What is parthenogenesis?

The Nature of Parthenogenesis

Parthenogenesis is derived from two Greek words that mean “virgin”
(parthenos) and “origin” (genesis) and
describes a form of reproduction in which females lay diploid eggs (containing two
sets of chromosomes) that develop into offspring without fertilization—there is no
fusion of a sperm nucleus with the ovum nucleus
to produce the new diploid individual. This is a form of clonal reproduction
because all of the offspring are genetically identical to the mother and to each
other. The mechanisms of parthenogenesis do not show any single pattern and have
evolved independently in different groups of organisms. In some organisms, such as
rotifers and aphids, parthenogenesis alternates with
normal sexual reproduction. When there is a rich food source, such as new rose
bushes emerging in the early spring, aphids reproduce by parthenogenesis; late in
the summer, however, as the food source is decreasing, sexually reproducing
females appear. The same pattern has been observed in rotifers, in which a
decrease in the quality of the food supply leads to the appearance of females that
produce haploid eggs by normal meiosis that require fertilization for development.
The strategy appears to involve the clonal production of large numbers of
genetically identical individuals that are well suited to the environment when the
conditions are favorable and the production of a variety of different types, by
the recombination that occurs during normal meiosis and the
mixing of alleles from two individuals in sexual reproduction, when
the conditions are less favorable. In social insects, such as bees, wasps, and
ants, parthenogenesis is a major factor in sex determination, although it may not
be the only factor. In these insects, eggs that develop by parthenogenesis remain
haploid and develop into males, while fertilized eggs develop into diploid,
sexually reproducing females.

In algae and some forms of plants, parthenogenesis also allows rapid reproduction when conditions are favorable. In citrus, seed development by parthenogenesis maintains the favorable characteristics of each plant. For this reason, most commercial citrus plants are propagated by asexual means, such as grafting. Parthenogenesis has also been induced in organisms that do not show the process in natural populations. In sea urchins, for example, development can be induced by mechanical stimulation of the egg or by changes in the chemistry of the medium. Even some vertebrate eggs have shown signs of early development when artificially stimulated, but haploid vertebrate cells lack all of the information required for normal development, so such “zygotes” cease development very early.

Parthenogenesis in Vertebrates

Parthenogenesis has been observed in vertebrates such as fish, frogs, and
lizards. In these parthenogenetic populations, all organisms are female, so
reproduction of the clone is restricted to parthenogenesis. Parthenogenetic fish
often occur in populations along with sexually reproducing individuals. The
parthenogenetic forms produce diploid eggs that develop without fertilization; in
rare cases, however, fertilization of a parthenogenetic egg gives rise to a
triploid individual that has three sets of chromosomes rather than the normal two
sets (two from the diploid egg and one from the sperm). In some groups,
penetration of a sperm is necessary to activate development of the zygote, but the
sperm nucleus is not incorporated into the zygote.

Evidence indicates that in each of these vertebrate situations, the
parthenogenetic populations have resulted from a hybridization between two
different species. The parthenogenetic forms always occur in regions where the two
parental species overlap in their distribution, often an area that is not the most
favorable habitat for either species. The hybrid origin has been confirmed by the
demonstration that the animals have two different forms of an enzyme that
have been derived from the two different species in the region. Genetic identity
has also been confirmed using skin graft studies. In unrelated organisms, skin
grafts are quickly rejected because of genetic incompatibilities; clonal animals,
on the other hand, readily accept grafts from related donors. Parthenogenetic fish
from the same clone accept grafts that confirm their genetic identity, but
rejection of grafts by other parthenogenetic forms from different populations
shows that they are different clones and must have a different origin. This makes
it possible to better understand the structure of the populations and helps in the
study of the origins of parthenogenesis within those populations. Comparisons
using nuclear and mitochondrial DNA also allow the determination of species origin
and the maternal species of the parthenogenetic form since the mitochondria are
almost exclusively transmitted through the vertebrate egg. Within the hybrid, a
mechanism has originated that allows the egg to develop without fertilization,
although, as already noted, penetration by a sperm may be required to activate
development in some of the species.

The advantage of parthenogenesis appears to be the production of individuals
that are genetically identical. Since the parthenogenetic form may, at least in
vertebrates, be a hybrid, it is heterozygous at most of its genetic loci. This
provides greater variation that may provide the animal with a greater range of
responses to the environment. Maintaining this heterozygous genotype may give the
animals an advantage in environments where the parental species are not able to
reproduce successfully and may be a major reason for the persistence of this form
of reproduction. Many vertebrate parthenogenetic populations are found in
disturbed habitats, so their unique genetic composition may allow for adaptation
to these unusual conditions.

Mechanisms of Development

The mechanisms of diploid egg development are as diverse as the organisms in
which this form of reproduction is found. In normal meiosis, the like chromosomes
of each pair separate at the first division and the copies of each chromosome
separate at the second division (producing four haploid cells). During the meiotic
process in the egg, three small cells (the polar bodies), each with one set of
chromosomes, are produced, and one set of chromosomes remains as the egg nucleus.
In parthenogenetic organisms, some modification of this process occurs that
results in an egg nucleus with two sets of chromosomes—the diploid state. In some
forms, the first meiotic division does not occur, so two chromosome sets remain in
the egg following the second division. In other forms, one of the polar bodies
fuses back into the cell so that there are two sets of chromosomes in the final
egg. In another variation, there is a replication of chromosomes after the first
division, but no second division takes place in the egg, so the chromosome number
is again diploid. In all of these mechanisms, the genetic content of the egg is
derived from the mother’s genetic content, and there is no contribution to the
genetic content from male material.

The situation may be even more complex, however, because some hybrid individuals may retain the chromosomal identity of one species by a selective loss of the chromosomes of the other species during meiosis. The eggs may carry the chromosomes of one species but the mitochondria of the other species. The haploid eggs must be fertilized, so these individuals are not parthenogenetic, but their presence in the population shows how complex reproductive strategies can be and how important it is to study the entire population in order to understand its dynamics fully: A single population may contain individuals of the two sexual species, true parthenogenetic individuals, and triploid individuals resulting from fertilization of a diploid egg.

Key Terms



adaptive advantage

:

increased fertility in offspring as a result of passing on favorable genetic information




diploid

:

having two sets of homologous chromosomes





fertilization


:

the fusion of two cells (egg and sperm) in sexual reproduction




haploid

:

having one set of chromosomes





meiosis


:

nuclear division that reduces the chromosome number from diploid to
haploid in the production of the sperm and the egg





zygote


:

the product of fertilization in sexually reproducing organisms





Bibliography

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Print.

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