A sexual system is a pattern of sex allocation[1] or a distribution of male and female function across organisms in a species.[2] Terms like reproductive system and mating system have also been used as synonyms.[3]

The distinction between sexual systems is not always clear due to phenotypic plasticity.[1] Sexual systems are viewed as a key factor for genetic variation and reproductive success, and may have also led to the origin or extinction of certain species.[4]

Interests in sexual systems go back to Darwin, who found that barnacles contain species that are androdioecious and some that are dioecious.[5]

Types of sexual systems

In angiosperms there are monomorphic sexual systems where a species has combination of hermaphrodite, male, and/or female flowers on the same plant. Monomorphic sexual systems include monoecy, gynomonoecy, andromonoecy, and trimonoecy. There are also dimorphic sexual systems where individual plants within a species only produce one sort of flower — hermaphrodite, male, or female. Dimorphic sexual systems include dioecy, gynodioecy, androdioecy, and trioecy.[6] Mixed sexual systems are where hermaphrodites coexist with single sexed individuals.[7] This includes androdioecy, gynodioecy, and trioecy.[8]

What determines whether a flower is male, female, or hermaphroditic is the presence of a stamen — which contains male gametes — and/or pistil — which contains female gametes. Male (a.k.a. staminate) flowers only have a stamen. 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.[9]

(See Plant reproductive morphology for further details on plant sexual systems.)

List of sexual systems

Sexual system Description
Androdioecy males and hermaphrodites coexist in a population.[10] It is rare in both plants and animals.[11]
Andromonoecy rare sexual system in angiosperms, in which a plant has both male and hermaphroditic flowers.[12] It has been a subject of interest regarding the mechanism of sex expression.[13]
Dichogamy an individual plant produces either exclusively male or exclusively female flowers at different points in time.[14] It is thought the temporal separation of producing male and female flowers occurs to prevent self-fertilization,[15] however this is debatable as dichogamy occurs in similar frequency among species which are self-compatible and self-incompatible.[16]
Dioicy one of the two main sexual systems in bryophytes.[17] In dioicy male and female sex organs are on separate gametophytes.[18]
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).[19]
Gonochorism individuals are either male or female.[19]

The term "gonochorism" is usually applied to animals while "dioecy" is applied to plants.[20] Gonochorism is the most common sexual system in animals, occurring in 95% of animal species.[21]

Gynodioecy females and hermaphrodites coexist in the same population.[10]
Gynomonoecy defined as the presence of both female and hermaphrodite flowers on the same individual of a plant species.[22] It is prevalent in Asteraceae but is poorly understood.[23]
Gynodioecy-Gynomonoecy a sexual system for plants when female, hermaphrodite, and gynomonoecious plants coexist in the same population.[24]: 360 
Monoicy one of the two main sexual systems in bryophytes.[17] In monoicy male and female sex organs are present in the same gametophyte.[18]
Monoecy a sexual system in which male and female flowers are present on the same plant. It is common in angiosperms,[25] and occurs in 10% of all plant species.[26][dubious – discuss]
Sequential hermaphroditism individuals start their adult lives as one sex, and change to the other sex at a later age.[27]
Sequential monoecy a confusing sexual system,[28] in which the combination of male, female, and hermaphrodite flowers presented changes over time.[29] For example, some conifers produce exclusively either male or female cones when young, then both when older.[30] Sequential monoecy can be difficult to differentiate from dioecy.[31] 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).[32]
Simultaneous hermaphroditism an individual can produce both gamete types in the same breeding season.[33] 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.[34]
Synoecy all individuals in a population of flowering plants bear solely hermaphrodite flowers.[27]
Trioecy males, females, and hermaphrodites exist in the same population.[8] It is present in both plants and animals but is always extremely rare.[35] Trioecy occurs in about 3.6% of flowering plants.[36] Trioecy may infrequently be referred to as tridioecy.[37]
Trimonoecy (also called androgynomonoecy) is when male, female, and hermaphrodite flowers are present on the same plant.[27][38] Triomonoecy is rare.[39]

References

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