Sexual system

For Linnaeus' sexual system, a classification of plants, see Linnaean taxonomy § Classification for plants.

A sexual system is a distribution of male and female functions across organisms in a species.^[1][2] The terms reproductive system and mating system have also been used as synonyms.^[3]

Barnacles have a variety of sexual systems.

Sexual systems play a key role in genetic variation and reproductive success, and may also have led to the origin or extinction of certain species.^[4] In flowering plants and animals, sexual reproduction involves meiosis, an adaptive process for repairing damage in the germline DNA transmitted to progeny.^[5] The distinctions between different sexual systems is not always clear due to phenotypic plasticity.^[2]

Interest in sexual systems goes back to Charles Darwin, who found that barnacles include some species that are androdioecious and some that are dioecious.^[6]

Types of sexual systems

See also: Plant reproductive morphology

The life cycle of an angiosperm.

Flowering plants may have dimorphic or monomorphic sexual systems. In monomorphic sexual systems, a combination of hermaphrodite, male, and/or female flowers may be present on the same plant. Monomorphic sexual systems include monoecy, gynomonoecy, andromonoecy, and trimonoecy. In dimorphic sexual systems, individual plants within a species only produce one sort of flower, either hermaphrodite or male, or female. Dimorphic sexual systems include dioecy, gynodioecy, androdioecy, and trioecy.^[7]

Male (a.k.a. staminate) flowers have a stamen but no pistil and produce only male gametes. Female (a.k.a. pistillate) flowers only have a pistil. Hermaphrodite (a.k.a. perfect, or bisexual) flowers have both a stamen and pistil. The sex of a single flower may differ from the sex of the whole organism: for example, a plant may have both staminate and pistillate flowers, making the plant as a whole a hermaphrodite. Hence although all monomorphic plants are hermaphrodites, different combinations of flower types (staminate, pistillate, or perfect) produces distinct monomorphic sexual systems.^[8]

In animals, androdioecy, gynodioecy, and trioecy are referred to as mixed sexual systems;^[9]where hermaphrodites coexist with single sexed individuals.^[10]

List of sexual systems

Sexual system	Description
Androdioecy	males and hermaphrodites coexist in a population.[11] It is rare in both plants and animals.[12]
Andromonoecy	rare sexual system in angiosperms, in which a plant has both male and hermaphroditic flowers.[13] It has been a subject of interest regarding the mechanism of sex expression.[14]
Dichogamy	an individual plant produces either exclusively male or exclusively female flowers at different points in time.[15] It is thought the temporal separation of producing male and female flowers occurs to prevent self-fertilization,[16] however this is debatable as dichogamy occurs in similar frequency among species which are self-compatible and self-incompatible.[17]
Dioicy	one of the main sexual systems in bryophytes.[18] In dioicy male and female sex organs are on separate gametophytes.[19]
Dioecy	a species has distinct individual organisms that are either male or female, i.e., they produce only male or only female gametes, either directly (in animals) or indirectly (in plants).[20]
Gonochorism	individuals are either male or female.[20] The term "gonochorism" is usually applied to animals while "dioecy" is applied to plants.[21] Gonochorism is the most common sexual system in animals, occurring in 95% of animal species.[22]
Gynodioecy	females and hermaphrodites coexist in the same population.[11]
Gynomonoecy	defined as the presence of both female and hermaphrodite flowers on the same individual of a plant species.[23] It is prevalent in Asteraceae but is poorly understood.[24]
Gynodioecy-Gynomonoecy	a sexual system for plants when female, hermaphrodite, and gynomonoecious plants coexist in the same population.[25]: 360
Monoicy	one of the main sexual systems in bryophytes.[18] In monoicy male and female sex organs are present in the same gametophyte.[19]
Monoecy	a sexual system in which male and female flowers are present on the same plant. It is common in angiosperms,[26] and occurs in 10% of all plant species.[27][dubious – discuss]
Sequential hermaphroditism	individuals start their adult lives as one sex, and change to the other sex at a later age.[28]
Sequential monoecy	a confusing sexual system,[29] in which the combination of male, female, and hermaphrodite flowers presented changes over time.[30] For example, some conifers produce exclusively either male or female cones when young, then both when older.[31] Sequential monoecy can be difficult to differentiate from dioecy.[32] Several alternative terms may be used in reference to sexual systems involving temporal changes to sex presentation of a plant species (e.g. dichogamy, sequential hermaphroditism, sex change, paradioecy, diphasy).[33]
Simultaneous hermaphroditism	an individual can produce both gamete types in the same breeding season.[34] Simultaneous hermaphroditism is one of the most common sexual systems in animals (though far less common than gonochorism) and is one of the most stable.[35]
Synoecy	all individuals in a population of flowering plants bear solely hermaphrodite flowers.[28]
Trioecy	males, females, and hermaphrodites exist in the same population.[9] It is present in both plants and animals but is always extremely rare.[36] Trioecy occurs in about 3.6% of flowering plants.[37] Trioecy may infrequently be referred to as tridioecy.[38]
Trimonoecy	(also called androgynomonoecy) is when male, female, and hermaphrodite flowers are present on the same plant.[28][39] Triomonoecy is rare.[40]