Dioecy

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Dioecy (Greek: διοικία "two households"; adjective form: dioecious) is a characteristic of a species, meaning that it has distinct individual organisms that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is biparental reproduction. Dioecy has costs, since only about half the population directly produces offspring. It is one method that excludes self-fertilization and promotes 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.

Contents

Dioecy is a dimorphic sexual system alongside gynodioecy, and androdioecy. [1]

In zoology

Physalia physalis, Portuguese man o' war, is a dioecious colonial marine animal; the reproductive medusae within the colony are all of the same sex. Physalia.jpg
Physalia physalis, Portuguese man o' war, is a dioecious colonial marine animal; the reproductive medusae within the colony are all of the same sex.

In zoology, dioecious species may be opposed to hermaphroditic species, meaning that an individual is either male or female, in which case the synonym gonochory is more often used. [3] [ page needed ] Most animal species are dioecious (gonochoric). [4] Dioecy may also describe colonies within a species, such as the colonies of Siphonophorae (Portuguese man-of-war), which may be either dioecious or monoecious. [5]

In botany

Land plants (embryophytes) differ from animals in that their life cycle involves alternation of generations. In animals, typically an individual produces haploid gametes of one kind, either sperms or egg cells. A sperm and an egg cell fuse to form a zygote that develops into a new individual. In land plants, by contrast, one generation – the sporophyte generation – consists of individuals that produce haploid spores rather than haploid gametes. Spores do not fuse, but germinate by dividing repeatedly by mitosis to give rise to haploid multicellular individuals which produce gametes – the gametophyte generation. A male gamete and a female gamete then fuse to produce a new diploid sporophyte. [6]

In bryophytes (mosses, liverworts and hornworts), the gametophytes are fully independent plants, and produce spores of only one type (isospores). [7] Seed plants (spermatophytes), are heterosporic, producing spores of two different sizes (heterospores). [8] 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 stamens, and the female gametophytes develop within ovules produced by the sporophyte's carpels. [6]

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. Alternation of generations simpler still.svg
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.

The sporophyte generation of a seed plant is called "monoecious" when each sporophyte plant has both kinds of spore-producing organ, so ultimately produces both male and female gametophytes and hence both male and female gametes. For example, a single flowering plant of a monoecious species has both functional stamens and carpels, either in separate flowers [9] or in the same flower. [10]

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 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. [9] [11] (See Plant reproductive morphology for further details, including more complex cases.)

In dioecious holly, some plants only have male flowers that produce pollen. Ilex aquifolium male HC1.JPG
In dioecious holly, some plants only have male flowers that produce pollen.
Other holly plants only have female flowers that produce ovules. Ilex aquifolium female HC1.JPG
Other holly plants only have female flowers that produce ovules.

Slightly different terms, dioicous and monoicous , may be used for the gametophyte generation, although dioecious and monoecious 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 include ginkgos, willows, cannabis and African teak. As its specific name implies, the perennial stinging nettle Urtica dioica is dioecious, [15] :305 while the annual nettle Urtica urens is monoecious. [15] :305 Dioecious flora are predominant in tropical environments. [16]

About 65% of gymnosperm 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 of flowering plant species are entirely dioecious and about 7% of angiosperm genera contain some dioecious species. [20] Dioecy is more common in woody plants, [21] and heterotrophic 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 in Arisaema species. [23]

Certain algae are dioecious.[ clarification needed ] [24] Dioecy is prevalent in the brown algae (Phaeophyceae) and may have been the ancestral state in that group. [25]

Evolution of dioecy

In plants, dioecious species generally evolve either from hermaphroditic species or from monoecious species with a previously untested hypothesis stating that this is in order to escape inbreeding. [26] However dioecy has been shown to be associated with increased genetic diversity and greater protection against deleterious mutations. [27] Regardless of the evolutionary pathway the intermediate states need to have fitness advantages compared to homostylous cosexual flowers in order to survive. [28]

Dioecy evolves due to male or female sterility, [29] although it is unlikely that mutations for male and female sterility occurred at the same time. [30] In angiosperms unisexual flowers evolve from bisexual ones. [31] Dioecy occurs in almost half of plant families, but only in a minority of genera, suggesting recent evolution. [32]

From monoecy

Dioecious flowering plants can evolve from monoecious ancestors that have flowers containing both functional stamens and functional carpels. [33] In the genus Sagittaria , since there is a distribution of sexual systems, it has been postulated that dioecy evolved from monoecy [34] through gynodioecy mainly from mutations that resulted in male sterility. [35] :478 However, since the ancestral state is unclear, more work is needed to clarify the evolution of dioecy via monoecy. [35] :478

From hermaphroditism

Dioecy usually evolves from hermaphroditism through gynodioecy but may also evolve through androdioecy, [36] through distyly [37] or through heterostyly. [27] In the Asteraceae, dioecy may have evolved 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. [38]

In mycology

Very few dioecious fungi have been discovered. [39]

Monoecy and dioecy in fungi 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. [40] The definition avoids reference to male and female reproductive structures, which are rare in fungi. [40] 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. [40]

Adaptive benefit

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 excludes self-fertilization and promotes allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. [41] 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. [42] Dioecy may also accelerate or retard lineage diversification in angiosperms. 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. [43]

See also

Related Research Articles

Gametophyte Haploid stage in the life cycle of plants and algae

A gametophyte is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The sporophyte can produce haploid spores by meiosis that on germination produce a new generation of gametophytes.

Sex Trait that determines an individuals reproductive function, male or female, in sexually reproducing organisms

Sex is a trait that determines an individual's reproductive function, male or female, in animals and plants that propagate their species through sexual reproduction. The type of gametes produced by an organism define its sex. Commonly in plants and animals, male organisms produce smaller gametes while female organisms produce larger gametes. Organisms that produce both types of gametes are called hermaphrodites. During sexual reproduction, male and female gametes fuse to form zygotes that develop into offspring that inherit a selection of the traits of each parent.

Spore Unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavorable conditions. Spores form part of the life cycles of many plants, algae, fungi and protozoa

In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, fungi and protozoa.

Alternation of generations Reproductive cycle of plants and algae

Alternation of generations is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct haploid sexual and diploid asexual stages. In these groups, a multicellular haploid gametophyte with n chromosomes alternates with a multicellular diploid sporophyte with 2n chromosomes, made up of n pairs. A mature sporophyte produces haploid spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n.

Gymnosperm Clade of non-flowering, naked-seeded vascular plants

The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and gnetophytes. The term gymnosperm comes from the composite word in Greek: γυμνόσπερμος, literally meaning 'naked seeds'. The name is based on the unenclosed condition of their seeds. The non-encased condition of their seeds contrasts with the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones, or solitary as in yew, Torreya, Ginkgo.

Bryophyte Terrestrial plants that lack vascular tissue

Bryophytes are a hypothetical taxonomic division containing three groups of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses. They are characteristically limited in size and prefer moist habitats although they can survive in drier environments. The bryophytes consist of about 20,000 plant species. Bryophytes produce enclosed reproductive structures, but they do not produce flowers or seeds. They reproduce via spores. Though bryophytes were considered a paraphyletic group in recent years, almost all of the most recent phylogenetic evidence supports the monophyly of this group, as originally classified by Wilhelm Schimper in 1879. The term "bryophyte" comes from Greek βρύον, bryon "tree-moss, oyster-green" and φυτόν, phyton "plant".

Biological life cycle Life cycle of living species

In biology, a biological life cycle is a series of changes in form that an organism undergoes, returning to the starting state. "The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.

Sporophyte Diploid multicellular stage in the life cycle of a plant or alga

A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. All land plants, and most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase. In the seed plants, the largest groups of which are the gymnosperms and flowering plants (angiosperms), the sporophyte phase is more prominent than the gametophyte, and is the familiar green plant with its roots, stem, leaves and cones or flowers. In flowering plants the gametophytes are very reduced in size, and are represented by the germinated pollen and the embryo sac.

Plant reproductive morphology Study of the physical form and structure (the morphology) of those parts of plants directly or indirectly concerned with sexual reproduction

Plant reproductive morphology is the study of the physical form and structure of those parts of plants directly or indirectly concerned with sexual reproduction.

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.

Pteridophyte Paraphyletic group of spore-bearing vascular plants

A pteridophyte is a vascular plant that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden. Ferns, horsetails, and lycophytes are all pteridophytes. However, they do not form a monophyletic group because ferns are more closely related to seed plants than to lycophytes. "Pteridophyta" is thus no longer a widely accepted taxon, but the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, respectively. Ferns and lycophytes share a life cycle and are often collectively treated or studied, for example by the International Association of Pteridologists and the Pteridophyte Phylogeny Group.

Monoicy is a sexual system in haploid plants where both sperm and eggs are produced on the same gametophyte, in contrast with dioicy, where gametophytes produce only sperm or eggs but never both. Both monoicous and dioicous gametophytes produce gametes in gametangia by mitosis rather than meiosis, so that sperm and eggs are genetically identical with their parent gametophyte.

Prothallus Gametophyte stage in the fern life cycle

A prothallus, or prothallium, is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well.

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from the parent or parents. Asexual reproduction produces new individuals without the fusion of gametes, genetically identical to the parent plants and each other, except when mutations occur.

Environmental sex determination

Environmental sex determination is the establishment of sex by a non-genetic cue, such as nutrient availability, experienced within a discrete period after fertilization. Environmental factors which often influence sex determination during development or sexual maturation include light intensity and photoperiod, temperature, nutrient availability, and pheromones emitted by surrounding plants or animals. This is in contrast to genotypic sex determination, which establishes sex at fertilization by genetic factors such as sex chromosomes. Under true environmental sex determination, once sex is determined, it is fixed and cannot be switched again. Environmental sex determination is different from some forms of sequential hermaphroditism in which the sex is determined flexibly after fertilization throughout the organism’s life.

Heterospory The production of spores of two different sizes and sexes by several groups of land plants

Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation.

Sexual reproduction Reproduction process that creates a new organism by combining the genetic material of two organisms

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete with a single set of chromosomes (haploid) combines with another to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid). Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction does not occur in prokaryotes, but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to sexual reproduction in early eukaryotes.

Monoecy

Monoecy is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system alongside gynomonoecy, andromonoecy and trimonoecy.

A sexual system is a pattern of sex allocation or a distribution of male and female function across organisms in a species. The borders between sexual systems are not always clear due to phenotypic plasticity. Sexual systems are viewed as a key factor for genetic variation and reproductive success. Sexual systems may have also lead to the origin or extinction of certain species.

Dioicy is a sexual system where archegonia and antheridia are produced on separate gametophytes. It is one of the two main sexual systems in bryophytes. One biologist stated there is a good reason for the distinction between dioecy and dioicy. Others have stated that the term dioecy is meaningless for bryophytes. Nonetheless dioecy and dioicy are comparable in many respects.

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Bibliography