Since human cultures first began cultivating cannabis, selective breeding has been employed to improve wild cannabis as a source of seeds, fiber and drugs. However, cannabis is not a simple plant to breed, as two primary complications have made controlled selective breeding a challenge. Firstly, cannabis is typically a dioecious plant, indicating that individual plants are distinctly male or female. Therefore, cannabis plants are predisposed to outcrossing as opposed to self-pollination, which is the primary means of fixing desirable traits in other species. In addition to this, the valuable components of cannabis are produced and harvested from female plants, and thus it is a challenge to identify males with favourable genetically regulated traits. Secondly, cannabis is a wind-pollinated plant and therefore will very easily pollinate surrounding females, making selective crosses difficult to control.
Due to the challenges outlined above, it is typical of cannabis growers to utilize clonal propagation as opposed to seeds, as this will produce identical “offspring.” Regardless of these limitations, cannabis breeders have improved upon the concentration of psychotropic compounds and yield, whereas hemp breeders have continuously worked to improve the textile characteristics of fibre-type cannabis cultivars. Understanding the inheritance of chemical phenotype (chemotype) for the most clinically relevant cannabinoids has been central to modern medicinal cannabis and hemp breeding. Modern molecular techniques have allowed for a history and cannabis’ greater ability to screen for elite cultivars, increasing the rate at which desired traits can be identified and bred into new cultivated varieties.
Primarily through the research of de Meijer at HortaPharm B.V., four loci, O, A, B and C, have been found to genetically regulate cannabinoid content. Cannabinoids are terpenophenolic compounds, produced primarily with the monoterpenoid precursor geranylpyrophosphate (GPP), and one of two phenolic precursors, olivetolic acid or divarinolic acid, both of which are resorcinolic acid homologs produced by the polyketide pathway. Production of the phenolic precursors can be disrupted by a mutant null allele o, at locus O. In a homozygous state, synthesis of either resorcinolic acid precursor is blocked, while O/o heterozygous phenotypes typically have one-tenth the cannabinoid content. This indicates that allele o acts as a dominant repressor of the polyketide pathway that generates both olivetolic acid and divarinolic acid.
Synthesis of either olivetolic acid or divarinolic acid is regulated by locus A, which according to de Meijer is polygenic, with the alleles Ape 1 to n encoding olivetolic acid synthase, and alleles Apr 1 to n encoding divarinolic acid synthase. These phenolic precursors, along with GPP, are utilized by the enzyme geranylpyrophosphate: olivetolate transferase to produce either CBGA or CBGVA depending on the phenolic precursor present. The synthesis of the two most clinically relevant cannabinoids, THC and CBD, is then controlled by co-dominant alleles present at Locus B. THCA/THCVA or CBDA/CBDVA will be produced if alleles BT or BD is present and functional, respectively, while homozygous individuals will produce significant quantities of both metabolites. Variations in the sequence of BT and BD can lead to enzymes with reduced function, so THC:CBD ratios are commonly found to deviate from 1:1. Mutant alleles BT0 and BD0 significantly reduce THCA and CBDA production, while leading to considerable accumulation of the precursor CBGA. Lastly, an independent gene at Locus C produces the enzyme CBCA synthase, which competes with CBDA synthase and THCA synthase for CBGA precursor, producing the cannabinoid CBCA or CBCVA.
Many of the genes mentioned above have been sequenced, and molecular markers detectable via PCR have been developed and validated to correlate with specific chemotypes. Modern breeders can take advantage of this simple molecular technique to expedite breeding objectives, while using classical breeding techniques to select for other favourable traits, such as yield, disease resistance and flowering time requirements, all aspects that impact the output of a medical cannabis facility. Soon, more advanced molecular breeding techniques, such as transgenic gene expression or substitution of gene promoters with knockdown/overexpression variants could yield dramatically different chemotypes with potentially novel medical applications.