Parthenogenesis

Parthenogenesis (from the Greek parthenos, meaning "virgin," and genesis, meaning "creation" or "origin") is a form of asexual reproduction in which an unfertilized female gamete (ovum) undergoes embryogenesis and develops into a viable offspring without fertilization by a male spermatozoon. Unlike somatic budding, fission, or fragmentation—in which new individuals arise from multicellular somatic tissues—parthenogenesis is strictly agamospermy of the animal germ line, initiating development from a single germ cell inside the ovary. Across the animal kingdom, parthenogenesis occurs naturally in rotifers, nematodes, tardigrades, gastrotrichs, mollusks, crustaceans (such as water fleas, Daphnia), arachnids, hexapods (including aphids, stick insects, and hymenopterans), and roughly 80–100 species of cartilaginous fishes, bony fishes, amphibians, squamate reptiles, and birds—though it is completely absent as a natural viable reproductive strategy in mammals due to parent-of-origin genomic imprinting.

The first experimental demonstration of parthenogenesis in animals was conducted in 1740 by the Genevan naturalist Charles Bonnet, who isolated a newly birthed female spindle-tree aphid (Aphis euonymi) on a potted plant inside a sealed glass vessel and documented 9 successive generations of all-female viviparous offspring produced without male contact. A century later, the British anatomist Richard Owen (1849) coined the term parthenogenesis in his treatise On Parthenogenesis, or the Successive Production of Procreating Individuals from a Single Ovum, and Jan Dzierżon (1845) discovered that unfertilized queen honeybee (Apis mellifera) eggs develop into haploid drone males while fertilized eggs develop into diploid female workers and queens.

In evolutionary biology, as formalized by John Maynard Smith (1978), a parthenogenetic female avoids the "two-fold cost of sex" (or "cost of males"): in an anisogamous sexual population with a 1:1 sex ratio where males contribute only DNA and zero parental investment, a sexual female passes her genome to only 50% of her reproductive output per generation (the other 50% being males), whereas an all-female parthenogenetic mutant investing the same resources into daughters produces twice as many egg-laying females per generation (r_asexual = 2 r_sexual). That sexual reproduction nonetheless dominates eukaryotic diversity reflects the long-term evolutionary advantages of genetic recombination in purging deleterious mutations (Muller's ratchet) and generating rare genotypes to resist coevolutionary parasites (Red Queen hypothesis).

Because somatic cells in most animals are diploid (2*n*) while standard mature ova produced by meiosis are haploid (n), parthenogenetic lineages that produce diploid daughters must either suppress meiosis entirely or restore diploidy before or after meiotic division. These cytological pathways exert distinct effects on offspring heterozygosity (H):

Apomictic (Ameiotic) Parthenogenesis

In apomixis (mitotic parthenogenesis), practiced by cyclical aphids, Daphnia, bdelloid rotifers, and many polyploid weevils, meiotic reduction division (Meiosis I homologous chromosome pairing and segregation) is suppressed during oogenesis. Primary oocytes divide by mitosis, producing diploid (2*n*) eggs that are genetically identical clones of the mother (barring de novo somatic/germline mutations and rare mitotic gene conversion). Consequently, 100% of maternal heterozygosity (H_t+1 = H_t) is preserved at every locus across generations; indeed, because homologous chromosomes accumulate independent point mutations over evolutionary time without recombining, ancient apomicts often exhibit pronounced intragenomic divergence between alleles (the Meselson effect).

Automictic (Meiotic) Parthenogenesis

In automixis, the primary oocyte undergoes complete meiosis (including homologous synapsis, crossing over, and reduction to haploidy n), and diploidy (2*n*) is subsequently restored by one of four cytological fusion or duplication mechanisms:

1. Premeiotic Endomitosis (Endoduplication): Before entering Meiosis I, the oogonium undergoes an extra round of DNA replication without cytokinesis, doubling its chromosome number from 2*n* to tetraploid (4*n*). During Meiosis I, identical sister chromatids (rather than homologous chromosomes) pair as pseudobivalents and recombine; two normal meiotic divisions then reduce the 4*n* oocyte back to a diploid (2*n*) ovum that retains 100% of maternal heterozygosity across the entire genome. This mechanism is used by unisexual hybrid whiptail lizards (Aspidoscelis [= Cnemidophorus] neomexicanus and A. tesselatus) and the parthenogenetic gecko Lepidodactylus lugubris.
2. Central Fusion: Following meiosis, the haploid ovum fuses with one of the two nuclei derived from the first polar body (separated at Meiosis I). Because homologous centromeres segregated into the oocyte and first polar body at Anaphase I, central fusion restores 100% of maternal heterozygosity for loci proximal to the centromere (where no crossover occurred), while loci distal to chiasmata lose 0–50% of heterozygosity per generation depending on recombination frequency. Central fusion occurs in the Cape honeybee (Apis mellifera capensis) and certain Drosophila strains.
3. Terminal Fusion: The haploid ovum fuses with its sister nucleus, the second polar body (separated at Meiosis II). Because sister chromatids share identical centromeres, terminal fusion produces 100% homozygosity near the centromere and retains maternal heterozygosity only at loci distal to a Meiosis I crossover event, rapidly eroding genome-wide heterozygosity (H_t+1 ≈ [0 to 1/3]*H*_t). Terminal fusion is the predominant mechanism of facultative parthenogenesis in vertebrates (sharks, Komodo dragons, snakes, and birds).
4. Gamete Duplication: The haploid (n) egg completes meiosis, initiates its first mitotic cleavage S-phase replication, and suppresses the first cleavage cytokinesis (or fuses the first two cleavage pronuclei), instantaneously producing a 100% homozygous diploid (2*n*) embryo (H = 0 at every locus across the genome). Gamete duplication is the mechanism induced by endosymbiotic Wolbachia bacteria in parasitoid wasps.

Parthenogenetic modes are also classified by the sex of the resulting progeny:

• Thelytoky: Unfertilized eggs develop exclusively into females (found in obligate asexual rotifers, aphids, stick insects, whiptail lizards, and XX facultative sharks).
• Arrhenotoky (Haplodiploidy): Unfertilized eggs remain haploid (n) and develop into viable males, whereas fertilized eggs are diploid (2*n*) and develop into females. Arrhenotoky characterizes the entire insect order Hymenoptera (ants, bees, and wasps, >150,000 species), Thysanoptera (thrips), scale insects, whiteflies, bark beetles, and monogonont rotifers. Because a haplodiploid male passes 100% of his genome to every daughter, full sisters sharing a singly mated mother share an average coefficient of relatedness of r = 0.75 (50% identical paternal genome + 50% chance of sharing the maternal allele) rather than r = 0.50 with their own offspring, a genetic asymmetry analysed by W. D. Hamilton (1964) in the theory of kin selection and eusociality.
• Deuterotoky (Amphitoky): Unfertilized eggs can develop into either males or females, as occurs in cyclical aphids during the autumn transition when apomictic XX mothers undergo non-disjunction of a single X chromosome during oogenesis to produce XO sexual males alongside XX oviparous females.

In facultative vertebrate parthenogenesis, the sex of the offspring depends on whether the clade possesses an XX/XY (male heterogametic) or ZZ/ZW (female heterogametic) sex-determination system:

• In elasmobranch sharks (e.g., bonnethead shark Sphyrna tiburo, zebra shark Stegostoma tigrinum, white-spotted bamboo shark) and pit vipers that use XX/XY or terminal-fusion XX mechanisms, a female (XX) produces exclusively X ova, so terminal fusion yields 100% XX female progeny.
• Conversely, in Komodo dragons (Varanus komodoensis), boas and pythons (historically, though some use XY), and birds (domestic turkeys Meleagris gallopavo and critically endangered California condors Gymnogyps californianus), females are heterogametic (ZW) and males are homogametic (ZZ). Meiosis I in a ZW female yields 50% Z oocytes and 50% W oocytes; terminal fusion or gamete duplication converts these into 50% ZZ viable males and 50% WW embryos (which are inviable and abort early in cleavage). Thus, a virgin female Komodo dragon stranded on an uninhabited island can produce a clutch of all-male ZZ sons, and later mate sexually with her own grown sons to re-establish a mixed-sex population.

Cyclical Parthenogenesis vs. Obligate Ancient Asexuals

1. Cyclical Parthenogenesis (Heterogony): Practiced by ~5,000 species of aphids (Aphididae), cladoceran crustaceans (Daphnia), monogonont rotifers (Brachionus), and cynipid gall wasps. During spring and summer when algal or phloem resources are abundant, females reproduce via apomictic parthenogenesis—with telescoping generations in viviparous aphids, where a newborn female already carries developing granddaughters inside her embryonic ovaries—achieving rapid Malthusian population growth (N(t) = N₀ e^rt). When autumn photoperiod shortening, crowding pheromones, or food depletion signal winter onset, maternal endocrine signals (juvenile hormone decline) induce a switch to sexual meiosis, producing males and sexual females whose fertilized diapause eggs (ephippia in Daphnia) survive winter freezing and desiccation while restoring genetic variation via recombination.
2. Bdelloid Rotifers (Ancient Asexuals): The ~450 species of freshwater bdelloid rotifers (Class Bdelloidea, e.g., Adineta vaga) have reproduced by obligate apomictic parthenogenesis for >40 million years without any known males or canonical meiosis. Bdelloids evade Muller's ratchet and parasite extinction through two specialized adaptations: anhydrobiosis (surviving desiccation at any life stage, allowing wind dispersal away from fungal parasites while repairing hundreds of desiccation-induced DNA double-strand breaks via homologous gene conversion that purges deleterious mutations) and extensive horizontal gene transfer (HGT) (~8–10% of active bdelloid genes are acquired from bacteria, fungi, and plants).

Hybridogenesis, Gynogenesis, and Microbial Manipulation

Among unisexual vertebrates, obligate parthenogenetic lineages arose via interspecific hybridization between two diverged sexual parental species (which disrupts normal meiotic homologous pairing in the F₁ hybrid):

• True Unisexual Hybrid Reptiles (Aspidoscelis): The New Mexico whiptail lizard (Aspidoscelis neomexicanus, 2*n*) arose as a natural F₁ hybrid between a maternal A. inornatus and a paternal A. tigris, locking in interspecific fixed heterozygosity across its genome via premeiotic endomitosis. Notably, although no males exist, ovulation in A. neomexicanus is stimulated by pseudocopulation, in which post-ovulatory females with elevated progesterone titers mount and court pre-ovulatory females with elevated estrogen titers.
• Gynogenesis (Sperm-Dependent Parthenogenesis): The all-female Amazon molly (Poecilia formosa, a hybrid fish between P. mexicana and P. latipinna) and the triploid salamander Ambystoma platineum produce diploid unreduced eggs that require penetration by a spermatozoon from a closely related sexual male species merely to mechanically/chemically activate cortical-granule exocytosis and embryogenesis; the paternal sperm nucleus is subsequently destroyed and excluded without contributing any genes to the clonal daughter (kleptogenesis when occasional paternal DNA is incorporated).
• Hybridogenesis (Hemiclonal Reproduction): In the European edible frog (Pelophylax [= Rana] esculentus, a hybrid carrying one P. lessonae [L] genome and one P. ridibundus [R] genome), the germline selectively eliminates the entire paternal L genome prior to meiosis and transmits only the maternal R genome clonally into the egg, requiring fertilization by an L male every generation to reconstitute the somatic LR hybrid.
• Microbial Wolbachia-Induced Parthenogenesis: Vertically transmitted intracellular α-proteobacteria of the genus Wolbachia (alongside Cardinium and Rickettsia) infect the ovaries of haplodiploid parasitoid wasps (Trichogramma). Because Wolbachia is transmitted strictly through egg cytoplasm and dies in males, the bacterium manipulates host mitosis via gamete duplication to convert unfertilized haploid (n) male-destined eggs into diploid (2*n*) infected females; treating these wasps with tetracycline antibiotics cures the infection and immediately restores sexual male production.

Although mammalian oocytes can be artificially activated in the laboratory via electrical pulses or Sr²⁺/ethanol treatment to form diploid parthenotes, every mammalian parthenote arrests and dies around embryonic day 9.5–10.5 in mice. As demonstrated by Davor Solter and Azim Surani (1984) via pronuclear transplantation experiments, eutherian and marsupial mammals require both a maternal genome and a paternal genome due to parent-of-origin genomic imprinting (~150–200 imprinted genes marked by differential DNA CpG methylation during spermatogenesis vs. oogenesis). Under the parental conflict (kinship) hypothesis of David Haig, paternally expressed genes—such as Igf2 (insulin-like growth factor 2) and Peg1/Peg3—are silenced on the maternal chromosome and drive placental trophoblast invasion and fetal growth, whereas maternally expressed genes—such as the growth-suppressing receptor Igf2r and the non-coding RNA H19—are silenced on the paternal chromosome. A diploid mammalian parthenote carrying two maternal genomes (gynogenote) lacks Igf2 expression and suffers lethal extraembryonic/placental failure, whereas an embryo with two paternal genomes (androgenote) forms a disorganized trophoblastic hydatidiform mole with virtually no embryo proper. Only in 2004 did Tomohiro Kono and colleagues engineer a viable bimaternal mouse ("Kaguya") by fusing a mature oocyte with a nongrowing oocyte carrying a targeted deletion of the H19/Igf2 imprinting control region.

How does parthenogenesis in Komodo dragons produce only male offspring?

Unlike mammals, which have XX/XY sex determination, Komodo dragons (Varanus komodoensis) have a ZZ/ZW sex-determination system in which females are heterogametic (ZW) and males are homogametic (ZZ). When an unfertilized female Komodo dragon undergoes automictic parthenogenesis via terminal fusion or gamete duplication, her haploid Z and W oocytes are duplicated into diploid ZZ and WW embryos. Because WW embryos lack essential Z-linked genes and fail to develop, 100% of viable parthenogenetic offspring hatch as ZZ males.

Why can't mammals reproduce by natural parthenogenesis?

Mammalian development requires both a maternal and a paternal genome due to genomic imprinting, an epigenetic process in which ~150–200 genes are differentially methylated and silenced depending on parental origin. Paternally expressed genes (such as Igf2) are required to build the placenta, while maternally expressed genes (such as H19 and Igf2r) restrain fetal growth and direct embryogenesis. A mammalian parthenote carrying two maternal genomes lacks paternal Igf2 expression and dies mid-gestation from placental failure.

What is the difference between apomixis and automixis?

Apomixis is ameiotic parthenogenesis: the primary oocyte skips meiotic reduction division entirely and divides by mitosis, producing diploid daughters that are 100% genetically identical clones of the mother and retain all maternal heterozygosity. Automixis is meiotic parthenogenesis: the oocyte completes meiosis and restores diploidy by fusing with a polar body or duplicating its haploid chromosomes, which alters or eliminates heterozygosity depending on whether central fusion, terminal fusion, or gamete duplication occurs.

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2. Neaves, W. B., & Baumann, P. (2011). "Unisexual reproduction among vertebrates." Trends in Genetics, 27(3), 81–88. https://doi.org/10.1016/j.tig.2010.12.002
3. Watts, P. C., Buley, K. R., Sanderson, S., Boardman, W., Ciofi, C., & Gibson, R. (2006). "Parthenogenesis in Komodo dragons." Nature, 444(7122), 1021–1022. https://doi.org/10.1038/4441021a
4. Kono, T., Obata, Y., Wu, Q., Niwa, K., Ono, Y., Yamamoto, Y., Park, E. S., Seo, J.-S., & Ogawa, H. (2004). "Birth of parthenogenetic mice that can develop to adulthood." Nature, 428(6985), 860–864. https://doi.org/10.1038/nature02402