Parthenogenesis is reproduction without fertilization, where an offspring develops from an unfertilized egg. It occurs naturally across a surprisingly wide range of animals and plants, from water fleas and aphids to sharks and Komodo dragons. Far from a biological curiosity, parthenogenesis shapes ecosystems, complicates conservation efforts, and has even inspired approaches to stem cell research and crop breeding. The mechanics are more varied than most people realize, and the evolutionary reasons it persists alongside sexual reproduction remain one of biology’s most actively debated questions.
How an Egg Develops Without Sperm
There are two broad ways an egg can develop into a viable offspring on its own, and the genetic consequences of each are very different. In the first, called apomixis, meiosis is skipped entirely. The egg is produced through a process resembling ordinary cell division, so the offspring is essentially a genetic clone of the mother. In the second, called automixis, the egg still goes through meiosis but then restores its full chromosome count, either by fusing two of its own meiotic products or by duplicating the remaining set after division.1PubMed Central. The evolution of meiotic sex and its alternatives
The distinction matters because automixis tends to produce offspring with increased homozygosity, meaning the two copies of each gene become more alike with each generation. In some forms of automixis, offspring become completely homozygous at regions near the center of the chromosome in a single generation.2PubMed Central. The evolution of meiotic sex and its alternatives Apomixis, by contrast, can preserve the mother’s genetic diversity intact, passing along all her variation to the next generation.3Encyclopedia of Insects. Parthenogenesis in Insects and Mites These two pathways create very different evolutionary trajectories, which is why lumping all parthenogenesis together as “cloning” misses important nuance.
Where Parthenogenesis Shows Up in Nature
Parthenogenesis is far more common than most people assume. Among invertebrates, it is practically routine. Aphids reproduce parthenogenetically for most of the growing season, cranking out genetically identical daughters at astonishing speed, then switch to sexual reproduction when conditions deteriorate. Water fleas in the genus Daphnia follow a similar pattern, called cyclical parthenogenesis, where environmental cues like food quality or day length trigger the shift from asexual to sexual reproduction.4PubMed. Food quality triggers the reproductive mode in the cyclical parthenogen Daphnia (Cladocera) This strategy lets populations explode when resources are abundant, then generate the genetic diversity of sex right before winter or drought hits. Gall wasps, gall midges, and many rotifers use this same alternating approach.
Among vertebrates, parthenogenesis is rarer but far from absent. Several species of whiptail lizards in the American Southwest reproduce exclusively through parthenogenesis. These all-female species arose through hybridization between two sexually reproducing parent species, and they maintain the genetic diversity that came from that original cross.5Oxford Academic (Genome Biology and Evolution). Ancestral Chromosome-Level Assemblies Reveal Posthybridization Genome Evolution in the New Mexico Whiptail Lizard (Aspidoscelis neomexicanus) However, even in these supposedly clonal lizards, recent genomic work has found population-specific losses of heterozygosity across multiple chromosomes, showing that the two parental genomes occasionally pair up and recombine, slowly eroding the original hybrid diversity.
In Hymenoptera, the insect order that includes ants, bees, and wasps, parthenogenesis is baked into the sex-determination system. Unfertilized eggs develop into haploid males, while fertilized eggs produce diploid females. This is not considered “true” parthenogenesis in most discussions because it is how males are normally made, but some hymenopteran species have taken it further, producing diploid females from unfertilized eggs as well. In the solitary wasp Euodynerus foraminatus, both diploid and haploid males have been found to be fertile, adding complexity to how sex determination and parthenogenesis interact.6PubMed Central. Functionally reproductive diploid and haploid males in an inbreeding hymenopteran with complementary sex determination
Sharks and Snakes That Skip the Male
Some of the most headline-grabbing cases of parthenogenesis have come from vertebrate species that normally reproduce sexually but turn out to be capable of the occasional “virgin birth.” The first genetic confirmation in a cartilaginous fish came from a bonnethead shark in captivity that produced a pup without any contact with a male. Genetic testing confirmed the pup was parthenogenetic, produced through automictic parthenogenesis.7PubMed Central. Virgin birth in a hammerhead shark
Since that discovery, similar cases have turned up in other shark species. A zebra shark in an Australian aquarium began producing offspring years after being separated from any male, ruling out stored sperm. Genetic analysis of the pups showed elevated homozygosity consistent with terminal fusion automixis, which is thought to be the dominant mechanism for facultative parthenogenesis in vertebrates.8Scientific Reports. Switch from sexual to parthenogenetic reproduction in a zebra shark Captive boas and pythons have also been documented producing offspring without mating. These cases raise a practical concern: in species that are already threatened, parthenogenetic reproduction reduces genetic diversity in offspring, which could compound the problems facing small or fragmented populations.9PubMed Central. Virgin birth in a hammerhead shark
This worry extends to wild populations. Recurrent parthenogenesis has been documented in the endangered common smooth-hound shark, where the phenomenon is not just an oddity of captivity but appears to function as an adaptive strategy when mating opportunities are scarce.10Scientific Reports. First report of recurrent parthenogenesis as an adaptive reproductive strategy in the endangered common smooth-hound shark Mustelus mustelus Whether this helps or harms the species long-term depends on the balance between the immediate advantage of producing any offspring at all and the long-term cost of reduced genetic variety.
When a Bacterium Flips the Switch
Not all parthenogenesis is under the animal’s own control. Wolbachia, a bacterium that lives inside the cells of a huge number of insect species, can manipulate its host’s reproduction to favor its own transmission. Since Wolbachia is passed from mother to offspring through the egg cytoplasm, males are a dead end for the bacterium. One of Wolbachia’s strategies is to induce parthenogenesis, effectively eliminating males from the population so that every individual can transmit the infection.11PubMed Central. Wolbachia-induced parthenogenesis in a genus of phytophagous mites
This has been documented in phytophagous mites, certain parasitoid wasps, and other arthropods. When researchers treat infected populations with antibiotics to clear the Wolbachia infection, males reappear and sexual reproduction resumes. It is a vivid example of how the boundary between sexual and asexual reproduction can be shaped by something other than the host’s own genome. Estimates suggest Wolbachia infects somewhere between 40% and 65% of all insect species, though not all infections lead to parthenogenesis; the bacterium has several other reproductive tricks in its toolkit, including killing male embryos and feminizing genetic males.
Why Mammals Cannot Do This
If parthenogenesis works for lizards, sharks, and insects, why not mammals? The primary barrier is genomic imprinting, a process where certain genes are chemically tagged so that only the copy inherited from one parent is active. Some imprinted genes are switched on only when they come from the father; others are active only from the mother. A parthenogenetic mammalian embryo would have two maternal copies of every gene and zero paternal copies, meaning all the genes that require paternal activation would be silent, and all the genes that are normally silenced when inherited maternally would be doubly active.12PubMed. Constraints on the evolution of asexual reproduction
This imbalance is lethal in normal circumstances. However, researchers have managed to work around it in the lab. A Japanese team showed that deleting the imprinting-control regions on two specific chromosomes in an immature mouse egg could mimic the effect of paternal silencing. Embryos carrying both deletions developed into healthy adult mice at rates comparable to standard in vitro fertilization.13Nature Reports Stem Cells. Making healthy mice with two moms The experiment demonstrated that genomic imprinting is the wall, not some deeper incompatibility with parthenogenesis itself. In principle, if enough imprinting could be overridden, mammalian parthenogenesis is not biologically impossible. In practice, the number of imprinted genes and the complexity of the regulatory system make it an engineering challenge rather than a flip-of-the-switch solution.
Gynogenesis and Other Sperm-Dependent Twists
Parthenogenesis exists on a spectrum of unisexual reproduction strategies that blur familiar categories. In gynogenesis, females still need sperm to trigger egg development, but the sperm’s DNA is discarded and does not contribute genetically to the offspring.14Trends in Genetics. Unisexual vertebrates: challenges to the evolutionary ‘dead-end’ dogma The Amazon molly, a small freshwater fish, reproduces this way, mating with males of closely related species and using their sperm purely as a starter pistol. Hybridogenesis is another variation: half the genome is discarded each generation and replaced by mating with a sexual species, so the lineage is half clonal, half sexual. Kleptogenesis, documented in some salamanders, involves occasionally incorporating genetic material from a host species into the unisexual lineage.15PubMed Central. Evolutionary perspectives on clonal reproduction in vertebrate animals
These strategies show that nature has found many intermediate solutions between fully sexual and fully asexual reproduction. The old textbook framing of sex versus no-sex is really a continuum, and a surprising number of vertebrate species sit somewhere in the middle.
The Evolutionary Cost of Going It Alone
If parthenogenesis lets a population grow twice as fast, since every individual produces offspring rather than “wasting” half the population on males, why does sexual reproduction dominate the animal kingdom? This is one of the oldest puzzles in evolutionary biology, sometimes called the “cost of males” or the “paradox of sex.”
One major problem with permanent asexuality is the irreversible accumulation of harmful mutations, a process known as Muller’s ratchet. In a sexual population, recombination can combine bad mutations from two parents into one unlucky offspring while producing others that are mutation-free. Without recombination, every new harmful mutation that arises is stuck in the lineage forever. Over time, the genome degrades. Evidence for this process has been found in mitochondrial genomes of nematodes evolving in small natural populations.16PubMed Central. Muller’s Ratchet and compensatory mutation in Caenorhabditis briggsae mitochondrial genome evolution And a recent genomic study of parthenogenetic whiptail lizards found accelerated accumulation of protein-altering mutations in both parental subgenomes, with genes involved in core cellular functions particularly affected, providing direct support for the prediction that Muller’s ratchet operates in real asexual vertebrate lineages.17PubMed Central. Mutation accumulation in a hybrid parthenogenetic vertebrate
The other major explanation is the Red Queen hypothesis: parasites and pathogens constantly evolve to exploit the most common host genotypes, and sexual reproduction generates rare genotypes that parasites have not yet adapted to. A long-running study of freshwater snails in New Zealand, where sexual and asexual individuals of the same species coexist, found that the sexual population was consistently less infected by a sterilizing trematode parasite than the asexual population. At some sites and years, the frequency of uninfected sexual females was more than double that of uninfected asexual females.18PubMed. Infection dynamics in coexisting sexual and asexual host populations: support for the Red Queen hypothesis Complementary work at the same lake system showed that clonal turnover was faster in habitats with high parasite pressure, meaning the dominant asexual genotypes were being replaced more rapidly where parasites hit hardest, exactly as the Red Queen hypothesis predicts.19PubMed. Faster clonal turnover in high-infection habitats provides evidence for parasite-mediated selection
Ancient Asexuals and Their Workarounds
Bdelloid rotifers are microscopic freshwater animals that have apparently reproduced without sex for tens of millions of years, earning them the label “ancient asexuals.” Their survival seems to defy the predictions of both Muller’s ratchet and the Red Queen. Researchers have found a likely explanation: bdelloids regularly take up DNA from their environment and incorporate it into their own genomes, a process called horizontal gene transfer that is common in bacteria but extremely rare in animals.
Experiments with the bdelloid Philodina roseola showed that these rotifers readily internalize environmental DNA, and indirect evidence suggested the foreign DNA can be incorporated into the genome during desiccation and passed on to offspring. Facultatively sexual rotifers that cannot survive drying out did not show the same uptake ability.20PubMed Central. Evidence Supporting the Uptake and Genomic Incorporation of Environmental DNA in the “Ancient Asexual” Bdelloid Rotifer Philodina roseola The ability to withstand complete desiccation, which bdelloids use to survive in temporary puddles and moss patches, may have inadvertently opened a door to genetic exchange that partially compensates for the lack of sex. It is an elegant example of how evolution can stumble into alternative solutions to the same problem.
Clonal Seeds and the Future of Crop Breeding
The plant equivalent of parthenogenesis is called apomixis: producing seeds that are genetic clones of the mother plant without fertilization. This happens naturally in hundreds of plant species, including dandelions and some grasses. For agriculture, apomixis would be transformative, because hybrid crop varieties that show enhanced yield and resilience could be propagated indefinitely through seed without losing their favorable gene combinations.21PubMed Central. Engineering apomixis in crops
Right now, hybrid rice and corn seeds must be produced fresh every generation by crossing the parent lines, which is expensive and logistically demanding. Synthetic apomixis aims to bypass this. Researchers have achieved it in hybrid rice by making targeted genetic modifications: inactivating three genes that convert meiosis into a mitosis-like division, and activating a parthenogenesis trigger gene called BBM1 in the egg cell. The result is hybrid plants that produce more than 95% clonal seeds across multiple generations while maintaining normal fertility and the yield characteristics of the original hybrid.22Nature Communications. High-frequency synthetic apomixis in hybrid rice A separate group replicated this achievement in elite hybrid rice varieties, confirming that the approach works in commercially relevant genetic backgrounds and produces plants that retain the agronomic traits of the parent hybrid.23iScience. Engineering high-frequency apomixis with normal seed production in hybrid rice
This is still in the research phase and faces regulatory hurdles, since the plants are genetically modified. But if synthetic apomixis can be deployed at scale, it could reshape how hybrid crops are delivered to farmers worldwide, particularly smallholder farmers who currently cannot afford to buy fresh hybrid seed every planting season.
Human Medicine and Parthenogenetic Cells
Parthenogenesis intersects with human biology in two very different ways. The first is medical pathology. Ovarian teratomas, benign tumors containing a chaotic mix of tissues like hair, teeth, and bone, have been shown to arise from a single egg cell that begins developing parthenogenetically after the first meiotic division. The tumors are homozygous at chromosomal regions near the centromere while the surrounding normal tissue is heterozygous, a genetic fingerprint that confirms their parthenogenetic origin.24New England Journal of Medicine. Parthenogenic origin of benign ovarian teratomas These tumors are not pregnancies gone wrong; they are examples of an egg cell launching into development without any of the organizational signals that would produce a coherent embryo.
The second intersection is regenerative medicine. Researchers have derived pluripotent stem cell lines from human parthenogenetic blastocysts, embryos created by chemically activating unfertilized eggs. These cells behave much like conventional embryonic stem cells: they express the right markers, have normal chromosomes, and can differentiate into cell types from all three embryonic germ layers.25PubMed. Patient-specific stem cell lines derived from human parthenogenetic blastocysts Because they are derived from a single woman’s egg, the resulting cells are genetically matched to the donor, which could reduce immune rejection in transplantation. However, because of genomic imprinting, there are concerns about whether parthenogenetic stem cells can fully differentiate and function the same as conventional embryonic stem cells across all tissue types, and this remains an active area of investigation.26PubMed. Human parthenogenetic embryonic stem cells: one potential resource for cell therapy
Invasive Clones
Parthenogenesis can also turn an organism into an ecological nightmare. The marbled crayfish, first noticed in the German aquarium trade in the 1990s, reproduces exclusively through parthenogenesis. Every individual is female, every offspring is a clone, and the species is extraordinarily prolific. After being released into the wild, marbled crayfish spread rapidly across freshwater habitats in Madagascar and parts of mainland Europe. Genetic analysis confirmed that the wild populations are clonal, descended from a single lineage that apparently arose through a spontaneous genome duplication event.27PubMed. Clonal genome evolution and rapid invasive spread of the marbled crayfish
The marbled crayfish illustrates the short-term ecological power of parthenogenesis: a single individual released into a new environment can establish an entire population. It does not need to find a mate. It just starts reproducing. This advantage, combined with high fecundity and tolerance for a range of water conditions, has made the marbled crayfish a potent invader that threatens native crayfish and the broader freshwater ecosystems it colonizes. Whether the species will eventually hit the evolutionary dead end predicted by Muller’s ratchet is an open question, but in the meantime, the ecological damage is real and ongoing.