Scientists Did a Massive DNA Study on Global Weed Strains. Here’s What They Found
Cannabis sativa, the plant that has gifted the world hemp, pot, and countless other earthly delights, is a modern cash crop with a rich history of cultivation that dates back thousands of years. But even as new cannabis strains are constantly bred for psychoactive or medicinal effects, the origins of domesticated cannabis remain murky, in part due to legal restrictions that prohibit scientists from acquiring plant samples.
An international team of scientists has spent years overcoming the red tape on these green weeds in order to present “a unique global view of the domestication of C. sativa,” along with “valuable genomic resources” for research into its modern breeds, according to a study published on Friday in Science Advances.
By analyzing 110 whole genomes of C. sativa sourced from all around the world, the researchers traced the plant’s cultivation back more than 12,000 years. Their results reveal that four genetic groups of domesticated cannabis stemmed from a single ancestral origin in northwest China, which challenges a widely held view that C. sativa emerged from a Central Asian center of crop domestication.
The new research is “the largest survey with genomes from such geographically diverse origins and such diverse domestication types”—including feral plants, traditional landraces, and historical and modern cultivated varieties—”which is necessary for such a comparative investigation of the domestication origin of a cultivated species,” said senior author Luca Fumagalli, a professor of ecology and evolution at the University of Lausanne, in an email.
Of the 110 genomes collected for the study, 82 were new samples sourced from field sites and commercial stores in Switzerland, China, India, Pakistan, and Peru, as well as from the Vavilov Institute of Plant Genetic Resources in Saint Petersburg, Russia. The team, which was led by Lanzhou University biologist Guangpeng Ren, also re-analyzed 28 publicly available genomes, most of which belonged to North American breeds.
“It is very difficult to obtain hemp and especially drug-type samples and feral plants” which are “ancient domesticated plants readapted to the wild environment” from “non-Western countries, due to legal restrictions,” Fumagalli noted. He added that the team “had to convince and establish collaborations with local scientists in several key countries” and rely on the Vavilov Institute “to get seeds originating from many countries where field collection of cannabis plants is difficult.”
“All in all, this took several years,” he said.
Fortunately, the team’s time and energy paid off, as the varied set of genomes tell a fascinating tale about the domestication and cultivation history of C. sativa. According to the study, two lineages diverged from wild cannabis plants about 12,000 years ago: Basal cannabis, which still exists as a feral species in China and the US today, and a second group that was probably bred as a multipurpose crop with textile, nutritional, and medicinal properties.
About 4,000 years ago, that second lineage split again into two distinct groups: hemp breeds that were selected for fiber production and marijuana breeds that were selected for the production of cannabinoids such as tetrahydrocannabinol (THC), the plant’s primary psychoactive compound. The fourth lineage identified by the team is a branch of the drug-type breed that went feral and now grows south of the Himalayas. Meanwhile, the ancestral wild cannabis plant that gave rise to these four lineages has likely become extinct.
Interesting insights were hidden in the genes of the hemp and marijuana groups, which both lost functional genes that had been present in their multipurpose ancestors as a result of their increasingly specialized use as either a fiber source or as a psychoactive substance.
“This suggests an ancestral situation (in the wild species) with both genes in a functional state, an intermediate situation just before or after the beginning of domestication (similar to what observed today in plants from the Basal group), and a progressive loss of one or another gene after strong divergent selection started for either fiber-type plants or drug-type plants,” Fumagalli explained.
Perhaps the biggest surprise, however, was the discovery of the Basal group, which includes 14 feral plants and landraces collected in China and two feral plants from the United States that likely migrated across the ocean sometime in the 19th century.
“An unexpected result was the presence of this Basal genetic group, which did not comprise any of the domesticated plants we consider today as Cannabis sativa (hemp or marijuana),” Fumagalli said. “It’s a bit like discovering a dog breed unknown to date and genetically independent from the genetic cluster grouping all dog breeds described today worldwide.”
Previous studies based on feral plant studies have suggested that C. sativa was first domesticated by peoples living in Central Asia, but Fumagalli and his colleagues propose that the crop emerged from a fairly narrow region of East Asia in northwest China, where the elusive Basal group still grows. In addition to conducting further genomic analyses on this controversial, beloved, and increasingly influential plant, the researchers hope to track down the exact region where its many powers were first discovered and harnessed by humans. “Clearly, this is a key geographical area, and future additional sampling here could more precisely show where plants from the Basal group are distributed,” Fumagalli concluded.
By Becky Ferreira