Parthenogenesis [Gr., =virgin birth]
In parthenogenesis ("virgin birth"), the females produce eggs, but these develop into young without ever being fertilized. In cases of parthenogenesis an ovum starts to divide by itself without fertilization, producing an embryo in which the paternal chromosomes may be replaced by a duplication of maternal ones. This asexual reproductive method is rare among warm-blooded vertebrates but more common among invertebrates. Pathological parthenogenesis has been observed in higher animals, such as the frog, fowl, and certain mammals. Parthenogenesis usually gives rise to female offspring. Attempts at artificial parthenogenesis in humans have not yet been successful. The first cloned human embryo was produced in October 2001. Eggs had their own genetic material removed and were injected with the nucleus of a donor cell. They were then incubated under special conditions to prompt them to divide and grow. One embryo grew to six cells before it stopped dividing. The same experimenters also tried to induce human eggs to divide into early embryos parthenogenetically -- without being fertilized by a sperm or enucleated and injected with a donor cell -- but their efforts met with only limited success. There is some evidence, however, that natural parthenogenesis does occasionally occur in humans. There are many instances in which impregnation have allegedly taken place in women without there being any possibility of the semen entering. In some cases it was found either in the course of pregnancy or at the time of childbirth that the female passages were obstructed. In 1956 the medical journal Lancet published a report concerning 19 alleged cases of virgin birth among women in England, who were studied by members of the British Medical Association. The six-month study convinced the investigators that human parthenogenesis was physiologically possible and had actually occurred in some of the women studied. Many primitive peoples believe that there are two methods of human reproduction: the ordinary animal one and a higher one rarely employed -- virgin birth. One belief is that the rays of the sun can fertilize women. In this regard, it is interesting that ultraviolet rays can cause parthenogenesis in unfertilized eggs of sea-urchins. It is also believed that moon rays, wind, rain, and certain types of food can cause impregnation. In the 19th century the Trobriand Islanders of the western Pacific insisted that cases of virgin birth still occurred among them.
Why choose asexual reproduction?Perhaps the better question is: Why not? After all, asexual reproduction would seem a more efficient way to reproduce and avoids all sorts of problems. Perhaps sexual reproduction has kept in style because it provides a mechanism to weed out harmful mutations that arise in the population (through the recombination process of meiosis). Some strains of the water flea Daphnia pulex reproduce sexually, others asexually. The asexual strains accumulate deleterious mutations in their mitochondrial genes four times as fast as the sexual strains. But there are many examples of populations that thrive without sex, at least while they live in a stable environment. Perhaps it is the ability to adapt to a changing environment that has caused sex to remain the method of choice for most living things. An asexual population tends to be genetically static. Mutant alleles appear but remain forever associated with the particular alleles present in the rest of that genome. Even a beneficial mutation will be doomed to extinction if trapped along with genes that reduce the fitness of that population. But with the genetic recombination provided by sex, new alleles can be shuffled into different combinations with all the other alleles available to the genome of that species. A beneficial mutation that first appears alongside harmful alleles can, with recombination, soon find itself in more fit genomes that will enable it to spread through a sexual population. The speed with which parasites like bacteria and viruses can change their virulence may provide the strongest need for their hosts to have the ability to make new gene combinations. So sex may be virtually universal because of the never-ending need to keep up with changes in parasites.The idea that a constantly-changing environment, especially with respect to parasites, drives evolution is often called the Red Queen hypothesis. It comes from Lewis Carroll's book Through the Looking Glass, where the Red Queen says "Now here, you see, it takes all the running you can do to keep in the same place".
In parthenogenesis ("virgin birth"), the females produce eggs, but these develop into young without ever being fertilized. In cases of parthenogenesis an ovum starts to divide by itself without fertilization, producing an embryo in which the paternal chromosomes may be replaced by a duplication of maternal ones. This asexual reproductive method is rare among warm-blooded vertebrates but more common among invertebrates. Pathological parthenogenesis has been observed in higher animals, such as the frog, fowl, and certain mammals. Parthenogenesis usually gives rise to female offspring. Attempts at artificial parthenogenesis in humans have not yet been successful. The first cloned human embryo was produced in October 2001. Eggs had their own genetic material removed and were injected with the nucleus of a donor cell. They were then incubated under special conditions to prompt them to divide and grow. One embryo grew to six cells before it stopped dividing. The same experimenters also tried to induce human eggs to divide into early embryos parthenogenetically -- without being fertilized by a sperm or enucleated and injected with a donor cell -- but their efforts met with only limited success. There is some evidence, however, that natural parthenogenesis does occasionally occur in humans. There are many instances in which impregnation have allegedly taken place in women without there being any possibility of the semen entering. In some cases it was found either in the course of pregnancy or at the time of childbirth that the female passages were obstructed. In 1956 the medical journal Lancet published a report concerning 19 alleged cases of virgin birth among women in England, who were studied by members of the British Medical Association. The six-month study convinced the investigators that human parthenogenesis was physiologically possible and had actually occurred in some of the women studied. Many primitive peoples believe that there are two methods of human reproduction: the ordinary animal one and a higher one rarely employed -- virgin birth. One belief is that the rays of the sun can fertilize women. In this regard, it is interesting that ultraviolet rays can cause parthenogenesis in unfertilized eggs of sea-urchins. It is also believed that moon rays, wind, rain, and certain types of food can cause impregnation. In the 19th century the Trobriand Islanders of the western Pacific insisted that cases of virgin birth still occurred among them.
Why choose asexual reproduction?Perhaps the better question is: Why not? After all, asexual reproduction would seem a more efficient way to reproduce and avoids all sorts of problems. Perhaps sexual reproduction has kept in style because it provides a mechanism to weed out harmful mutations that arise in the population (through the recombination process of meiosis). Some strains of the water flea Daphnia pulex reproduce sexually, others asexually. The asexual strains accumulate deleterious mutations in their mitochondrial genes four times as fast as the sexual strains. But there are many examples of populations that thrive without sex, at least while they live in a stable environment. Perhaps it is the ability to adapt to a changing environment that has caused sex to remain the method of choice for most living things. An asexual population tends to be genetically static. Mutant alleles appear but remain forever associated with the particular alleles present in the rest of that genome. Even a beneficial mutation will be doomed to extinction if trapped along with genes that reduce the fitness of that population. But with the genetic recombination provided by sex, new alleles can be shuffled into different combinations with all the other alleles available to the genome of that species. A beneficial mutation that first appears alongside harmful alleles can, with recombination, soon find itself in more fit genomes that will enable it to spread through a sexual population. The speed with which parasites like bacteria and viruses can change their virulence may provide the strongest need for their hosts to have the ability to make new gene combinations. So sex may be virtually universal because of the never-ending need to keep up with changes in parasites.The idea that a constantly-changing environment, especially with respect to parasites, drives evolution is often called the Red Queen hypothesis. It comes from Lewis Carroll's book Through the Looking Glass, where the Red Queen says "Now here, you see, it takes all the running you can do to keep in the same place".
Image source- asymptotia.com/2007/05/26/parthenogenesis-ii/
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