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 systemDescription
Androdioecymales and hermaphrodites coexist in a population.[10] It is rare in both plants and animals.[11]
Andromonoecyrare 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]
Dichogamyan 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]
Dioicyone of the two main sexual systems in bryophytes.[17] In dioicy male and female sex organs are on separate gametophytes.[18]
Dioecya 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]
Gonochorismindividuals 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]

Gynodioecyfemales and hermaphrodites coexist in the same population.[10]
Gynomonoecydefined 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-Gynomonoecya sexual system for plants when female, hermaphrodite, and gynomonoecious plants coexist in the same population.[24]:360
Monoicyone of the two main sexual systems in bryophytes.[17] In monoicy male and female sex organs are present in the same gametophyte.[18]
Monoecya 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]
Sequential hermaphroditismindividuals start their adult lives as one sex, and change to the other sex at a later age.[27]
Sequential monoecya 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 hermaphroditisman 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]
Synoecyall individuals in a population of flowering plants bear solely hermaphrodite flowers.[27]
Trioecymales, 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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  5. Yusa Y, Yoshikawa M, Kitaura J, Kawane M, Ozaki Y, Yamato S, Høeg JT (March 2012). "Adaptive evolution of sexual systems in pedunculate barnacles". Proceedings. Biological Sciences. 279 (1730): 959–66. doi:10.1098/rspb.2011.1554. PMC 3259936. PMID 21881138.
  6. Torices R, Méndez M, Gómez JM (April 2011). "Where do monomorphic sexual systems fit in the evolution of dioecy? Insights from the largest family of angiosperms". The New Phytologist. 190 (1): 234–248. doi:10.1111/j.1469-8137.2010.03609.x. PMID 21219336.
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