Cannabis Domestication, Breeding History, Present-day Genetic Diversity, and Future Prospects
Robert C. Clarke and Mark D. Merlin
CRITICAL REVIEWS IN PLANT SCIENCES, 2016, VOL. 35, NOS. 5–6, 293–327
Doi : 10.1080/07352689.2016.1267498
Humans and the Cannabis plant share an intimate history spanning millennia. Humans spread Cannabis from its Eurasian homelands throughout much of the world, and, in concert with local climatic and human cultural parameters, created traditional landrace varieties (cultivars resulting from a combination of natural and farmer selection) with few apparent signs of domestication. Cannabis breeders combined populations from widely divergent geographical regions and gene pools to develop economically valuable fiber, seed, and drug cultivars, and several approaches were used with varying results. The widespread use of single plant selections in cultivar breeding, inbreeding, and the adoption of asexual reproduction for commercial drug production, reduced genetic diversity and made many present-day cultivars susceptible to pathogens and pests. The great majority of drug Cannabis cultivars are now completely domesticated, and thus are entirely dependent on humans for their survival. Future ramifications remain to be realized.
KEYWORDS : Afghanistan; Africa; cannabinoids; Europe; fiber; hashish; hemp; India; indica; marijuana; New World; ruderalis; sativa; seed oil; sinsemilla; terpenoids
I. Cannabis botany and ecology
The ecological requirements and genetic inheritance of plants determine where they grow naturally. Cannabis plants require well-drained soils, adequate sunlight, warmth, and moisture, so most naturally growing popu- lations are found seasonally across accommodating northern temperate latitudes. Under natural conditions, Cannabis grows well along exposed riverbanks, lakesides, the margins of agricultural land, and other areas dis- turbed by humans (Merlin, 1972; Clarke, 1977, 1981; Clarke and Merlin, 2013; Small 2015). Based on ecologi- cal constraints, Cannabis evolved somewhere in temper- ate latitudes of the northern hemisphere, and Eurasia is favored as its primary region of origin (Clarke and Merlin, 2013). Seminal uses, early cultivation, worldwide dissemination, and eventual domestication of Cannabis all began within this natural biogeographical range.
Cannabis plants are usually dioecious and produce either male (pollen) flowers or female (seed) flowers. An individual Cannabis plant’s gender is determined by X and Y chromosomes, and monoecious plants of this genus producing flowers of both sexes occur only rarely in nature. Cannabis also relies on air currents to spread pollen grains from male plants to female seed plants. Wind pollination, dioecious sexuality, and X/Y sexual inheritance are each relatively rare in plants, yet all three are characteristics of Cannabis.
Cannabis plants grow and develop within their annual life cycle of sexual reproduction. Moistened seeds germi- nate as spring weather warms, and juvenile plants grow rapidly through the summer. Fast-growing juvenile plants appear much alike, but as autumn day length decreases, populations begin to flower and plants express individual phenotypic differences. Male plants within a single population are often slightly taller than female plants. Male flowers hang from branches with few leaflets and are exposed to the wind (which facilitates pollen dis- persal), whereas flowers on female plants are tightly clus- tered with small leaflets to trap male pollen grains that fertilize the female ovules. Soon after males shed their pollen they die. Before the arrival of killing frosts, the fer- tilized female plants ripen viable seeds. These dissemi- nules fall to the ground via the wind or feeding birds and other animals that disperse the seeds inadvertently; and then they overwinter in the soil ready to initiate another life cycle the following spring. During their fertilized development, each enlarging seed is surrounded by a bract covered with thousands of secretory hairs called glandular trichomes. These hairs produce a resinous blend of cannabinoids (chemically related compounds found in Cannabis) and aromatic compounds (mostly terpene compounds common in many plants); and these secondary metabolites are believed to protect the devel- oping seed by repelling pests and pathogens (Clarke, 1977, 1981; McPartland et al., 2000). Psychoactive delta- 9-tetrahydrocannabinol (THC) and nonpsychoactive cannabidiol (CBD) are the primary cannabinoid constit- uents in almost all Cannabis. THC and aromatic psycho- active secretions are evolutionarily significant as they attracted early human attention, and at least in modern times they remain the primary impetus for the continued breeding and worldwide dispersal of Cannabis cultivars (Pollan, 2001; also see Merlin, 1972; Clarke, 1998; Clarke and Merlin, 2013).
Cannabis favors a mild climate with sufficient water and sunlight, and early humans spread it into a range of favorable temperate and sub-tropical niches where it became naturalized (feral) throughout Eurasia, in parts of Africa, and more recently in the New World. Canna- bis thrives on the nutrient-rich dump heaps near human occupation and has readily apparent agronomic traits, and it was therefore preadapted to cultivation (Ander- son, 1967; Merlin, 1972; Clarke and Merlin, 2013). Annual plants of this genus branch freely when culti- vated in open areas; when grown in dense stands, they suppress branching, forming a single central stalk (Iltis, 1983). This naturally adapts Cannabis to high density sowing for fiber production, as well as low density plant- ings that encourage branching and flower formation for seed and drug production.