Dioecy (/daɪˈiːsi/dy-EE-see;[1] from Ancient Greekδιοικίαdioikía'two households'; adj.dioecious,/daɪˈiːʃ(i)əs/dy-EE-sh(ee-)əs)[2][3] is a characteristic of certainspecies that have distinct unisexual individuals, each producing either male or femalegametes, either directly (in animals) or indirectly (inseed plants).Dioecious reproduction is biparental reproduction. Dioecy has costs, since only the female part of the population directly produces offspring. It is one method for excludingself-fertilization and promotingallogamy (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, andself-incompatibility.
In zoology, dioecy means that an animal is either male or female, in which case the synonymgonochory is more often used.[5][page needed] Most animal species are gonochoric, almost all vertebrate species are gonochoric, and all bird and mammal species are gonochoric.[6] Dioecy may also describe colonies within ananimal species, such as the colonies ofSiphonophorae (Portuguese man-of-war), which may be either dioecious ormonoecious.[7]
Land plants (embryophytes) differ from animals in that theirlife cycle involvesalternation of generations. In animals, typically an individual producesgametes of one kind, eithersperm oregg cells. The gametes have half the number ofchromosomes of the individual producing them, so arehaploid. Without further dividing, a sperm and an egg cell fuse to form azygote that develops into a new individual. In land plants, by contrast, one generation – thesporophyte generation – consists of individuals that produce haploidspores rather than haploidgametes. Spores do not fuse, butgerminate by dividing repeatedly bymitosis to give rise to haploidmulticellular individuals, thegametophytes, which produce gametes. A male gamete and a female gamete then fuse to produce a newdiploid sporophyte.[8]
Alternation of generations in plants: the sporophyte generation produces spores that give rise to the gametophyte generation, which produces gametes that fuse to give rise to a new sporophyte generation.
Inbryophytes (mosses,liverworts andhornworts), the gametophytes are fully independent plants.[9] Seed plant gametophytes are dependent on the sporophyte and develop within the spores, a condition known asendospory. In flowering plants, the male gametophytes develop withinpollen grains produced by the sporophyte'sstamens, and the female gametophytes develop withinovules produced by the sporophyte'scarpels.[8]
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 singleflowering plant of a monoecious species has both functional stamens and carpels, in separate flowers.[10]
The sporophyte generation of seed plants is calleddioecious 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 likeholly 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.[10][11] There are other, more complexreproductive schemes such asgynodioecy andandrodioecy.
Slightly different terms,dioicous andmonoicous, may be used for thegametophyte generation of non-vascular plants, althoughdioecious andmonoecious are also used.[12][13] A dioicous gametophyte either produces only male gametes (sperm) or produces only female gametes (egg cells). About 60% of liverworts are dioicous.[14]: 52
Dioecy occurs in a wide variety of plant groups. Examples of dioecious plant species includeginkgos,willows,cannabis andAfrican teak. As its specific name implies, the perennial stinging nettleUrtica dioica is dioecious,[15]: 305 while the annual nettleUrtica urens is monoecious.[15]: 305 Dioeciousflora are predominant intropical environments.[16]
About 65% ofgymnosperm species are dioecious,[17] but almost all conifers are monoecious.[18]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).[19]
About 6 percent offlowering plant species are entirely dioecious and about 7% of angiospermgenera contain some dioecious species.[20] Dioecy is more common inwoody plants,[21] andheterotrophic species.[22] 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 inArisaema species.[23]
In dioecious holly, some plants only have 'male' flowers with stamens producing pollen.
Other holly plants only have 'female' flowers that produce ovules.
Each bisexual (perfect) tulip flower has both stamens and carpels.
Certainalgae, such as some species ofPolysiphonia, are dioecious.[24] Dioecy is prevalent in the brown algae (Phaeophyceae) and may have been the ancestral state in that group.[25]
In plants, dioecy hasevolved independently multiple times[26] either from hermaphroditic species or from monoecious species. A previously untested hypothesis is that this reduces inbreeding;[27] dioecy has been shown to be associated with increasedgenetic diversity and greater protection against deleterious mutations.[28] Regardless of the evolutionary pathway the intermediate states need to have fitness advantages compared to cosexual flowers in order to survive.[29]
Dioecy evolves due to male or female sterility,[30] although it is unlikely that mutations for male and female sterility occurred at the same time.[31] In angiosperms unisexual flowers evolve from bisexual ones.[32] Dioecy occurs in almost half of plant families, but only in a minority of genera, suggesting recent evolution.[33] For 160 families that have dioecious species, dioecy is thought to have evolved more than 100 times.[34]
In the familyCaricaceae, dioecy is likely the ancestral sexual system.[35]
Dioecious flowering plants can evolve frommonoecious ancestors that have flowers containing both functional stamens and functional carpels.[36] Some authors argue monoecy and dioecy are related.[37]
In the genusSagittaria, since there is a distribution of sexual systems, it has been postulated that dioecy evolved from monoecy[38] throughgynodioecy mainly from mutations that resulted in male sterility.[39]: 478 However, since the ancestral state is unclear, more work is needed to clarify the evolution of dioecy via monoecy.[39]: 478
Dioecy usually evolves fromhermaphroditism throughgynodioecy but may also evolve throughandrodioecy,[40] throughdistyly[41] or throughheterostyly.[28] In theAsteraceae, dioecy may haveevolved independently from hermaphroditism at least 5 or 9 times. The reverse transition, from dioecy back to hermaphroditism has also been observed, both in Asteraceae and in bryophytes, with a frequency about half of that for the forward transition.[42]
InSilene, since there is no monoecy, it is suggested that dioecy evolved through gynodioecy.[43]
Very few dioecious fungi have been discovered.[44]
Monoecy and dioecy infungi 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 bykaryogamy to form azygote.[45] The definition avoids reference to male and female reproductive structures, which are rare in fungi.[45] 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.[45]
Dioecy has the demographic disadvantage compared with hermaphroditism that only about half of reproductive adults are able to produce offspring. Dioecious species must therefore have fitness advantages to compensate for this cost through increased survival, growth, or reproduction. Dioecy excludesself-fertilization and promotesallogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population.[46] In trees, compensation is realized mainly through increased seed production by females. This in turn is facilitated by a lower contribution of reproduction to population growth, which results in no demonstrable net costs of having males in the population compared to being hermaphroditic.[47] Dioecy may also accelerate or retard lineage diversification inangiosperms. Dioecious lineages are more diversified in certain genera, but less in others. An analysis suggested that dioecy neither consistently places a strong brake on diversification, nor strongly drives it.[48]
^Dunn, C.W.; Pugh, P.R.;Haddock, S.H.D. (2005). "Molecular Phylogenetics of the Siphonophora (Cnidaria), with Implications for the Evolution of Functional Specialization".Systematic Biology.54 (6):916–935.doi:10.1080/10635150500354837.PMID16338764.
^Maggs, C.A. and Hommersand, M.H. 1993.Seaweeds of the British Isles Volume 1 Rhodophyta Part 3A Ceramiales. The Natural History Museum, London.ISBN0-11-310045-0
^LuthringerR, Cormier A, Ahmed S, Peters AF, Cock JM, Coelho, SM (2014). "Sexual dimorphism in the brown algae".Perspectives in Phycology.1 (1):11–25.doi:10.1127/2198-011X/2014/0002.
^abcEsser, K. (1971). "Breeding systems in fungi and their significance for genetic recombination".Molecular and General Genetics.110 (1):86–100.doi:10.1007/bf00276051.PMID5102399.S2CID11353336.
^Bruijning, Marjolein; Visser, Marco D.; Muller-Landau, Helene C.; Wright, S. Joseph; Comita, Liza S.; Hubbell, Stephen P.; de Kroon, Hans; Jongejans, Eelke (2017). "Surviving in a Cosexual World: A Cost-Benefit Analysis of Dioecy in Tropical Trees".The American Naturalist.189 (3):297–314.Bibcode:2017ANat..189..297B.doi:10.1086/690137.hdl:2066/168955.ISSN0003-0147.PMID28221824.S2CID6839285.
