Asexual reproduction

Cellular basis and mechanisms

Many forms of asexual reproduction depend on ordinary cell division processes such as mitosis or binary fission. A single organism or cell may grow and divide to form genetically similar offspring. Distinct mechanisms by which whole organisms reproduce asexually include:

• Binary fission — a cell divides into two roughly equal daughter cells; common in prokaryotes and many single-celled eukaryotes.
• Budding — a new individual grows from an outgrowth of the parent and later detaches, seen in some fungi, hydroids and yeasts.
• Fragmentation — parts of an organism break off and regenerate into complete individuals, typical of some worms and algae.
• Spore formation — production of specialized dispersal cells that can develop into new individuals under suitable conditions.
• Vegetative propagation — plants produce new individuals from stems, roots, tubers or runners.
• Parthenogenesis — development of embryos from unfertilised eggs in some animals and in certain plant forms (see below).

Occurrence and examples

Asexual reproduction is widespread across the tree of life. Many single-celled organisms, including amoeboid protists such as the Amoeba, reproduce by simple division. Prokaryotes commonly use binary fission to multiply. Numerous plants spread clonally via runners, tubers or cuttings; horticulture and agriculture exploit such vegetative propagation in many plants. In colonial animals, the individual zooids that make up a colony are often genetically identical because they arise by repeated asexual budding or fragmentation; this is seen in some zooids, coral colonies and bryozoans. Some lineages show long-term absence of conventional sex: for example, an entire order of rotifers known as bdelloid rotifers has been studied for its apparent long-term asexuality.

Parthenogenesis and special reproductive cycles

Parthenogenesis is a form of asexual reproduction in which an egg develops into an individual without fertilization. It occurs in various invertebrates and some vertebrates, either facultatively or as the usual mode. Some insects, such as aphids, alternate between sexual and asexual generations depending on season and environment. In hymenopteran insects (bees, ants and wasps) unfertilised eggs can develop into males (a system called haplodiploidy); for example, in honey bees certain males (drones) arise from unfertilised eggs while females develop from fertilised eggs in that group.

Genetic consequences and evolutionary notes

Asexual reproduction preserves successful genotypes and allows rapid population increase, but it reduces the rate at which genetic recombination generates new combinations of alleles. Populations that reproduce only asexually rely on mutation and occasional genomic events for variation, which can make them vulnerable to environmental change, pathogens or accumulation of deleterious mutations. For this reason, many species use a mix of sexual and asexual strategies, switching between them when selective conditions favor one or the other. Evolutionary biologists study factors that maintain sexual reproduction despite its costs, and why asexuality arises or persists in particular lineages.

Ecological and practical importance

Asexual reproduction has important ecological implications: it facilitates rapid colonization, clonal spread and the formation of large genetically uniform populations or stands. In agriculture and horticulture, vegetative propagation produces uniform crops and cultivars. In research, clonal cell lines and model organisms allow reproducible experiments. At the same time, clonal outbreaks can pose conservation challenges because low genetic diversity may reduce resilience to disease or environmental change.

Further reading and resources

For concise introductions and specific topics, consult general overviews and resources on cell division and organismal examples: general overview, cell division and mitosis, and organismal examples such as Amoeba, coral colonies and bryozoans. Additional discussion of clones and mutation processes is available in articles on clonal reproduction and mutation, while information on genetic concepts can be found under genes and heredity. Practical guides to vegetative propagation and horticultural techniques refer to methods for many plants. For cases of long-term asexuality and its study, see resources on bdelloid rotifers and related research. Overviews of reproductive strategies also discuss how individual organisms or cells reproduce without a mate and how colonial animals form via asexual processes (zooids).