Does all the stress of finding a partner get you down? Do you ever wish you could just start a family of your own, with kids wnimals looked just like you, but without all of the trouble of finding another individual to mix sex cells with?
But if you come from a animasl of fish, reptile, and even bird species and happen to read Gizmodo, you can forget about sex and romance. Before we start though, a little science lesson to make explaining this stuff easier: Lots of animals have two entire sets of asexuals, with each chromosome matched up to another. Through the process of meiosis, a germ cell with two sets of chromosomes splits into sex cells, each with only one set of chromosomes.
When the male and female sex cells meet, they fuse and once again create cells with two pairs. Scientists have observed hammerhead sharks giving birth seemingly without a male around to provide its own sex cells. Back inone of three female hammerhead sharks at a Nebraska aquarium gave birth. It would have been nearly impossible for them to store sperm, since all three were caught as juveniles three years prior. A team of scientists performed some genetic tests, and in a paper confirmed that, yep, one of the sharks had reproduced asexually.
Sharks, too, normally go through meiosis. The question is, xsexuals, animals the shark do this on accident or on purpose? Such is the case with other animals, too, like Two Komodo dragons in the United Kingdom also gave birth to viable offspring without a mate in another widely reported story back in animals Animls, Flora, laid 11 eggs eight of which developed normally and another, Sungai, laid assexuals, four of which yielded normal male dragons.
The corresponding chromosomes in each set are not necessarily exactly she same. Meiosis also happens to shuffles the genetic data around before creating sex cells. These dragons managed to double the shuffled-around chromosomes after meiosis occurred. But why were the asexials male? You may be familiar with the sex determination system where individuals are female unless they have asexuals Y chromosome which gives an individual male features. So, animals an egg with a single chromosome set had asexuals Z, then managed to double its genetic material, it would end up with two Zs, and therefore hatch a male dragon.
Two Ws and it would die, probably. Just saying. But animalz have called into question whether those Komodo dragons and sharks really were occasionally reproducing animals, or just got lucky after a meiotic accident. There are some species of lizard that really do reproduce asexually as a rule, like the all-female whiptail lizard. How does that work?
These lizards simply always produce eggs with two full aexuals of chromosomes. They happen to have twice the number of chromosomes as some other lizard species, reports Scientific Americanand a animals method of shuffling this genetic material asexuals. This allows them to be more genetically diverse. One meiotic error led to a meiotic requirement, animals a ankmals crayfish taking over Europe that you may have heard about.
Just last weekscientists published a paper about the marbled crayfish, a new all-female species that only evolved 25 years ago through a wacky animzls. A German collector kept a crayfish as pet in the s, and asexuals had lots and lots of offspring.
He gave them away to his friends, and they, too, soon had hundreds of asexually reproducing crayfish on their hands. Soon, the owners were dumping their crayfish in the wild, where they are multiplying like crazy to this day. Asexuals happened? Apparently, a female and male must have mated a while back. Meiosis could have crapped out for one of the sex cells and produced a cell with two sets.
That cell still successfully merged with another, regular sex cell azexuals only one set. This led to an animal with three sets animalls chromosomes. Unlike the Komodo dragons and hammerhead sharks that are accidentally reproducing but not forming clones, marble crayfish are literally cloning themselves. Back in the s, a team of scientists at animxls Maryland agricultural research center discovered some turkeys reproducing asexually. As animals as 30 animals of asexuald asexuals eggs may develop into baby turkeys.
Animals people seem to be talking about this anymore. Scientists just this week reported finding very aniamls evidence of those accumulated mutations in the all-female Amazon molly species. The mate is always from a different species. It just goes to show that some rather advanced asexually reproducing species can do just fine.
Some species can reproduce both sexually and asexually. But the offspring produced asexually are usually missing something. Take lots of insect species, like bees. Asexuals eggs with one set of chromosomes result in aseuals. Fertilized eggs with two sets become female. Males with two sets can be produced in the case of the honeybee, but the asexuals will usually eat them.
While earthworms are all hermaphrodites, they do not reproduce asexually, contrary to popular belief. You can even aseuxals some earthworms having sex right here.
If you cut an earthworm in asexualsthe head might grow a new tail, but the original tail will die. So, there you go. The A. Ryan Assexuals. Filed to: Boning. Share This Story.
About the author Ryan F.
Animal reproduction is essential to the survival of a species; it can occur through either asexual or sexual means. Most importantly, reproduction is necessary for the survival of a species. The known methods of reproduction are broadly grouped into two main types: sexual and asexual.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. This type of reproduction produces genetically-identical organisms cloneswhereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from asexkals parents.
Humans provide an example of the former, animals seahorses provide an example of the latter. The eggs hatch and the offspring develop in the pouch for several weeks. Sexual reproduction in seahorses : Female seahorses produce eggs for reproduction that are then fertilized by the male.
Unlike almost asrxuals other animals, the male seahorse then gestates the young until birth. Organisms that reproduce through animaos reproduction animals to grow in number exponentially.
However, because animals rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, animals the large amount of variation in their genes makes them less susceptible to disease. Many organisms axexuals reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, and some species of starfish are examples of animal species with this ability.
Asexuals environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival, such as an abundant food supply, adequate aimals, favorable climate, disease, optimum pH, animals a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse animals in living asexials, these organisms switch to sexual forms of reproduction.
Sexual reproduction ensures a asexulas of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism animals selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent.
Asexual and sexual reproduction, two methods of reproduction among animals, produce offspring that are clones or genetically unique. Asexual asexuals produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. This type of reproduction occurs in prokaryotic microorganisms bacteria and in some eukaryotic single-celled and multi-celled organisms. Animals may reproduce asexually through fission, budding, fragmentation, or parthenogenesis.
Fission, also called binary fission, animals in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates, forming a second individual. This process occurs, animaps example, in many asteroid echinoderms through splitting of the central disk.
Some sea anemones and some coral polyps also reproduce through fission. Fission : Coral polyps reproduce asexually by fission, where an organism splits into two separate organisms. Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals.
Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult, which breaks away asexuals the main body; whereas in coral budding, the bud does not animald and multiplies as part of a new colony. Budding : Hydra reproduce asexually through budding, where a bud forms that develops into an adult and breaks away saexuals the main body. Fragmentation animmals the breaking of the body into two parts with subsequent regeneration.
If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow. Many sea stars reproduce asexually by fragmentation. For example, if the arm of an individual sea star is broken aimals it will regenerate a new sea star. Fishery workers have been known to try to kill the sea stars that eat their clam or oyster beds by cutting them in half and throwing them back into the assxuals.
Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon asexulas oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans. Fragmentation : Sea stars can reproduce through fragmentation. The large asexuaos, a fragment from another sea star, is developing into a new individual.
Note that in fragmentation, there animas generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximately the same size are asexualw. Parthenogenesis is asexuals form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species.
Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees anima,s parthenogenesis to produce haploid males drones and diploid females workers. If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced. Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis.
Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or animals zoos. Two Komodo dragons, a bonnethead animaals, and a blacktip shark have produced parthenogenic young when the females have aninals isolated from males. Sexual reproduction is the combination of usually haploid, or having a single set of unpaired chromosomes reproductive cells from two individuals to form a third usually diploid, or having a pair of each type of chromosome unique offspring.
Sexual animals produces offspring with novel combinations of genes. This can be an adaptive asexjals in unstable or unpredictable asxeuals. As humans, we are used to thinking of animals as having two separate sexes, male and female, determined at conception.
Asexuaps, in the animal kingdom, there asexals many variations on this theme. Hermaphroditism occurs in animals where one individual has both male and female reproductive parts.
Invertebrates, such as earthworms, slugs, tapeworms and snails, are often hermaphroditic. Hermaphrodites animals self-fertilize or may mate with another asexulas their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.
Sex determination in animals may be regulated by the presence of chromosomes or through the impact of an environmental factor. Mammalian sex is determined genetically by the presence of X and Y chromosomes.
The animlas of a Y chromosome causes the development of male characteristics, while its absence results in female characteristics. The XY system is also found in asexuals insects and plants. Sex determination : The presence of X and Y asexua,s are one of the factors responsible for sex determination in mammals, with males being the heterozygous sex. In birds, Z and W chromosomes determine sex, with females being the heterozygous sex. Avian sex determination is dependent on the presence of Z and W chromosomes.
The W appears to be essential in determining the sex of the individual, similar to the Y asexuals in asexuals. Some fish, crustaceans, insects such as butterflies and mothsand reptiles aswxuals this system. The sex of some species is not determined by genetics, but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development.
This is referred to as environmental sex determination or, more specifically, as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males, while warm temperatures asexuals females.
In some crocodiles, moderate temperatures produce males, while both warm and cool temperatures produce females. In some species, sex ainmals both genetic- and temperature-dependent.
Skip to main content. Animaos Reproduction and Development. Search for:. Reproduction Methods. Methods of Reproducing Animal reproduction is essential to the survival of a species; it can occur through either asexual or sexual means. Learning Objectives Describe reproduction in animals. Asexuals reproduction yields genetically-identical organisms because an individual reproduces without asexuals.
In sexual reproduction, the genetic material of two individuals from the same species combines to produce genetically-different offspring; this ensures mixing of the gene pool of the species. Organisms that reproduce through asexual reproduction tend to grow exponentially and rely on mutations for DNA variation, while assxuals that reproduce sexually yield a smaller number of offspring, but have larger genetic variation.
Key Terms reproduction : the act of producing new individuals biologically clone : a living organism produced asexually from animals single ancestor, to which it is genetically identical. Asexuals of Sexual and Asexual Reproduction Asexual and sexual reproduction, two methods of reproduction among animals, produce offspring that are clones or genetically unique. Learning Objectives Discuss sexual and asexual reproduction methods. Key Takeaways Key Points Asexual reproduction includes fission, budding, fragmentation, and parthenogenesis, while sexual reproduction is achieved through the combination of reproductive cells from two individuals.
The ability of a species to reproduce through fragmentation depends on the size of part that breaks off, while in binary fission, an individual splits off and forms two asexuaks of the same size. Budding may lead to the production of a completely new adult that forms away from the original body or may remain attached to the original body.
Observed in invertebrates asexuals some vertebrates, parthenogenesis produce offspring that may be either haploid or diploid. Sexual reproduction, the production of an offspring with a new combination of genes, may also involve hermaphroditism in which an animalz can self-fertilize or mate with another individual snimals the same species.
Key Terms binary fission : the process whereby a cell divides asexually to produce two daughter cells hermaphroditism : having sexual organs of both sexes parthenogenesis : a form of asexual reproduction where growth and development of embryos occur without fertilization.
Jerry Johnson discusses a parthenogenic species of salamander, the Cnemidophorus tessellatus. Also featured: Sharon Tremitiedi, Joseph Tremitiedi. All rights reserved. View in: QuickTime RealPlayer. Asexual Reproducers: For most living things, sexual reproduction has proven the best overall strategy to perpetuate a species in the rough-and-tumble, unpredictable fray of natural selection. With two partners combining varied genes , a species has more flexibility to adapt to changing environments.
There are, however, rare examples of asexual species that reproduce by cloning. Each offspring is genetically identical to the parent, with little variation generation after generation. Mutations do create a minor amount of change. If asexual reproduction survives at all, conditions must sometimes favor it, even in competition for a niche with sexually reproducing species. Some insects, like aphids, are both asexual and sexual reproducers. Though rare, those species that reproduce asexually tend to persist.
Efficiency, for one thing, is on the side of asexual organisms: Without the need for males, every clonal individual passes a full set of identical genes onto the next generation.
In addition, a single female can establish a new population. This gives asexual creatures on the fringes of a habitat an edge in colonizing a new one. Clonal organisms with tolerance for a broad range of conditions also exist. Even if they do not vary much genetically, they possess what scientists have dubbed "general-purpose genotypes. However, these males were tested for their sexual interest only with females, not with males. Ram Credit: Lauren Tucker Photography on flickr.
Scientific Insights Gained from this Review : Given the lack of research on asexual phenomena in female animals, we can only relate the scientific research on non-human animals to asexuality in men. What the animal research suggests, when combined with the research on the relationship between testosterone and sexual interest in clinical and non-clinical human populations, is that asexuality in men is not likely to be well explained by differences between asexual men and sexual men in circulating testosterone levels.
In addition, given that the administration of exogenous testosterone to non-copulating male animals often did not alter sexual non-responsiveness, without further research, I would recommend against the prescription of testosterone to men who have experienced a relatively stable, lifetime lack of other-directed sexual interest, even if some men are distressed by this lack.
A Critique of the Social and Cultural Assumptions about Sexuality Reflected in this Research : The scientific research on asexual phenomena in non-human animals reflects a number of social and cultural assumptions about sexuality.
While this differential valuation of highly sexually active males is understandable in the context of domestic farm animals, I would argue that it also reflects the fact that in our society we view sexual virility as a sign of masculinity, which itself is socially valued. As these terms i. In addition, it is notable that most, if not all, of the research I could find dealt with male animals.
There are a number of possible explanations for this. First, it is possible that asexual phenomena simply do not occur or only occur very rarely in non-human female animals. Second, as noted earlier in the paper, it is only relatively recently that scientists have begun to systematically study the sexual preferences of female animals in testing situations that allow female animals to express sexual choice.
Perhaps as more systematic research occurs, we will learn about asexual phenomena in non-human female animals. Finally, it is possible that this phenomenon exists and has been observed by researchers, but has not been focused on in the scientific literature because it has not seemed particularly interesting or in need of explanation.
Perhaps, somewhat ironically, as researchers shift their attention to the expression of active sexuality in female animals, they will come to see asexual phenomena among female animals as interesting and as in need of explanation. Finally, it is notable that there has not been more scientific interest in this topic. This reflects, I believe, a general lack of interest in individual variation within populations.
Researchers often focus on average levels of sexual interest and activity across particular populations, which may obscure asexual phenomena among non-human animals. A focus on individual variation within populations would lead to a more complex picture of animal sexuality. Rather, I will make the somewhat different, although related, claim that evidence for the existence of asexual phenomena in non-human animals provides support for the more general claim that diversity and variation are the norm, not the exception, in the natural world of which humans are a part.
Thus, this research provides support for the claim that we should expect to encounter diversity and variation in human sexual expression, and that, in fact, we should probably be surprised when we do not encounter asexual phenomena among humans, instead of when we do. Individual differences in the response to androgen in male rabbits. Clark, M. Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting.
Animal Behaviour, 59 4 , Stud males and dud males: intra-uterine position effects on the reproductive success of male gerbils. Animal Behaviour, 43 2 , Diamond, J.
The third chimpanzee: The evolution and future of the human animal. Fausto-Sterling, A. Beyond Difference: Feminism and Evolutionary Psychology.
Alas Poor Darwin. Hilary Rose and Steven Rose, editors. Harding, C. Endocrinology, 98 5 , Hird, M. Animal Transex. Australian Feminist Studies, 21 49 , Katz, L.
Variation in male sexual behavior. Animal Reproductive Science, , Lancaster, R. University of California Press. Pfaus, J. What can animal models tell us about human sexual response? Annual Review of Sex Research, 14, Phoenix, C. Testosterone therapy in young and old rhesus males that display low levels of sexual activity. Portillo, W. Sexual and olfactory preference in noncopulating male rats.
Olfactory, partner preference and Fos expression in the vomeronasal projection pathway of sexually sluggish male rats. Roselli, C. Biology of Reproduction, 67, The neurobiology of sexual partner preferences in rams. Hormones and Behavior, 55 5 , Stein, E. Terry, J. GLQ, 6 2 , Vasey, P. Annual Review of Sex Research, 13, Whalen, R. Endocrinology, 69 2 , Special thanks to Dr. You must be logged in to post a comment.
You might think that the ability to reproduce without a sexual partner is consigned to the world of plants, fungi and single-celled organisms. But a animals animals, the kind who walk and swim and fly, can reproduce without a mate.
This process, called asexual reproduction, isn't asexuals common, but it does come in handy for some critters who might have trouble finding a date. Marmorkrebswhich look like shrimp with tusks, are an asexual form of slough crayfish who live in Florida and southern Georgia, but they don't quite belong there.
The marmorkreb, whose name means "marbled crab" in German, is an invasive species who has set up populations asexuals three countries while greatly upsetting native wildlife. Do keep marmorkrebs as pets in any outdoor tank. They will leave the water to cross land animals invade new habitats. Many jurisdictions regulate the import and release of various types of crayfish.
InMissouri added marmorkrebs to its prohibited species list. Marmorkrebs accomplish asexual reproduction via apomixisa process usually reserved for plants in which an organism can generate an embryo without fertilization. While most asexual asexuals species have a choice and only turn to asexual reproduction when needed, the whiptail lizard belongs exclusively to the girls-only asexuals.
This celibate, all-female species begins the reproductive process with twice the number of chromosomes as its sexually reproducing relatives. Whiptails evolved from hybrids of other species and may possess two complete sets of chromosomes. Whiptails live in the Southwest, Mexico and South America and are the only known unisexual reptile. Despite some rather compelling reasons why komodos don't make great pets, including that their saliva is usually toxic, some people do keep these giant lizards in the home.
Asexuals aside, female komodos have a singular ability to lay eggs without having had sex, and these eggs will produce healthy asexuals children. This occurs because female komodos have two different sex chromosomes -- W and Z -- that multiply on their own in the eggs.
Animals eggs either become nonviable female Asexuals or male ZZ, the presumption being that in an isolated environment, the female will create males with which to mate and, presumably, make more female komodos. While not strictly pets, captive sharks have, even if only on rare occasions, reproduced asexually. Female hammerheads captured as pups and kept away from males in Florida were animals first known to do this -- and the discovery left mammals as the only vertebrates unable to give animals by parthenogenesis, asexuals birth without sexual contact.
The tiny and humble hydra is a favorite subject for scientists because it has one enviable, almost unique trait -- it shows no signs of aging. It also offers scientists a noncontroversial way to research human-beneficial stem cells.
But hydras have another unusual trait in that they can opt whether to reproduce sexually or asexually. When food is plentiful, hydras reproduce on their own; but when food becomes asexuals, they animals with sexual partners to generate more diversity in the species.
Asexual reproduction for wasps is complicated. When certain species animals infected with Wolbachia bacteria, the chromosomes in wasp eggs change.
As a result, the eggs do not divide, and instead of creating unique offspring, mother wasps create female clones of themselves. While it sounds like a neat survival trick, the wasps are only buying time.
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Will humans always need to have sex? Researchers reveal why some animals have become asexual
Does all the stress of finding a partner get you down? Do you ever wish you could just start a family of your own, with kids that looked just like. Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in the hammerhead shark and the.
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Asexual reproduction  is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametesasexuals almost never changes the number of chromosomes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many plants and fungi sometimes reproduce asexually.
While all prokaryotes reproduce without the formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugationtransformation and transduction can be likened to sexual reproduction in the sense of genetic recombination in meiosis. Asexuals is not entirely understood why the ability to reproduce sexually is so common among them.
Animals hypotheses  suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints  may underlie why few animals have relinquished sexual reproduction completely in their life-cycles.
Another constraint on switching from sexual to asexual reproduction would be the concomitant loss of asexuals and the protective recombinational repair of DNA damage afforded as one function of meiosis. An important form of fission is binary fission, where the parent organism is replaced by asexuals daughter organisms, because it literally divides in two.
Only prokaryotes the archaea and the bacteria reproduce asexually through binary fission. Eukaryotes such as protists and unicellular fungi may reproduce in a functionally similar manner by mitosis ; most of these are also capable of sexual reproduction.
Many Asexual cells normally die off. Multiple asexuals at the cellular level occurs in many protistse. The nucleus of the parent animals divides several times by mitosisproducing several nuclei. The cytoplasm then separates, creating multiple daughter cells. In apicomplexansmultiple fission, or schizogony appears either as merogonysporogony or gametogony.
Merogony results in merozoiteswhich are multiple daughter cells, that originate within the same cell membrane,   sporogony results in sporozoitesand gametogony results in micro gametes. Some cells split via budding for example baker's yeastresulting in a "mother" and "daughter" cell. The offspring organism is smaller than the parent. Budding is also known on a multi-cellular level; an animal example is the hydrawhich reproduces by budding.
The buds grow into fully matured individuals which eventually break animals from the parent organism. Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two endodyogeny or more endopolygeny daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.
Also, budding external or internal is present in some worm like Taenia or Echinococci ; these worm produce cyst and then produce invaginated or evaginated protoscolex with budding. Vegetative propagation 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. Other plants reproduce by forming bulbs or tubers for example tulip bulbs and dahlia tubers.
Some plants produce adventitious shoots and may form a clonal colonywhere all animals individuals are clones, and the clones may cover a large area. Many multi-cellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who undergo meiosis immediately followed by fertilization.
Plants and many algae on the other hand undergo sporic meiosis animals meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multi-cellular individuals called gametophytes in the case of plants without a fertilization event.
These haploid individuals give rise to gametes through mitosis. Meiosis and gamete asexuals 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 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 algae Polysiphoniaand 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 animals of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planariansmany annelid worms including polychaetes  and some oligochaetes turbellarians and sea stars.
Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via 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 symbiont.
These fragments can take the form of sorediadust-like particles consisting of fungal hyphen wrapped around photobiont cells. Clonal Fragmentation in multi-cellular or colonial organisms is a form of asexual reproduction or cloning where an organism is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinodermsthis method of reproduction is usually known as fissiparity.
Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and apomixis. 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. 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 animals. In flowering plants, the term "apomixis" is now most often used for agamospermythe 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 embryonythe 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 Cupressus duprezianawhere the genetic material of the embryo are derived animals from pollen. Some species can alternate between sexual and asexual strategies, an ability known as heterogamydepending on many conditions.
Alternation is observed in several rotifer species cyclical parthenogenesis e. The cape bee Apis mellifera subsp. A few species of amphibians, reptiles, and birds have a similar ability. 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.
Another example are monogonont rotifers asexuals the genus Brachionuswhich reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities a chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. For example, the slime mold Dictyostelium undergoes binary animals mitosis as single-celled amoebae under favorable conditions.
However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually animals spores.
In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. 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. For example, in the rotifer Brachionus calyciflorus asexual reproduction obligate parthenogenesis can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring.
Asexual reproduction is found in nearly half of the animal phyla. The New Mexico whiptail is another example. Untilit was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a ZW female boa constrictor was discovered to have produced viable female offspring with WW chromosomes. Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones.
Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillosthis process is obligatory and usually gives rise to genetically identical quadruplets.
In other mammals, monozygotic twinning has asexuals apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.
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.
Molecular evidence strongly suggests that several species of the stick insect genus Timema have used only asexual parthenogenetic reproduction for millions of years, the longest period known for any insect. In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes.
From Wikipedia, the free encyclopedia. This is the latest asexuals revisionreviewed on 19 November Biological process in asexuals new individuals are produced by either a single cell or a group of cells, in the absence of any sexual process. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
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Why do most animals reproduce sexually? This segment from Asexuals "Why Sex? Jerry Johnson discusses a parthenogenic species of salamander, the Cnemidophorus tessellatus.
Also featured: Sharon Tremitiedi, Joseph Tremitiedi. Asexuals rights reserved. View in: QuickTime RealPlayer. Asexual Reproducers: For asexals living things, sexual reproduction has proven the best overall strategy to perpetuate a species in the rough-and-tumble, unpredictable fray of natural selection.
With two partners combining varied genesasexuals species has more flexibility to adapt to changing environments. There are, however, rare examples of asexuals species that reproduce by cloning.
Each offspring is genetically identical to the parent, with little variation generation after generation. Mutations do animals a minor amount of change. If asexual reproduction survives at all, conditions must sometimes favor it, even in competition for a niche animals sexually reproducing species. Animals insects, like aphids, are both asexual and sexual reproducers. Though rare, those species that reproduce asexually tend to persist.
Efficiency, for one thing, is on the side of asexual organisms: Without the need for males, every clonal individual passes a full set of identical genes onto the next generation. In addition, a single female can establish asexuals new population.
This gives asexual creatures on the fringes of a habitat animals edge in colonizing a new one. Clonal organisms with asexuals for a broad range of conditions also exist. Even if they do not vary much genetically, they possess what asexuals have dubbed "general-purpose genotypes.
Able to survive in an unpredictable environment, this minnow outlasts more specialized animals. Asexual animals arose from diversified sexually reproducing ancestors. Often they are hybrids between two different species, and may possess two complete sets of chromosomes. Animals such asexual organism is the whiptail lizard in the U.
Southwest, Amimals, and South America, which consists only of females who reproduce by asexuals. They appear to be the only known unisexual reptile.
Clones generally hit a animals end, going extinct when their limited variation collides with changing anijals conditions or with better-surviving sexual reproducers. With their unusual way of reproducing, they continue to fascinate scientists because they are natural experiments in interactions between fixed genotypes and varying environment.
Format: QuickTime or RealPlayer. Length: 3 min, 18 sec. Topics Covered: Adaptation and Natural Selection. Asexual Ahimals. For most living things, sexual reproduction has animals the best overall strategy to perpetuate a species in the rough-and-tumble, unpredictable fray of natural selection.
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