Genetic contamination
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Genetic contamination is the entry of foreign alleles, through stray pollen or mixed seed, into a population whose genetic make-up breeders, seed producers or conservationists want to keep.[1] It is also called genetic pollution.[2] At its furthest it becomes genetic swamping, in which hybrids replace the pure forms of a population.[3]
Cannabis is pollinated by wind and its forms cross freely.[4] John McPartland and Ernest Small wrote in 2020 that the germplasm of Central and South Asian landraces outside their centres of diversity had become "genetically contaminated" by widespread crossbreeding.[5] Fatima Bachir and colleagues used the same word in 2022 for the traditional kif of Morocco.[6] How alleles move between populations is treated under gene flow. How they come to stay in a lineage is treated under introgression.
From seed purity to extinction risk
Plant breeders spoke first of "pollen contamination". Extraneous pollen frustrated selection in the field and produced unwanted hybrid seed in plots grown for sale. Seed producers still aim to keep the genetic purity of seed at a set level.[1]
Conservation biologists took up the problem for wild plants and animals. Judith Rhymer and Daniel Simberloff argued in 1996 that nonindigenous species can bring about a form of extinction in native species by hybridisation and introgression, above all where a rare species meets an abundant one. Molecular methods were showing cases that morphology alone did not reveal.[7] Potts and colleagues wrote of genetic pollution in 2003, when eucalypt plantations were spreading rapidly in Australia. They judged naturally small or remnant populations to be at particular risk from non-local stock planted within the range of native populations. They expected the risk to be small where barriers to hybridisation were strong.[2]
Some gene flow, Rhymer and Simberloff wrote, is a normal and constructive evolutionary process, and not every combination of genes can be preserved.[7] For Norman Ellstrand gene flow is "not a problem in itself", and a change in gene flow can alter an endangered population for better or worse.[1]
How hybridisation can remove a population
Hybridisation can lead to extinction by two routes. In demographic swamping, hybrids are so unfit that a parental lineage wastes its reproduction on them and falls below replacement. In genetic swamping, hybrids are at least partly fertile and viable and replace the pure parental genotypes. Genetic swamping ends the pure genome but not necessarily its alleles or traits, which live on in the hybrids.[3] Genetic swamping is also called genetic assimilation.[1] Crosses between hemp and Moroccan drug-type cannabis are fully fertile.[6]
Hybridisation was judged an extinction threat in 69 of the 143 cases Marco Todesco and colleagues surveyed in 2016, and genetic swamping was the more common cause, in 87 per cent of those 69. Local loss was predicted more often than global loss. A threat was reported in 72 per cent of cases with human involvement and 46 per cent of those without. Husbandry and agriculture figured in 55 per cent of the human cases. Most of the studies concerned crosses between species, although whether the parents were one species or two did not appear to change the predicted outcome. The authors warn that their search terms and scoring may have biased these figures.[3]
Reduced fitness in hybrids is called outbreeding depression, a risk sometimes set on a par with inbreeding depression. It can come from the disruption of local adaptation. The fitness loss shows most strongly in early generations and can be short-lived: in long-term experiments on plants, selection typically purged it after five to ten generations. Todesco and colleagues found little evidence that it often drives extinction and called the fitness consequences of hybridisation "surprisingly benign". Hybridisation can also raise the fitness of a small inbred population, an effect called genetic rescue, though it was a rare outcome in their survey.[3]
Landraces beside modern varieties
Gene flow between a landrace and a modern variety is expected to run one way. Seed companies multiply their varieties in isolation, while farmers multiply the landrace themselves.[8]
Studies of maize and barley measured this over time by genotyping collections made at different dates. In central Italy, flint maize landraces collected in the 1950s were compared with those grown 50 years later, after dent hybrids had come into wide use. The recent landraces carried a significant amount of introgression from the hybrids, at levels that varied widely and were mostly low. They still descended from the older landrace gene pool, with no sign of genetic erosion.[9]
In Sardinia, barley landraces sampled in 1990 and 1999 showed a low level of introgression that rose over the decade. Barley sets its own seed and its pollen travels short distances, so the authors attributed the gene flow mainly to seed, for instance through a contractor's combine harvester shared among farmers. The level differed from one farmer's population to the next. Both research groups concluded that landraces can be conserved on the farm alongside modern agriculture if practices limit the entry of unwanted seed and pollen.[8][9]
Drug-type landraces of Central and South Asia
McPartland and Small date the crossing of Central and South Asian landraces to breeders in the West from the early 1970s. By their account the crossing largely obliterated the differences between the two groups, and unhybridised landraces were hard to obtain in the United States and Europe by the mid-1980s. They attribute the loss of differences reported in recent genetic surveys to the hybrid "strains" those surveys sampled. Older studies of landraces collected from the 1970s to the 1990s found clearer differences.[5]
In the account of the same authors, foreign germplasm also reached the landraces' centres of diversity:
| Place | Material | Date |
|---|---|---|
| Afghanistan | "Mexican Gold" | about 1972 |
| Nepal, Jamaica, Thailand | Central Asian landraces | 1970s |
| Southern Kashmir | Afghani landraces, grown for sieved hashish, with escapes near crop fields | by 1980 |
| South Africa, Morocco | Central Asian genetics | 1980s |
| Kullu, Himachal Pradesh | "hybrid Cannabis", attributed to foreign nationals | by 1988 |
Citing also a 2005 report that Afghani landraces were disappearing, McPartland and Small concluded that these landraces face extinction through introgressive hybridisation and that the survival of unaltered wild populations in Asia is precarious. They called for collecting germplasm in the centres of diversity and held that formal names for the four varieties they describe may help to prevent their extinction.[5]
Replacement and pollen in the Rif
The first change in the Rif is a change of seed. Pierre-Arnaud Chouvy and Kenza Afsahi surveyed the region in July 2013 and estimated from what they saw in the fields that hybrids with much larger resin yields had largely replaced the kif landrace. Growers named about ten new varieties, khardala above all, and said they grew them to meet outside demand.[10]
The second is a change in the plant. M'hamed Boudouah wrote in 2017 that the pollen of the new plants affects the old local one and that farmers were close to giving kif up, since buyers no longer saw it as pure and growers no longer found it competitive.[11] Bachir and colleagues describe the local varieties as genetically contaminated, acquiring the traits of the imported ones and disappearing rapidly.[6]
Kif itself may carry older crosses. Bachir and colleagues see a considerable chance that European hemp genes entered the plant long before the hybrids, and they think Lebanese seed may have arrived with the sieving technique in the 1960s.[6] Chouvy accepts that foreign landraces probably modified kif by introgression and holds that it remains distinct. In his view a landrace keeps evolving and should not be frozen by strict conservation, though it can disappear if left unprotected. He argues that the switch to modern hybrids has produced a very different hashish, and he proposes an appellation of origin to protect the landrace.[12]
Hemp seed and feral stands
In hemp the concern is also legal, since crops are tested against a limit on THC. Seven of 108 plants grown in Minnesota from Canadian certified seed were heterozygous for the drug-type form of the gene that sets the THC to cannabidiol ratio. All seven exceeded 0.3 per cent THC. Jonathan Wenger and colleagues warned that pollen drifting from feral or cultivated plants could defeat even strict testing during seed multiplication. They proposed screening seed lots by genotype at each stage of breeding and multiplication.[13]
Feral populations in the Midwestern United States include plants with THC-type alleles. Whether these came with impure early hemp or from later drug cultivation is debated, a question treated under introgression.[13]
Keeping populations apart
Breeders have relied most on isolation distance, set for each crop from experiments showing cross-fertilisation falling off rapidly away from a pollen source.[1] For cannabis the distance required to prevent cross-pollination may be as much as 5 km. Pollen may still cross it.[4] Wenger and colleagues suggest testing in the field whether 5 km is enough to keep a crop pure.[13]
The same difficulty arises inside collections. Accessions regrown near one another can cross, which Matthew Welling and colleagues consider a possible reason for reports of a widely shared cannabis gene pool.[4] Preventing such crossing is an important consideration in ex situ conservation. Welling and colleagues suggest freezing seed to reduce the crossing that regrowing allows.[4]
Advice for populations in the field is more general. Todesco and colleagues recommend barriers to the introduction of exotic relatives prone to hybridise, more care over releases and the upkeep of mature and diverse habitats that resist hybrids.[3] For eucalypts, a published framework assesses the risk from plantations.[2] In the barley study the uneven level of introgression was attributed to farmers' practices, such as seed source and harvesting.[8]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 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 Potts, B.M.; Barbour, R.C.; Hingston, A.B.; Vaillancourt, R.E. (2003). "Genetic pollution of native eucalypt gene pools—identifying the risks". Australian Journal of Botany. 51 (1): 1–25. doi:10.1071/BT02035.
- ↑ 3.0 3.1 3.2 3.3 3.4 Todesco, M.; Pascual, M.A.; Owens, G.L.; Ostevik, K.L.; Moyers, B.T.; Hübner, S.; Heredia, S.M.; Hahn, M.A.; Caseys, C.; Bock, D.G.; Rieseberg, L.H. (2016). "Hybridization and extinction". Evolutionary Applications. 9 (7): 892–908. doi:10.1111/eva.12367. PMC 4947151.
- ↑ 4.0 4.1 4.2 4.3 Welling, M.T.; Shapter, T.; Rose, T.J.; Liu, L.; Stanger, R.; King, G.J. (2016). "A belated green revolution for Cannabis: virtual genetic resources to fast-track cultivar development". Frontiers in Plant Science. 7 1113. doi:10.3389/fpls.2016.01113.
- ↑ 5.0 5.1 5.2 McPartland, J.M.; Small, E. (2020). "A classification of endangered high-THC cannabis (Cannabis sativa subsp. indica) domesticates and their wild relatives". PhytoKeys. 144: 81–112. doi:10.3897/phytokeys.144.46700.
- ↑ 6.0 6.1 6.2 6.3 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 Rhymer, J.M.; Simberloff, D. (1996). "Extinction by hybridization and introgression". Annual Review of Ecology and Systematics. 27: 83–109. doi:10.1146/annurev.ecolsys.27.1.83.
- ↑ 8.0 8.1 8.2 Bellucci, E.; Bitocchi, E.; Rau, D.; Nanni, L.; Ferradini, N.; Giardini, A.; Rodriguez, M.; Attene, G.; Papa, R. (2013). "Population structure of barley landrace populations and gene-flow with modern varieties". PLOS ONE. 8 (12) e83891. doi:10.1371/journal.pone.0083891. PMC 3873955.
- ↑ 9.0 9.1 Bitocchi, E.; Nanni, L.; Rossi, M.; Rau, D.; Bellucci, E.; Giardini, A.; Buonamici, A.; Vendramin, G.G.; Papa, R. (2009). "Introgression from modern hybrid varieties into landrace populations of maize (Zea mays ssp. mays L.) in central Italy". Molecular Ecology. 18 (4): 603–621. doi:10.1111/j.1365-294X.2008.04064.x.
- ↑ Chouvy, Pierre-Arnaud; Afsahi, Kenza (2014). "Hashish revival in Morocco". International Journal of Drug Policy. 25 (3): 416–423. doi:10.1016/j.drugpo.2014.01.001.
- ↑ Boudouah, M'hamed (2017). "Evolution de l'économie du Rif Central (Senhaja Sraîr) et problématique d'alternatives". Revue Tidighin des Recherches Amazighes et Développement (6). ISSN 2028-9782.
- ↑ Chouvy, Pierre-Arnaud (2024). "International property rights for Cannabis landraces and terroir products. The case of Moroccan Cannabis and hashish". International Journal of Drug Policy. 129 104479. doi:10.1016/j.drugpo.2024.104479.
- ↑ 13.0 13.1 13.2 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.