The evolution of Cannabis sativa: hemp and marijuana
Last updated: 17 Jul, 2015
A history of gene duplication and divergence explains the differences in drug content between marijuana and hemp, a new study published in Early View form in New Phytologist has shown.
Marijuana and hemp are two cultivated forms of the same plant, Cannabis sativa. Marijuana produces the psychoactive drug tetrahydrocannabinolic acid (THCA), while hemp produces the pharmaceutically important cannabidiolic acid (CBDA). Both THCA and CBDA are produced from the same precursor compound, cannabigerolic acid.
The enzymes that produce THCA or CBDA (THCA synthase and CBDA synthase, respectively) were believed to be two alleles of the same gene. New research from George Weiblen (University of Minnesota, USA) and colleagues has shown that in fact, THCA synthase and CBDA synthase are encoded by two separate but linked regions of the Cannabis genome. Marijuana plants simply have a non-functional copy of CBDA, so convert all cannabigerolic acid into THCA.
Image: The relative levels of THCA and CBDA in marijuana, hemp, and the first (F1) and second (F2) generation plants following a marijuana x hemp cross. The dashed line represents the expected ratio of THCA to CBDA if their respective synthase enzymes were equally competitive for the common precursor, cannabigerolic acid, so it is clear that the F1 and F2 intermediates have a significant skew towards producing CBDA. Image credit: Weiblen et al., 2015.
This research has important implications for the regulation of Cannabis cultivation. In many countries, all Cannabis plants are classified as controlled substances, but legislation in Europe, Canada and the USA still distinguishes hemp from marijuana, defining hemp as containing <0.3% THCA by dry weight.
Weiblen hopes that his research will enable the commercial production of hemp, a plant whose fibre and seeds are in huge demand internationally: "It’s a plant of major economic importance that is very poorly understood scientifically. With this study, we have indisputable evidence for a genetic basis of differences among Cannabis varieties, further challenging the position that all Cannabis should be regulated as a drug”.
Weiblen, GD, Wenger, JP, Craft, KJ, ElSohly, MA, Medmedic, Z, Treiber, EL, Marks, MD, 2015. Gene duplication and divergence affecting drug content in Cannabis sativa. New Phytologist: Early View.
Edited 24th July 2015: The original version of this post stated that hemp is defined as containing '<0.03%' THCA by dry weight, which has now been corrected to '<0.3%'.