Asexual reproduction includes all forms of reproduction that do not involve the fusion of male and female gametes. The asexual reproduction of a cell consists in the formation of genetically identical daughter cells; in the eukaryotes, this implies nuclear division via mitosis. Multicellular organisms are formed through repeated cycles of asexual reproduction from an initial zygote (see Development and Embryology). The daughter cells need not be completely identical, because cells with the same genotypes may differ in the quantitative distribution of transcription factors (see Differentiation). The genotypes of the daughter cells may differ because of mutation, mitotic recombination, chromosomal nondisjunction, and other infrequent events.
There are two kinds of asexual reproduction of multicellular organisms, namely, parthenogenesis and vegetative reproduction. Parthenogenesis imitates sexual reproduction, in that embryogenesis is apparently normal, but without participation of a male gamete. Haploid individuals are produced through the parthenogenetic development of a normal female gamete. Diploid individuals are produced through the fusion of the haploid nuclei of a female gamete and a neighboring haploid cell, such as the synergids of plants and the polar bodies of animals. Other diploid individuals are produced through the parthenogenetic development of a diploid cell after failure of meiosis. In vegetative reproduction, individuals are derived from one or many somatic cells. It is a common natural process in many plants and lower animals, and it is the only multiplication mechanism in some of them.
Artificial interventions permit new forms of asexual reproduction. These include (a) the production of haploid plants from the culture of anthers and (b) the production of diploid animals through the replacement of the nucleus of the ovum by a somatic nucleus. Vegetative reproduction and ameiotic parthenogenesis lead to the formation of clones, which are sets of individuals with identical genotype. Clonal propagation is usually carried out through vegetative reproduction from a single cell or a group of identical cells.
The parasexual cycle may be seen as a form of asexual reproduction in the lower eukaryotes and in cultured cells of multicellular organisms. Haploid cells, not necessarily of different mating type, fuse to form heterokaryons, and the nuclei of the heterokaryon fuse to produce diploid nuclei; these revert to haploidy, not by meiosis, but by random chromosome losses during mitosis, accompanied by mitotic recombination.
The sexual processes of bacteria imply the conjugation of two compatible cells and the transfer of part of the genetic information of one to the other (see Hfr’S And F-Primes; F Plasmid). Formally similar are asexual processes in which some genetic information from a cell is transferred to another via a virus or via naked DNA (see Transformation). These processes are not exclusive to the prokaryotes.
In evolutionary terms and in comparison with the various sexual processes, asexual reproduction has advantages of speed, economy, and the conservation of certain genotypes that may be lost or made less frequent after meiosis (see Heterosis, and Aneuploidy). The main drawback of asexual reproduction is the limited creation of new genetic variation (see Genetic Diversity).
