|Dictionary / A|
Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as the archaea, bacteria, and protists. Many plants and fungi reproduce asexually as well. While all prokaryotes reproduce asexually (without the formation and fusion of gametes), mechanisms for lateral gene transfer such as conjugation, transformation and transduction are sometimes likened to sexual reproduction. A lack of sexual reproduction is relatively rare among multicellular organisms, for reasons that are not completely understood. 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.
Costs and benefits
Because asexual reproduction does not require the formation of gametes (often in separate individuals) and bringing them together for fertilization, it occurs much faster 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 lizards), reducing the chance of finding a mate, or during colonisation of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population.
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 if 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. A 2004 article in the journal Nature reported that the modern arbuscular mycorrhizal fungi, which reproduce asexually, are identical to fossil records dating back to the Ordovician period, 460 million years ago.
Types of asexual reproduction
Many single-celled organisms (unicellular), such as archaea, bacteria, and protists, reproduce asexually through binary fission. An exception to the rule are unicellular fungi such as fission yeast, unicellular algae such as Chlamydomonas, and ciliates and some other protists, which reproduce both sexually and asexually. Some single-celled organisms (unicellular) rely on one or more host organisms in order to reproduce, but most literally divide into two organisms.
Some cells split via budding (for example baker's yeast), resulting in a 'mother' and 'daughter' cell. The offspring organism 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 individuals are formed without the production of seeds or spores by meiosis or syngamy. Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves (for example in kalanchoe), and some produce new plants out of rhizomes or stolon (for example in strawberry). Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and dahlia tubers). Some plants produce adventitious shoots and suckers that form along their lateral roots. Plants that reproduce vegetatively may form a clonal colony, where all the individuals are clones, and the clones may cover a large area.
Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who 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 "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore 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 Basidiomycota, and 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 sea stars), fungi, and plants. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which 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.g. water fleas, aphids, stick insects, some ants, bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, fish, very rarely birds). In plants, apomixis may or may not involve parthenogenesis.
Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and apomixis.
Apomixis and nucellar embryony
Apomixis in plants is the formation of a new sporophyte without fertilization. It is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy, the formation of seeds without fertilization, but was once used to 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 was 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 rare 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.
Alternation between sexual and asexual reproduction
Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring 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. For example, the slime mold Dictyostelium undergoes binary fission as single-celled amoebae 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. Asexual reproduction is far less complicated than sexual reproduction. In sexual reproduction one must find a mate.
Examples in animals
A number of invertebrates and some less advanced vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual. Alternation is observed in a few types of insects, such as aphids (which will, under favourable 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 behaviour can interfere with the incubation of eggs in turkey farming.
There are examples of parthenogenesis in the hammerhead shark and the blacktip shark. 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.
Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration. (Wikipedia)