Parthenogenesis

What is Parthenogenesis?

parthenogenesis is a type of asexual reproduction where an organism develops from an unfertilized egg cell, without the involvement of a male parent.

• Parthenogenesis is the moodification of sexual reproduction
• An egg develops into a complete offspring without fertilization.
• It is monoparental.Parthenogenesis was discovered by Bonnet (1745).
• Parthenogenesis is found in many non vertebrates such as rotifers, aphids, bees and crustaceans. It also occur in a few vertebrates.

Types of parthenogenesis

Natural parthenogenesis :

• It is a regular phenomenon in the life history at some animals.It may be three type.
  • Complete (Obligatory) parthenogenesis : Males are absent, females develop parthenogenetically, e.g., rotifers, Typhlina brahmina (small lizard, 15 cm long), Lacerta saxicola- armeniaca (Caucasian Rock Lizard), Cnemidophorus (Whiptail Lizards of America).
  • Incomplete (cyclic) parthenogenesis : Some animals have both sexual and parthenogenetic individuals, which may alternate.
    • In these animals, female can produce unfertilized or fertilized eggs, depending upon environmental conditions.
    • In Daphnia, a fresh water crustacean, female lays unfertilized eggs that develop parthenogenetically under favourable conditions, and fertilized eggs during times of environmental stress.
    • In honeybee, unfertilized eggs develop into male bees (drones) with haploid cells, and fertilized eggs give rise to females (queen bees and worker bees) with diploid cells.
  • Paedogenetic parthenogenesis : In certain insects, larvae lay eggs which develop parthenogenetically into a new generation of larvae. Parthenogenesis in larvae is called paedogenesis.

Artificial parthenogenesis :

• Eggs of certain animals, such as annelids, mollusks, starfish, frog, hen, rabbit, etc., can be induced to develop parthenogenetically by artificial stimuli.
• Artificial stimuli may be (i) physical, viz., prick of a needle, electric shock, change in temperature or pH; or (ii) chemical such as addition of urea, fatty acids, ether, chloroform, to water.
• On the basis of chromosome sets parthenogenesis is of two types –
  • 1.Arrhenotoky (Haploid parthenogenesis) : Haploid eggs grow to form haploid males e.g., Arachnids, some insects (honey bees).
  • 2. Thelotoky (Diploid parthenogenesis) : Diploid eggs grow without fertilization in to diploid individuals, generally females. e.g., Gall fly.

Advantages of parthenogenesis

• This avoids the wastage of germplasm as sperms and ova.
• A mode of high reproduction rate e.g., aphids.
• There is no chance of separation of useful combination of genes by crossing over and are transmitted as such.
• The offsprings are exactly similar to parents.Haploid parthenogenesis is the direct proof of chromosomal theory of sex-determination

Disadvantages of parthenogenesis

It stops the chances of new combinations of genes and thus avoids selection in population.It decreases the chances of adaptability followed by extinction.