Dioecy
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Dioecy is the condition in which male and female flowers are borne on separate individuals, so that any one plant is either pollen-bearing (staminate) or seed-bearing (pistillate). Cannabis is normally dioecious: a crop raised from seed separates into male and female plants in roughly equal numbers.[1] The two sexes differ in the structure of their flowers and, less sharply, in growth habit.[2] Dioecy is the ancestral and prevailing condition in the family Cannabaceae, shared with the sister genus Humulus, the hops; a minority of cultivated hemp lines are instead monoecious, carrying both kinds of flower on one plant.[3][4]
The separation of the sexes shapes how cannabis is grown and how its populations are structured. Because cannabinoids accumulate mainly in the resin of the unfertilised female inflorescence, drug-type and cannabinoid crops are grown as female-only stands.[2] Excluding pollen is central to their cultivation, since a pollinated inflorescence sets seed in place of resin.[5] Dioecy also enforces outcrossing: no plant can fertilise itself, so every seed descends from two separate parents, a mating system that maintains genetic variation within a population.[2][1]
Chromosomal sex in cannabis is set by a pair of X and Y chromosomes. The genetic and molecular detail of that system is treated in Sex determination in cannabis. This article concerns dioecy as a reproductive system: the form and behaviour of male and female plants, the ratio in which the sexes occur, the monoecious exception and the consequences of separate sexes for breeding and for wild and landrace populations.
Dioecy in cannabis
In its wild and most of its cultivated forms cannabis is dioecious. The sexes cannot be told apart during vegetative growth and diverge only at flowering, when the distinct male and female flowers appear.[1]
Staminate and pistillate plants
Staminate plants bear their flowers in loose, branched clusters. Each male flower has five tepals and five pendant anthers that split to release pollen, which is carried on the wind.[1] Pistillate plants carry their flowers singly in the axils of small leaves and bracts, each flower reduced to an ovary wrapped in a bract and topped by two long stigmas that catch airborne pollen.[1] Males are commonly taller and more sparsely branched, flower slightly earlier and die back after shedding pollen, while females stay in flower for longer and, if no pollen reaches them, continue to produce resin instead of setting seed.[1][2] Because the sexes are alike until flowering, a grower cannot separate them during vegetative growth; in a female-only crop the males are identified and removed once their flowers appear, a routine known as roguing.[2]
Monoecy and continuous sex expression
The main departure from dioecy is found in cultivated hemp. Monoecious hemp cultivars carry both male and female flowers on the same plant, a trait bred into fibre and grain lines so that a crop flowers and ripens evenly and can be harvested in a single pass.[4][5] Monoecy in cannabis is not a separate chromosomal sex. Cytogenetic and marker studies show that monoecious cultivars carry the female XX chromosome constitution and lack a Y, so their bisexual flowering is a genetic modification acting on an otherwise female plant.[6][4] Monoecy is valued in fibre and grain hemp, where a crop of uniform flowering behaviour ripens together and needs no removal of males, which suits mechanised harvest.[5]
Monoecious hemp shows a graded range of sex expression: some plants are almost entirely male, some evenly mixed and others almost entirely female.[4] Breeders score this on a scale and cultivars differ in their average position on it, so monoecy behaves as a heritable, quantitative character.[4] Because expression grades continuously and is only partly heritable, monoecious lines are less stable than dioecious ones and throw a proportion of strongly male or female plants that breeders select against.[4] Dioecious plants can also stray across the divide: under stress or hormone treatment a plant of one sex may form a few flowers of the other, whether a single hermaphroditic flower or the full reversals used in seed production. Reviews describe the sexual system of cannabis as mixed and modifiable, under both genetic and environmental control.[7][8]
Sex ratios
A cross between an XX female and an XY male predicts a primary sex ratio near one to one, the even split expected of such a cross. Cannabis approaches this ratio when nothing disturbs it.[3][8] Departures from equal numbers in a crop arise mainly from the lability of sex expression, not from any distortion of chromosome segregation: conditions during early growth, among them photoperiod, nutrient supply and stress, can bias the flowers a stand produces toward one sex.[7][1] Reported ratios in cultivation and in wild stands vary accordingly. Reviews treat the balance of the sexes as an outcome of genetic and environmental control acting together.[7] In a drug-type crop the aim is to remove the males altogether, so the sex ratio of a seed batch is a practical concern. It is one reason growers turn to feminised seed or to clonal propagation, both of which avoid the segregation of the sexes.[2]
Environmental sex reversal
A plant's chromosomal sex and the sex of the flowers it produces can be uncoupled. Hormonal, nutritional and environmental signals shift expressed sex without changing the chromosomes.[8] The commitment to a male or female pathway is made early, before any flower is visible, so a plant's eventual sex can be redirected during that early window.[9] This lability is put to direct use in seed production. Treating a genetically female plant with a silver compound makes it form functional male flowers and shed pollen; because that pollen carries only X chromosomes, crossing it onto another female yields seed that is entirely female, the basis of feminised seed.[10][8] The same responsiveness of expressed sex to growing conditions is one instance of the plant's broader phenotypic plasticity, so the sex a cannabis plant shows is not a wholly reliable guide to the chromosomes it carries.[7]
Genetic basis of sex
Sex in dioecious cannabis is carried on a pair of heteromorphic sex chromosomes: females are XX and males XY, so the male is the heterogametic sex; monoecious cultivars, being XX, carry no Y.[3] How the chromosomes specify sex is not settled: the literature discusses both a dominant male-determining Y and a balance between the X chromosomes and the autosomes, with cannabis showing features cited for each.[3][4] The sex chromosomes were localised to a single chromosome pair in 2020.[11] The genetics and molecular biology of the system, including the structure and evolution of the chromosomes and the markers used to read them, are set out in Sex determination in cannabis.
Ecology and evolution of dioecy
Dioecy occurs in only about six per cent of flowering-plant species and has arisen independently many times; it is generally understood as one of the ways a plant lineage enforces outcrossing.[12] In cannabis the same effect follows from the separation of the sexes: because a plant cannot pollinate itself, matings are obligate crosses, which keeps populations genetically diverse and heterozygous.[2][1] Pollen is carried on the wind and can travel long distances, so a female stand is difficult to isolate from unwanted pollen and gene flow between neighbouring populations and crops is easy.[1][2]
These properties matter for the maintenance of landrace and wild populations. A dioecious landrace cannot be reproduced from a single plant. Because pollen moves so freely, a traditional population grown near hybrid crops is readily contaminated by their pollen.[2][13] Conserving such populations therefore means sampling both sexes and enough individuals to hold their effective population size and standing diversity, whether in the field or in an ex situ collection.[13][2]
Separate sexes carry a cost, since only the female plants of a stand set seed, yet the outcrossing that dioecy enforces keeps populations genetically variable and prevents self-fertilisation.[12][2] The same feature makes uniform types harder to fix in cultivation, one reason fibre and grain hemp have been bred toward monoecy and drug cannabis toward feminised, clonally propagated lines.[5][2]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Small, E.; Pocock, T.; Cavers, P.B. (2003). "The biology of Canadian weeds. 119. Cannabis sativa L.". Canadian Journal of Plant Science. 83 (1): 217–237. doi:10.4141/P02-021.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Clarke, R.C.; Merlin, M.D. (2013). Cannabis: Evolution and Ethnobotany. University of California Press. ISBN 978-0-520-27048-0.
- ↑ 3.0 3.1 3.2 3.3 Divashuk, M.G.; Alexandrov, O.S.; Razumova, O.V.; Kirov, I.V.; Karlov, G.I. (2014). "Molecular cytogenetic characterization of the dioecious Cannabis sativa with an XY chromosome sex determination system". PLoS ONE. 9 (1) e85118. doi:10.1371/journal.pone.0085118.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Faux, A.-M.; Berhin, A.; Dauguet, N.; Bertin, P. (2014). "Sex chromosomes and quantitative sex expression in monoecious hemp (Cannabis sativa L.)". Euphytica. 196 (2): 183–197. doi:10.1007/s10681-013-1023-y.
- ↑ 5.0 5.1 5.2 5.3 Small, E. (2015). "Evolution and classification of Cannabis sativa (marijuana, hemp) in relation to human utilization". The Botanical Review. 81 (3): 189–294. doi:10.1007/s12229-015-9157-3.
- ↑ Razumova, O.V.; Alexandrov, O.S.; Divashuk, M.G.; Sukhorada, T.I.; Karlov, G.I. (2016). "Molecular cytogenetic analysis of monoecious hemp (Cannabis sativa L.) cultivars reveals its karyotype variations and sex chromosomes constitution". Protoplasma. 253 (3): 895–901. doi:10.1007/s00709-015-0851-0.
- ↑ 7.0 7.1 7.2 7.3 Baek, Y.; Vergara, D. (2025). "A review of sexual strategies in Cannabis sativa L. under genomic and environmental controls". Agrosystems, Geosciences & Environment. 8 (1) e70050. doi:10.1002/agg2.70050.
- ↑ 8.0 8.1 8.2 8.3 Owen, L.C.; Suchoff, D.H.; Chen, H. (2023). "A novel method for stimulating Cannabis sativa L. male flowers from female plants". Plants. 12 (19) 3371. doi:10.3390/plants12193371.
- ↑ Moliterni, V.M.C.; Cattivelli, L.; Ranalli, P.; Mandolino, G. (2004). "The sexual differentiation of Cannabis sativa L.: a morphological and molecular study". Euphytica. 140 (1–2): 95–106. doi:10.1007/s10681-004-4758-7.
- ↑ Mohan Ram, H.Y.; Sett, R. (1982). "Induction of fertile male flowers in genetically female Cannabis sativa plants by silver nitrate and silver thiosulphate anionic complex". Theoretical and Applied Genetics. 62 (4): 369–375. doi:10.1007/BF00275107.
- ↑ Prentout, D.; Razumova, O.; Rhoné, B.; Badouin, H.; Henri, H.; Feng, C.; Käfer, J.; Karlov, G.; Marais, G.A.B. (2020). "An efficient RNA-seq-based segregation analysis identifies the sex chromosomes of Cannabis sativa". Genome Research. 30 (2): 164–172. doi:10.1101/gr.251207.119.
- ↑ 12.0 12.1 Renner, S.S. (2014). "The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database". American Journal of Botany. 101 (10): 1588–1596. doi:10.3732/ajb.1400196.
- ↑ 13.0 13.1 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.