Hemp, a crop grown for durable fiber and nutritious seed, and marijuana, the most abundant illegal drug of abuse in the United States, both belong to the species Cannabis sativa. They differ in levels of the psychoactive drug tetrahydrocannabinol (THC) but are otherwise difficult to tell apart. The technique holds promise for distinguishing different cultivars (domesticated plant lines) in U.S. criminal cases. It may also prove useful in countries where the cultivation of hemp is permitted but marijuana is illegal, as in Canada and Europe. The work appears in the March issue (volume 51, No. 2) of the Journal of Forensic Science.
The new technique is an improvement on previous means of separating the two types of Cannabis, said author George Weiblen, an assistant professor of plant biology in the university's College of Biological Sciences and College of Food, Agricultural and Natural Resource Sciences. For decades it has been possible to identify THC chemically, but the drug is not present in all plant tissues or throughout a plant's life cycle. And other researchers have found that genetic markers known as "short tandem repeats," which are used to identify individuals in paternity and criminal cases, lack the power to distinguish Cannabis cultivars unequivocally.
In tests with three different cultivars of hemp and one of marijuana, the DNA fingerprints of all the cultivars were distinct and nonoverlapping. Weiblen and Shannon L. Datwyler, a postdoctoral associate who is now on the faculty of California State University, Sacramento, found that the AFLP (amplified fragment length polymorphism) technique generated hundreds of genetic markers that together established separate identities for each of the four cultivars.
"We think this technique has the potential to distinguish marijuana varieties as well," said Weiblen. "It has implications not just for separating hemp from marijuana in countries where hemp cultivation is permitted, but in establishing origins of seized drugs and, therefore, conspiracy in drug distribution networks. It also could be used in criminal defenses against claims of conspiracy."
The technique chops up DNA and generates numerous fragments of DNA, each defined by particular "marker" DNA sequences that act like bookends. The lengths of the fragments within the bookends were found to vary according to the cultivar. Thus, the pattern of fragment lengths adds up to a composite picture of each cultivar.
"With this technique, we find hundreds of markers scattered across the genome," said Weiblen. "The larger number of markers, compared to other techniques, gives us the power to separate the cultivars."
The Cannabis plant has been cultivated for millennia and is important in the global economy as both a licit and an illicit crop, said Weiblen. Hemp is a source of durable fiber that provides an alternative to cotton fabric, among other uses. Cotton requires pesticide application and a hot climate, whereas hemp does not, which makes it suitable for local Minnesota agriculture. Weiblen seeks to screen a wider range of Cannabis cultivars to refine the technique. He is also working to identify regions of the Cannabis genome responsible for drug content in marijuana. If enough can be learned about the genome, it may one day be possible to produce an entirely drug-free hemp plant that looks different from marijuana. Currently, all hemp products are imported into the United States. Developing a new variety that could be cultivated in the United States would reduce American dependence on foreign products while creating a new alternative crop for American farmers.