The asexual reproduction is a form of reproduction in which the descendants come from a single progenitor of which they inherit their genes without the fusion of gametes and, with few exceptions, without there being changes in the number of chromosomes . In most cases of asexual reproduction there is no genetic recombination and the offspring are equal to the parent.
Asexual reproduction is the main form of reproduction of unicellular organisms and is the one that all prokaryotes follow . It also occurs in many plants and fungi , often with alternation between sexual and asexual reproduction. Asexual reproduction also occurs in some animals of primitive organization, such as sponges or coelenterates.
In biology, fission is the division of a unicellular organism into two or more parts and regeneration of those parts into separate individuals similar to the original. One of the most important forms of fission is binary fission . In binary fission , an organism is divided into two and gives rise to two identical descendants clones of the parent .
Only prokaryotes (bacteria, archaea) reproduce by binary fission. In eukaryotes there is a similar mechanism of cell division, mitosis, but mitosis is a mechanism primarily of growth and development , not of reproduction.
Another type of fission is the multiple fission that occurs in many protists and unicellular algae. It is similar to binary fission but more than two daughter cells are produced; first, the cell nucleus is divided into several nuclei by mitosis and, subsequently, the cytoplasm surrounds each nucleus and separates creating multiple daughter cells.
Budding is a type of asexual reproduction that is produced by asymmetric mitosis and is characterized because the new organism grows together with the parent in an area called yolk . The new organism is detached from the parent when it reaches maturity and continues its life as an independent individual. The same adult can have numerous yolks and descendants at the same time.
As in fission, the descendants are clones identical to each other and equal to the progenitor, unless some mutation occurs during the copying of the DNA. The yeasts are a typical example of reproduction by budding, although it also occurs in animals, for example , sponges, starfish larvae or cnidarians ( genus Hydra ).
Vegetative reproduction, also called vegetative propagation, is a very striking type of reproduction that occurs in many types of plants. It is characterized by the formation of a new individual without the formation of spores or seeds .
There are different types of vegetative reproduction:
- Seedling formation on specialized leaves.
- Formation of new individuals from specialized rhizomes, for example bulbs or tubers.
- Formation of new individuals by cuttings or formation of stolons .
By vegetative reproduction, a plant can grow rapidly and form a monoclonal colony and eventually cover large areas where all individuals are genetic clones.
Sporogenesis is the formation of reproductive spores that is observed in many plants and fungi. Spores, like gametes in sexual reproduction, are formed by meiosis, but gametes are joined in pairs ( fertilization ) to give a new individual, while each of the spores will directly form a new individual.
The individual formed from spores will produce gametes to reproduce. In this way, sporogenesis alternates with diploid generations that produce spores (asexual reproduction) and haploid generations that produce gametes (sexual reproduction). For example, in the ferns.
Many unicellular organisms form spores as part of their life cycle but not as a mechanism of reproduction, because an individual re-forms from the spore of an individual. This is the case of sporulation in bacteria to enter the endospore phase , considered a specialized cell type and not a true reproduction mechanism.
Fragmentation is a form of asexual reproduction in which new organisms develop from fragments of a parent. Fragmentation can be observed a wide variety of life forms, such as animals, plants or lichens . Some plants have specialized structures for reproduction by fragmentation, such as the gems of hepatic plants .
In animals it is typical in echinoderms (starfish) and in many types of worms, such as planarians, annelids or polychaetes. Reproduction by fragmentation in echinoderms is often referred to as fisitionition . In the fisitionición, the parent loses a fragment of his body, around which a complete organism develops while the parent regenerates the lost fragment.
Agamogenesis is a form of asexual reproduction from female gametes without the participation of male gametes . That is, without fertilization. The two most common types of agamogenesis are parthenogenesis and apomixia.
The parthenogenesis occurs when a new individual develops from an egg or unfertilized egg . It can be observed in many plants, in many types of insects and also in some vertebrates such as snakes and other reptiles , amphibians, and even in some birds.
The apomixis called a form of reproduction in plants through seed unfertilized . It is mostly produced by parthenogenesis, that is, from an unfertilized ovule, but some cases of male apomixia such as the cypress of the Sahara ( Cupressus dupreziana ) are also known.