Parthenogenesis
Parthenogenesis
Parthenogenesis (“virgin origin”) is the development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilization. There are many patterns of parthenogenesis. In one type, called ameiotic parthenogenesis, no meiosis occurs, and the egg is formed by mitotic cell division. This “asexual” form of parthenogenesis is known to occur in some species of flatworms, rotifers, crustaceans, insects, and probably others. In these cases, the offspring are clones of the parent because, without meiosis, the parent’s chromosomal complement is passed intact to offspring.
In meiotic parthenogenesis a haploid ovum is formed by meiosis, and it may or may not be activated by the influence of a male. For example, in some species of fishes, a female may be inseminated by a male of the same or related species, but the sperm serves only to activate the egg; the male’s genome is rejected before it can penetrate the egg. In several species of flatworms, rotifers, annelids, mites, and insects, the haploid egg begins development spontaneously; no males are required to stimulate activation of an ovum. The diploid condition is restored by chromosomal duplication. A variant of this type of parthenogenesis occurs in many bees, wasps, and ants. In honey bees, for example, the queen bee can either fertilize the eggs as she lays them or allow them to pass unfertilized. Fertilized eggs become diploid females (queens or workers), and unfertilized eggs develop parthenogenetically to become haploid males (drones); this type of sex determination is known as haplodiploidy. In some animals meiosis may be so severely modified that the offspring are clones of the parent. This happens in certain populations of whiptail lizards of the American southwest, which are clones consisting solely of females (Cole, 1984).
Parthenogenesis is surprisingly widespread in animals. It is an abbreviation of the usual steps required of bisexual reproduction. It may have evolved to avoid the problem—which may be great in some animals—of bringing together males and females at the right moment for successful fertilization. The disadvantage of parthenogenesis is that if the environment should suddenly change, as it often does, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions. Bisexual species, by recombining parental characteristics, have a better chance of producing variant offspring that can utilize new environments.
From time to time claims arise that spontaneous parthenogenetic development to term has occurred in humans.A British investigation of about 100 cases in which the mother denied having had intercourse revealed that in nearly every case the child possessed characteristics not present in the mother, and consequently must have had a father.Nevertheless, mammalian eggs very rarely will spontaneously start developing into embryos without fertilization. In certain strains of mice, such embryos will develop into fetuses and then die.The most remarkable instance of parthenogenetic development among the higher vertebrates has been found in turkeys in which ova of certain strains, selected for their ability to develop without sperm, grow to reproducing adults.
Parthenogenesis (“virgin origin”) is the development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilization. There are many patterns of parthenogenesis. In one type, called ameiotic parthenogenesis, no meiosis occurs, and the egg is formed by mitotic cell division. This “asexual” form of parthenogenesis is known to occur in some species of flatworms, rotifers, crustaceans, insects, and probably others. In these cases, the offspring are clones of the parent because, without meiosis, the parent’s chromosomal complement is passed intact to offspring.
In meiotic parthenogenesis a haploid ovum is formed by meiosis, and it may or may not be activated by the influence of a male. For example, in some species of fishes, a female may be inseminated by a male of the same or related species, but the sperm serves only to activate the egg; the male’s genome is rejected before it can penetrate the egg. In several species of flatworms, rotifers, annelids, mites, and insects, the haploid egg begins development spontaneously; no males are required to stimulate activation of an ovum. The diploid condition is restored by chromosomal duplication. A variant of this type of parthenogenesis occurs in many bees, wasps, and ants. In honey bees, for example, the queen bee can either fertilize the eggs as she lays them or allow them to pass unfertilized. Fertilized eggs become diploid females (queens or workers), and unfertilized eggs develop parthenogenetically to become haploid males (drones); this type of sex determination is known as haplodiploidy. In some animals meiosis may be so severely modified that the offspring are clones of the parent. This happens in certain populations of whiptail lizards of the American southwest, which are clones consisting solely of females (Cole, 1984).
Parthenogenesis is surprisingly widespread in animals. It is an abbreviation of the usual steps required of bisexual reproduction. It may have evolved to avoid the problem—which may be great in some animals—of bringing together males and females at the right moment for successful fertilization. The disadvantage of parthenogenesis is that if the environment should suddenly change, as it often does, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions. Bisexual species, by recombining parental characteristics, have a better chance of producing variant offspring that can utilize new environments.
From time to time claims arise that spontaneous parthenogenetic development to term has occurred in humans.A British investigation of about 100 cases in which the mother denied having had intercourse revealed that in nearly every case the child possessed characteristics not present in the mother, and consequently must have had a father.Nevertheless, mammalian eggs very rarely will spontaneously start developing into embryos without fertilization. In certain strains of mice, such embryos will develop into fetuses and then die.The most remarkable instance of parthenogenetic development among the higher vertebrates has been found in turkeys in which ova of certain strains, selected for their ability to develop without sperm, grow to reproducing adults.
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