Monoicy () is a

sexual system A sexual system is a distribution of male and female functions across organisms in a species. The terms reproductive system and mating system have also been used as synonyms. Sexual systems play a key role in genetic variation and reproductive ...

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

plants (mainly

bryophyte Bryophytes () are a group of embryophyte, land plants (embryophytes), sometimes treated as a taxonomic Division (taxonomy), division referred to as Bryophyta ''Sensu#Common qualifiers, sensu lato'', that contains three groups of non-vascular pla ...

s) where both sperm and eggs are produced on the same

gametophyte 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 se ...

, in contrast with dioicy, where each gametophyte produces only sperm or eggs but never both.Crandall-Stotler, B.J. & Bartholomew-Began, S.E. (2007). ''Morphology of Mosses (Phylum Bryophyta)''. In: Flora of North America Editorial Committee, eds. (1993+). ''Flora of North America North of Mexico''. 16+ vols. New York and Oxford. Volume 27, 2007.Bell, P.R. & Helmsley, A.R. (2000). ''Green plants, their origin and diversity'' (2nd ed.). Cambridge University Press. Both monoicous () and

dioicous Dioicy () is a sexual system in non-vascular plants where archegonia (female organs) and antheridia (male organs) are produced on separate plants in the gametophyte phase. It is one of the two main sexual systems in bryophytes, the other being ...

gametophytes produce gametes in gametangia by

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

rather than

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

, so that sperm and eggs are genetically identical with their parent gametophyte. It has been suggested that monoicy may have benefits in dry habitats where the ability to produce sporophytes is limited due to lack of water. Monoicy is similar to, and often conflated with,

monoecy Monoecy (; adj. monoecious ) 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 comparable with gynomonoecy, andromonoecy and trimonoecy, and con ...

, which applies to seed plants (

spermatophytes A seed plant or spermatophyte (; New Latin ''spermat-'' and Greek ' (phytón), plant), also known as a phanerogam (taxon Phanerogamae) or a phaenogam (taxon Phaenogamae), is any plant that produces seeds. It is a category of embryophyte (i.e. la ...

) and refers to separate male and female cones or flowers on the same plant.

Etymology and history

The word monoicous and the related forms mon(o)ecious are derived from the Greek mόνος (''mónos''), ''single'', and οἶκος (''oîkos'') or οἰκία (''oikía''), ''house''. The words dioicous and di(o)ecious are derived from οἶκος or οἰκία and δι- (di-), twice, double. (''(o)e'' is the Latin way of transliterating Greek οι, whereas ''oi'' is a more straightforward modern way.) Generally, the terms "monoicous" and "dioicous" have been restricted to description of haploid sexuality (

gametophytic 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 sex ...

sexuality), and are thus used primarily to describe

bryophytes Bryophytes () are a group of land plants ( embryophytes), sometimes treated as a taxonomic division referred to as Bryophyta '' sensu lato'', that contains three groups of non-vascular land plants: the liverworts, hornworts, and mosses. In t ...

in which the gametophyte is the dominant generation. Meanwhile, "monoecious" and "dioecious" are used to describe diploid sexuality ( sporophytic sexuality), and thus are used to describe

tracheophytes Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue ( ...

(vascular plants) in which the

sporophyte A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...

is the dominant generation. However, this usage, although precise, is not universal, and "monoecious" and "dioecious" are still used by some bryologists for the gametophyte.

Occurrence

40% of mosses are monoicious.

Bryophyte sexuality

Bryophytes have life cycles that are

dominated. The longer lived, more prominent

autotrophic An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs produce complex organic compounds (such as carbohydrates, fats, and proteins) us ...

plant is the gametophyte. The sporophyte in mosses and liverworts consists of an unbranched stalk (a seta) bearing a single

sporangium A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

or spore-producing capsule. Even when capable of photosynthesis, as in mosses and hornworts, bryophyte sporophytes require additional photosynthate from the gametophyte to sustain growth and spore development and are dependent on the gametophyte for their supplies of water, mineral nutrients and nitrogen.Glime, J.M. (2007). ''Bryophyte Ecology''. Vol. 1. ''Physiological Ecology''. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Accessed on 4 March 2013 at http://www.bryoecol.mtu.edu/chapters/5-9Sporophyte.pdf . Antheridia and archegonia are often clustered. A cluster of antheridia is called an androecium while a cluster of archegonia is called a gynoecium. (Note these terms have a different meaning when used to refer to flower structures.) Bryophytes have the most elaborate gametophytes of all living land plants, and thus have a wide variety of gametangium positions and developmental patterns. Gametangia are typically borne on the tips of shoots, but may also be found in the axils of leaves, under thalli or on elaborate structures called gametangiophores. Bryophyte species may be: * Autoicous meaning that androecia and gynoecia are found on the same individual (monoicous) but in distinctly separate locations. If these are on separate branches, the term cladautoicous can be applied. * Synoicous (also called androgynous) bryophytes produce antheridia and archegonia interspersed in the same cluster. *Paroicous bryophytes produce antheridia and archegonia in separate clusters in different leaf axils. * Heteroicous bryophyte species may be either monoicous or sequentially dioicous depending on environmental conditions. This condition is also called polygamous or polyoicous.

Role in survival

There can be both selective advantages and selective disadvantages for organisms that are monoicous or dioicous. Monoicous bryophytes can easily reproduce sexually, since both sexes can be found on the same organism. On the other hand, this can lead to

inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely genetic distance, related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genet ...

and reduce genetic variation within populations. Dioicous organisms necessarily exchange

genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

with other organisms of the species during sexual reproduction, increasing

heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

and variability (given a sufficiently large variable mating population). If isolated, however, organisms may only reproduce asexually, which could present a severe selective disadvantage over time. Bryophyte sperm dispersal can therefore be key to species longevity, particularly in dioicous species. While sperm dispersal is typically passive, with sperm dispersing through water, certain species exhibit very active dispersal mechanisms, such as aerial dispersal recently described in the liverwort '' Conocephalum conicum''.

Evolution

The ancestral sexual system in bryophytes is unknown but it has been suggested monoicy and dioicy evolved several times. In the liverwort genus ''

Radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

'' it was found that monoicy was a recent evolutionary acquisition connected to epiphytism, arising 6 times.

References