Parthenogenesis in Animals: How Some Species Reproduce Without Mating

Parthenogenesis is a reproductive mode in which an egg develops into an offspring without fertilization by a male gamete. This article explains the developmental mechanisms that make parthenogenesis possible, how it differs from sexual reproduction, and which animal groups exhibit this trait. Readers will gain a working understanding of obligate and facultative parthenogenesis, the genetic consequences of asexual reproduction, and practical guidance for identifying parthenogenetic reproduction in research and captive management settings.

At a Glance: Parthenogenesis vs. Sexual Reproduction

Defining Parthenogenesis and Its Scope

Parthenogenesis describes a reproductive strategy where a female produces offspring without any paternal contribution. The term derives from Greek roots meaning virgin birth. In animals, this mode of reproduction appears across a broad taxonomic range, though its frequency varies greatly between groups. Invertebrates such as aphids, rotifers, and some insects use parthenogenesis regularly. Vertebrates show the trait far less often, with obligate parthenogenesis documented in roughly 100 vertebrate species and about 1,000 invertebrate species 15.

The developmental challenge of parthenogenesis is substantial. A fertilized egg normally receives two essential contributions from the male gamete: a set of chromosomes and a centrosome, the organelle that organizes microtubules during cell division. An unfertilized egg lacks both. The absence of a paternal chromosome set forces the egg to restore diploidy through modified meiosis. The absence of a centrosome requires the egg to assemble this structure through alternative pathways 3 5.

These two constraints form what researchers describe as a near-absolute barrier against the gradual evolution from sporadic to obligate parthenogenesis. The barrier helps explain why regular facultative parthenogenesis, where females can switch between sexual and asexual reproduction, is so rare and entirely absent in vertebrates 3.

How Parthenogenesis Differs from Sexual Reproduction

Sexual reproduction in animals typically requires two differently shaped haploid cells, the male and female gametes. Each gamete provides genetic material to restore diploidy in the zygote. The process depends on a defined sequence of steps completed in the correct order and location. Pronuclear migration and formation of the first mitotic spindle promote the mixing of parental chromosomes and formation of the zygotic nucleus. A complex microtubule network ensures proper execution of these steps 5.

Centrosome inheritance during fertilization is biparental. Both gametes provide essential components to build a functional centrosome. Cells actively check for the presence of only one centrosome to avoid multipolar spindle formation. Parthenogenetic development breaks this model. In insects, parthenogenetic eggs are naturally depleted of centrosomes, and development is ensured by the de novo assembly of multiple centrosomes 5.

The genetic consequences of parthenogenesis differ from sexual reproduction in predictable ways. When recombination is preserved in parthenogens, most meiotic modifications result in a loss of heterozygosity. This loss can be prevented if recombination is abolished or strongly reduced during meiotic prophase. Recent examples have demonstrated that loss of heterozygosity can be prevented despite recombination, illustrating the constraints underlying the origin and evolution of asexuality 8.

Types of Parthenogenesis

Obligate Parthenogenesis

Obligate parthenogenesis describes species that reproduce exclusively through this mode. Females produce offspring without mating, and sexual reproduction does not occur. This form is relatively rare, found in roughly 100 vertebrate species and about 1,000 invertebrate species 15. Obligate parthenogens have evolved the genetic and developmental machinery required for regular parthenogenesis, and shifts between different types of parthenogenesis can occur relatively easily once this machinery is in place 3.

Facultative Parthenogenesis

Facultative parthenogenesis describes species where females can reproduce both sexually and parthenogenetically. This flexibility allows females to produce offspring without a mate when conditions demand it, while retaining the option of sexual reproduction. In vertebrates, facultative parthenogenesis is observed in some elasmobranchs, including sharks 15. The phenomenon is mainly documented in captivity, where long-term observation is possible 12 15.

Automixis and Terminal Fusion

Parthenogenetic mechanisms vary in how they restore diploidy. Terminal fusion automixis is one mechanism where the egg nucleus fuses with a polar body, restoring diploidy while producing homozygosity at genetic markers. Genetic analysis of parthenogenetic common smooth-hound sharks showed homozygosity at each genetic marker, consistent with terminal fusion automixis 15.

Developmental Constraints on Parthenogenesis

The evolution of parthenogenesis faces two major developmental constraints. The first constraint comes from the absence of a centrosome in an unfertilized ovum. The second comes from the missing set of chromosomes. Both constraints must be broken for parthenogenesis to evolve 3.

Breaking the constraint of missing chromosomes is the best understood process. It generally involves rare occasions of drastic changes in meiosis. These changes can be induced or facilitated by sudden cytological events, including repeated rounds of hybridization, endosymbiont infections, and contagious infections 3.

Once the genetic and developmental machinery is in place for regular or obligate parthenogenesis, shifts to other types can evolve relatively easily. Examples include shifts from facultative to obligate parthenogenesis, or from pseudoarrhenotoky to haplodiploidy 3.

Parthenogenesis in Insects

Insects provide some of the best-studied examples of parthenogenesis. The migratory locust (Locusta migratoria) shows a clear relationship between parthenogenesis and phase change, a phenomenon where locusts shift between solitary and gregarious forms. Research comparing the two phases found that solitary locusts exhibit higher parthenogenetic capacity than gregarious locusts, as shown by greater total oviposition quantity and higher hatching rates. However, parthenogenesis resulted in significantly lower hatching rates compared to sexually fertilized females 13.

The study also found that gregarious locusts isolated after eclosion showed increased parthenogenetic capacity, depending on their juvenile density. This finding suggests that environmental conditions experienced during development influence the capacity for parthenogenetic reproduction 13.

Insects also demonstrate the cellular mechanisms that make parthenogenesis possible. Parthenogenetic development in insects is ensured by the de novo assembly of multiple centrosomes. This process provides a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation 5.

Parthenogenesis in Reptiles

Reptiles include some of the most notable vertebrate examples of parthenogenesis. Research on parthenogenesis in reptiles has examined both constraints and correlations associated with the loss of sexual reproduction 16. Molecular genetic evidence has confirmed parthenogenesis in the Burmese python (Python molurus bivittatus), demonstrating that this reproductive mode occurs even in large constricting snakes 17.

The study of parthenogenesis in reptiles has informed broader questions about gene versus environment interactions. Reproductive traits such as parthenogenesis in planaria and polyembryony in armadillos have been used to investigate classic questions about the relative contributions of genes and environment to development 6.

Parthenogenesis in Sharks and Other Fish

Elasmobranchs, the group containing sharks and rays, show a notable pattern of facultative parthenogenesis. The common smooth-hound shark (Mustelus mustelus), classified as endangered by the IUCN, has been documented reproducing parthenogenetically in captivity. A study at the Cala Gonone Aquarium in Sardinia, Italy recorded a parthenogenetic reproductive episode in April 2024 that produced two offspring. Genetic analyses excluded paternal contribution, supporting the occurrence of facultative parthenogenesis 12.

The recurring pattern of parthenogenesis in this species is remarkable. Research has shown that parthenogenesis can occur annually in these sharks, alternating between two females. The evidence conclusively excludes long-term sperm storage as a cause. This recurring pattern supports the hypothesis of a structured reproductive dynamic and suggests the involvement of individual-specific or environmentally mediated factors 12 15.

The findings have implications for conservation. Understanding the reproductive flexibility of endangered species like M. mustelus could inform conservation efforts for captive populations 15.

In fish, genomic evidence has revealed alternative reproductive modes that may be overlooked instead of truly absent. A study of the cichlid fish Cyphotilapia frontosa investigated an unusual reproductive event where a female produced offspring in the absence of a male. Whole-genome sequencing analysis compared patterns of heterozygosity with those from a wild family and a closely related inbred family. The uniparental family exhibited reduced genetic diversity, elevated relatedness, and genome-wide patterns of homozygosity distinct from those expected under parthenogenesis or inbreeding, but consistent with self-fertilization 11.

Parthenogenesis in Mammals

Parthenogenesis is not a form of natural reproduction in mammals. However, mammalian oocytes can undergo parthenogenetic activation under appropriate stimuli. This process produces parthenotes, which differ fundamentally from embryos produced by fertilization 10.

The biological mechanisms regulating parthenogenetic activation in mammals have been studied for their research applications. Based on the differences between parthenotes and embryos, parthenogenesis has been proposed as an experimental tool to investigate embryo development. This approach may address some ethical concerns associated with the use of human embryos for experimental purposes. Potential applications include in vitro assays to study aspects of assisted reproductive technologies, toxicology, and stem cell research 10.

Research on mammalian oocytes has also examined the role of nucleoli in oocyte maturation and early embryonic development. The germinal vesicles and pronuclei of some mammals contain clearly visible nucleoli with atypical morphological structure. These nucleoli can be manipulated using micromanipulation techniques, allowing researchers to uncover their functions in oocyte maturation and early embryonic development 7.

Microbial and Endosymbiont Influences on Parthenogenesis

Microbial infections can influence reproductive strategies in insects. Sex ratio bias in insects can be introduced by feminization, parthenogenesis, cytoplasmic incompatibility, or male-killing. Intracellular bacteria such as Wolbachia and Spiroplasma are known male-killing agents. A non-bacterial male-killing agent, the Osugoroshi virus, has been found in the oriental tea tortrix 4.

Rickettsia species are arthropod-hosted endosymbionts with a wide range of ecologies, including induction of parthenogenesis and male-killing. These bacteria attract attention because many are vertebrate pathogens, but their ecological roles in arthropod reproduction are significant 14.

Endosymbiont infections are among the sudden cytological events that can facilitate the drastic meiotic changes required for parthenogenesis to evolve 3.

Practical Assessment: Identifying Parthenogenesis in a Population

For researchers, wildlife managers, and aquaculture professionals, identifying parthenogenesis requires systematic observation and genetic confirmation. The following steps provide a practical framework.

Step 1: Establish the Reproductive Context

Document the housing or environmental conditions. Note whether females had any contact with males, the duration of isolation, and the reproductive history of each female. In captive settings, verify that sperm storage cannot explain the observed reproduction. For the common smooth-hound shark, genetic evidence was required to conclusively exclude long-term sperm storage as a cause of parthenogenetic reproduction 15.

Step 2: Observe Reproductive Output

Record the number of offspring produced, the timing of reproductive events, and whether reproduction occurs repeatedly. In M. mustelus, parthenogenesis occurred annually and alternated between two females, a pattern that would be visible in long-term records 12 15.

Step 3: Collect Genetic Samples

Obtain tissue samples from the mother and offspring. Genetic analysis should examine multiple markers across the genome. Homozygosity at each genetic marker is consistent with terminal fusion automixis, a mechanism observed in parthenogenetic sharks 15. Whole-genome sequencing provides the highest resolution for distinguishing parthenogenesis from self-fertilization and other alternative reproductive modes 11.

Step 4: Compare with Expected Patterns

Compare observed patterns of heterozygosity with those expected under different reproductive modes. Parthenogenesis, self-fertilization, and inbreeding produce distinct genomic signatures. The uniparental cichlid family showed patterns consistent with self-fertilization instead of parthenogenesis, demonstrating the importance of genetic discrimination 11.

Step 5: Document and Report

Maintain detailed records of reproductive events, including dates, parentage, offspring counts, and genetic results. This information supports conservation management decisions for captive populations of endangered species 12 15.

Records and Measurements for Parthenogenesis Studies

Common Failure Patterns in Parthenogenesis Research

Misattributing Sperm Storage as Parthenogenesis

A common error is concluding parthenogenesis occurred when long-term sperm storage explains the reproduction. Genetic evidence is required to exclude this possibility. In the common smooth-hound shark, genetic analysis conclusively excluded long-term sperm storage as a cause of parthenogenetic reproduction 15.

Confusing Parthenogenesis with Self-Fertilization

Parthenogenesis and self-fertilization both produce offspring from a single parent, but they are genetically distinct. Parthenogenesis involves an egg developing without fertilization, while self-fertilization involves fusion of male and female gametes from the same individual. Genomic analysis of the cichlid fish C. frontosa distinguished these modes by comparing patterns of homozygosity 11.

Assuming Parthenogenesis Is Always Obligate

Facultative parthenogenesis, where females can reproduce both sexually and parthenogenetically, is more common in vertebrates than obligate forms. Researchers should not assume that a species capable of parthenogenesis has lost the capacity for sexual reproduction 15.

Overlooking Environmental Influences

Parthenogenetic capacity can vary with environmental conditions. In migratory locusts, solitary locusts exhibited higher parthenogenetic capacity than gregarious locusts, and gregarious locusts isolated after eclosion showed increased parthenogenetic capacity depending on their juvenile density 13. Studies that ignore these environmental influences may produce misleading conclusions.

Limitations of Current Knowledge

The majority of mechanisms underlying parthenogenesis remain unknown in many species. Researchers emphasize the need to revisit older literature using state-of-the-art cytological imaging and genomic techniques to shed light on unexplored processes 8.

The combination of the two developmental constraints forms a near-absolute barrier against the gradual evolution from sporadic to obligate or regular facultative parthenogenesis. This barrier explains why regular facultative parthenogenesis is so rare and entirely absent in vertebrates 3.

Parthenogenesis in elasmobranchs is mainly documented in captivity, where detailed long-term observation is possible. Whether the patterns observed in captivity reflect natural reproductive behavior in the wild remains uncertain 12 15.

Welfare and Safety Context

For professionals managing captive animals, parthenogenesis has practical welfare implications. Parthenogenetic offspring may have reduced genetic diversity and elevated homozygosity, which can affect health and viability 15. Managers should monitor parthenogenetic offspring for developmental abnormalities and reduced fitness.

In aquaculture and conservation breeding programs, understanding parthenogenesis informs breeding decisions. For endangered species like M. mustelus, the capacity for facultative parthenogenesis provides a reproductive option when mates are unavailable, but it should not replace genetic management through sexual reproduction 12 15.

For researchers working with mammalian oocytes, parthenogenetic activation raises ethical and legal considerations. The widespread belief that human embryos should not be created and studied for research purposes only regulates and limits human embryological studies. Parthenotes differ from embryos, and their use as an experimental tool may address some ethical concerns, but institutional policies and jurisdiction-specific requirements apply 10.

Professional Escalation Criteria

Consult a specialist in reproductive biology or genetics when any of the following situations arise:

• Genetic analysis is needed to distinguish parthenogenesis from self-fertilization or sperm storage
• Parthenogenetic reproduction occurs in a species of conservation concern
• Parthenogenetic offspring show reduced viability or developmental abnormalities
• Endosymbiont or viral infections are suspected of influencing reproductive patterns
• Research involving mammalian oocyte activation is being planned
• Management decisions for captive breeding programs depend on understanding reproductive mode

Decision Framework: Choosing Between Parthenogenesis and Sexual Reproduction in Captive Management

For professionals managing captive animal populations, the appearance of offspring without observed mating presents a practical decision point. The choice between allowing parthenogenetic reproduction, encouraging sexual reproduction, or intervening to prevent either outcome depends on species status, genetic management goals, and the developmental viability of parthenogenetic offspring. This framework provides a structured approach for making those decisions.

Step 1: Confirm the Reproductive Mode Before Acting

The first decision gate requires genetic confirmation of the reproductive mode. Visual observation of offspring without a male present is insufficient evidence for parthenogenesis. Long-term sperm storage can produce offspring long after male contact, and self-fertilization in hermaphroditic species produces genetically distinct patterns. In the common smooth-hound shark, genetic analysis was required to conclusively exclude long-term sperm storage as a cause of parthenogenetic reproduction 15. Whole-genome sequencing provides the resolution needed to distinguish parthenogenesis from self-fertilization, as demonstrated in the cichlid fish Cyphotilapia frontosa where genomic patterns were consistent with selfing instead of parthenogenesis 11.

Collect tissue samples from the mother and all offspring. Genotype offspring at multiple markers across the genome. Homozygosity at each genetic marker is consistent with terminal fusion automixis, the mechanism observed in parthenogenetic sharks 15. Compare these patterns with those expected under inbreeding and self-fertilization before concluding that parthenogenesis occurred.

Step 2: Assess Species Conservation Status and Breeding Goals

The management response differs based on whether the species is of conservation concern. For endangered species like the common smooth-hound shark, classified as endangered by the IUCN, facultative parthenogenesis provides a reproductive option when mates are unavailable 15. This capacity may support population persistence in captive settings where mate availability is limited. However, parthenogenesis should not replace genetic management through sexual reproduction because parthenogenetic offspring show reduced genetic diversity and elevated homozygosity 15.

For species not of conservation concern, the decision focuses on research value and colony management. Parthenogenetic lines can serve as experimental systems for studying centriole assembly, centrosome formation, and meiotic modifications 5 8. The August Krogh principle supports using unusual reproductive traits to understand developmental phenomena more generally, as demonstrated with parthenogenesis in planaria informing gene versus environment questions 6.

Step 3: Evaluate Parthenogenetic Offspring Viability

Parthenogenetic offspring may show reduced viability compared with sexually produced offspring. In migratory locusts, parthenogenesis resulted in significantly lower hatching rates compared with sexually fertilized females 13. Document hatching rates, developmental abnormalities, and long-term survival for parthenogenetic offspring. Compare these metrics with those from sexually produced offspring in the same facility.

The genetic mechanism matters for viability predictions. Terminal fusion automixis produces homozygosity at genetic markers, which can expose recessive deleterious alleles 15. Mechanisms that prevent loss of heterozygosity despite recombination may produce more viable offspring, though the majority of these mechanisms remain unknown in many species 8.

Step 4: Determine Whether to Intervene

Intervention decisions depend on the answers to the first three steps. If genetic confirmation shows parthenogenesis in a conservation priority species with viable offspring, continued monitoring without intervention may be appropriate. The recurring pattern in common smooth-hound sharks, where parthenogenesis occurred annually and alternated between two females, suggests a structured reproductive dynamic that may be adaptive under limited mate availability 12 15.

If parthenogenetic offspring show poor viability or if genetic diversity goals require sexual reproduction, intervention is indicated. Introducing a genetically compatible male or using assisted reproductive techniques may restore sexual reproduction. For species where parthenogenesis is facultative, females retain the capacity for sexual reproduction, so mate introduction can shift reproductive mode 15.

Step 5: Document Decisions and Outcomes

Record the rationale for each management decision, the genetic evidence supporting the reproductive mode determination, and the outcomes of any intervention. This documentation supports adaptive management as new information becomes available. The finding that parthenogenesis in elasmobranchs is mainly documented in captivity highlights the importance of detailed long-term observation for detecting and understanding this reproductive mode 12 15.

Decision Matrix for Captive Population Management

Environmental Factors in Decision Making

Environmental conditions during development influence parthenogenetic capacity. In migratory locusts, solitary locusts exhibited higher parthenogenetic capacity than gregarious locusts, and gregarious locusts isolated after eclosion showed increased parthenogenetic capacity depending on their juvenile density 13. Managers should record density conditions and social environment when interpreting parthenogenetic events and predicting future reproductive patterns.

The phase change phenomenon in locusts demonstrates that parthenogenetic capacity is not fixed within a species. Management decisions should account for the possibility that environmental manipulation could influence the frequency of parthenogenetic reproduction. Conversely, environmental conditions that reduce parthenogenetic capacity may be used to encourage sexual reproduction when genetic diversity goals require it.

Escalation Criteria for Specialist Consultation

Consult a reproductive biologist or geneticist when the decision framework reaches any of the following points:

• Genetic analysis is required to distinguish parthenogenesis from self-fertilization or sperm storage
• Parthenogenetic offspring show unexplained developmental abnormalities
• The species is of conservation concern and parthenogenesis affects breeding program design
• Endosymbiont or viral infections are suspected of influencing reproductive patterns
• The decision involves introducing or removing males from a breeding population
• Parthenogenetic reproduction recurs across multiple reproductive events in the same individuals

Frequently Asked Questions

What is the difference between parthenogenesis and sexual reproduction?

Parthenogenesis produces offspring from an unfertilized egg with no paternal genetic contribution. Sexual reproduction requires two gametes, each providing genetic material to restore diploidy in the zygote. Parthenogenesis requires modified meiosis to produce unreduced gametes and compensate for the lack of outcrossing 5 8.

Which animal groups exhibit parthenogenesis?

Parthenogenesis occurs across invertebrates such as aphids, rotifers, and some insects, and in vertebrates including some reptiles, sharks, and fish. Obligate parthenogenesis is found in roughly 100 vertebrate species and about 1,000 invertebrate species. Facultative parthenogenesis, where females can reproduce both sexually and parthenogenetically, is observed in some vertebrates including elasmobranchs 15.

Can mammals reproduce through parthenogenesis?

Parthenogenesis is not a form of natural reproduction in mammals. However, mammalian oocytes can undergo parthenogenetic activation under appropriate stimuli, producing parthenotes that differ fundamentally from embryos produced by fertilization. These parthenotes have been proposed as experimental tools for research 10.

How do parthenogenetic eggs overcome the missing centrosome problem?

In insects, parthenogenetic eggs are naturally depleted of centrosomes, and development is ensured by the de novo assembly of multiple centrosomes. This process provides a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation 5.

What is facultative parthenogenesis?

Facultative parthenogenesis describes species where females can reproduce both sexually and parthenogenetically. In vertebrates, this phenomenon is mainly documented in captivity. The common smooth-hound shark shows facultative parthenogenesis, with genetic evidence excluding long-term sperm storage as a cause 12 15.

How is parthenogenesis confirmed genetically?

Genetic analysis examines patterns of heterozygosity in offspring compared with the mother. Homozygosity at each genetic marker is consistent with terminal fusion automixis, a mechanism observed in parthenogenetic sharks. Whole-genome sequencing can distinguish parthenogenesis from self-fertilization and other alternative reproductive modes 11 15.

Can environmental conditions influence parthenogenetic capacity?

Yes. In migratory locusts, solitary locusts exhibit higher parthenogenetic capacity than gregarious locusts. Gregarious locusts isolated after eclosion show increased parthenogenetic capacity depending on their juvenile density. These findings demonstrate that environmental conditions during development influence parthenogenetic reproduction 13.

Why is parthenogenesis rare in vertebrates?

The evolution of parthenogenesis faces two major developmental constraints: the absence of a centrosome and the missing set of chromosomes in an unfertilized ovum. These constraints form a near-absolute barrier against the gradual evolution from sporadic to obligate or regular facultative parthenogenesis, explaining why regular facultative parthenogenesis is entirely absent in vertebrates 3.

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Parthenogenesis and developmental constraints.. Evolution & development, 2020.
• [Osugoroshi virus, a male-killer virus].. Uirusu, 2021.
• Parthenogenesis in Insects: The Centriole Renaissance.. Results and problems in cell differentiation, 2017.
• Putting the August Krogh principle to work in developmental physiology.. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 2021.
• Enucleolation and nucleolus transfer in mammalian oocytes and zygotes.. The International journal of developmental biology, 2019.
• Cracking the meiotic secrets behind animals asexuality: when cytology seduces genomics to decode asexual meiosis.. Journal of evolutionary biology, 2025.
• E. E. Just's broad, yet hidden, influence on modern cell and developmental biology.. Molecular reproduction and development, 2020.
• Parthenogenetic activation: biology and applications in the ART laboratory.. Placenta, 2008.
• Genomic evidence for facultative selfing in the cichlid fish Cyphotilapia frontosa. 2026.
• Recurrent Alternate Parthenogenesis in the Common Smooth-Hound Shark (&lt,i&gt,Mustelus mustelus&lt,/i&gt,) with Additional Cases and Further Evidence for a Putative Adaptive Reproductive Strategy.. 2026.
• Influence of Phase Change on Parthenogenesis in the Migratory Locust: A Behavioral Analysis.. 2025.
• Molecular detection of Rickettsia spp. in ticks (Ixodida: Argasidae and Ixodidae) and mites (Trombiculoidae) from a brazilian taxonomic collection. 2025.
• First report of recurrent parthenogenesis as an adaptive reproductive strategy in the endangered common smooth-hound shark Mustelus mustelus. Scientific Reports, 2024.
• Lost sex in the reptiles: Constraints and correlations. Lost Sex the Evolutionary Biology of Parthenogenesis, 2009.
• Molecular genetic evidence for parthenogenesis in the Burmese python, Python molurus bivittatus. Heredity, 2003.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.