patient prescribed cannabis for medicinal purposes, the researchers scanned the assembled genome for mutations that differed from hemp strains. They reasoned that such mutations might be why Purple Kush produces THCA, which is the active ingredient of cannabis, while hemp strains do not.

Frustrated in their search for mutations, which didn't turn up a smoking gun, the researchers instead analyzed the transcriptome sequence – and struck the jackpot.

The transcriptome showed that, while both hemp and marijuana have the THCA synthase genes in their genomes, the gene is strongly expressed in marijuana but not transcribed in hemp. THCA synthase is a gene that codes for an essential enzyme in THCA production; thanks to the Genome Biology article, we now know that making more of this enzyme is how hemp got high.

See also: our editorial