Parthenogenesis Of Asexual Reproduction
Parthenogenesis /ˌpɑrθənoʊˈdʒɛnəsɨs/
is a form of asexual reproduction in which growth and development of
embryos occur without fertilization. In animals, parthenogenesis means
development of an embryo from an unfertilized egg cell and is a
component process of apomixis.
Gynogenesis
and pseudogamy are closely related phenomena in which a sperm or pollen
triggers the development of the egg cell into an embryo but makes no
genetic contribution to the embryo. The rest of the cytology and
genetics of these phenomena are mostly identical to that of
parthenogenesis.
The word parthenogenesis comes from the Greek παρθένος, parthenos,
meaning "virgin" and γένεσις, genesis, meaning "birth". The term is
sometimes used inaccurately to describe reproduction modes in
hermaphroditic species that can reproduce by themselves because they
contain reproductive organs of both sexes in a single individual's body.
Parthenogenesis occurs naturally in many plants, some invertebrate
animal species (including nematodes, water fleas, some scorpions,
aphids, some bees, some Phasmida and parasitic wasps) and a few
vertebrates (such as some fish, amphibians, reptiles 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 haploid, with half as many
chromosomes as their mother's body cells. Haploid individuals, however,
are usually non-viable, and parthenogenetic offspring usually have the
diploid chromosome number. 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.
arthenogenesis 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,
rotifers, aphids, stick insects, some ants, bees and parasitic wasps),
and vertebrates (e.g. some reptiles, amphibians,rarely birds). In
plants, apomixis may or may not involve parthenogenesis.
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