Gene flow
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Gene flow is the movement of alleles from one population into another, counted when pollen or seed from outside a population leaves offspring within it. Pollen that lands without fertilising a plant or seed that never grows to breed has been dispersed but has added nothing to the population it reached.[1] In Cannabis alleles travel as wind-borne pollen and as seed carried by birds and other animals. People have also taken the plant with them across continents.[2][3]
Gene flow is one of the four processes that change allele frequencies from one generation to the next, with mutation, genetic drift and natural selection. Low levels of gene flow are enough to offset drift, mutation and moderate selection in the other direction.[1] When gene flow joins two populations that have diverged into distinct lineages, it produces hybrids. Where alleles of one lineage persist in the other after the hybrids have crossed back into it over several generations, the result is introgression.[4]
In cannabis, gene flow runs between hemp and drug-type crops, between feral populations and the fields around them and between traditional landraces and the cultivars introduced among them.[5][2][6] How the variation it moves is arranged among populations is set out under genetic diversity.
Pollen, seed and people
Cannabis is wind-pollinated and mostly dioecious, with pollen and seed borne on separate plants.[7] In flowering plants generally, pollen moves genes farther than seed does. Where the two have been compared within a species, gene flow by pollen has exceeded gene flow by seed in the great majority of cases. Wind-pollinated species also average more gene flow than species pollinated by animals.[1]
Cannabis pollen can travel a long way. A pollen trap at Tétouan in northern Morocco caught cannabis pollen mainly from early April to late August in each year from 2008 to 2010. Stations in southern Spain, where no hemp was grown, caught small amounts only on days when the wind blew from North Africa. An earlier study of wind trajectories traced that pollen to the plantations of the Rif.[8] Pollen traps measure dispersal. Most studies of pollen or seed leaving a source measure only its departure, without showing whether the pollen fertilised a plant or the seed established, so they do not measure gene flow itself.[1]
Seed carries genes too. Wenger and colleagues consider long-distance dispersal of cannabis seed by birds to have even greater potential than pollen to move genes among populations.[2] Feral cannabis in the United States grows along roadsides, railway rights-of-way and at livestock feedlots, sites where its establishment beyond the old hemp districts may reflect seed carried by birds, animals and people.[9]
Cannabis has travelled with people for thousands of years. That transport, together with hybridisation, has left the evolutionary history of the species tangled.[3] The high-potency drug-type cannabis of North America is thought to have come from Southeast and Central Asia by way of the Caribbean and South America, a route known mainly from limited historical accounts.[5] About 400,000 acres of hemp were grown in the United States between 1942 and 1945. Woods and colleagues trace the country's present feral populations to that crop.[3]
What gene flow does to populations
Gene flow tends to make the populations it connects alike. One or a few successful immigrants per generation are enough to counter the divergence that drift produces, whatever the size of the populations, so that most alleles come to be present in most populations. Against selection, gene flow at a rate equal to or above the strength of selection prevents a population from evolving local adaptation. Working with selection, it speeds the spread of a favoured allele from one population to the next. At rates above the mutation rate it becomes the main immediate source of new alleles in a population.[1]
Hemp has been grown in Morocco since before the 10th century, in the same Mediterranean region as the local drug-type plant of the Rif. Cannabis pollen is abundant and carried far. Crosses between hemp and the drug-type plant are fully fertile. Bachir and colleagues describe the local varieties acquiring the traits of the hybrid cultivars introduced since 1980.[6] The consequences of gene flow from introduced cultivars for traditional populations are treated under genetic contamination.
Hemp and drug-type cannabis
Hemp and drug-type cannabis are differentiated across the whole genome, a structure described under genetic diversity. A survey of 110 whole genomes found hemp-type and drug-type samples each forming their own branch of the family tree, with little evidence of gene flow between the use types as the crop spread across the world. The model of the crop's history that fitted the data best nevertheless included continuing gene flow after domestication into both the hemp and the drug types from a group of feral plants and landraces in China, the closest descendants of the ancestral gene pool.[7]
In a pangenome of 193 genomes, FST between drug-type and hemp populations was 0.20, a degree of differentiation the authors compare to that between species of strawberry. The study's model of ancestry inferred six gene-flow events among Asian hemp, European hemp, drug-type cannabis and cannabidiol-rich lines, which the authors relate to breeding that crossed Asian and European hemp.[5] Breeding has kept the use types apart: hemp and drug cultivars have histories of independent breeding and reduced gene flow between populations selected for different traits.[10] Breeders' crosses between hemp and drug-type plants made for cannabidiol content are a case of deliberate gene flow. The hemp segments those cultivars carry are described under introgression.[10]
Feral plants and crops
Feral cannabis in the United States descends from hemp grown for fibre and other industrial uses between the 18th and 20th centuries.[9] In Minnesota a federal agency licensed and subsidised hemp growing during the Second World War. Escapes from cultivation formed feral populations across the upper Midwest.[2] Some of these stands carry drug-type alleles and pass them on in their pollen. Among 317 feral plants genotyped in Minnesota, four were of the THC-dominant type. Seedlings raised from a feral plant of the intermediate type included THC-dominant plants, which indicated that pollen donors carrying THC-type alleles grew in the stand.[2] Of 53 feral populations in which at least ten plants were genotyped, about a third carried only the functional allele of CBDA synthase, the form typical of hemp. The others also held the non-functional allele typical of drug-type plants, at frequencies from about 0.1 to 0.6.[9][2]
Pollen also moves from feral and cultivated plants into hemp seed crops. Seven of 108 plants grown from certified Canadian hemp seed were of the intermediate type and exceeded the legal limit of 0.3% THC. Wenger and colleagues note that pollen drifting from nearby feral or cultivated plants that carry THC-type alleles could defeat even strict testing during seed multiplication. They suggest field tests of whether the recommended isolation distance of 5 km is enough to keep a crop pure.[2]
Measuring gene flow
Gene flow is measured directly or inferred from genetic structure. Plant breeders measured it first. They called it "pollen contamination", since stray pollen spoilt selection plots and seed stocks. To measure it they planted a source of plants carrying a dominant marker surrounded by recipient plants at increasing distances and scored the recipients' offspring for the marker. Such experiments showed cross-fertilisation falling off rapidly with distance and gave each crop a working isolation distance. Parentage analysis with genetic markers later made it possible to assign seeds to the plants that fathered them.[1]
Indirect methods estimate the average number of migrants per generation, Nm, from the differentiation of allele frequencies among populations, as measured by FST. Values well above 0.5 are high enough to counter drift. The models behind these estimates rest on simple assumptions, so a single estimate may be inaccurate, though comparisons across many species remain useful.[1] Genome sequencing has added models that fit migration between populations into a reconstruction of their history, the approach behind the estimates of gene flow among use types of cannabis.[7][5]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Ellstrand, N.C. (2014). "Is gene flow the most important evolutionary force in plants?". American Journal of Botany. 101 (5): 737–753. doi:10.3732/ajb.1400024.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Wenger, J.P.; Dabney, C.J.; ElSohly, M.A.; Chandra, S.; Radwan, M.M.; Majumdar, C.G.; Weiblen, G.D. (2020). "Validating a predictive model of cannabinoid inheritance with feral, clinical, and industrial Cannabis sativa". American Journal of Botany. 107 (10): 1423–1432. doi:10.1002/ajb2.1550. PMC 7702092.
- ↑ 3.0 3.1 3.2 Woods, P.; Price, N.; Matthews, P.; McKay, J.K. (2023). "Genome-wide polymorphism and genic selection in feral and domesticated lineages of Cannabis sativa". G3. 13 (2) jkac209. doi:10.1093/g3journal/jkac209.
- ↑ Suarez-Gonzalez, A.; Lexer, C.; Cronk, Q.C.B. (2018). "Adaptive introgression: a plant perspective". Biology Letters. 14 (3) 20170688. doi:10.1098/rsbl.2017.0688. PMC 5897607.
- ↑ 5.0 5.1 5.2 5.3 Lynch, R.C.; Padgitt-Cobb, L.K.; Garfinkel, A.R.; Knaus, B.J.; Hartwick, N.T.; et al. (2025). "Domesticated cannabinoid synthases amid a wild mosaic cannabis pangenome". Nature. 643 (8073): 1001–1010. doi:10.1038/s41586-025-09065-0. PMC 12286863.
- ↑ 6.0 6.1 Bachir, Fatima; Eddouks, Mohamed; Arahou, Mohamed; Fekhaoui, Mohammed (2022). "Origin, Early History, Cultivation, and Characteristics of the Traditional Varieties of Moroccan Cannabis sativa L.". Cannabis and Cannabinoid Research. 7 (5): 603–615. doi:10.1089/can.2021.0020.
- ↑ 7.0 7.1 7.2 Ren, G.; Zhang, X.; Li, Y.; Ridout, K.; Serrano-Serrano, M.L.; Yang, Y.; Liu, A.; Ravikanth, G.; Nawaz, M.A.; Mumtaz, A.S.; Salamin, N.; Fumagalli, L. (2021). "Large-scale whole-genome resequencing unravels the domestication history of Cannabis sativa". Science Advances. 7 (29) eabg2286. doi:10.1126/sciadv.abg2286.
- ↑ Aboulaich, Nadia; Trigo, M. Mar; Bouziane, Hassan; Cabezudo, Baltasar; Recio, Marta; El Kadiri, Mohamed; Ater, Mohammed (2013). "Variations and origin of the atmospheric pollen of Cannabis detected in the province of Tetouan (NW Morocco): 2008–2010". Science of the Total Environment. 443: 413–419. doi:10.1016/j.scitotenv.2012.10.075.
- ↑ 9.0 9.1 9.2 Aina, Ademola; Wenger, Jonathan P.; Stanton, Eliot; Majumdar, Chandrani Gon; ElSohly, Mahmoud; Weiblen, George D.; Ellison, Shelby (1 July 2025). "Genetic diversity, population structure, and cannabinoid variation in feral Cannabis sativa germplasm from the United States". Scientific Reports. 15 (1) 20423. doi:10.1038/s41598-025-07912-8. PMC 12215616.
- ↑ 10.0 10.1 Grassa, C.J.; Weiblen, G.D.; Wenger, J.P.; Dabney, C.; et al. (2021). "A new Cannabis genome assembly associates elevated cannabidiol (CBD) with hemp introgressed into marijuana". New Phytologist. 230 (4): 1665–1679. doi:10.1111/nph.17243. PMC 8248131.