Parthenogenesis-A Type Of Asexual Reproduction
Parthenogenesis-A Type Of Asexual Reproduction
In biological sense, reproduction means producing of offspring that may or may not be the exact copy of the parents. On of the major and very common type of reproduction in lower invertebrates is asexual reproduction.
Asexual reproduction is very common among the lower vertebrates like sponges, jellyfish, worms and many segmented worms. In the lower vertebrates the most common form of asexual reproduction are fission, budding, fragmentation and parthenogenesis.
Certain flatworms, rotifers, roundworms, insects, lobsters, some lizards and some fishes can reproduce without sperm and normal fertilization. These animals carry out what is called parthenogenesis. However, most of the parthenogenetic animals also reproduce sexually at some stages in their life history.
Parthenogenesis is spontaneous activation of a mature egg, followed by normal egg division and subsequent embryonic development. In fact, mature eggs of species that do not undergo parthenogenesis can sometimes be activated to develop without fertilization by pricking them with a needle, by exposing them to high concentrations of calcium, or by altering their temperature.
Because parthenogenetic eggs are not fertilized, they do not receive male chromosomes. The offspring would thus be expected to have only haploid set of chromosomes. In some animals, however, meiotic division is suppressed, so the diploid number is conserved. In other animals, meiosis occurs, but an unusual mitosis returns the haploid embryonic cells to the diploid condition.
Overall, animals that reproduce parthenogenetically have substantially less genetic variability than do animals with chromosomes sets from two parents. This condition may be an advantage for animals that are well adapted to a relatively stable environment. However, in meeting the challenges of a changing environment, parthenogentic animals may have less flexibility, which may explain why this form of reproduction is relatively uncommon.
Parthenogenesis also play an important role in social organization in colonies of certain bees, wasps and ants. In these insects, large numbers of male (drones) are produced parthenogenetically, whereas sterile female workers and reproductive females (queens) are produced sexually.
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