What is Parthenogenesis?
Parthenogenesis is the process where here is a natural form of asexual reproduction where the embryo grows and develops without any fertilisation by the male sperm. In case of animals this process means the embryo will develop for an embryo from an egg cell which is not fertilised. In case of plants however there is apomixis. This process naturally occurs in some plants, animals (also scorpions, nematodes, bees, mites, wasps etc). Also it is present in some vertebrate animals for example fishes, reptiles, amphibians etc. This process has been introduced artificially in some fishes and amphibians.
Process of Parthenogenesis-
Here the eggs which are formed by the process are normal and this happens by meiosis and is haploid and the chromosomes are half to the cells in the mother. They are not viable and the offsprings have chromosomes which are diploid. The number of diploid chromosomes is haploid and hence the offsprings might have all or just some of the alleles of the mother. The genetic material are all carried out and are therefore called full clones and those having having half are called half clones. They are formed without the process of meiosis. The process of meiosis occurs and then the offsprings only get half of the genetic material forms the mother and this takes places because the DNA division takes places which create variation.
This process will yield offsprings that will have either XY or XO sex system which determines and they have two chromosomes which are X and are females. In some species that use the ZW sex determination system there are two Z chromosomes which are male or two W chromosomes which are not viable. Not necessarily females. They also could posses one Z and one W chromosome.
This process will not happen to isogamous or monogamous species.
MECHANISM:
The process that involves meiosis is very complicated and thus in this cases the offsprings are haploid for example male ants. In some other cases, there is natural and automatic parthenogenesis and the ploidy state gets back to diploid state by various means. Haploids are not viable that is why. Since there is meiosis the offsprings have half the clones from the mother.
The process without meiosis happens through mitosis and is called APOMICTIC parthenogenesis. Here the egg cells become matures and produced by mitosis and these cells become into embryos. In some flowering plants the cells will go through this process and the offsprings will fully clone the mother. Example- Aphids.
What is its natural occurrence?
They happen naturally in Aphids, rotifers, nematodes and other invertebrates as well in plants. In Vertebrates, this happens in lizards, snakes, birds, etc. In some cases like fishes, amphibians, will show different form of gynogenesis which is like an incomplete form of parthenogenesis.
Lets see how this process occurs in different species:
- Rotifers: Here the females will reproduce exclusively and while the others will toggle between asexual and sexual reproduction. Here any allele who were recessive leads to loss of sexual reproduction in some offsprings.
- Velvet Worms: There are some species who were found in Trinidad that showed this process. This particular species is the only one known to have reproduced via parthenogenesis.
- Flatworms: Some species of Flatworms will show that individuals will reproduce through this process. This process requires mating process and here the sperm does not contribute to the offspring genetics. A cycle of mating process between the asexual and polyploid individuals who show the parthenogenesis process will produces new offsprings.
- Snails: Some species in the country of New Zealand where there are mud snails will show this process.
- Insects : They cover many mechanisms, here the offsprings produced may be either of the sexes or only females, or only males. Here true parthenogenesis are known to occur. Here the egg cells which belongs to a particular species without meiosis or any other automatic mechanism.
This process if different from other artificial animal cloning and the latter process is one where new organism is genetically made identical to the cell donor. Here the nucleus of the cell from the donor organism is thus kept into the enucleated egg cell and then the cell will go through the mitosis process which results in the offspring being identical to the donor. The process of parthenogenesis can be made possible from an artificial process. There are some species who reproduce occasionally and for example the Komodo dragon are list added to the vertebrates who show the process.
Learn more concepts fromReproductive in Organisms from Class 12 Biology