Heterosis
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Heterosis, also called hybrid vigour, is the superiority of hybrid offspring over their parents.[1] It appears when two inbred lines or two populations are crossed. It shows in greater biomass, faster development and higher fertility.[2][3] The word describes the hybrid and leaves the cause open: overdominance at a locus is one possible explanation among several.[1] An F1, the first generation of a cross between two inbred lines, is heterozygous at every locus where the lines differ. In the next generation of random mating its advantage falls halfway back towards the mean of the parents.[1] Breeders use heterosis in F1 hybrid varieties of many crops.[4]
In Cannabis, crosses of hemp varieties and selfed lines in Ukraine and Russia have exceeded their parents in seed yield, while heterosis for fibre has been harder to obtain.[5][6] Part of the seed advantage in crosses of dioecious and monoecious hemp comes from an F1 that is nearly all female.[6] In a trial of crosses between inbred lines, plants in four of five F1 lines were larger than their inbred parents. Two of the five exceeded the open-pollinated stock the lines came from.[7]
Measuring heterosis
Mid-parent heterosis measures a hybrid against the average of its two parents. On a purely additive model that difference is zero, so it comes from dominance and epistatic deviations.[8] Better-parent heterosis is a hybrid's excess over the higher parent. For a trait such as days to flowering the better parent is the lower.[9] Ukrainian hemp breeding adds competitive heterosis, the excess over a standard variety, and calls mid-parent heterosis hypothetical and better-parent heterosis true.[5] Dimitriev and colleagues reject the mid-parent as a measure, since a cross can beat the mean of its parents without beating the better one. Their hemp crosses ran 16.9 to 38.5 per cent above the better parent and 26.7 to 44.3 per cent above the mean.[10]
| Reference | Seed mass (g) | Heterosis (per cent) |
|---|---|---|
| Mean of the varieties Afina and Ionino (mid-parent) | 9.29 | 28.7 |
| Afina, the higher parent | 9.60 | 24.6 |
| Hliana, the standard variety | 5.61 | 113.2[5] |
In 293 maize hybrids with the inbred line B73, average better-parent heterosis ran from 5 per cent for cob diameter to 185 per cent for plant yield. More than 90 per cent of the hybrids showed better-parent heterosis for 10 of the 17 traits measured and fewer than half for two of them. Its levels in different traits were only weakly correlated with one another.[9] A trait that is the product of several others can show heterosis while its components stay near the mid-parent. Oil palm is one case: one pool bears few heavy bunches and another many light ones, and their hybrids give about 25 per cent more fruit with bunch number and bunch weight near the mid-parent. No dominance is needed to explain it.[8] The sign can also differ between traits: mules and hybrids of indica and japonica rice gain vigour and lose fertility. Stress usually increases heterosis in maize, though maize hybrids then perform worse overall.[8]
Crossing two randomly mating populations gives panmictic-midparent heterosis, measured against the populations themselves. Crossing inbred lines drawn from those populations gives inbred-midparent heterosis, measured against the lines. The difference between the two is baseline heterosis, the recovery of what inbreeding took away.[8] A hybrid of inbred lines has long been read as inbreeding depression in reverse.[1] On this partition, which is Lamkey and Edwards's, only baseline heterosis is the exact reverse of inbreeding depression, because heterosis between populations can arise from additive-by-additive epistasis without dominance.[8] Whitlock and colleagues calculated that heterosis between populations has a different genetic basis from inbreeding depression within them and is much more likely to come from alleles of intermediate effect.[11]
After the first generation
When two inbred lines are fixed for different alleles at a locus, every F1 plant is Aa. Without a general rule about dominance, the means of the parents do not predict the mean of the F1. They do predict the next generation. The F2 is one-quarter AA, one-half Aa and one-quarter aa, so its mean lies halfway between the F1 and the midparent:[1]
- F2 = ½ F1 + ½ (½ P1 + ½ P2)
The rule carries over to the mean of a whole plant when the effects of loci add, and to parents that are populations in Hardy–Weinberg proportions.[1] Heterosis between two populations halves in the F2 in the same way.[8] Where a maize F1 far outyields its parent lines, the F2 falls far back towards them, chiefly through inferior homozygotes. Random mating among F2 plants leaves the mean unchanged in the F3. Selfing carries it further, halfway towards its limit in each generation. A backcross to one parent has a mean halfway between that parent and the F1. The fit of F2 and backcross means to these rules can show on which scale gene effects add.[1]
Hybrid maize and the name
Charles Darwin described the effect in 1876 in his work on cross and self fertilisation. In 1909 George Harrison Shull found higher grain yields in hybrid corn and proposed crossing as a method of breeding it. He introduced the word heterosis in 1914 to replace "heterozygosis", which did not express the hybrid's superior performance.[4] Hybrid corn went from under 10 per cent of the crop in Iowa in 1935 to over 90 per cent four years later. Yield drove the change, along with an evenness of growth and flowering that suited machine harvesting. Yuan Longping developed hybrid rice in the 1970s, with a yield advantage of 10 to 20 per cent over inbred parental varieties. Hybrids covered about half of China's rice area by the early 2000s.[4]
Heterosis may or may not be due to overdominance, the superiority of a heterozygote at one locus, although overdominance is sometimes miscalled heterosis.[2][1]
Why hybrids exceed their parents
On the dominance model, recessive alleles at different loci in the two parents are complemented in the hybrid. On the overdominance model, different alleles at one locus interact in the hybrid and raise its vigour.[3] Both date from 1908, dominance from C.B. Davenport and overdominance from East and Shull.[12] D.F. Jones set out the dominance view for maize yield in 1917 and E.M. East still favoured overdominance in 1936. The mean of a cross cannot distinguish them.[1] Pseudo-overdominance is a variant of the dominance model: alleles of opposite effect linked in repulsion are never seen apart and so behave as a single overdominant locus.[8] Dominance and overdominance are argued over in the same terms for inbreeding depression.
A single gene can produce heterosis: in tomato, one heterozygous loss-of-function allele of SINGLE FLOWER TRUSS, the gene behind the flowering hormone florigen, raised yield by up to 60 per cent in distinct genetic backgrounds and environments.[13] Two studies of rice worked from the parents of the hybrid Shanyou 63. In 250 F3 families, most yield QTLs showed overdominance, overall marker heterozygosity had very little relation to yield and two-locus interactions were widespread, which the authors took as strong evidence for epistasis. Family means of that kind halve dominance.[12] The 2003 study measured heterosis directly in hybrids. Partial dominance, full dominance, overdominance and all three forms of two-locus interaction contributed in that population. For Shanyou 63 itself the authors held single-locus effects and dominance-by-dominance interactions sufficient, and in one year these summed to 86 per cent of its heterosis for grains per panicle.[14] Across 10,074 F2 lines from 17 crosses no locus was shared by all, though within each of three groups of hybrid a few loci from the female parents explained much of the yield advantage over the male parents, some of them through partial dominance.[15] Work on maize points to incomplete dominance. In a diallel, putatively deleterious sites showed it widely and the more damaging variants tended to be more recessive. The elite inbred lines carried fewer such sites than a comparable set of traditional maize landraces.[16]
Birchler and colleagues argued in 2010 that complementing recessive alleles cannot account for all of heterosis. Inbred maize lines selected for decades at one seed company gained yield with little change in the size of their heterosis. Crosses between species can show more heterosis than crosses within them. Tetraploids gain more as they combine more diverse genomes. They proposed dropping the classical terms for a quantitative framework of dosage-sensitive regulatory networks, which they said was not yet a complete model.[3] Schnable and Springer concluded in 2013 that heterosis has no single simple explanation and that different loci affect it for different traits and in different hybrids.[17] Proposed general mechanisms include changes in circadian rhythm, hormones, chromatin and small RNAs. None has been shown to explain heterosis universally.[8] Chen (2013) attributes heterosis to allelic interactions between the parental genomes, with epigenetic changes that alter regulatory networks.[18]
Divergence between parents
East concluded in 1936, from data on many species, that heterosis rises on average with the genetic disparity of the parents.[3] Heterosis in the 293 maize hybrids was prevalent even in crosses with non-adapted tropical lines. Genetic distance between parents correlated with better-parent heterosis at 0.57 for plant yield and 0.41 for plant height, with no correlation for tassel branch count. Flint-Garcia and colleagues found distance a poor predictor of the most heterotic combinations.[9] Maize breeders sort inbred lines into heterotic groups, derive new lines within a group and test them against another. Crosses between groups raised average better-parent heterosis.[9] A parent's mean heterosis across all its crosses is its general combining ability. The parent's heterosis in one particular cross is its specific combining ability.[19]
Hybrids of small populations linked by low to moderate migration can have large positive heterosis, in the model of Whitlock and colleagues. Genetic drift leaves different partially recessive deleterious alleles in different populations, and a cross masks them. Hybrid vigour between two populations is therefore no evidence that they were completely isolated.[11]
Heterosis between populations can be negative and is then sometimes called outbreeding depression. It is attributed mainly to lost favourable epistatic combinations of alleles and not to dominance.[8] Lynch's model of 1991 covers the shift from outbreeding enhancement to outbreeding depression as populations diverge.[20]
Heterosis measured in cannabis
Barcaccia and colleagues wrote in 2020 that heterosis had been important in breeding modern fibre hemp and was still largely unexplored or undocumented in drug cultivars.[21] In 2021 Carlson and colleagues noted a deficiency of published studies on hemp hybrids and left open whether hemp has heterotic groups.[22]
Hemp breeding programmes
In 1965 French breeders evaluated about 150 fibre hemp strains to find parents for F1 cultivars. They crossed the chosen strains with Fibrimon, then abandoned F1 breeding and stabilised the best hybrids by backcrossing into new monoecious cultivars. De Meijer described Hungary in 1994 as the only country where heterosis breeding of hemp had become important. Hungarian hybrids included Kompolti Hybrid TC, a three-way cross of two selections of Chinese origin with Kompolti. An all-female F1 served as the seed parent of these three-way crosses and took the place of male sterility in large-scale seed production.[23]
Russian work on hemp hybrids began in the 1930s and has practically stopped since 2010, because male sterility for crossing monoecious and dioecious varieties remains unsolved. Five of the 33 hemp entries on the Russian variety register in 2024 were F1 hybrids. Hand removal of male plants from the seed parent limits the use of F1 seed.[6] Mishchenko and colleagues wrote in 2025 that no cytoplasmic male sterility has been found in hemp and that heterotic hybrids were absent from production.[24] Hybrid hemp has meanwhile become more common in the United States, since limited legalisation in 2014.[25]
What the trials measured
| Plants | Cross | Compared with | Result |
|---|---|---|---|
| Hemp, seed types | Lines selfed four and six generations, Afina × Ionino | Mid-parent, higher parent, standard variety | Seed mass +28.7, +24.6 and +113.2 per cent in that order; little for height and stem diameter[5] |
| Hemp, fibre types | Lines selfed six and four generations, Hlukhivski 51 × Nika | Mid-parent, higher parent, standard variety | No heterosis for height, stem mass, fibre mass or fibre content; below both parents for most[5] |
| Hemp, dioecious × monoecious, field plots, means for 2000–2015 | Varieties; Glukhovskaya-10 × YSO-1 yielded most | Higher parent | That cross 15.1 c/ha of seed against 10.9, +38.5 per cent. Of all combinations, 66.9 per cent exceeded the higher parent for seed and none for straw or fibre; 57.2 per cent equalled it for straw and 42.8 per cent for fibre[10] |
| Hemp, southern dioecious × central Russian monoecious | Over 100 F1 and backcross combinations | Each parent | F1 seed yield 1.5 to 2.2 times the mothers and 1.5 to 1.7 times the fathers[6] |
| Accessions of varied origin, in pots under controlled light | Five crosses of lines selfed six generations | Inbred parents, mean of the original lines | Larger than inbred parents in four of five; seed yield +3.9 to +155 per cent; biomass above both original lines in two[7] |
| Wife × Wilhelmina, in a greenhouse | Lines selfed one to three generations | Selfed lines of each parent, in separate studies | More plant and flower dry weight than Wife lines, and similar height despite a dwarf parent[26] |
| Two Thai accessions of Hang Kra Rok, under artificial light | Reciprocal crosses, Bangkok and Sakon Nakhon | First selfed generation of each | Taller, about 87 to 89 cm against 75 to 79 cm; flowering about seven days later; no difference in dry inflorescence weight[27] |
In Russian crosses of a dioecious mother with a monoecious father, the F1 is 95 to 99 per cent female and the first backcross to the father has 60 to 70 per cent females. Serkov and Uschapovsky ascribe the higher seed yield to this sexual uniformity together with heterosis. In their own hybrids with a low-yielding mother, the F1 gave 2.1 to 2.5 times her seed yield and the backcross 45 to 95 per cent more than she did.[6] In a second Russian series of the same kind of cross, seed weight per plant exceeded the higher parent in 92.0 per cent of combinations and 1000-seed weight in 65.2 per cent.[10]
In the Ukrainian crosses, heterosis for seed appeared in nearly all kinds of cross, while fibre content mostly fell or stayed intermediate, even between fibre varieties. Earlier crosses of varieties and selfed lines of Hlukhivski 58, Hlesiia and Zolotoniski 15 reached 159.6 per cent over the mid-parent for fibre mass and 220.3 per cent for seed mass per plant, with no heterosis for cannabinoid content.[5] Those papers call a trait inherited by overdominance when its dominance coefficient, calculated from the means of the hybrid and its parents, exceeds 1.[5] Means alone do not separate overdominance at a locus from dominance.[1] Selfed lines of Zolotoniski 15 had high general combining ability for almost all traits in a topcross.[19]
Each F1 line in the 2026 trial by García-de Heer and colleagues has two references: the mean of its direct inbred parents and the mean of the original open-pollinated lines. Only EDA × No. 8 and IPK_36 × No. 8 exceeded both original lines in total biomass, although all but Bern × IPK_36 were larger than their inbred parents. The original lines may already have been heterozygous enough to narrow the gap between them and the hybrids, the authors suggest. All F1 lines flowered within four days of the change to short days, while the original and inbred lines took more than 20. Cannabinoid concentrations mostly fell between those of the parents.[7] McDonald and Lubell-Brand judged it likely that heterosis explained the growth of the Wife × Wilhelmina hybrids.[26] The authors of the Thai trial credit hybridisation between divergent accessions for the extra height and conclude that selfing cost those plants neither yield nor THC.[27]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. pp. 411–414.
- ↑ 2.0 2.1 Felsenstein, J. (December 2019). Theoretical evolutionary genetics. Seattle: Department of Genome Sciences, University of Washington. p. 85.
- ↑ 3.0 3.1 3.2 3.3 Birchler, J.A.; Yao, H.; Chudalayandi, S.; Vaiman, D.; Veitia, R.A. (2010). "Heterosis". The Plant Cell. 22 (7): 2105–2112. doi:10.1105/tpc.110.076133.
- ↑ 4.0 4.1 4.2 Fujimoto, R.; Uezono, K.; Ishikura, S.; Osabe, K.; Peacock, W.J.; Dennis, E.S. (2018). "Recent research on the mechanism of heterosis is important for crop and vegetable breeding systems". Breeding Science. 68 (2): 145–158. doi:10.1270/jsbbs.17155.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Міщенко, С.В.; Марченко, Т.Ю.; Лавриненко, Ю.О.; Ткаченко, С.М. (2022). "Рівень прояву та успадкування селекційних ознак у міжлінійних гібридів конопель насіннєвого та волокнистого напрямів використання" [The level of expression and inheritance of breeding traits in interlinear hybrids of hemp of seed and fiber directions of economic use]. Таврійський науковий вісник (in українська) (125): 84–90. doi:10.32851/2226-0099.2022.125.12.
- ↑ 6.0 6.1 6.2 6.3 6.4 Серков, В.А.; Ущаповский, И.В. (2024). "Селекционная оценка методов гибридизации при создании гибридов конопли посевной (Cannabis sativa L.)" [Selection evaluation of hybridization methods when creating hybrids of Cannabis sativa L.]. Technical Crops. Scientific Agricultural Journal (in русский). 4 (4): 32–42. doi:10.54016/svitok.2024.99.23.005.
- ↑ 7.0 7.1 7.2 Garcia-de Heer, L.; Mieog, J.; Burn, A.; Nolan, M.; Liu, L.; et al. (2026). "Weeding out variability: a proof-of-concept for producing uniform F1 hybrid Cannabis sativa L. using single-seed descent". Horticulture Research. 13 (5) uhag038. doi:10.1093/hr/uhag038. PMC 13188225.
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Labroo, M.R.; Studer, A.J.; Rutkoski, J.E. (2021). "Heterosis and hybrid crop breeding: a multidisciplinary review". Frontiers in Genetics. 12 643761. doi:10.3389/fgene.2021.643761.
- ↑ 9.0 9.1 9.2 9.3 Flint-Garcia, S.A.; Buckler, E.S.; Tiffin, P.; Ersoz, E.; Springer, N.M. (2009). "Heterosis is prevalent for multiple traits in diverse maize germplasm". PLoS ONE. 4 (10) e7433. doi:10.1371/journal.pone.0007433.
- ↑ 10.0 10.1 10.2 Dimitriev, V.L.; Shashkarov, L.G.; Yeliseeva, L.V.; Yeliseev, I.P.; Yakovleva, M.I.; Chernov, A.V. (2022). "Features of the heterosis manifestation in monoecious hybrids of cannabis-free hemp and their resistance to adverse environmental conditions". IOP Conference Series: Earth and Environmental Science. 981 (2) 022013. doi:10.1088/1755-1315/981/2/022013.
- ↑ 11.0 11.1 Whitlock, M.C.; Ingvarsson, P.K.; Hatfield, T. (2000). "Local drift load and the heterosis of interconnected populations". Heredity. 84 (4): 452–457. doi:10.1046/j.1365-2540.2000.00693.x.
- ↑ 12.0 12.1 Yu, S.B.; Li, J.X.; Xu, C.G.; Tan, Y.F.; Gao, Y.J.; Li, X.H.; Zhang, Q.; Maroof, M.A.S. (1997). "Importance of epistasis as the genetic basis of heterosis in an elite rice hybrid". Proceedings of the National Academy of Sciences. 94 (17): 9226–9231. doi:10.1073/pnas.94.17.9226.
- ↑ Krieger, U.; Lippman, Z.B.; Zamir, D. (2010). "The flowering gene SINGLE FLOWER TRUSS drives heterosis for yield in tomato". Nature Genetics. 42 (5): 459–463. doi:10.1038/ng.550.
- ↑ Hua, J.; Xing, Y.; Wu, W.; Xu, C.; Sun, X.; Yu, S.; Zhang, Q. (2003). "Single-locus heterotic effects and dominance by dominance interactions can adequately explain the genetic basis of heterosis in an elite rice hybrid". Proceedings of the National Academy of Sciences. 100 (5): 2574–2579. doi:10.1073/pnas.0437907100.
- ↑ Huang, X.; Yang, S.; Gong, J.; Zhao, Q.; Feng, Q.; et al. (2016). "Genomic architecture of heterosis for yield traits in rice". Nature. 537 (7622): 629–633. doi:10.1038/nature19760.
- ↑ Yang, J.; Mezmouk, S.; Baumgarten, A.; Buckler, E.S.; Guill, K.E.; et al. (2017). "Incomplete dominance of deleterious alleles contributes substantially to trait variation and heterosis in maize". PLOS Genetics. 13 (9) e1007019. doi:10.1371/journal.pgen.1007019. PMC 5633198.
- ↑ Schnable, P.S.; Springer, N.M. (2013). "Progress toward understanding heterosis in crop plants". Annual Review of Plant Biology. 64 (1): 71–88. doi:10.1146/annurev-arplant-042110-103827.
- ↑ Chen, Z.J. (2013). "Genomic and epigenetic insights into the molecular bases of heterosis". Nature Reviews Genetics. 14 (7): 471–482. doi:10.1038/nrg3503.
- ↑ 19.0 19.1 Міщенко, С.В. (2017). "Ефекти загальної та варіанси специфічної комбінаційної здатності самозапилених ліній і сортів конопель у системі топкросів" [Effects of total and variances specific combining ability of hemp inbred lines and varieties in topcross systems]. Faktori eksperimental'noi evolucii organizmiv (in українська). 21: 62–67. doi:10.7124/feeo.v21.808.
- ↑ Lynch, M. (1991). "The genetic interpretation of inbreeding depression and outbreeding depression". Evolution. 45 (3): 622–629. doi:10.1111/j.1558-5646.1991.tb04333.x.
- ↑ 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.
- ↑ 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.
- ↑ de Meijer, E. (1994). Diversity in Cannabis (PhD thesis). Wageningen: Landbouwuniversiteit Wageningen. pp. 8–9. doi:10.18174/207065. ISBN 90-5485-338-7.
- ↑ 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.
- ↑ Baluch, S. (2026). "Advances in hemp breeding". Achieving sustainable cultivation of hemp. Burleigh Dodds Science Publishing. pp. 49–70. doi:10.19103/as.2025.0161.04.
- ↑ 26.0 26.1 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.
- ↑ 27.0 27.1 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.