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Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female , either directly (in animals) or indirectly (in Spermatophyte). Dioecious reproduction is biparental reproduction. Dioecy has costs, since only the female part of the population directly produces offspring. It is one method for excluding self-fertilization and promoting allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Plants have several other methods of preventing self-fertilization including, for example, dichogamy, herkogamy, and self-incompatibility.
In (, Marchantiophyta and ), the gametophytes are fully independent plants. Seed plant gametophytes are dependent on the sporophyte and develop within the spores, a condition known as endospory. In flowering plants, the male gametophytes develop within pollen grains produced by the sporophyte's , and the female gametophytes develop within produced by the sporophyte's carpels.
The sporophyte generation of a seed plant is called "monoecious" when each sporophyte plant has both kinds of spore-producing organ but in separate flowers or cones. For example, a single flowering plant of a monoecious species has both functional stamens and carpels, in separate flowers.
The sporophyte generation of seed plants is called dioecious when each sporophyte plant has only one kind of spore-producing organ, all of whose spores give rise either to male gametophytes, which produce only male gametes (sperm), or to female gametophytes, which produce only female gametes (egg cells). For example, a single flowering plant sporophyte of a fully dioecious species like Ilex aquifolium has either flowers with functional stamens producing pollen containing male gametes (staminate or 'male' flowers), or flowers with functional carpels producing female gametes (carpellate or 'female' flowers), but not both. There are other, more complex reproductive schemes such as gynodioecy and androdioecy.
Slightly different terms, dioicous and monoicous, may be used for the gametophyte generation of non-vascular plants, although dioecious and monoecious are also used. A dioicous gametophyte either produces only male gametes (sperm) or produces only female gametes (egg cells). About 60% of liverworts are dioicous. (2025). 9780521700733, Cambridge University Press. ISBN 9780521700733
Dioecy occurs in a wide variety of plant groups. Examples of dioecious plant species include , , cannabis and Milicia excelsa. As its specific name implies, the perennial stinging nettle Urtica dioica is dioecious, while the annual nettle Urtica urens is monoecious. Dioecious flora are predominant in Tropics environments.
About 65% of gymnosperm species are dioecious, but almost all conifers are monoecious. In gymnosperms, the sexual systems dioecy and monoecy are strongly correlated with the mode of pollen dispersal, monoecious species are predominantly wind dispersed (anemophily) and dioecious species animal-dispersed (zoophily).
About 6 percent of Angiosperm species are entirely dioecious and about 7% of angiosperm Genus contain some dioecious species. Dioecy is more common in , and heterotrophic species. In most dioecious plants, whether male or female gametophytes are produced is determined genetically, but in some cases it can be determined by the environment, as in Arisaema species.
Certain algae, such as some species of Polysiphonia, are dioecious.Maggs, C.A. and Hommersand, M.H. 1993. Seaweeds of the British Isles Volume 1 Rhodophyta Part 3A Ceramiales. The Natural History Museum, London. Dioecy is prevalent in the brown algae (Phaeophyceae) and may have been the ancestral state in that group.
Dioecy evolves due to male or female sterility, although it is unlikely that mutations for male and female sterility occurred at the same time. In angiosperms unisexual flowers evolve from bisexual ones. Dioecy occurs in almost half of plant families, but only in a minority of genera, suggesting recent evolution. For 160 families that have dioecious species, dioecy is thought to have evolved more than 100 times.
In the family Caricaceae, dioecy is likely the ancestral sexual system.
In the genus Sagittaria, since there is a distribution of sexual systems, it has been postulated that dioecy evolved from monoecy through gynodioecy mainly from mutations that resulted in male sterility. However, since the ancestral state is unclear, more work is needed to clarify the evolution of dioecy via monoecy.
In Silene, since there is no monoecy, it is suggested that dioecy evolved through gynodioecy.
Monoecy and dioecy in fungus refer to the donor and recipient roles in mating, where a nucleus is transferred from one haploid hypha to another, and the two nuclei then present in the same cell merge by karyogamy to form a zygote. The definition avoids reference to male and female reproductive structures, which are rare in fungi. An individual of a dioecious fungal species not only requires a partner for mating, but performs only one of the roles in nuclear transfer, as either the donor or the recipient. A monoecious fungal species can perform both roles, but may not be self-compatible.
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