Trioecy, or subdioecy, is a rare

sexual system A sexual system is a pattern of sex allocation or a distribution of male and female function across organisms in a species. Terms like reproductive system and mating system have also been used as synonyms. The distinction between sexual systems ...

characterized by the coexistence of

males Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...

females Female (symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females ...

, and

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...

s. It has been found in both

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

and

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

s. Trioecy is sometimes referred to as a mixed mating system alongside

androdioecy Androdioecy is a reproductive system characterized by the coexistence of males and hermaphrodites. Androdioecy is rare in comparison with the other major reproductive systems: dioecy, gynodioecy and hermaphroditism. In animals, androdioecy has be ...

and

gynodioecy Gynodioecy is a rare breeding system that is found in certain flowering plant species in which female and hermaphroditic plants coexist within a population. Gynodioecy is the evolutionary intermediate between hermaphroditism (exhibiting both fem ...

Evolution of trioecy

Many speculate trioecy is a transient state and is often associated with evolutionary transitioning from

dioecy Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproducti ...

. Other studies show that trioecious populations originated from

gonochoristic In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism c ...

ancestors which were invaded by a mutant selfing hermaphrodite, creating a trioecious population. It has been suggested that

chromosomal duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene ...

is an important part in the evolution of trioecy.

Evolutionary stability

Trioecy is usually viewed as evolutionarily unstable, but its exact stability is unclear. Like in

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

species trioecy usually breaks into androdioecy or gynodioecy. But one study found that trioecy can be stable under nucleocytoplasmic sex determination. Another theoretical analysis indicates that trioecy could be evolutionary stable in plant species if a large amount of

pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the m ...

s vary geographically.

Occurrence

Trioecy is a relatively common sexual system in plants. Trioecy has been estimated to occur in about 3.6% of

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...

species, although most reports of trioecy could be misinterpretations of

. It is rare as well as poorly understood in animals.

Species that exhibit trioecy

The following species have been observed to exhibit a trioecious breeding system.

Plants

* ''

Buddleja sessiliflora ''Buddleja sessiliflora'', commonly known as Rio Grande butterfly-bush or tepozán, native to southern Arizona and the lower Rio Grande Valley of Texas in the United States as well as much of central and northern Mexico excluding the Chihuahua De ...

'' * '' Buddleja americana'' *''

Coccoloba cereifera ''Coccoloba cereifera'' is a rare species of flowering plant in the knotweed or buckwheat family Polygonaceae. The species is restricted to a single mountain, the Serra do Cipó, in southern Brazil. The species is notable for its expression of a ...

'' *''

Garcinia indica ''Garcinia indica'', a plant in the mangosteen family (Clusiaceae), commonly known as ''kokum'', is a fruit-bearing tree that has culinary, pharmaceutical, and industrial uses. It primarily grows in the Western Ghats, especially the Goa and Konka ...

'' *''

Fraxinus excelsior ''Fraxinus excelsior'', known as the ash, or European ash or common ash to distinguish it from other types of ash, is a flowering plant species in the olive family Oleaceae. It is native throughout mainland Europe east to the Caucasus and Albor ...

'' * ''

Mercurialis annua ''Mercurialis annua'' is a species of flowering plant in the spurge family Euphorbiaceae known by the common name annual mercury or (rarely) French mercury. It is native to Europe, North Africa, and the Middle East but it is known on many other c ...

'' * ''

Opuntia robusta ''Opuntia robusta'', the wheel cactus, nopal tapon, or camuesa, is a species of cactus in the family Cactaceae. It is native and endemic to central and northern Mexico to within of the Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ...

'' * '' Pachycereus pringlei''

Animals

* '' Aiptasia diaphana'' *'' Auanema rhodensis'' *'' Auanema freiburgensis'' *'' Hydra viridissima'' *'' Thor manningi'' *'' Semimytilus algosus'' Pacific mussel

References

{{Reflist Reproductive system Fertility Sex Sexual system