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Parthenogenesis

Parthenogenesis is a form of asexual reproduction in which an embryo develops from an unfertilized egg cell, resulting in offspring that are genetically identical to the mother or with limited variation, without any genetic contribution from a male.^[1] This process occurs naturally across diverse taxa, including invertebrates such as aphids, rotifers, and certain hymenopterans like bees and ants, as well as vertebrates like some lizards, sharks, and fish.^[2] It also appears in some plants, where unfertilized eggs spontaneously form embryos, though it is rarer and often linked to apomixis.^[3] Parthenogenesis can be classified into several types based on its occurrence and mechanism. Obligate parthenogenesis is the sole reproductive mode in certain species, such as all-female whiptail lizards (Aspidoscelis spp.), where females produce clones of themselves.^[4] Facultative parthenogenesis allows females to reproduce either asexually or sexually when males are available, as observed in captive sharks like the blacktip and hammerhead, which have produced viable young in isolation.^[5] Cyclical parthenogenesis alternates between asexual and sexual phases, common in aphids during favorable conditions for rapid population growth.^[6] Other variations include gynogenesis, where sperm triggers egg development but contributes no genes, seen in some salamanders, frogs, and fish.^[7] The biological mechanisms underlying parthenogenesis involve modifications to meiosis and egg activation, often suppressing the second polar body extrusion to restore diploidy.^[8] In animals, it provides a reproductive advantage in environments with scarce mates, such as isolated habitats, but typically results in reduced genetic diversity, increasing vulnerability to diseases and environmental changes.^[9] Recent genetic studies have identified key regulators, like the Polo and Myc genes in Drosophila, that can induce parthenogenesis, offering insights into evolutionary transitions between sexual and asexual reproduction.^[10] In plants, engineering parthenogenesis genes holds potential for crop breeding to fix desirable traits without segregation.^[3]

Definition and Basic Concepts

Definition

Parthenogenesis is a form of asexual reproduction in which an embryo develops from an unfertilized ovum, resulting in offspring that are genetically identical or nearly identical clones of the mother.^[1] The term derives from the Greek words parthenos, meaning "virgin," and genesis, meaning "birth" or "origin," reflecting its characterization as reproduction without male fertilization.^[11] It was first coined in 1849 by British anatomist Richard Owen, who described it as the successive production of procreating individuals from a single ovum.^[12] Unlike other modes of asexual reproduction such as budding or fission, which involve mitotic division of somatic cells to produce new individuals, parthenogenesis specifically entails the development of egg cells through oogenesis without subsequent fertilization by sperm.^[13] This process requires two fundamental biological prerequisites: activation of the unfertilized egg to initiate embryonic development, often mimicking fertilization-induced calcium signaling, and either suppression of meiosis to maintain diploidy or mechanisms to restore the diploid chromosome number, such as fusion of polar bodies.^[10] These steps ensure the viability of the offspring in species where haploid development would otherwise be lethal. The outcomes of parthenogenesis typically yield diploid female offspring that are genetically similar to the mother, though variations occur depending on the mechanism involved, such as the production of haploid males in certain insects.^[10] For instance, in apomictic parthenogenesis, the egg retains the full maternal genome without reduction, producing exact clones, while automictic forms may introduce limited genetic recombination.^[14]

Historical Background

The concept of parthenogenesis, or reproduction without fertilization, has roots in ancient observations of what was often interpreted as spontaneous generation. Aristotle, in his biological writings around 350 BCE, described various forms of animal reproduction, including asexual processes akin to parthenogenesis in certain invertebrates, though he framed them within broader theories of spontaneous emergence from non-living matter.^[15] These early ideas laid groundwork for later scientific inquiry but lacked empirical detail on unfertilized egg development. The first modern observation of parthenogenesis came in 1740 when Swiss naturalist Charles Bonnet documented the phenomenon in aphids (Aphididae), noting that unmated females produced live offspring that were genetic clones of the mother.^[16] Bonnet's detailed accounts, published in 1745, challenged prevailing views on reproduction and highlighted the process's role in insect population dynamics.^[17] In the 19th century, British anatomist Richard Owen advanced the understanding through his 1849 monograph "On Parthenogenesis," where he systematically reviewed cases in invertebrates and plants, emphasizing the successive generations arising from a single ovum and integrating it into evolutionary discussions.^[18] Owen's work helped establish parthenogenesis as a legitimate biological mode rather than mere anomaly.^[19] Experimental breakthroughs occurred in 1899 when German-American physiologist Jacques Loeb induced artificial parthenogenesis in sea urchins (Arbacia punctulata) by treating unfertilized eggs with magnesium chloride solutions, triggering normal embryonic development and larval formation.^[20] This demonstration proved parthenogenesis could be mechanistically replicated in the lab, influencing debates on fertilization's necessity.^[21] Early 20th-century scientists often debated whether parthenogenesis represented true reproduction or a pathological deviation, particularly in vertebrates where it was rare. The discovery of obligate parthenogenesis in whiptail lizards (genus Aspidoscelis) in the 1960s, through observations of all-female populations lacking males yet producing viable offspring, shifted perceptions toward its viability as an evolutionary strategy.^[22] By the mid-20th century, cytological studies using microscopy and chromosome analysis confirmed the genetic mechanisms of parthenogenesis, such as automixis and apomixis, in diverse taxa, solidifying its status as a natural reproductive adaptation rather than a curiosity or defect.^[23] These investigations, building on earlier work, revealed how parthenogenetic lineages maintained stability across generations, paving the way for broader ecological and evolutionary interpretations.^[24]

Types and Mechanisms

Apomixis and Automixis

Apomixis, also known as ameiotic parthenogenesis, is a form of asexual reproduction in which the oocyte undergoes a mitotic-like division rather than meiosis, producing unreduced diploid eggs that develop into offspring genetically identical to the mother.^[25] This process bypasses the reduction division of meiosis entirely, maintaining the maternal ploidy level such that a diploid (2n) mother produces diploid (2n) offspring without any genetic recombination or reduction in chromosome number.^[26] Cytologically, apomixis involves the suppression of both meiotic divisions, preventing the formation of polar bodies and ensuring the oocyte retains the full maternal genome as a clone.^[25] The genetic outcome is complete preservation of the maternal genotype, with no introduction of homozygosity or variation beyond mutations, which supports long-term clonal lineages but limits adaptability.^[27] In contrast, automixis, or meiotic parthenogenesis, involves a standard meiotic division of the oocyte followed by restoration of diploidy through the fusion of meiotic products, such as the egg pronucleus with a polar body, leading to offspring that are not perfect clones but exhibit some degree of genetic recombination.^[27] This mechanism produces polar bodies during meiosis I and II, with diploidy restored either by central fusion—where the egg pronucleus fuses with the meiosis I polar body—or terminal fusion, where it fuses with the meiosis II polar body.^[28] Central fusion tends to maintain maternal heterozygosity, particularly near centromeres, as the fusing nuclei share more genetic similarity from earlier meiotic stages, while terminal fusion increases the risk of inbreeding depression by promoting homozygosity across the genome.^[27] The genetic outcomes of automixis differ markedly from apomixis due to the involvement of recombination during meiosis. In apomixis, the full maternal genome is preserved without alteration, resulting in identical clones.^[25] In automixis with terminal fusion, heterozygosity is typically halved in each generation (H' = H/2), as the fusion of sister products after the second meiotic division leads to widespread homozygosity, especially in the absence of crossovers, though recombination can partially mitigate this.^[29] Central fusion, however, better preserves heterozygosity by fusing nuclei that retain more diverse chromatid combinations from meiosis I, reducing the rate of homozygosity accumulation and allowing limited genetic shuffling.^[27] These processes ensure diploidy restoration (2n mother → 2n offspring) but introduce variable levels of inbreeding, influencing the evolutionary stability of automictic lineages.^[28]

Facultative and Obligate Parthenogenesis

Facultative parthenogenesis refers to a reproductive strategy in which organisms can alternate between sexual and asexual (parthenogenetic) reproduction depending on environmental or physiological conditions.^[30] In this mode, females produce offspring asexually when conditions favor rapid population growth, such as abundant resources, but switch to sexual reproduction to generate genetic diversity when cues like shorter day lengths or crowding signal impending stress.^[31] A classic example occurs in aphids, such as the pea aphid (Acyrthosiphon pisum), where parthenogenesis dominates in spring and summer, yielding live-born, genetically identical daughters through apomictic processes, while sexual reproduction is triggered in autumn to produce hardy overwintering eggs.^[31] This flexibility allows facultative parthenogens to exploit mate scarcity or favorable conditions for clonal expansion, yet still form hybrids with males when they are available, enhancing adaptability.^[32] In contrast, obligate parthenogenesis is an exclusive asexual reproductive mode with no sexual phase, typically observed in isolated, all-female lineages that have lost the capacity for meiosis or male production.^[30] These organisms rely solely on unfertilized eggs to propagate, often stabilized by mechanisms that maintain genomic integrity, such as hybrid origins providing initial heterozygosity or occasional gene flow from related sexual species to counteract heterozygosity loss.^[33] Bdelloid rotifers exemplify this, reproducing parthenogenetically for millions of years without evidence of sex or males, while certain lizard species, like those in the genus Darevskia, form all-female populations through obligate parthenogenesis, deriving from interspecific hybridization that sustains evolutionary persistence.^[34] In facultative cases, automixis may occasionally contribute to parthenogenetic offspring production, but obligate forms depend on consistent asexual mechanisms without such alternation.^[30] The comparative advantages of these modes highlight trade-offs in reproductive strategy. Facultative parthenogenesis offers genetic diversity through periodic sexual recombination, enabling faster adaptation to changing environments and mitigating the accumulation of deleterious mutations, while still permitting explosive clonal growth when advantageous.^[35] Obligate parthenogenesis, however, ensures consistent reproductive assurance and higher fecundity in stable or male-scarce habitats, promoting rapid population increases—such as slightly elevated offspring production rates in obligate rotifer clones compared to cyclical ones—but at the risk of unchecked mutation buildup due to the absence of genetic mixing.^[36] These benefits are evident in ecological contexts, where obligate forms may competitively displace facultative relatives by avoiding the "cost of sex," including resources wasted on non-reproductive males.^[36] Evolutionary evidence suggests transitions from facultative or cyclical parthenogenesis to obligate forms, often driven by genetic mutations that eliminate sexual capabilities. In rotifers like Brachionus calyciflorus, obligate parthenogenesis emerges from cyclical ancestors via a single recessive allele, leading to loss of male and sexual female production, as demonstrated in lab-derived clones where obligate lines outcompeted cyclical ones.^[36] Such shifts, including ancient ones inferred in bdelloid rotifers, underscore how obligate parthenogenesis can evolve for long-term stability in isolated lineages, though it may limit broader adaptability compared to facultative flexibility.^[36]

Sex Determination in Offspring

In parthenogenesis, offspring sex is primarily determined by the species' chromosomal sex-determination system and the type of parthenogenetic mechanism employed, resulting in predominantly female progeny across most taxa. In systems utilizing XX/XY sex determination, such as many vertebrates and some invertebrates, unfertilized eggs develop into diploid XX females through processes like automixis, where meiotic products fuse to restore diploidy, ensuring female development.^[37] This reliance on XX chromosomes leads to all-female lineages in obligate parthenogens, including whiptail lizards (Aspidoscelis spp.), where premeiotic endomitosis doubles chromosomes before meiosis, producing only female offspring.^[24] Environmental factors, such as temperature-dependent sex determination in some reptiles, can influence outcomes in facultative cases but typically do not override the female bias in established parthenogenetic lines.^[38] Variations in sex determination arise from different parthenogenetic modes, notably thelytoky, arrhenotoky, and rare androtoky. Thelytoky, the production of females from unfertilized eggs, predominates in diploid systems and is facilitated by mechanisms that maintain diploidy, such as central fusion automixis.^[39] In contrast, arrhenotoky—common in haplodiploid Hymenoptera like bees—involves unfertilized eggs developing into haploid males (drones) via complementary sex determination, where heterozygosity at sex loci promotes female development from fertilized eggs, while haploids become males.^[40] Haplodiploidy integrates with parthenogenesis in some cases, allowing unfertilized eggs to yield males, though thelytokous variants in wasps produce females by manipulating ploidy.^[41] Androtoky, the rare development of diploid males from unfertilized eggs, occurs sporadically in systems like XO insects, where automixis can segregate chromosomes to produce XO males through random fusion of meiotic products. In automictic parthenogenesis, sex determination can yield rare males via chromosomal segregation, particularly in XO systems of insects like stick insects, where unbalanced meiosis occasionally results in XO individuals viable as males.^[42] For instance, in the parthenogenetic stick insect Ramulus mikado, rare males arise but are often sterile, limiting their role. These mechanisms contrast with standard automixis, which favors females, and briefly reference the fusion or segregation processes outlined in broader automixis descriptions. The predominance of female offspring in parthenogenetic populations constrains genetic diversity by perpetuating clonal lineages, as recombination is limited without male input.^[43] However, the occasional production of rare males enables sporadic outcrossing with sexual populations, introducing genetic variation and potentially stabilizing population dynamics, as observed in diploid parthenogenetic Artemia where functional rare males fertilize sexual females.^[44] This intermittent male production thus serves as a bridge for gene flow, mitigating the risks of unchecked asexuality in otherwise all-female clades.^[45]

Natural Occurrence

In Invertebrates

Parthenogenesis is widespread among invertebrates, occurring in diverse phyla and enabling rapid population growth in various ecological contexts. In insects, aphids exemplify facultative cyclical parthenogenesis, where females produce genetically identical daughters asexually during favorable conditions, switching to sexual reproduction for males and overwintering eggs when environments deteriorate.^[46] This strategy, classified as apomixis, allows aphids to exploit ephemeral resources like spring foliage, achieving up to 20 generations per year in some species.^[47] Honeybees demonstrate arrhenotokous parthenogenesis within their haplodiploid sex determination system, where unfertilized eggs develop into haploid males (drones) that serve reproductive roles in the colony.^[48] In contrast, some stick insects, such as certain lineages in the genus Timema, exhibit obligate parthenogenesis, producing only females through mechanisms like automixis, which has evolved repeatedly and leads to low genetic diversity but stable all-female populations.^[49] Among other arthropods, bdelloid rotifers represent an extreme case of ancient obligate parthenogenesis, with the class Bdelloidea comprising over 450 all-female species that have persisted without males or meiosis for >60 million years, as evidenced by fossil records and genomic analyses.^[50] These microscopic aquatic animals reproduce via ameiotic parthenogenesis, maintaining genetic stability through horizontal gene transfer rather than recombination.^[51] Parthenogenesis also occurs in mites (Acari), where arrhenotoky predominates in families like Tetranychidae, producing haploid males from unfertilized eggs to facilitate rapid colonization of host plants.^[52] In spiders, rare thelytokous parthenogenesis is documented in species like the oonopid Triaeris stenaspis, yielding diploid female offspring without fertilization, though this mode is uncommon and often linked to isolated populations.^[53] Scorpions provide further examples, with Tityus serrulatus in Brazil reproducing parthenogenetically to form all-female broods, a trait potentially influenced by endosymbiotic bacteria like Wolbachia.^[54] In nematodes, parthenogenesis is observed in certain plant-parasitic species within orders like Tylenchida, where mitotic or meiotic processes produce polyploid females, enabling asexual proliferation in soil and root environments.^[55] Flatworms, particularly in the Digenea (parasitic flukes), employ parthenogenesis alongside asexual multiplication in intermediate hosts, generating infective larvae from unfertilized eggs to sustain complex life cycles in vertebrate definitive hosts.^[56] Ecologically, parthenogenesis in these invertebrates promotes rapid colonization of unstable or mate-scarce habitats, such as temporary pools for rotifers or seasonal crops for aphids, enhancing invasion success and population persistence.^[34] In aphids, hybrid origins of some parthenogenetic lineages further contribute to adaptive radiation, allowing exploitation of novel host plants.^[43]

In Vertebrates

Parthenogenesis in vertebrates is exceptionally rare, occurring primarily in select reptile and fish species, often as an adaptation to isolated or low-male-density environments. Unlike the widespread asexual reproduction in invertebrates, vertebrate parthenogens typically arise from hybrid origins, leading to all-female lineages that bypass meiosis through specialized mechanisms like automixis or apomixis. These cases highlight evolutionary trade-offs, including reduced genetic diversity but enhanced colonization potential.^[57] In reptiles, parthenogenesis is best documented among squamates, with obligate forms dominating. Whiptail lizards of the genus Aspidoscelis represent a classic example of obligate parthenogenesis, where all individuals are female and reproduce via automixis following hybrid speciation from sexual ancestors. These lizards restore diploidy through premeiotic endomitosis, duplicating chromosomes before meiosis, which maintains heterozygosity but limits genetic variation; widespread meiotic failures do not impair their high fecundity.^[58] Recent studies have identified post-meiotic gametic duplication as a mechanism in facultative parthenogenetic whiptails, enabling occasional sexual reproduction while preserving clonal lineages.^[59] Caucasian rock lizards (Darevskia spp.) also exhibit obligate parthenogenesis in at least seven diploid species, originating from interspecific hybridization events post-Pleistocene glaciation. These parthenogens employ premeiotic endoreplication to produce unreduced eggs, avoiding hybrid sterility and facilitating rapid range expansion in heterogeneous Caucasian habitats; sexual parental species actively avoid interspecific mating, reinforcing parthenogen isolation.^[57]^[60] A groundbreaking 2025 study on the flowerpot snake (Indotyphlops braminus), the only known triploid parthenogenetic serpent, revealed genomic adaptations enabling stable asexual reproduction. This blindsnake's hybrid triploid genome, derived from ancient hybridization, features enhanced DNA repair pathways that suppress meiotic errors and aneuploidy, allowing viable clonal offspring without males; these mechanisms provide insights into bypassing meiosis in polyploid vertebrates.^[61] Among fish, facultative parthenogenesis occurs sporadically in elasmobranchs and poeciliids, often as a reproductive rescue in male-scarce conditions. In sharks, such as the endangered common smooth-hound (Mustelus mustelus), recurrent parthenogenesis was first documented in 2024, with captive females producing multiple litters over 13 years without male contact via automictic mechanisms that duplicate maternal genomes.^[62] The Amazon molly (Poecilia formosa), an all-female poeciliid, relies on gynogenesis—a sperm-dependent parthenogenesis—where males of related species trigger egg development without contributing DNA, resulting in clonal daughters and highlighting parasitic reproductive strategies in fish.^[63] Parthenogenesis remains undocumented in natural bird and amphibian populations, with reported cases limited to experimental or captive settings that yield non-viable embryos due to imprinting conflicts and meiotic barriers.^[64] In mammals, no confirmed natural instances exist, as genomic imprinting and placental requirements preclude viable parthenogenetic development beyond early embryos.^[34] Recent 2023–2025 research underscores genomic innovations, such as heterozygosity-maintaining duplications in snakes and error-tolerant ploidy in sharks, as key to vertebrate parthenogen persistence.^[61]

Artificial Induction

Methods of Induction

Artificial parthenogenesis was first successfully induced in 1899 by Jacques Loeb, who treated unfertilized sea urchin (Arbacia punctulata) eggs with a hypertonic solution of non-electrolyte seawater and magnesium chloride, leading to cleavage and development into pluteus larvae without fertilization.^[65] This chemical approach mimicked aspects of fertilization by altering the egg's osmotic environment and triggering activation. Early physical methods included pricking unfertilized eggs with fine glass needles to mechanically disrupt the plasma membrane and initiate development; such techniques achieved activation rates of up to 80% in hamster oocytes and were also applied to frog eggs, producing viable embryos.^[66] Electric stimulation emerged as another physical method, where short pulses of direct current (e.g., 1.0 kV/cm for 80 μsec) were applied to oocytes, inducing calcium transients similar to sperm entry and resulting in parthenogenetic activation in porcine oocytes, with blastocyst formation rates exceeding 50% in optimized protocols.^[67] Chemical induction has since become a cornerstone of laboratory protocols, particularly using ions to replicate sperm-induced calcium oscillations. Strontium chloride (SrCl₂), at concentrations of 5-10 mM for 2-10 minutes, effectively activates mouse and porcine oocytes by substituting for calcium in oscillatory release, yielding activation rates of 70-90% and subsequent diploid embryo development when combined with spindle inhibitors.^[68] Temperature shocks provide an alternative chemical-physical hybrid, where brief exposure to elevated temperatures (e.g., 44°C for 5-10 minutes) suppresses the second meiotic division in mouse oocytes, promoting diploid parthenogenesis with development to the blastocyst stage in over 30% of cases.^[69] These methods target primarily invertebrate models like sea urchins and vertebrate systems such as frogs (Rana spp.), mice, and pigs, where success rates vary from 20-90% depending on timing post-ovulation and species-specific sensitivities. Ploidy manipulation enhances the viability of induced parthenotes by preventing polar body extrusion and maintaining diploidy. Colchicine, an antimitotic agent at 0.1-0.5 μg/ml, inhibits microtubule assembly during meiosis, allowing chromosome retention in sea urchin and amphibian eggs and resulting in polyploid embryos suitable for developmental studies.^[70] In modern applications, CRISPR/Cas9 editing is integrated into parthenogenetic systems for targeted genetic modifications; for instance, injection into activated porcine parthenotes achieves biallelic mutations in up to 80% of embryos, facilitating gene function analysis without paternal contributions.^[71] These techniques, refined since Loeb's pioneering work, enable precise control over reproduction in model organisms for research into embryogenesis and genetics.

Applications in Research and Agriculture

Induced parthenogenesis serves as a valuable model in research for investigating meiosis, embryonic development, and genetic mechanisms underlying reproduction. In Drosophila, studies have identified specific genetic alterations that enable facultative parthenogenesis, allowing researchers to explore how organisms switch between sexual and asexual reproduction modes. For instance, a 2023 analysis revealed that parthenogenetic strains of Drosophila mercatorum exhibit distinct oogenic gene expression profiles compared to sexual strains, providing insights into the molecular basis of reproductive plasticity. Additionally, research on aneuploidy in facultatively parthenogenetic Drosophila has demonstrated that genetic changes promoting parthenogenesis lead to mosaic aneuploidy during larval development, highlighting potential evolutionary trade-offs in meiotic processes. Beyond development, parthenogenesis offers perspectives on oncogenesis, as p53 deficiency in mice induces abnormal gametogenesis culminating in parthenogenetic activation and teratoma-like tumors, linking reproductive errors to cancer initiation. Similarly, parthenogenetic programs in unfertilized oocytes have been observed to drive uterine teratoma formation through endoreduplication and incomplete embryogenesis. In agriculture, induced parthenogenesis facilitates the production of all-female stocks in aquaculture, enhancing growth uniformity and yield. In species like the crucian carp, massive production of all-female diploids and triploids via gynogenesis has been achieved, reducing unwanted reproduction and improving commercial farming efficiency. While direct parthenogenesis in tilapia is less common, related techniques drawing from parthenogenetic principles, such as gynogenesis, have been adapted to generate all-female populations, supporting sustainable fish production. For pest management, parthenogenesis intersects with sterile insect techniques (SIT) by enabling the propagation of unisexual lines in insects like parasitoid wasps, where induced thelytoky produces female-only offspring for release, potentially amplifying control efforts against agricultural pests. Apomictic plants, which reproduce via a parthenogenetic-like process, contribute to economic benefits by stabilizing hybrid vigor in crops; for example, engineering apomixis in rice and other cereals could cut seed production costs by up to 50% and accelerate breeding for higher yields, mirroring the clonal seed production seen in weeds like dandelions. Recent advances as of 2024 have achieved synthetic apomixis in rice with clonal seed production rates exceeding 95%, potentially eliminating the need for annual hybrid seed production.^[72] Despite these applications, induced parthenogenesis faces significant limitations, particularly in higher organisms where offspring viability is low due to genomic imprinting conflicts and reduced genetic diversity. In mammals, parthenogenetic embryos rarely develop beyond early stages, as paternal imprinting genes are absent, leading to developmental arrest. In vertebrates like reptiles, parthenogenesis is often self-destructive, with high extinction risks from accumulated mutations and inbreeding depression. Ethical concerns arise in vertebrate research and agriculture, including animal welfare issues from manipulative induction methods and broader implications for biodiversity if parthenogenetic strains disrupt natural populations. Overall, while promising for targeted uses, these challenges restrict widespread adoption beyond invertebrates and plants.

Parthenogenesis in Humans

Natural Instances

Natural instances of parthenogenesis in humans are exceedingly rare and typically manifest as pathological conditions rather than viable offspring. The most well-documented examples involve ovarian teratomas, which are benign tumors originating from the parthenogenetic activation of unfertilized oocytes. These tumors develop into disorganized masses containing differentiated tissues derived solely from the maternal genome, such as hair, teeth, skin, and sebaceous material, but they do not form functional embryos capable of independent development. Genetic analyses have confirmed their parthenogenetic origin through the demonstration of homozygous genotypes and absence of paternal alleles, indicating duplication of the maternal haploid set without fertilization.^[73]^[74] Ovarian teratomas, particularly mature cystic variants (also known as dermoid cysts), represent the primary natural expression of human parthenogenesis and account for approximately 15-20% of all ovarian neoplasms in women of reproductive age, with an overall incidence of about 1.2-14.2 cases per 100,000 individuals annually. Immature ovarian teratomas, which may also arise parthenogenetically, are far rarer, with an estimated incidence of 3.4 × 10^{-7}. These tumors are typically discovered incidentally during imaging or surgery and pose health risks primarily through complications like torsion or rupture rather than embryonic development. Recent reviews from 2023 highlight that such parthenogenetic events are more frequent than previously assumed, often going undetected unless they form macroscopic tumors.^[75]^[76]^[73] Historical reports from the 1950s, including claims of "virgin births" or spontaneous pregnancies without intercourse, were often misattributed to parthenogenesis but later identified as molar pregnancies or other gestational trophoblastic diseases, with no verified cases of viable human parthenotes emerging from these investigations. Genetic studies, including those published between 2023 and 2025, consistently show that ovarian teratomas derive exclusively from XX chromosomes of maternal origin, reinforcing their parthenogenetic etiology through microsatellite marker analysis and lack of heterozygosity patterns indicative of fertilization. To date, no natural instance has produced a live-born individual, underscoring the profound biological constraints on human parthenogenesis.^[77]^[73] A key biological barrier preventing full embryonic development in these parthenogenetic events is genomic imprinting, an epigenetic mechanism where certain genes are expressed differently depending on whether they are inherited from the mother or father. In humans and other mammals, parthenogenotes lack paternally imprinted genes essential for placental formation and fetal growth, leading to developmental arrest or tumorous overgrowth instead of viable embryos. This imprinting asymmetry enforces biparental reproduction and explains why spontaneous parthenogenesis results in non-viable outcomes like teratomas.^[78]^[77]

Artificial and Experimental Attempts

Artificial parthenogenesis in humans has been pursued through laboratory techniques to activate unfertilized oocytes, mimicking fertilization without sperm contribution. Early efforts drew from pioneering work in the 1930s, where researchers like Gregory Pincus and E.V. Enzmann induced parthenogenesis in rabbit oocytes using non-physiological stimuli such as temperature changes and chemical agents, achieving embryonic development up to the blastocyst stage in some cases.^[79] These animal models laid groundwork for mammalian studies but highlighted genomic imprinting barriers, where paternal gene expression is essential for full development. By the 2000s, human applications advanced with chemical activation methods; for instance, in 2001, researchers exposed human oocytes to ion-altering chemicals, resulting in cleavage-stage embryos from 22 eggs, though none progressed beyond early stages.^[80] Subsequent experiments in the mid-2000s focused on calcium signaling to trigger oocyte activation, a key step in parthenogenesis. Calcium ionophores, such as ionomycin combined with strontium chloride, were used to induce oscillations mimicking sperm-induced calcium release, enabling human metaphase II oocytes to form pronuclei and develop to the morula or blastocyst stage in vitro.^[81]^[82] In 2007, Revazova et al. derived the first human parthenogenetic embryonic stem cell (pESC) lines from activated oocytes, preventing polar body extrusion to retain a diploid genome; these cells exhibited pluripotency markers and normal karyotypes but were highly homozygous.^[83] Similar protocols in nonhuman primates, like rhesus monkeys, yielded pESCs in 2002, demonstrating feasibility across primates but confirming developmental arrest due to imprinting defects.^[84] Recent advances as of 2025 have emphasized parthenogenetic stem cells for regenerative medicine rather than reproduction. In early 2025, studies reviewed the derivation of human pESCs from activated oocytes, noting their potential to generate neural and cardiac lineages without ethical concerns over embryo destruction, as they bypass fertilization.^[85] Modifications to IVF protocols, including optimized activation with ionophores and inhibitors of cyclin-dependent kinases, have produced parthenogenetic blastoids—embryo-like structures reaching blastocyst equivalents—but these fail to implant or develop further in utero models.^[86] No full-term human parthenogenetic births have been achieved, with embryos typically arresting at preimplantation stages due to incomplete genomic activation and imprinting errors.^[87] These efforts face significant ethical hurdles, including debates over whether parthenogenesis equates to cloning, as it produces genetically identical offspring to the mother. Many countries, including those adhering to the 1997 Oviedo Convention, ban reproductive cloning and extend prohibitions to parthenogenetic embryo creation for gestation, citing risks to child welfare and human dignity.^[88] In the US, while federal law does not explicitly ban parthenogenesis, several states prohibit human cloning, encompassing similar techniques, and bioethicists argue it violates principles of reproductive justice by commodifying women's gametes.^[89]^[90] Despite this, parthenogenetic stem cells are pursued for research, offering histocompatible cells for autologous therapies without sperm donation, as demonstrated in derivations of functional neuronal cells from human pESCs.^[91]^[92]

Gynogenesis

Gynogenesis is a form of asexual reproduction in which sperm from a male activates the development of an unfertilized or unreduced egg, but the paternal genome is excluded, resulting in offspring that are genetically identical clones of the mother. This process ensures that all genetic material is maternally derived, with the sperm serving only as a trigger for embryogenesis without contributing DNA.^[93] Unlike true parthenogenesis, which proceeds without any male involvement, gynogenesis depends on the physical presence and activation signal from sperm, often leading to the maintenance of clonal lineages over generations.^[94] The underlying mechanisms of gynogenesis typically involve the sperm penetrating the egg and inducing physiological changes, such as an increase in intracellular calcium that resumes meiosis or initiates cleavage, while the sperm nucleus remains condensed or is actively discarded during early embryonic divisions. In many cases, this exclusion occurs through the failure of the sperm pronucleus to properly integrate with the maternal genome, sometimes forming an uncondensed chromatin clump that is eliminated at the first metaphase. For instance, in polyploid systems, premeiotic endomitosis may double the maternal chromosomes, allowing synapsis of sister chromatids to form pseudo-bivalents that bypass reduction, with the paternal contribution rejected post-activation.^[95] These processes prevent genetic recombination from the male, preserving maternal heterozygosity and ploidy.^[96] Natural gynogenesis is documented in select fish and amphibian lineages, where it sustains all-female populations reliant on sperm from related sexual species. A prominent example is the Amazon molly (Poecilia formosa), a triploid hybrid fish that originated from interspecific crosses but reproduces exclusively via gynogenesis, using sperm from sympatric species such as P. mexicana or P. latipinna to trigger intrafollicular egg development, yielding daughters identical to the mother.^[93] Similarly, in salamanders of the Ambystoma complex, unisexual triploid females (e.g., those with the LJJ genotype involving A. laterale, A. jeffersonianum, and A. tigrinum genomes) employ gynogenesis, where sperm from bisexual males stimulates unreduced eggs but is discarded, producing clonal offspring that perpetuate the maternal lineage.^[95] These systems highlight gynogenesis as a sperm-dependent strategy for clonal propagation in hybrid-derived taxa.

Hybridogenesis

Hybridogenesis is a specialized reproductive mode observed in certain hybrid organisms, particularly in amphibians and fish, where hybrid females produce gametes that clonally transmit one parental genome while eliminating the other during gametogenesis. This process ensures the viability of offspring by requiring fertilization from males of one parental species, which provides the discarded genome. Unlike fully asexual reproduction, hybridogenesis combines elements of clonality and sexuality, allowing hybrids to perpetuate their lineage without complete genetic recombination but with dependence on sexual parental populations.^[97] The mechanism involves premeiotic elimination of one parental genome in the hybrid's germ cells, followed by endoduplication of the retained genome to restore diploidy before meiosis. The resulting haploid gametes carry only the cloned parental set and must be fertilized by sperm from the species contributing the eliminated genome, typically through backcrossing to a parental species. This restores the hybrid genotype in the next generation, preventing the accumulation of deleterious mutations through occasional genetic input from the sexual partner. In some systems, the eliminated genome may vary, leading to intrapopulation diversity in transmission patterns.^[98] A prominent example occurs in the water frog complex Pelophylax esculentus (hybrids of P. lessonae and P. ridibundus), where hybrid females often eliminate the P. lessonae genome and clonally transmit the P. ridibundus genome in eggs, which are then fertilized by P. lessonae sperm to produce new hybrids. Similarly, in the fish genus Poeciliopsis, all-female hybrid lineages such as P. 2 monacha-lucida eliminate the paternal P. lucida genome during oogenesis, transmitting only the maternal P. monacha genome clonally; fertilization by P. lucida males renews the hybrid form. These systems demonstrate how hybridogenesis sustains unisexual populations in sympatry with parental species.^[97]^[99] Evolutionarily, hybridogenesis maintains hybrid lineages by avoiding the genetic stagnation of pure clonality while incorporating fresh genetic material via backcrossing, fostering diversity through multiple origins and interactions with sexual hosts. However, its long-term stability is limited, as hybrid populations rely on the persistence and availability of parental species for reproduction; disruptions in these dynamics can lead to lineage extinction. Compared to parthenogenesis, which enables autonomous development without sperm, hybridogenesis remains obligately sperm-dependent, classifying it as hemiclonal rather than fully asexual reproduction.^[99]^[100]

Evolutionary and Ecological Implications

Advantages and Disadvantages

Parthenogenesis offers several ecological and fitness advantages as a reproductive strategy, particularly in environments where sexual reproduction may be hindered. One key benefit is the potential for rapid population growth, as parthenogenetic females produce only female offspring, effectively doubling the reproductive rate compared to sexual populations where half the offspring are male. This transmission advantage allows for quicker colonization and expansion in suitable habitats. Additionally, parthenogenesis eliminates the costs associated with mate searching, courtship, and competition for mates, which can be energetically expensive and time-consuming in sexual species.^[101] It is especially effective in low-density populations or isolated settings, such as newly colonized islands or fragmented habitats, where finding a mate is unlikely, enabling isolated females to reproduce and establish populations without delay.^[102] Despite these benefits, parthenogenesis carries significant disadvantages related to genetic risks and long-term viability. The lack of genetic recombination results in reduced diversity among offspring, increasing susceptibility to inbreeding depression, where harmful recessive alleles become expressed and lower fitness.^[103] Parthenogenetic lineages are also more vulnerable to evolving parasites and pathogens, as predicted by the Red Queen hypothesis, which posits that sexual reproduction provides an advantage through genetic variability that helps hosts evade coevolving antagonists. Furthermore, without meiosis to purge deleterious mutations, parthenogens suffer from their accumulation via Muller's ratchet, leading to a progressive decline in fitness over generations.^[104] Empirical evidence highlights both the successes and limitations of parthenogenesis. Bdelloid rotifers, an ancient asexual clade persisting for over 40 million years, demonstrate thriving despite asexuality through enhanced DNA repair mechanisms that mitigate mutation accumulation and enable survival in harsh, variable environments.^[105] In vertebrates, such as parthenogenetic lizards and snakes, initial hybrid origins often confer hybrid vigor, boosting early fitness through heterozygosity, but populations typically exhibit declines and high extinction rates over time due to genetic constraints.^[106]^[103] Facultative parthenogenesis, where organisms can switch between asexual and sexual modes, represents a key trade-off that enhances adaptability by combining rapid asexual reproduction for immediate survival with periodic sexual reproduction to restore genetic diversity.^[107] This flexibility allows species to exploit transient opportunities, such as mate scarcity, while avoiding the long-term pitfalls of obligate asexuality.^[108]

Evolutionary Origins and Genetic Consequences

Parthenogenesis has evolved multiple times across animal lineages, often arising from hybridization events between closely related sexual species. In whiptail lizards (genus Aspidoscelis), diploid parthenogens originated through interspecific hybridization, where genomes from two divergent parental species combine, followed by genome duplication to restore fertility and maintain heterozygosity.^[109] This hybrid origin is evident in admixed nuclear genomes and distinct mitochondrial haplotypes tracing back to specific bisexual ancestors, as seen in species like A. uniparens.^[110] Such hybridization disrupts meiotic pairing but enables automictic parthenogenesis, where diploid eggs form via premeiotic endomitosis.^[109] Bdelloid rotifers, a clade exceeding 460 species, have long been considered to have reproduced asexually for over 40 million years, as inferred from phylogenetic analyses and fossil-calibrated trees, though recent studies suggest possible cryptic genetic exchange or rare sex.^[111] Their genomes show degeneration of meiosis-associated genes, including losses of Rad52 and Msh3, rendering conventional meiosis incompatible and supporting ameiotic reproduction via gene conversion and horizontal gene transfer.^[111] Genetic consequences of parthenogenesis include progressive increases in homozygosity, particularly under automixis, where central fusion or terminal fusion mechanisms restore diploidy but erode heterozygosity over generations. In parthenogenetic stick insects (Timema spp.), heterozygosity drops below 10⁻⁵—over 140 times lower than in sexual relatives—leading to reduced genetic diversity and weaker positive selection efficiency.^[49] Long-term asexual lineages also exhibit telomere shortening due to impaired maintenance, as observed in parthenogenetic nematodes where defective telomere function promotes chromosome fusions and subtelomeric expansions.^[112] Recent genomic studies illuminate polyploid parthenogenesis evolution, such as in the triploid flowerpot snake (Indotyphlops braminus). A 2025 analysis revealed its ~5.4 Gb genome comprises three subgenomes diverged ~41 million years ago, with triploidy arising from incomplete lineage sorting and chromosome fusions, including extras on chromosomes 17 and 37; premeiotic endoreplication enables meiosis-like pairing despite odd ploidy.^[61] These findings highlight hybridization and polyploidy as recurrent pathways stabilizing parthenogenesis. In stable environments, parthenogenesis may reverse the two-fold cost of sex by avoiding recombination disadvantages, allowing clonal lineages to persist without the fitness penalty of male production.^[113]

Cultural and Scientific Significance

Representations in Culture

Parthenogenesis has long served as a motif in mythology, symbolizing divine autonomy and creation without male involvement. In ancient Greek cosmology, as described in Hesiod's Theogony, several primordial deities reproduce parthenogenetically, including Gaea (Earth), who generates Uranus (Sky) and the mountains, and Night, who births Aether and Day without a consort.^[114] This asexual reproduction underscores themes of self-sufficiency among female entities in early cosmogonies, where female figures initiate the universe's order independently. Similarly, the birth of Athena from Zeus's head is interpreted as a form of pseudo-parthenogenesis, emerging fully armed and embodying wisdom without a maternal womb, though originating from the swallowed goddess Metis.^[115] In Christian tradition, the Virgin Birth of Jesus is sometimes analogized to parthenogenesis in theological and scientific discussions, portraying Mary's conception as a miraculous, unfertilized divine intervention that parallels natural asexual processes observed in other species.^[116] In literature and art, parthenogenesis frequently symbolizes female independence and challenges patriarchal norms. Charlotte Perkins Gilman's 1915 utopian novel Herland depicts an all-female society that sustains itself through parthenogenesis, where women produce offspring from unfertilized eggs, representing liberation from male dominance and the potential for self-reliant matriarchal communities.^[117] This motif extends to figures like Gilman, who drew on emerging biological concepts to advocate for women's reproductive autonomy and critique societal constraints on motherhood. In avant-garde fiction, parthenogenesis often explores ethical tensions around reproduction, as seen in works that duel between empowerment and isolation, highlighting fears of genetic uniformity while affirming women's agency over procreation.^[118] Modern media inverts or reimagines these themes to critique reproductive control. Margaret Atwood's The Handmaid's Tale (1985), adapted into film and television, portrays a dystopian regime enforcing coerced insemination on women amid fertility crises, emphasizing subjugation and the loss of bodily sovereignty.^[119] Bioethics fiction further engages parthenogenesis in narratives of technological intervention, such as speculative tales of artificial virgin births that provoke debates on cloning ethics and human dignity, often framing it as a tool for either emancipation or societal peril.^[120] Overall, parthenogenesis in culture embodies dual symbolism: divine intervention in mythic origins, where it signifies sacred creation, and female independence in contemporary works, underscoring autonomy amid reproductive debates.^[121]

Recent Research and Future Directions

Recent research on parthenogenesis has advanced our understanding of its genetic and evolutionary mechanisms, particularly through genomic analyses and experimental models in diverse taxa. In 2023, studies on facultative parthenogenesis in Drosophila identified key genes, such as those involved in meiotic suppression and diploidization, that enable the switch between sexual and asexual reproduction modes, providing insights into the minimal genetic changes required for this reproductive flexibility.^[122] Similarly, a 2024 investigation documented recurrent facultative parthenogenesis in the endangered common smooth-hound shark (Mustelus mustelus) as an adaptive strategy in male-scarce environments, where unfertilized eggs developed into viable offspring over multiple generations, highlighting its role in population persistence under ecological stress.^[62] Building on these, a 2025 genomic study of the triploid flowerpot snake (Indotyphlops braminus), the only known parthenogenetic serpent, revealed adaptations in DNA repair pathways and chromosome stability that sustain long-term asexuality, offering a model for triploid genome evolution.^[61] These findings address critical gaps in the evolutionary genomics of asexuality, demonstrating how parthenogenetic lineages accumulate mutations at rates comparable to sexual counterparts but with enhanced repair mechanisms to mitigate deleterious effects.^[61] Additionally, research has begun exploring climate change impacts on facultative parthenogenetic species, showing that warming temperatures enhance asexual fitness and survival in organisms like the freshwater cnidarian Hydra oligactis by favoring parthenogenetic reproduction during stressful conditions, potentially altering population dynamics in warming ecosystems.^[123] Looking ahead, parthenogenesis holds promise for therapeutic applications in treating human infertility, with artificial oocyte activation techniques—mimicking parthenogenetic processes—improving fertilization success in intracytoplasmic sperm injection (ICSI) cycles for couples with activation failures.^[124] In synthetic biology, efforts to engineer apomictic crops via parthenogenesis induction could revolutionize agriculture by enabling clonal seed production in hybrids, as demonstrated in recent sorghum models that maintain hybrid vigor across generations without traditional breeding.^[125] Emerging integrations of CRISPR technologies for controlled clonality, such as dCas9-mediated activation of parthenogenesis genes in maize egg cells, further support precise induction of asexual reproduction in plants.^[126] However, advancing vertebrate parthenogenesis induction raises ethical concerns, necessitating frameworks that address embryo model regulations, genetic diversity loss, and equitable access in reproductive technologies.

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The adaptability of facultative parthenogenesis and 'multiple ... - NIH
Apr 30, 2025 · While obligate parthenogenesis is somewhat rare among vertebrates, facultative parthenogenesis is more abundant and appears to be present in ...Missing: differences advantages
[107]
[PDF] Facultative parthenogenesis: a transient state in transitions between ...
Jul 13, 2023 · Automixis with “complete” heterozygosity loss in a single generation is also the proposed mechanism of parthenogenesis in obligate asexual ...
[108]
Origin and Evolution of Parthenogenetic Genomes in Lizards
Mar 9, 2010 · Squamate reptiles are the only vertebrates that exhibit true naturally occurring parthenogenesis, where mothers produce unreduced, genetically ...Missing: decline | Show results with:decline
[109]
The evolutionary network of whiptail lizards reveals predictable ...
Aug 11, 2022 · Hybridization between diverging lineages is associated with the generation and loss of species diversity, introgression, adaptation, and changes in ...
[110]
Genomic evidence for ameiotic evolution in the bdelloid rotifer ...
Jul 21, 2013 · Here we report the genome of a bdelloid rotifer, Adineta vaga (Davis, 1873) 3 , and show that its structure is incompatible with conventional meiosis.
[111]
The adaptability of facultative parthenogenesis and 'multiple ...
Apr 30, 2025 · Finally, parthenogenetic reproduction may be particularly beneficial where environmental conditions are stable, as it allows beneficial allelic ...
[112]
Parthenogenesis in Hesiod's Theogony - UA Campus Repository
Five of Hesiod's deities reproduce parthenogenetically: Chaos, Gaea, Night, Strife, and Hera, of whom all but the sexually indeterminate Chaos are female.
[113]
[PDF] The Generation of Monsters in Hesiod - Jenny Strauss Clay
Nov 13, 2003 · The most striking exception is Zeus himself with his pseudo-parthenogenesis of Athena and his various marriages to his sister Olympians. 13.
[114]
The Science Behind the Virgin Birth - VICE
Dec 25, 2015 · In parthenogenesis, a cell within an animal splits via meiosis, halving its genetic material. Then one of these split cells, essentially an egg, ...
[115]
Economics, Evolution, and Feminism in Charlotte Perkins Gilman's ...
See Sandra Gilbert's “She in Herland: Feminism as Fantasy” where Gilbert lists several possible reasons for Gilman's fantasy of woman-parthenogenesis in Herland ...<|separator|>
[116]
Sex and the sawfish: the world of wildlife virgin births - The Guardian
Jun 2, 2015 · Charlotte Perkins Gilman's 1915 novel, Herland, foresaw a female society that reproduced entirely through parthenogenesis, partly as a response ...
[117]
"The Duelling Fictions of Parthenogenesis." Yearbook of the Institute ...
Avant-garde fiction explores themes of parthenogenesis, highlighting societal fears and challenges to traditional reproductive norms. Parthenogenesis is ...
[118]
How Darwin's sexual selection theory co-stars in 'The Handmaid's ...
Jan 28, 2020 · The Handmaid's Tale presents a society that is reduced to the gender stereotypes that were inscribed in Darwin's theory of sexual selection.<|control11|><|separator|>
[119]
Moral and Fictional Discourses on Assisted Reproductive ...
Dec 11, 2019 · Moreover, in the future, it may be possible to create an embryo to use for reproduction through parthenogenesis ... Clearly, the relationship ...
[120]
[PDF] Parthenogenesis (“Virgin Birth”): A Woman's Futile Attempt at ...
The goal of this paper is to show the way in which a current fictional depiction of historical events occurring in 1955–1956 is influenced by fourth-wave ...
[121]
A genetic basis for facultative parthenogenesis in Drosophila
Facultative parthenogenesis enables sexually reproducing organisms to switch between sexual and asexual parthenogenetic reproduction.Missing: prerequisites | Show results with:prerequisites<|control11|><|separator|>
[122]
Warming increases survival and asexual fitness in a facultatively ...
Apr 10, 2023 · Increasing temperature due to recent global warming is likely to disrupt population dynamics of facultatively sexual animals because of the ...
[123]
Article Artificial oocyte activation improves ICSI outcomes following ...
AOA is a powerful tool that can transform clinical outcomes for couples experiencing apparently unexplained fertilization abnormalities.
[124]
CRISPR/dCas-mediated gene activation toolkit development and its ...
Sep 12, 2022 · This study describes a parthenogenesis engineering approach using a programmable CRISPR/dCas9 transcription factor to target ZmBBM2 in maize egg cells.Missing: assisted | Show results with:assisted