Asexual reproduction is a form of reproduction in which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. It does not involve meiosisploidy reduction, or fertilization, and only one parent is involved genetically. A more stringent definition is agamogenesiswhich refers to reproduction Define spore formation in asexual reproduction offspring the fusion of gametes.
Asexual reproduction is the primary form of reproduction for single-celled organisms such the archaeabacteriaand protists. However, while all prokaryotes reproduce asexually without the formation and fusion of gametesthere also exist mechanisms for lateral gene transfer, such as conjugation, transformation, and transduction, whereby genetic material is exchanged between organisms.
Biological processes involving lateral gene sometimes are likened to sexual reproduction Narra and Ochman Many plants and fungi reproduce asexually as well, and asexual reproduction has been cited in some animals, including bdelloid rotiferswhich only are known to reproduce asexually, and various animals that exhibit parthenogenesis under certain conditions. In parthenogenesis, such as found in some invertebrates and vertebratesan embryo is produced without fertilization by a male.
Generally, parthenogenesis is considered a form of asexual reproduction because it does not involve fusion of gametes of opposite sexes, nor any exchange of genetic material from two sources Mayr however, some authorities McGraw-Hill classify parthenogenesis as sexual reproduction on the basis that it involves gametes or does not produce an offspring genetically identical to the parent such as a female domestic turkey producing male offspring.
A wide spectrum of mechanisms may be exhibited. For example, many plants alternate between sexual and asexual reproduction see Alternation of generationsand the freshwater crustacean Daphnia reproduces by parthenogenesis in the
Define spore formation in asexual reproduction offspring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases.
Many protists and fungi alternate between sexual and asexual reproduction. A lack of sexual reproduction is relatively rare among multicellular organisms, which exhibit the characteristics of being male or female. Biological explanations for this phenomenon are not completely settled. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.
In asexual reproduction, an individual can reproduce without involvement with another individual, there is no fusion of gametesand the new organism produced inherits all of its chromosomes from one parent and thus is a genetically-similar or identical copy of Define spore formation in asexual reproduction offspring parent.
Because asexual reproduction does not require the formation of gametes often in separate individuals and bringing them together for fertilization, nor involvement of another organism, it occurs much than sexual reproduction and requires less energy.
Asexual lineages can increase their numbers rapidly because all members can reproduce viable offspring. In sexual populations with two genders, some of the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions when compared with a sexual population half composed of males.
This is known as the two-fold cost of sex. Other advantages include the ability to reproduce without a partner in situations where the population density is low such as for some desert lizardsreducing the chance of finding a mate, or during colonization of isolated habitats such as oceanic islands, where a single female member of
Define spore formation in asexual reproduction offspring species is enough to start a population.
There does not need to be energy spent in finding and courting Define spore formation in asexual reproduction offspring partner for reproduction. A consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically similar to their parent, with as broad a range as that individual receives from one parent. The lack of genetic recombination results in fewer genetic alternatives than with sexual reproduction.
Many forms of asexual reproduction, for example budding or fragmentation, produce an exact replica of the parent. This genetic similarity may be beneficial the genotype is well-suited to a stable environment, but disadvantageous if the environment is changing.
For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it.
In contrast, a lineage that reproduces sexually has a higher probability of having more members survive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment. From an evolutionary standpoint, one could thus argue that asexual reproduction is inferior because it stifles the potential for change. However, there is also a significantly reduced chance of mutation or other complications that can result from the mixing of genes.
Conversely, Heng proposes that the resolution to the "paradox of sex" is that sexual reproduction actually reduces the drastic genetic diversity at the genome or chromosome level, resulting in the preservation of species identity, rather than the provision of evolutionary diversity for future environmental challenges.
He maintains that while genetic recombination contributes to genetic diversity, it does so secondarily and within the framework of the chromosomally defined genome.
That is, the asexual process generates more diverse genomes because of the less controlled reproduction systems, while sexual reproduction generates more stable genomes. A article in the journal Nature reported that the modern arbuscular mycorrhizas fungi, which reproduces asexually, is identical to fossil records dating back to the Ordovician period, million years ago Pawlowska and Taylor Binary fission involves the reproduction of a living cell by division into two parts, which each have the potential to grow to the size of the original cell.
Many single-celled organisms unicellularsuch as archaeabacteriaand protistsreproduce asexually through binary fission. Exceptions are unicellular fungi such as fission yeast, unicellular algae such as Chlamydomonas, and ciliates and some other protists, which reproduce both sexually and asexually.
Define spore formation in asexual reproduction offspring unicellular rely on one or more Define spore formation in asexual reproduction offspring organisms in order to reproduce, but most literally divide into two organisms.
In addition, mitochondria and chloroplasts of eukaryote cells divide by binary fission. See also the description under sexual reproduction. Some cells split via budding for example baker's yeastresulting in a "mother" and "daughter" cell. Budding is the formation of a new organism by the protrusion of part of another organism, with the protrusion normally staying attached to the primary organism for a while, before becoming free.
The new organism is naturally genetically identical to the
Define spore formation in asexual reproduction offspring one a clone and initially is smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra, which reproduces by budding. The buds grow into fully matured individuals, which eventually break away from the parent organism. Vegetative reproduction is a type of asexual reproduction found in plants where new independent individuals are formed without the production of seeds or spores.
Examples for vegetative reproduction include the formation of plantlets on specialized leaves for example Define spore formation in asexual reproduction offspring kalanchoe Define spore formation in asexual reproduction offspring, the growth of new plants of rhizomes or stolons for example in strawberryor the formation of new bulbs for example in tulips.
Define spore formation in asexual reproduction offspring resulting plants form a clonal colony. Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Define spore formation in asexual reproduction offspring are animals and some
Define spore formation in asexual reproduction offspringwhich undergo gametic meiosis immediately followed by fertilization.
Plants and many algae on the other hand undergo sporic meiosis, where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals called gametophytes in the case of plants without a fertilization event. These haploid individuals give rise to gametes through mitosis.
Meiosis and gamete formation therefore occur in separate generations or of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes fertilizationspore formation in plant sporophytes and algae might be considered a form of asexual reproduction agamogenesis despite being the result of meiosis and undergoing a reduction in ploidy.
However, both events spore formation and fertilization are necessary to complete sexual reproduction in the plant life cycle. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found, for example, in conidial fungi and the red alga Polysiphonia, and involves sporogenesis without meiosis.
Thus, the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants, most Basidiomycotaand many algae, are produced by meiosis.
Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms, such as animals some annelid worms and starfishfungiand plants.
Some plants have specialized structures for reproduction Define spore formation in asexual reproduction offspring fragmentation, such as gemmae in liverworts. Most lichenswhich are a symbiotic union of a fungus and photosynthetic algae or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts.
These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells. Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates e. In plants, apomixis may or may not involve parthenogenesis. Parthenogenesis is one example of agamogenesis, the term for any form of reproduction that does not involve a male gamete.
An example is apomixis. Apomixis in plants is the formation of a new sporophyte without fertilization. It is important in ferns and in flowering plantsbut is very rare in other seed plants.
In flowering plants, the term "apomixis" is now Define spore formation in asexual reproduction offspring often used for agamospermy, the formation of seeds without fertilization, but was once used
Define spore formation in asexual reproduction offspring include vegetative reproduction.
An example of an apomictic plant would be the triploid European dandelion. Apomixis mainly occurs in two forms. In gametophytic apomixis, the embryo arises from an unfertilized egg within a diploid embryo sac that Define spore formation in asexual reproduction offspring formed without completing meiosis. In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac.
Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in Define spore formation in asexual reproduction offspring cases, such as the Saharan cypress where the genetic material of the embryo are derived entirely from pollen.
The term "apomixis" is also used for asexual reproduction in some animals, notably water-fleas, Daphnia. Some species alternate between the sexual and asexual strategies, an ability known as heterogamydepending on conditions.
Define spore formation in asexual reproduction offspring example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity Define spore formation in asexual reproduction offspring competition and predation increases. For example, the slime mold Dictyostelium undergoes binary fission as single-celled
Define spore formation in asexual reproduction offspring under favorable conditions.
However, when conditions turn unfavorable, the cells aggregate and switch to sexual reproduction leading to the formation of spores. The hyphae of the common mold Rhizopus are capable of producing both mitotic as well as meiotic spores.
Many algae similarly switch between sexual and asexual reproduction.
Define spore formation in asexual reproduction offspring reproduction is far less complicated than sexual reproduction. In sexual reproduction one must find a mate. A number of invertebrates and some less Define spore formation in asexual reproduction offspring vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual.
Alternation is observed in a few Define spore formation in asexual reproduction offspring of insectssuch as aphids which will, under favorable conditions, produce eggs that have not gone through meiosis, essentially cloning themselves and the cape bee Apis mellifera capensis which can reproduce asexually through a process called thelytoky. A few species of amphibians and reptiles have the same ability see parthenogenesis for concrete examples.
A very unusual case among more advanced vertebrates is the female turkey's ability to produce fertile eggs in the absence of a male. The eggs result in often sickly, and nearly always male turkeys. This behavior can interfere with the incubation of eggs in turkey farming Savage There are examples of parthenogenesis in the hammerhead shark Eilperin and the blacktip shark Chapman et al. In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers.