Asexual reproduction

• 1. Introduction

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  Asexual reproduction is the ability of a living thing to create a new individual without fusing two sex cells together. It sounds simple, but the implications run deep. A Komodo dragon, the largest lizard on Earth at ten feet long and over three hundred pounds, can produce offspring without ever encountering a male. A clam in the genus Corbicula can fertilize its own eggs using sperm it also produces, creating clones of itself without any partner at all. Bdelloid rotifers, microscopic animals that live in freshwater, have been reproducing this way for millions of years without a single recorded act of sex.

  What drives organisms as different as bacteria, strawberry plants, aphids, and sharks to skip the whole business of finding a mate? What do they gain, and what do they give up? And why, if asexual reproduction can be so efficient, do so few complex animals rely on it exclusively?

• 2. Splitting and Budding

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  Prokaryotes, the group that includes archaea and bacteria, divide by binary fission. One parent organism splits cleanly in two, producing a pair of genetically identical daughters. Eukaryotes like protists and unicellular fungi can achieve something functionally similar through mitosis.

  Multiple fission takes the process further. In sporozoans and algae, the nucleus of a parent cell divides several times before the cytoplasm separates at all, releasing a burst of daughter cells at once. In apicomplexans, this process has three distinct names depending on the outcome: merogony produces merozoites, sporogony produces sporozoites, and gametogony produces microgametes, each serving a different role in the organism's life.

  Budding operates differently. Baker's yeast is a well-studied example: a daughter cell forms as a small outgrowth on the mother and separates once it has grown. The hydra, a small freshwater animal, buds on a multicellular scale, with the new individual eventually breaking free as a fully formed organism.

  Some parasites use a stranger variant. Toxoplasma gondii practices internal budding, in which two or more daughter cells develop inside the mother cell and then consume it before separating. Worms like Taenia and Echinococcus produce cysts that then bud outward or inward to form new protoscolex structures, extending the logic of budding into a parasitic context.

• 3. Plants Without Seeds

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  Vegetative propagation is the version of asexual reproduction that most gardeners encounter without ever thinking of it by that name. Kalanchoe, specifically the species Bryophyllum daigremontianum, produces miniaturized plants called plantlets directly on its leaves. Strawberry plants send out stolons, horizontal runners that root at intervals and establish new individuals. Tulip bulbs and dahlia tubers store enough resources to generate entirely new plants from underground storage organs.

  In all of these cases, every new plant is a genetic clone of the one it came from, and a single clonal population can spread to cover a substantial area of ground.

  Spore formation works on a different genetic logic. Fungi and some algae produce mitospores through mitosis, meaning the spore carries the same chromosome count as the parent. The red alga Polysiphonia uses exactly this route. Plants and most algae, however, produce spores through meiosis, which halves the chromosome number. Those haploid spores develop into multicellular bodies called gametophytes, which then produce gametes through mitosis. The gametes fuse, restoring the full chromosome count. Because the endpoint of that chain is a fertilization event, scientists debate whether to call spore formation in plant sporophytes asexual at all, even though no fertilization is required to generate the spore itself.

  Lichens sidestep the debate entirely. A lichen is a partnership between a fungus and photosynthetic algae or cyanobacteria, and most lichens reproduce through fragmentation. The fragments, called soredia, are dust-like particles consisting of fungal hyphae wrapped around photobiont cells, ensuring that each new lichen starts life already containing both partners.

• 4. Parthenogenesis in the Wild

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  Parthenogenesis, in which an unfertilized egg develops into a new individual, has been documented in over two thousand species. That figure spans an enormous range: water fleas, rotifers, aphids, stick insects, some ants, bees, parasitic wasps, and a substantial number of fish, amphibians, and reptiles.

  For a long time, the assumption was that parthenogenesis in vertebrates was confined to very small animals. That picture has shifted. The Komodo dragon, at ten feet long and more than three hundred pounds, is now the largest documented vertebrate capable of parthenogenetic reproduction. Female zebra sharks held in captivity without males have also been observed reproducing this way, using asexual means only when a mate is genuinely unavailable.

  The desert grassland whiptail lizard offers a different case. It is a hybrid of two other species. Hybrids are typically infertile, but this lizard reproduces through obligate parthenogenesis, meaning asexual reproduction is its only method, and the species has established stable populations through it.

  Some species rely on an even more unusual arrangement. The Amazon molly is an obligate parthenote with no males in the population at all. To trigger development of their eggs, female Amazon mollies depend on sperm from males of a closely related species, the Sailfin molly. The sperm initiates reproduction without ever contributing its genetic material to the offspring. This process is called gynogenesis.

  Hammerhead sharks and blacktip sharks have both been documented reproducing parthenogenetically in captivity, in each case after reaching sexual maturity in the absence of males. Genetic testing confirmed the offspring were identical to their mothers.

• 5. Androgenesis and Male-Only Inheritance

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  Gynogenesis passes only maternal chromosomes to the next generation. Androgenesis runs in the opposite direction, producing offspring that carry exclusively paternal nuclear genes.

  This can happen in two ways. Either the maternal nuclear genome is destroyed in the zygote after fertilization, or the female produces an egg cell that contains no nucleus to begin with. In both cases, the resulting offspring carry the male parent's chromosomes but still inherit mitochondria from the mother, as is standard across most sexually reproducing species.

  Androgenesis occurs naturally in clams, stick insects, ants, bees, flies, and parasitic wasps, as well as in some amphibians and fish. The stick insects Bacillus rossius and Bassillus Grandii, the little fire ant Wasmannia auropunctata, and the ant species Vollenhovia emeryi and Paratrechina longicornis have all been observed using it. The parasitoid Venturia canescens and the fruit fly Drosophila melanogaster, when carrying a specific mutant allele, can also produce androgenetic offspring. Scientists have also induced the process artificially in crops and fish by irradiating eggs to destroy the maternal nuclear genome.

  Male apomixis is a plant-specific variant. In this process, a new plant develops from a sperm cell alone, with no female cell involved. The Saharan cypress Cupressus dupreziana reproduces this way as its regular method. A range of other plants, including species of Nicotiana, Capsicum frutescens, Cicer arietinum, and Zea mays, have also been documented producing embryos this way. The clonal trees Lomatia tasmanica and Pando carry exclusively male genetic material and propagate through vegetative reproduction, making them effectively male clones persisting through landscape-scale root systems.

• 6. Switching Strategies

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  Some organisms do not commit permanently to either reproductive mode. Aphids are a widely studied example of heterogony, the capacity to alternate between sexual and asexual reproduction at regular intervals. During the growing season, female aphids are born pregnant and produce only daughters asexually, cycling through generations rapidly. In the fall, environmental cues shift their development so they produce eggs instead of live young, and sexual reproduction introduces new genetic variation before winter. The result is polyphenism, with different generations producing morphologically distinct offspring suited to specific seasonal roles, including winged individuals that colonize new plants.

  The freshwater crustacean Daphnia follows a similar logic, switching from parthenogenesis in spring, when rapid population growth is advantageous, to sexual reproduction as predation and competition intensify later in the season. Monogonont rotifers of the genus Brachionus respond to population density rather than season: at low densities females reproduce asexually, but as density rises a chemical signal accumulates and triggers the shift to sexual reproduction.

  The slime mold Dictyostelium presents a more dramatic switch. As single-celled amoebae in favorable conditions, individual cells reproduce by binary fission. When conditions deteriorate, the cells aggregate and then diverge along one of two pathways. In the social pathway, the aggregate forms a slug and then a fruiting body that releases asexually produced spores. In the sexual pathway, two cells fuse into a giant cell that becomes a large cyst; when the cyst germinates, it releases hundreds of amoebic cells produced by meiotic recombination between the original two.

  The cape bee Apis mellifera subsp. capensis can reproduce asexually through a process called thelytoky, producing female offspring without fertilization, a capacity that distinguishes it from most other honeybee subspecies.

• 7. Clones Under Pressure

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  Bdelloid rotifers have reproduced asexually for millions of years, and every individual in the class Bdelloidea is female. Their survival through such a long span of asexual reproduction involves a remarkable adaptation: an exceptionally efficient mechanism for repairing DNA double-strand breaks. Scientists have studied this repair system in two species, Adineta vaga and Philodina roseola, and found it appears to involve mitotic recombination between homologous DNA regions. The same mechanism that allows bdelloid rotifers to survive periods of desiccation also makes them extraordinarily resistant to ionizing radiation.

  Long-term asexual reproduction also appears to have enabled bdelloid rotifers to evolve new proteins through what is called the Meselson effect, which may improve their survival under dehydrating conditions. This suggests that the absence of sexual recombination has not simply frozen their genomes in place but has opened a different adaptive path.

  Several stick insect species in the genus Timema provide the longest documented stretch of obligate asexual reproduction in any insect. Molecular evidence places their shift away from sexual reproduction millions of years ago. The mite species Oppiella nova may have maintained a similarly long asexual run.

  In the nine-banded armadillo, a form of asexual reproduction called polyembryony operates after fertilization. A single fertilized egg routinely splits to produce genetically identical quadruplets. The same basic process in humans, monozygotic twinning, has no clear genetic basis but is widespread enough that at least ten million identical twins and triplets exist in the world today, making polyembryony one of the most common forms of asexual reproduction in mammals.