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Monoecy
Monoecy
Monoecy
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Monoecy (/məˈniːsi/; adj. monoecious /məˈniːʃəs/)[1] is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant.[2] It is a monomorphic sexual system comparable with gynomonoecy, andromonoecy and trimonoecy, and contrasted with dioecy where individual plants produce cones or flowers of only one sex, and with bisexual or hermaphroditic plants in which male and female gametes are produced in the same flower.[3]

Monoecy often co-occurs with anemophily,[2] because it prevents self-pollination of individual flowers and reduces the probability of self-pollination between male and female flowers on the same plant.[4]: 32 

Monoecy in angiosperms has been of interest for evolutionary biologists since Charles Darwin.[5]

Terminology

[edit]

Monoecious comes from the Greek words for one house.[6]

History

[edit]

The term monoecy was first introduced in 1735 by Carl Linnaeus.[2] Darwin noted that the flowers of monoecious species sometimes showed traces of the opposite sex function, suggesting that they evolved via hermaphroditism.[7] Monoecious hemp was first reported in 1929.[8]

Occurrence

[edit]

Monoecy is most common in temperate climates[9] and is often associated with inefficient pollinators or wind-pollinated plants.[10][11] It may be beneficial to reducing pollen-stigma interference,[clarification needed] thus increasing seed production.[12]

Around 10% of all seed plant species are monoecious.[9] It is present in 7% of angiosperms.[4]: 8  Most Cucurbitaceae are monoecious[13] including most watermelon cultivars.[14] It is prevalent in Euphorbiaceae.[15][16] Dioecy is replaced by monoecy in polyploid populations of Mercurialis annua.[17]

Maize

[edit]

Maize is monoecious since both pistillate (female) and stamenate (male) flowers occur on the same plant. The pistillate flowers are present on the ears of corn and the stamenate flowers are in the tassel at the top of the stalk. In the ovules of the pistillate flowers, diploid cells called megaspore mother cells undergo meiosis to produce haploid megaspores. In the anthers of the stamenate flowers, diploid pollen mother cells undergo meiosis to produce pollen grains. Meiosis in maize requires gene product RAD51, a protein employed in recombinational repair of DNA double-strand breaks.[18]

Evolution

[edit]
Main article: Evolution of sexual reproduction

The evolution of monoecy has received little attention.[7]

Male and female flowers evolve from hermaphroditic flowers[19] via andromonoecy or gynomonoecy.[20]: 148 

In amaranths monoecy may have evolved from hermaphroditism through various processes caused by male sterility genes and female fertility genes.[20]: 150 

Monoecy may be an intermediate state between hermaphroditism and dioecy.[21] Evolution from dioecy to monoecy probably involves disruptive selection on floral sex ratios.[22]: 65  Monoecy is also considered to be a step in the evolutionary pathway from hermaphroditism towards dioecy.[23]: 91  Some authors even argue monoecy and dioecy are related.[2] But, there is also evidence that monoecy is a pathway from sequential hermaphroditism to dioecy.[23]: 8 

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Monoecy

View on Grokipedia
from Grokipedia
Monoecy is a in seed plants where individual plants bear separate unisexual flowers, with male flowers producing and female flowers containing ovules, both on the same plant but spatially separated. This condition, derived from words monos (one) and oikos (house), literally means "one house" for both sexes, distinguishing it from , where male and female flowers occur on separate plants. Monoecy occurs in approximately 7% of angiosperm , a frequency slightly higher than that of (5-6%), and is particularly prevalent in certain families such as , , and . Notable examples include (Zea mays), where male flowers (tassels) form at the top of the plant and female flowers (ears) lower down; cucumbers (Cucumis sativus) and squash (Cucurbita spp.), which produce distinct male and female blossoms on the same vine; and trees like oaks (Quercus spp.) and birches (Betula spp.), where catkins serve as the unisexual inflorescences. In these plants, only female flowers develop into fruits and seeds following , which can be self- or cross-pollination depending on environmental factors and activity. Evolutionarily, monoecy typically arises from hermaphroditic ancestors through genetic mutations that suppress one sex's function in specific flowers, such as the TASSEL SEED genes in that promote development in apical structures. This system can facilitate controlled sex allocation, allowing plants to optimize resources between ( production) and female ( development) functions, and it often serves as an intermediate stage in the transition to via further genetic divergence. Temporal separation of and female flowering (protandry or protogyny) in some monoecious species further reduces self-fertilization risks, promoting while retaining the flexibility of a single-plant reproductive unit.

Fundamentals

Definition

Monoecy is a sexual system characterized by the presence of separate male and female reproductive structures on the same individual organism. In plants, this condition involves the production of unisexual flowers on a single plant, where male flowers (staminate) and female flowers (pistillate) are spatially separated, often influenced by developmental and environmental factors. This system contrasts with dioecy, in which male and female flowers occur on distinct individuals, and hermaphroditism (also known as monocliny), where both male and female organs coexist within the same flower. Monoecy thus represents an intermediate form of sexual separation at the organismal level, promoting outcrossing while allowing potential self-fertilization under certain conditions. In terms of reproductive anatomy, staminate flowers contain functional stamens but lack pistils; the stamens consist of filaments and anthers that produce pollen grains carrying male gametes. Pistillate flowers, conversely, possess functional pistils—including ovaries, styles, and stigmas—but lack stamens, with the ovaries serving as sites for egg development and subsequent seed production after . Monoecious structures in commonly include tassel-like inflorescences for male flowers, which release , and ear-like structures for flowers, which bear ovules and develop into fruits or seeds, as observed in various crop species.

Terminology

The term monoecy originates from the Greek words monos (μονός), meaning "single" or "alone," and oikos (οἶκος), meaning "house" or "dwelling," signifying a single or "household" bearing both and reproductive structures. This nomenclature was first introduced by in his (1735), where he established Monoecia as a class for producing unisexual flowers of both sexes on the same individual. Species Plantarum (1753) further applied this classification system. In , monoecy is synonymous with the presence of unisexual flowers—staminate () and pistillate ()—on the same plant, distinguishing it from hermaphroditism where individual flowers contain both sexual functions. Historically, it contrasted with the term "" in , which denoted plants bearing a mix of unisexual and hermaphroditic flowers on the same or different individuals, a usage prevalent in pre-20th-century classifications. In modern , monoecy is categorized as a monomorphic within systems, encompassing scenarios where a single produces separate male and female flowers, often integrated into broader frameworks like those extending Linnaean classes to include subcategories such as andromonoecy or gynomonoecy. This classification emphasizes its role in mechanisms while allowing potential self-fertilization, as documented in analyses of angiosperm reproductive diversity. Regarding related terms, monoecious is the adjective form standardly applied to vascular plants (tracheophytes) exhibiting this condition at the sporophyte level, whereas monoicous is reserved for s, where it describes gametophytes producing both antheridia and archegonia on the same ; however, discussions of monoecy in vascular plants do not extend to this bryophyte-specific usage.

Occurrence

In

Monoecy is estimated to occur in approximately 7% of angiosperm species, making it a relatively uncommon but phylogenetically widespread within the plant kingdom. It is particularly prevalent in certain families, including (grasses), (gourds and cucurbits), and (oaks and beeches), where it has evolved multiple times independently. These families often feature herbaceous or woody growth forms adapted to diverse habitats, contributing to the overall distribution of monoecy across angiosperms. Ecologically, monoecy shows distinct patterns, being more frequent among wind-pollinated (anemophilous) species than in those reliant on animal vectors, as the spatial separation of unisexual flowers reduces self-pollen interference in airborne dispersal. It also appears more common in tropical environments compared to temperate regions, as evidenced by higher incidences in tropical Australian floras where monoecy exceeds at community levels. This distribution supports by minimizing self-fertilization, thereby enhancing while avoiding the costs of complete sex separation seen in . The forms of monoecy in plants vary considerably, with unisexual male and female flowers often clustered into specialized inflorescences, such as the pendulous catkins of oaks (Quercus spp.), where male catkins produce abundant and female ones develop into acorns. Alternatively, flowers may be scattered individually along stems or branches, as in many cucurbits. Some monoecious plants exhibit temporal separation of male and female flowering (protandry or protogyny), further promoting cross-pollination. In agricultural contexts, monoecy enables efficient controlled , as the distinct flowers allow breeders to selectively remove one —typically males—to prevent unwanted selfing and facilitate production, improving crop yields and uniformity without relying on full .

In Animals

In , the term monoecy is sometimes used analogously to describe the condition in which a single organism possesses both reproductive capabilities, often with functional or spatial separation of these roles, such as through distinct gonads or sequential changes. This contrasts with , where sexes are separate, and can manifest as simultaneous hermaphroditism (both functions active concurrently) or sequential (transition between male and female phases). Examples include certain fish like , which exhibit protogynous hermaphroditism (female-to-male transition), and such as pulmonate snails, which often have separate male and female reproductive structures despite being simultaneous hermaphrodites. Monoecy is less prevalent in animals than in , occurring in roughly 1-2% of species—461 documented cases across 41 families as of 2020—and sporadically in , where it is rare but present in groups like scale insects (e.g., ). In , sequential forms dominate, with protogyny in about 66% of cases, as seen in the Labridae family (), while protandry appears in clownfish (Amphiprion spp.), where smaller individuals start as males and larger ones become females. examples are limited, primarily to certain hemipterans with hermaphroditic alongside males. Mechanisms of monoecy in animals typically involve hormonal regulation, particularly in sequential hermaphrodites, where environmental or social cues modulate steroid hormones like estrogen via aromatase enzymes to trigger gonadal reorganization. For instance, in fish, removal of a dominant individual can induce sex change through elevated gonadotropin-releasing hormone (GnRH) and reduced aromatase activity. Genetic underpinnings include polygenic sex determination or environmental sex determination (ESD) in many species, though some retain sex chromosomes that predispose individuals to hermaphroditic potential. Ecologically, monoecy provides reproductive flexibility in dynamic habitats, such as reefs, where aligns with optimal size or social status to maximize mating opportunities and offspring production. In clownfish, protandry supports structures in sea anemones, ensuring breeding by the largest individual regardless of initial , thus buffering against environmental perturbations like predation or shifts. This adaptability contributes to population stability in variable conditions.

Evolutionary Aspects

Origins

Monoecy in has evolved multiple times independently from ancestral hermaphroditic (cosexual) states, particularly within angiosperms, as evidenced by comparative phylogenetic analyses across major clades. The common ancestor of angiosperms is reconstructed as hermaphroditic, with transitions to monoecy often proceeding through intermediate mixed mating systems such as gynomonoecy, where produce both female and hermaphroditic flowers. These evolutionary shifts are supported by fossil records of early angiosperms from the period (approximately 145–66 million years ago), which document the initial diversification of hermaphroditic flowers and the subsequent appearance of floral diversity that aligns with the emergence of unisexual structures in later deposits. At the genetic level, monoecy arises through mutations that disrupt the development of one sexual organ within flowers, leading to unisexual (staminate) or (pistillate) flowers on the same . Key mechanisms involve alterations in floral identity genes, such as transcription factors that regulate or carpel formation, resulting in selective abortion or suppression of reproductive organs during floral . For instance, in species like (Zea mays), mutations in genes like tassel seed cause the conversion of potentially hermaphroditic florets into unisexual ones, illustrating how simple genetic changes can promote monoecy from a cosexual background. Recent molecular studies, such as analyses in monoecious Cucurbita pepo (as of 2023), have further identified differentially expressed genes involved in the transition to flowering, enhancing understanding of these pathways. Comparative studies highlight repeated transitions in specific lineages, such as the (daisy family), where phylogenetic reconstructions indicate monoecy evolved from hermaphroditism via gynomonoecy, with at least nine inferred shifts supported by Bayesian ancestral state analysis. estimates place the divergence of major clades, during which these sexual system transitions occurred, between approximately 76 and 100 million years ago, aligning with the family's radiation in the . Earlier fossil insights into potential precursors come from progymnosperms (around 419–358 million years ago), which exhibit heteromorphic spores suggestive of rudimentary sexual dimorphism, though true monoecy is confined to later seed plant lineages.

Adaptive Significance

Monoecy confers several key adaptive advantages in the reproductive strategies of by balancing with reproductive assurance. In , it promotes outcrossing through the spatial or temporal separation of unisexual flowers on the same individual, which reduces and self-fertilization compared to cosexual hermaphroditism, while self-compatibility serves as a backup mechanism during shortages or sparse populations. This arrangement mitigates by maintaining higher levels of genetic diversity within populations, as demonstrated in monoecious like Cnidoscolus urens, where behavior favors cross-pollination at rates around 70% despite self-compatibility. Efficient is another benefit, as monoecious individuals can specialize male flowers for production and export—often investing in attractants like —while directing resources toward and seed development in female flowers, optimizing fitness under resource constraints. theory predicts that this specialization enhances overall reproductive output, particularly when male gametes are cheaper to produce than female ones, allowing flexible adjustments based on environmental conditions. Ecologically, monoecy thrives in unstable or variable environments, such as those faced by annual plants or clonal in patchy habitats, where labile sex expression enables rapid to fluctuating availability or resource levels—maleness often increases with plant vigor to maximize dispersal. Compared to , monoecy sustains through reliable without the dependency on balanced ratios across individuals, though may amplify diversity in stable, high-density populations by enforcing stricter separation of es. Fitness models indicate that monoecious systems can yield 20-50% higher seed set relative to obligate selfing hermaphrodites, underscoring their selective advantage in promoting viable offspring production. Despite these benefits, monoecy carries potential drawbacks that can limit its advantages in certain contexts. Pollen limitation poses a in pollinator-scarce environments, where female flowers may receive insufficient cross-pollen, leading to reduced production despite the selfing backup—self-interference from can further exacerbate this by discounting outcross pollen on stigmas. Additionally, biases in floral sex ratios, such as overproduction of male flowers under stress, can create allocation imbalances within individuals, potentially lowering overall if function is underrepresented in populations. These vulnerabilities highlight monoecy's evolutionary trade-offs, particularly in contrast to the more rigid but sometimes costlier dioecious systems.

Historical Development

Early Observations

In the 17th century, advances in microscopy enabled more detailed examinations of plant reproductive organs. Italian anatomist Marcello Malpighi, using early microscopes, observed pollen grains and floral structures in various species, contributing to the emerging recognition of unisexual flowers as natural features rather than mere curiosities. These observations, detailed in his Anatome Plantarum (1675–1679), marked a shift toward empirical study of plant sexuality. The formal scientific conceptualization of monoecy emerged in the through Carl Linnaeus's of . Linnaeus coined the term Monoecia in the 1750s, as part of his , to describe plants bearing unisexual male and female flowers on the same individual, exemplified by his studies of willows (Salix spp.), where he documented separate staminate and pistillate catkins. This , introduced in the tenth edition of (1758), integrated monoecy into a broader framework of plant sexuality, distinguishing it from hermaphroditism and . Linnaeus viewed monoecy as a legitimate reproductive strategy. By the 19th century, monoecy informed practical applications in agriculture, particularly in the breeding of squash (Cucurbita pepo). Horticulturists in Europe and North America utilized monoecious traits to enhance fruit yield and uniformity through controlled pollination. These efforts, documented in agricultural treatises, represented the application of monoecious characteristics for crop improvement, bridging observational botany with practical farming.

Modern Insights

In the mid-20th century, genetic research on monoecy advanced significantly with the discovery of tassel seed mutants in maize (Zea mays), first identified in the 1950s, which revealed key mechanisms suppressing pistil development in male inflorescences and provided foundational insights into sex organ differentiation. These mutants, such as ts1 and ts2, demonstrated how recessive alleles disrupt hormonal pathways like jasmonic acid signaling, leading to feminization of the tassel and offering early evidence of monoecy's genetic control in crops. Building on this, 21st-century genomics has elucidated further through CRISPR-Cas9 editing; for instance, targeted modifications of developmental genes in the 2010s and beyond have confirmed roles in inflorescence meristem maintenance, with studies on genes like ZmWUS1 enhancing transformation efficiency and indirectly informing sex determination by altering meristem identity in monoecious systems. Ecological investigations from the late 20th century onward have highlighted monoecy's contributions to biodiversity via pollination dynamics, particularly in wind-pollinated species like oaks (Quercus spp.). Field experiments in the 1990s and early 2000s, such as those on blue oak (Quercus douglasii) woodlands, showed that monoecious individuals rely on neighborhood pollen sources within 60 meters for acorn production, with fragmentation reducing female flower fertilization by up to 41% in low-density stands and underscoring monoecy's role in sustaining genetic diversity amid habitat pressures. These studies, using regression models of flower density and weather variables, illustrated how monoecy promotes outcrossing in fragmented landscapes, potentially buffering biodiversity loss by maintaining pollen flow in mast years. Applied research has leveraged monoecy for agricultural improvements, notably in hybrid breeding programs for crops like cucumbers (Cucumis sativus), where monoecious lines (genotype MMff) serve as stable male parents crossed with gynoecious females to produce F1 hybrids with enhanced yield and uniformity. Marker-assisted selection targeting loci like CsACS2 and CsACS11 has accelerated these programs since the 2000s, enabling precise manipulation of sex expression for commercial varieties. Additionally, post-2000 modeling and field trials have quantified climate change impacts on monoecious reproduction; for example, drought simulations reduced female flower production by 41% in zucchini (Cucurbita pepo), altering investment ratios and shortening phenological windows, which could diminish fecundity in rain-fed systems. Current research gaps persist regarding shifts in monoecy prevalence due to anthropogenic habitat loss, with global trait databases like TRY (initiated in the 2000s and expanded in the 2010s) providing datasets on sexual systems that reveal correlations between fragmentation and altered reproductive strategies in over 200,000 trait records from diverse taxa. Ongoing debates center on whether habitat degradation favors shifts toward or clonal propagation in monoecious species, as evidenced by meta-analyses showing pollen limitation increases of 26% in threatened , prompting calls for integrated conservation genetics to track these dynamics.

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