Parthenogenesis

*Mode of reproduction in which the female gamete develops into embryo without any genetic contribution from the male gamete is called parthenogenesis.

*The individual formed from parthenogenesis is called parthenot.

*It is also called virgin birth.

*It is always monoparental.

*Term parthenogenesis is coined by Richar Owen.

                          Occurence 

Parthenogenesis occur uncertain orders of  insecta: Hymenoptera, Coleoptera ,Homoptera,Rotifera and lower crustaceans .It is also reported in certain vertebrates; in domestic turkey.It occurs sporadically in large number of plants .

              Types of parthenogenesis

Two types:

1)Natural parthenogenesis

2) Artificial parthenogenesis

                               Natural 

First reported by Bonnet in aphids . In natural parthenogenesis, it occur as a natural phenomenon. It is of two types: Complete and Incomplete parthenogenesis.

*Complete parthenogenesis:  In some individuals like Rotifera,smallest Indian snake,causcasiqn Rock lizard,in some platyhelminthes etc. reproduce only by parthenogenesis (exclusively).This is called complete parthenogenesis.In these organisms,males are very rare or absent.Every individual arise from unfertilised egg.Also known as obligatory parthenogenesis.It occurs rarely in large groups.Its advantage is having high reproductive power and disadvantage is that only source of variation is mutation.

*Incomplete parthenogenesis: In this type,sexual reproduction alternates with parthenogenesis.For example- in bees and certain wasps. Unfertilised egg produce the male and fertilized egg produced the females .It is also called as cyclic parthenogenesis.

On basis of behaviour of chromosomes, natural parthenogenesis is of two types: 

Arrhenotokus : haploid;males develop from unfertilised egg and females develop from fertilised ova.So males are haploid and females are diploid.Arrhentoky is not only means of reproduction but also means of sex determination.It is recorded in seven groups of animals :five types of insects,one arachnid and one rotifer.

Thelytokus : Diploid; the embryo develops from diploid but unfertilised egg.It is of two types: Meiotic Thelytokus and Ameiotic Thelytokus.

                Artificial

If parthenogenesis is induced by the influence of external conditions like temperature, chemical and physical stimulus , called as artificial parthenogenesis.First time discovered by Boursier in silk moths in 1847.

Stimulus for Artificial parthenogenesis:

1) Physical stimulus: Provided by activities like micro pricking the egg witha fine needle.

2) Chemical stimulus: In this,eggs are treated by hyper or hypotonic sea water,chemicals like butyric acid,lactic acid ,oleic acid and fat solvents like ether,acetone and Toluene.

3) Temperature: The eggs may divide if given temperature shock treatment.

4) Stimulus  by entry of sperm only: The sperms which are not in fuse with egg can stimulate cleavage.Example: Sea urchin eggs start cleavage if starfish sperms enter in egg walls only.

Significance of parthenogenesis:

*It is easier and simplier method of reproduction.

*It helps in establishment of triploid and aneuploid forms 

*The male is not required for mating.

*Haploid parthenogenesis and diploid parthenogenesis in Aphids provide a proof of chromosomal theory of inheritance.

*Parthenogenesis is very fast mode of reproduction as it avoids the wastage of germplasm.

              Disadvantages

Due to lack of new combination of genes and  variation due to parthenogenesis, adaptability is decreased As it us faster mode of reproduction,it generally leads to overcrowding and competition.