To help with the understanding of the look up Parthenogenesis.......http://en.wikipedia.org/wiki/Parthenogenesis
Parthenogenesis is a form of
asexual reproduction found in
females, where growth and development of
embryos occur without
fertilization by a male. In
plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of
apomixis.
The word "parthenogenesis" comes from the
Greek παρθένος,
parthenos, meaning "virgin", and γένεσις,
genesis, meaning "birth".
[1] The term is sometimes used inaccurately to describe reproduction modes in
hermaphroditic species which can reproduce by themselves because they contain reproductive organs of both sexes in a single individual's body.
Parthenogenesis occurs naturally in some
invertebrate animal species (e.g.,
water fleas,
aphids,
nematodes, some
bees, some
Phasmida, some
scorpion species, and
parasitic wasps) and some
vertebrates (e.g., some
reptiles,
[2][3] fish, and very rarely
birds[4] and
sharks[5]). This type of reproduction has been induced artificially in fish and amphibians.
[6]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. If the chromosome number of the haploid egg cell is doubled during development, the offspring is
"half a clone" of its mother. If the egg cell was formed without meiosis, it is a
full clone of its mother.
The offspring produced by parthenogenesis in species that use the
XY sex-determination system have two X chromosomes and are female. In species that use the
ZW sex-determination system they have either two Z chromosomes (male) or two W chromosomes
(non-viable or female), or (theoretically) if clonal parthenogenesis was
involved (also called apomixis), they could have one Z and one W
chromosome (female).
Parthenogenesis is seen to occur naturally in
aphids,
Daphnia,
rotifers,
nematodes and some other invertebrates, as well as in many plants and certain lizards.
Komodo dragons and the
hammerhead- and
blacktip sharks have recently been added to the list of vertebrates—along with several
genera of fish, amphibians, and reptiles—that exhibit differing forms of
asexual reproduction, including true parthenogenesis,
gynogenesis, and
hybridogenesis (an incomplete form of parthenogenesis). As with all types of
asexual reproduction,
there 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.
The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as the
XY sex-determination system), but they will be male if two like chromosomes determine the male sex (such as the
ZW sex-determination system),
because the process involves the inheritance and subsequent duplication
of only a single sex chromosome. The offspring may be capable of sexual
reproduction, if this mode exists in the species. In many cases,
parthenogenesis occurs when one gender (typically the male) is
unavailable in the general vicinity. Once males are again available, the
parthenogenesis-created females would be capable of mating with the
males and creating normal offspring.
[7]Parthenogenesis is distinct from artificial
animal cloning, a process where the new organism is necessarily genetically identical to the cell donor. In cloning, the
nucleus of a
diploid cell from a donor organism is inserted into an enucleated egg cell and the cell is then stimulated to undergo continued
mitosis,
resulting 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. Egg cells may be produced via
meiosis or
mitosis oogenesis.
[8] If produced by mitosis, the egg that undergoes parthenogenesis can be either
haploid or
diploid,
leading to a number of possible outcomes in terms of the genetic
fingerprint of the parthenogen. A diploid parent organism that undergoes
parthenogenesis via meiosis will create haploid offspring with a new
genetic fingerprint due to
crossing over of the chromosomes in the parent. Because there are so many variables
in parthenogenesis, there is little that can be said for sure unless the
specific methods of the particular parthenogenetic tendencies of an
organism are known.
A litter of offspring resulting from parthenogenesis may contain
genetically identical siblings. In organisms possessing an XY chromosome
system, parthenogenic offspring are always female, but they are not
necessarily genetically identical to one another or to their mother
(some chromosome segments may differ because of meiosis).
Parthenogenesis may be achieved through an artificial process as described below under the discussion of mammals.
Alternation between parthenogenesis and
sexual reproduction is called cyclical parthenogenesis or
heterogamy.
A form of reproduction related to parthenogenesis, but that only
requires the presence of sperm that do not fertilize an egg, is known as
gynogenesis. In
hybridogenesis the sperm fertilizes the egg, but its chromosomes are not carried to subsequent generations.
It's more of a complex form of this is gynogenesis andhybridogenesis.
http://en.wikipedia....sis#GynogenesisA form of asexual reproduction related to parthenogenesis is
gynogenesis. Here offspring are produced by the same mechanism as in
parthenogenesis, but with the requirement that the egg merely be
stimulated by the
presence of
sperm in order to develop. However, the sperm cell does not contribute any
genetic material to the offspring. Since gynogenetic species are all
female, activation of their eggs requires mating with males of a closely
related species for the needed stimulus. Some
salamanders of the genus
Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed[
who?]
that the success of those salamanders may be due to rare fertilization
of eggs by males, introducing new material to the gene pool, which may
result from perhaps only one mating out of a million. In addition the
amazon molly is known to reproduce by gynogenesis.
In hybridogenesis, reproduction is not completely asexual, but
instead hemiclonal: half the genome is passed intact to the next
generation, while the other half is discarded. It occurs in some animals
that are themselves hybrids between two different species.
Hybridogenetic females can mate with males of a "donor" species and
both will contribute genetic material to the offspring. When each female
offspring produces her own eggs, however, the eggs will contain no
genetic material from her father (the donor), only the chromosomes from
her own mother; the set of genes from the father is invariably
discarded. This process continues, so that each generation is half (or
hemi-) clonal on the mother's side and has half new genetic material
from the father's side. This form of reproduction is seen in some
live-bearing fish of the genus
Poeciliopsis[44] as well as in the waterfrog
Rana esculenta.
A graphical representation of this can be seen
through this link
Modifié par dreman9999, 21 octobre 2011 - 04:11 .