Gene Duplication and Divergence Affecting Drug Content in Cannabis Sativa

• Cannabis sativa is an economically important source of durable fibers, nutritious seeds, and psychoactive drugs but few economically important plants are so poorly understood from a molecular genetic standpoint.
• We crossed marijuana and hemp cultivars to evaluate competing models of cannabinoid inheritance and to explain the predominance of tetrahydrocannabinolic acid (THCA) in marijuana compared to cannabidiolic acid (CBDA) in hemp. We measured the differential expression of cannabinoid synthase genes, constructed a linkage map, and documented the associations of genetic markers with divergent cannabinoid phenotypes in marijuana and hemp.
• Although patterns of phenotypic segregation and a major quantitative trait locus (QTL) for the THCA/CBDA ratio were consistent with a simple model of co-dominant alleles at a single locus, the diversity of THCA and CBDA synthase sequences observed in the mapping population, the position of the enzyme coding loci on the map, and their patterns of expression reveal the existence of multiple linked loci. Phylogenetic analysis of cannabinoid synthase sequences and gene expression patterns suggest an evolutionary history of gene duplication and divergence affecting drug content.
• Marijuana is distinguished from hemp by a non-functional cannabinoid synthase that appears to have been positively selected to enhance psychoactivity. An unlinked QTL for cannabinoid quantity may also have played a role in the recent escalation of drug potency in Cannabis.