Outcrossing
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Outcrossing is mating between two different plants, in which the egg cells of one are fertilised by pollen from another. Its alternative is self-fertilisation, or selfing, in which a plant's egg cells are fertilised by its own pollen or by pollen of a clone of the same plant.[1] Cannabis bears its male and female flowers on separate plants and is pollinated by the wind, so in nature it reproduces only by outcrossing.[2][3]
Outcrossing keeps cannabis plants highly heterozygous. Natural populations and those bred by mass selection are made up of heterozygous plants that share a common gene pool.[2] Selfing becomes possible when a female plant is made to form male flowers, when a plant bears flowers of both sexes and in monoecious hemp.[2][3] Repeated selfing raises homozygosity and can bring on inbreeding depression: selfed lines of hemp, both of cultivars grown for flower and of monoecious cultivars grown for fibre and seed, have lost size and yield.[4][1][5] Crossing two inbred lines restores vigour through heterosis, the basis of F1 hybrid seed.[5][2]
The separate sexes that enforce outcrossing in cannabis are described under Dioecy and Sex determination in cannabis, and the movement of pollen between populations under gene flow.
Outcrossing and selfing
An outcrossed plant takes its two sets of genes from two parents. A selfed plant takes both from one, and a line kept by selfing loses heterozygosity quickly: each generation halves the heterozygosity that remains, so that after seven generations less than one per cent of it is left.[6] Mating between relatives also inbreeds a line, more slowly. Lines kept by mating full siblings come to lose nearly a fifth of their heterozygosity each generation.[7] How inbred a plant is can be measured by its inbreeding coefficient, the probability that the two copies of a gene it carries at one locus are identical by descent.[8]
Outcrossing in cannabis
Cannabis is dioecious and wind-pollinated. Stamens and pistils form in separate flowers on male and female plants. Outcrossing by wind-borne pollen is the plant's only natural means of reproduction.[2] As a dioecious plant pollinated by the wind, cannabis is highly heterozygous. Many drug-type strains are propagated as clones to keep their genetic identity.[9] Plants raised from seed segregate and vary, as clones do not.[2] In a trial of 23 hemp families from genetically varied parents, the variation within each family in form and in cannabinoids followed the heterozygosity of its parents.[10] Open pollination also brings about some hybridisation between improved and wild populations.[2]
Commercial drug-type crops are all female. Male plants are destroyed because seed formation lowers the quality of the flower. Seed still forms on plants that develop hermaphroditic flowers, whose anthers shed viable pollen and which can pollinate themselves. Where no male plants grow, as in indoor production, most such seed is likely to come from selfing. Seed from hermaphroditic flowers grown indoors gave only female seedlings. Outdoors seed can come from both selfing and cross-fertilisation. Of 16 seedlings raised from seed collected at an outdoor site, two were male, which only pollen from a male plant could have produced.[3]
Inbreeding depression
Inbreeding depression is the reduced fitness of inbred offspring that comes from the greater expression of recessive deleterious alleles. Inbred individuals are less fertile, die more often and fall to disease more readily.[11] Severely deleterious mutant alleles tend to be recessive. Inbreeding raises the share of plants homozygous for them.[12] Fitness also falls under inbreeding at a locus where the heterozygote is fitter than either homozygote, a condition called overdominance, because heterozygotes grow rarer.[12] The two explanations were set against each other by maize geneticists. D.F. Jones proposed in 1917 that at each locus the allele of the better homozygote tends to be dominant. E.M. East held that the loci concerned with yield were overdominant. The mean of a trait cannot tell the two apart. Traits that go with vigour, size or fitness tend to fall under inbreeding.[13]
Inbreeding can also rid a population of some of its deleterious recessive alleles, a process called purging. Inbred plants that express such alleles leave fewer offspring, so the alleles pass less often to the next generation. A population that is too small or too uniform may not survive the depression long enough to gain from purging.[11]
Selfing in cannabis
Selfed feminised seed
Treating a female cannabis plant with silver thiosulfate makes the treated branches form male flowers with viable pollen while the rest of the plant stays female. When that pollen fertilises the plant's own pistils, the plant is selfed. The pollen carries only X chromosomes, so the offspring are all female.[2] Producers of feminised seed use selfing to keep a strain name: a chosen female is propagated as clones, some clones are induced to form male flowers and their pollen is used on the others. The seed from this self-cross is called S1, the first self-generation.[1]
In a 2020 study, twelve plants each were grown from three batches of feminised hemp seed: two crosses between strains and a self-cross of the strain Candida (CD-1). The selfed plants grew to half the height and half the leaf area and total dry weight of the outcrossed plants. They began to flower seven days later and produced 63 per cent less dry flower. All of them had variegated leaves, which their parent did not. They varied two to three times as much in total and floral dry weight. Their expected heterozygosity at genetic markers was 0.085, against about 0.10 in the outcrossed progeny. The authors suggested that the selfed plants showed inbreeding depression from the expression of homozygous recessive traits and that seed producers outcross instead.[1]
A 2024 study selfed single plants of two cultivars for four generations, S1 to S4. In the dwarf autoflowering cultivar Wilhelmina the plants grew less variable with each round. S4 plants were uniformly small, which the authors took to be likely inbreeding depression. In the earlier generations of the same cultivar, 40 per cent of S1 plants, 40 per cent of S2 plants and 10 per cent of S3 plants had yellow, malformed leaves and a severely dwarfed habit and were infertile. No S4 plant showed this. The authors suggested that the trait might have been eliminated by the fourth generation and called for more seedlings to be grown to confirm it. In the taller cultivar Wife, 38 per cent of S1 plants, 25 per cent of S3 plants and half of the S4 plants produced no more than 1 gram of dried flower.[5] In the trial of 23 hemp families, the two families from a second generation of selfing grew the slowest, at 1.46 and 1.03 centimetres a day against 2.75 centimetres in the fastest family. The authors concluded that selfed populations suffered from inbreeding depression.[10]
Seedlings from seed set by hermaphroditic flowers on drug-type plants, the product of one generation of selfing, were as genetically varied at markers as seedlings from cross-fertilised seed. The authors expected a single cycle of selfing to have little effect in a plant with the heterozygosity of cannabis.[3] In a study of the Thai Hang Kra Rok landrace published in 2026, S1 progeny of plants from accessions held in Bangkok and Sakon Nakhon were compared with F1 progeny of crosses between the two accessions, grown in a plant factory under artificial light. The selfed plants flowered about a week earlier and were shorter: 78.94 and 74.83 centimetres against 87.22 and 88.78 centimetres. Their dry flower weight did not differ from that of the F1 plants and their THC content was not lower. The authors found no inbreeding depression in flower yield, which they suggested might reflect the purging of deleterious alleles in the landrace's past. They put the greater height of the F1 plants down to heterosis from combining the two accessions.[14]
Monoecious hemp
Monoecious hemp cultivars bear male and female flowers on one plant. They are chromosomally female (XX) and were first developed in European breeding programmes to raise the yield of grain.[10] Enclosed alone in an isolator, a monoecious plant can be self-pollinated.[4]
At the Institute of Bast Crops in Hlukhiv, Ukraine, single plants of three monoecious cultivars were self-pollinated for up to twelve generations between 2008 and 2023. In Hlukhivski 58 the first selfed generation already set 11.3 per cent less seed per plant. Its seeds were 8.1 per cent lighter. The depression deepened until the traits settled in the fourth to sixth generations, by which time seed weight per plant had fallen by about half. After twelve generations stem weight was 47.0 per cent lower than in the starting plants, fibre weight 55.0 per cent and seed yield 74.9 per cent. From the sixth generation most offspring bore mostly male flowers. The southern cultivar Zolotoniski 15 tended to show less depression, losing about a third of its stem and fibre weight and half of its seed weight by the twelfth generation.[4]
Inbred lines and hybrid seed
When two inbred lines are crossed, the hybrid is often superior to both. This hybrid vigour, which G.H. Shull named heterosis in 1914, is inbreeding depression in reverse. The same two explanations, dominance and overdominance, are given for it.[13] Crossing different inbred lines restores a good part of the heterozygosity of the population they came from.[12] In the generation after the first cross the advantage falls halfway back towards the mean of the two parent lines, as it does in hybrid maize.[13]
In the review by Barcaccia and colleagues, heterosis has been important in breeding modern fibre cultivars of hemp but has been little explored or recorded in drug-type cannabis. They describe drug-type cultivars as mostly made by crossing a single male of one landrace with a single female of another, giving a heterozygous and vigorous offspring. Such a cross is not a true F1 hybrid, since its parents were not inbred lines. A cultivar bred in this way keeps its uniformity only when it is propagated as a clone. True F1 hybrids need inbred parents, bred by repeated selfing or by mating full siblings. A masculinised inbred female used to pollinate another inbred female gives F1 seed that is all female.[2]
In the 2024 study, F1 seed from crossing selfed lines of Wife and Wilhelmina gave plants that were more uniform than the selfed lines of Wife and carried more plant and flower dry weight and higher percentages of CBD and THC. The hybrids grew as tall as the Wife lines although one parent was dwarf, which the authors attributed to heterosis.[5] The authors of the trial of hemp families concluded that hybrid hemp will need at least one inbred parent to grow and yield uniformly.[10] Breeders of monoecious hemp in Ukraine bring inbred lines into crosses from the fourth to sixth selfed generation, when their traits have settled. Crosses of inbred lines with each other and with cultivars show heterosis. The monoecious cultivars Artemida, Harmoniia, Demetra, Afina and Vik 2020 were bred in this way.[4]
See also
References
- ↑ 1.0 1.1 1.2 1.3 Kurtz, L.E.; Mahoney, J.D.; Brand, M.H.; Lubell-Brand, J.D. (2020). "Comparing genotypic and phenotypic variation of selfed and outcrossed progeny of hemp". HortScience. 55 (8): 1206–1209. doi:10.21273/HORTSCI15061-20.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Barcaccia, G.; Palumbo, F.; Scariolo, F.; Vannozzi, A.; Borin, M.; Bona, S. (2020). "Potentials and challenges of genomics for breeding cannabis cultivars". Frontiers in Plant Science. 11 573299. doi:10.3389/fpls.2020.573299.
- ↑ 3.0 3.1 3.2 3.3 Punja, Z.K.; Holmes, J.E. (2020). "Hermaphroditism in marijuana (Cannabis sativa L.) inflorescences – impact on floral morphology, seed formation, progeny sex ratios, and genetic variation". Frontiers in Plant Science. 11 718. doi:10.3389/fpls.2020.00718.
- ↑ 4.0 4.1 4.2 4.3 Міщенко, С.В.; Кабанець, В.М.; Кириченко, Г.І.; Марченко, Т.Ю.; Мачульський, Г.М. (2025). "Особливості прояву ознак однодомних конопель в процесі інбридингу та його використання в селекції" [Features of the expression of the traits of monoecious hemp in the inbreeding process and its use in breeding]. Faktori eksperimental'noi evolucii organizmiv (in українська). 37: 29–34. doi:10.7124/FEEO.v37.1742.
- ↑ 5.0 5.1 5.2 5.3 McDonald, M.M.; Lubell-Brand, J.D. (2024). "F1 hybrid seed can enhance cannabis crop uniformity and yield". HortScience. 59 (12): 1795–1799. doi:10.21273/HORTSCI18197-24.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 231–233.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. p. 237.
- ↑ Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. p. 219.
- ↑ Sawler, J.; Stout, J.M.; Gardner, K.M.; Hudson, D.; Vidmar, J.; Butler, L.; Page, J.E.; Myles, S. (2015). "The genetic structure of marijuana and hemp". PLoS ONE. 10 (8) e0133292. doi:10.1371/journal.pone.0133292.
- ↑ 10.0 10.1 10.2 10.3 Carlson, C.H.; Stack, G.M.; Jiang, Y.; Taşkıran, B.; Cala, A.R.; et al. (2021). "Morphometric relationships and their contribution to biomass and cannabinoid yield in hybrids of hemp (Cannabis sativa)". Journal of Experimental Botany. 72 (22): 7694–7709. doi:10.1093/jxb/erab346.
- ↑ 11.0 11.1 Davinack, D. (2024). Molecular Ecology & Evolution: An Introduction. Norton, Massachusetts: Wheaton College. pp. 172–174.
- ↑ 12.0 12.1 12.2 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 242–244.
- ↑ 13.0 13.1 13.2 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 411–414.
- ↑ Srikul, Pichsiri; Mayong, Amonrat; Matthayatthaworn, Weerachai; Sreewongchai, Tanee (2026). "Comparison of self-pollination and intercrossing for feminized seed production in Thai Hang Kra Rok cannabis (Cannabis sativa L.) landrace". Agriculture and Natural Resources. 60 (5) 600512. doi:10.34044/j.anres.2026.60.5.12.