In animalsparthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.
Parthenogenesis occurs naturally in some plants, some invertebrate animal species including nematodeswater fleassome scorpionsaphidssome mites, some beessome Phasmida and parasitic wasps and a few vertebrates such as some fish amphibiansreptiles   and very rarely birds . This type of reproduction has been induced artificially in a few species including fish and amphibians. Normal egg cells form after meiosis and are haploidwith half as many chromosomes as their mother's body cells.
Depending on the mechanism involved in restoring the diploid number of chromosomes, parthenogenetic offspring may have anywhere between all and half of the mother's alleles. The offspring having all of the mother's genetic material are called full clones and those having only half are called half clones.
Full clones are usually formed without meiosis.
If meiosis occurs, the offspring will get only a fraction of the mother's alleles since crossing over of DNA takes place during meiosis, creating variation. Parthenogenetic offspring in species that use either the XY or the X0 sex-determination system have two X chromosomes and are female.
In species that use the ZW sex-determination systemthey have either two Z chromosomes male or two W chromosomes mostly non-viable but rarely a femaleor they could have one Z and one W chromosome female. Some species reproduce exclusively by parthenogenesis such as the Bdelloid rotiferswhile others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis other terms are cyclical parthenogenesis, heterogamy   or heterogony  .
The switch between sexuality and
Asexual reproduction in humans pdf converter in such species may be triggered by the season aphidsome gall waspsor by a lack of males or by conditions that favour rapid population growth rotifers and cladocerans like daphnia.
In these species asexual reproduction occurs either in summer aphids or as long as conditions are favourable. This is because in asexual reproduction a successful genotype can spread quickly without being modified by sex or wasting resources on male offspring who won't give birth. In times of stress,
Asexual reproduction in humans pdf converter produced by sexual reproduction may be fitter as they have new, possibly beneficial gene combinations. In addition, sexual reproduction provides the benefit of meiotic recombination between non-sister chromosomes, a process associated with repair of DNA double-strand breaks and other DNA damages that may be induced by stressful conditions.
Origin and function of meiosis. Many taxa with heterogony have within them species that have lost the sexual phase and are now completely asexual. Many other cases of obligate parthenogenesis or gynogenesis are found among polyploids and hybrids where the chromosomes cannot pair for meiosis.
The production of female offspring by parthenogenesis is referred to as thelytoky e. When unfertilized eggs develop into both males and females, the phenomenon is called deuterotoky. Parthenogenesis can occur without meiosis through mitotic oogenesis.
This is called apomictic parthenogenesis. Mature egg cells are produced by mitotic divisions, and these cells directly develop into embryos.
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In flowering plants, cells of the gametophyte can undergo this process. The offspring produced
Asexual reproduction in humans pdf converter apomictic parthenogenesis are full clones of their mother. Parthenogenesis involving meiosis is more complicated. In some cases, the offspring are haploid e. In other cases, collectively called automictic parthenogenesisthe ploidy is restored to diploidy by various means.
This is because haploid individuals are not viable in most species. In automictic parthenogenesis the offspring differ from one another and from their mother. They are called half clones of their mother. Automixis  is a term that covers several reproductive mechanisms, some of which are parthenogenetic. Diploidy might be restored by the doubling of the chromosomes without cell division before meiosis begins or after meiosis is completed. This is referred to as an endomitotic cycle.
This may also happen by the fusion of the first two blastomeres. Other species restore their ploidy by the fusion of the meiotic products. The chromosomes may not separate at one of the two anaphases called restitutional meiosisor the nuclei produced may fuse or one of the polar bodies may fuse with the egg cell at some stage during its maturation.
Some authors consider all forms of automixis sexual as they involve recombination. Many others classify the endomitotic variants as asexual, and consider the resulting embryos parthenogenetic.
Among these authors the threshold for classifying automixis as a sexual process depends on when the products of anaphase I or of anaphase II are joined together. The criterion for "sexuality" varies from all cases of restitutional meiosis, "Asexual reproduction in humans pdf converter" to those where the nuclei fuse or to only those where gametes are mature at the time of fusion.
The genetic composition of the offspring depends on what type of apomixis takes
Asexual reproduction in humans pdf converter. When endomitosis occurs before meiosis   or when central fusion occurs restitutional meiosis of anaphase I or the fusion of its productsthe offspring get all   to more than half of the mother's genetic material and heterozygosity is mostly preserved  if the mother has two alleles for a locus, it is likely that the offspring will get both.
This is because in anaphase I the homologous chromosomes are separated. Heterozygosity is not completely preserved when crossing over occurs in central fusion. If terminal fusion restitutional meiosis of anaphase II or the fusion of its products occurs, a little over half the mother's genetic material is
Asexual reproduction in humans pdf converter in the offspring and the offspring are mostly homozygous.
In the case of endomitosis after meiosis the offspring is completely homozygous and has only half the mother's genetic material. In apomictic parthenogenesis, the offspring are clones of the mother and hence except for aphids are usually female.
In the case of aphids, parthenogenetically produced males and females are clones of their mother except that the males lack one of the X chromosomes XO.
When meiosis is involved, the sex of the offspring will depend on the type of sex determination system and the type of apomixis. In species that use the XY sex-determination systemparthenogenetic offspring will have two X chromosomes and are female. In species that use the ZW sex-determination system the offspring genotype may be one of ZW female  ZZ maleor WW non-viable in most species  but a fertile, [ dubious — discuss ] viable female in a few e.
In polyploid obligate parthenogens like the whiptail lizard, all the offspring are female. In many hymenopteran insects such as honeybees, female eggs are produced sexually, using sperm from a drone father, while the production of further drones males depends on the queen and occasionally workers producing unfertilised eggs.
This means that females workers and queens are always diploid, while males drones are always haploid, and produced parthenogenetically. Facultative parthenogenesis is the term for when a female can produce offspring either sexually or via asexual reproduction.
A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring. Facultative parthenogenesis is often incorrectly used to describe cases of accidental or spontaneous parthenogenesis in normally sexual animals. Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means.
Among these species, one of the most well documented transitions to obligate parthenogenesis was found in almost all metazoan taxa, albeit through highly diverse mechanisms. These transitions often occur as a result of inbreeding or mutation within large populations. As such, there are over 80 species of unisex reptiles mostly lizards but including a single snake speciesamphibians and fishes in nature for which males are no longer a part of the reproductive process.
Thus, a male is not needed to provide sperm to fertilize the egg. This form of asexual reproduction is thought in some cases to be a serious
Asexual reproduction in humans pdf converter to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring. Some invertebrate species that feature partial sexual reproduction in their native range are found to reproduce solely by parthenogenesis in areas to which they have been introduced.
Examples include several aphid species  and the willow sawfly, Nematus oligospiluswhich is sexual in its native Holarctic habitat but parthenogenetic where it has been introduced into the Southern Hemisphere. Parthenogenesis is seen to occur naturally in aphidsDaphniarotifersnematodes and some other invertebrates, as well as in many plants. Among vertebratesstrict parthenogenesis is only known to occur in lizards, snakes,  birds  and sharks,  with fish, amphibians and reptiles exhibiting various forms of gynogenesis and hybridogenesis an incomplete form of parthenogenesis.
As with all types of asexual reproductionthere are both costs low genetic diversity and therefore susceptibility to adverse mutations that might occur and benefits reproduction without the need for a male associated with parthenogenesis. Parthenogenesis is distinct from artificial animal cloninga process where the new organism is necessarily genetically identical to the cell donor.
In cloning, the nucleus of a diploid
Asexual reproduction in humans pdf converter from a donor organism is inserted into an enucleated egg cell and the cell is then stimulated to undergo continued mitosisresulting in an organism that is genetically identical to the donor.
Parthenogenesis is different, in that it originates from the genetic material contained within an egg cell and the new organism is not necessarily genetically identical to the parent.
Parthenogenesis may be achieved through an artificial process as described below under the discussion of mammals. Parthenogenesis in insects can cover a wide range of mechanisms. Both true parthenogenesis and pseudogamy "Asexual reproduction in humans pdf converter" or sperm-dependent parthenogenesis are known to occur. A related phenomenon, polyembryony is a process that produces multiple clonal offspring from a single egg cell. This is known in some hymenopteran parasitoids and in Strepsiptera.
In automictic species the offspring can be haploid or diploid. Diploids are produced by doubling or fusion of gametes after meiosis. In addition to these forms is hermaphroditism, where both the eggs and sperm are produced by the same individual, but is not a type of parthenogenesis.
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This is seen in three species of Icerya scale insects. Parasitic bacteria like Wolbachia have been noted to induce automictic thelytoky in many insect species with haplodiploid systems. They also cause gamete duplication in unfertilized eggs causing them to develop into female offspring. Among species with the haplo-diploid sex-determination systemsuch as hymenopterans ants, bees and wasps and thysanopterans thripshaploid males are produced from unfertilized eggs.
Usually eggs are laid only by the queen, but
Asexual reproduction in humans pdf converter unmated workers may also lay haploid, male eggs either regularly e. An example of non-viable parthenogenesis is common among domesticated honey bees. The queen bee is the only fertile female in the hive; if she dies without the possibility for a viable replacement queen, it is not uncommon for the worker bees to lay eggs. fold advantage for asexual reproduction,...
This is a result of the lack of the queen's pheromones and the pheromones secreted by uncapped broodwhich normally suppress ovarian development in workers. Worker bees are unable to mate, and the unfertilized eggs produce only drones maleswhich can mate only with a queen. Thus, in a relatively short period, all the worker bees die off, and the new drones follow if they have not been able to mate before the collapse of the colony. This behaviour is believed to have evolved to allow a doomed colony to produce drones which may mate with a virgin queen and thus preserve the colony's genetic progeny.
A few ants and bees are capable of producing diploid female offspring parthenogenetically. These include a honey bee subspecies from South Africa, Apis mellifera capensiswhere workers are capable of producing diploid eggs parthenogenetically, and replacing the queen if she dies; other examples include some species
Asexual reproduction in humans pdf converter small carpenter bee, genus Ceratina.
Many parasitic wasps are known to be parthenogenetic, sometimes due to infections by Wolbachia. The workers in five  ant species and the queens in some ants are known to reproduce by parthenogenesis. Article; Figures & Data; Info & Metrics; eLetters; PDF. We then discuss the costs and benefits of asexual reproduction. .
such as the basidiomycete pathogen of humans Cryptococcus neoformans where the alpha mating type is found in Additionally, gene conversion will leave a similar signature . review focuses on the role of asexual reproduction for eukaryotic such as gene conversion, that can strongly modify the.
humans in the United States. Clonal and sexy: The dynamics of sexual and asexual reproduction in dwarf. that 65% of the world's seagrass ecosystems have been impacted by human. conversion was halted in the late s (WolfReise ), construction of quepenser.com pdf.
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