Parthenogenesis
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Parthenogenesis is a form of asexual reproduction where growth and development of embryos can occur without fertilization. It was first recognized in 1745 and experiments in the early 1900s and 1930s stimulated both natural and artificial parthenogenesis in various species. There are two main types of natural parthenogenesis - complete (obligate) where reproduction relies exclusively on parthenogenesis, and incomplete (cyclic) where sexual and parthenogenetic generations alternate. Parthenogenesis occurs naturally in many invertebrates as well as some vertebrates and can be induced artificially.
Parthenogenesis
- 1. PARTHENOGENESIS _ Dr Zaid Wani
- 2. History • Parthenogenesis was first recognized in 1745 by CHARLES BONNET • JACQUES LOEB stimulated parthenogenesis in sea urchin and frog eggs by artificial means in 1900. • G.PINCUS in 1936 artificially produced the first known parthenogenetic offspring in a mammal {rabbit}.
- 3. Definition • Parthenogenesis is a natural form of asexual reproduction. • Growth and development of embryos occur without fertilization. • It is a component of apomixis.
- 4. Occurrence • Parthenogenesis is found in many invertebrates such as rotifers and arthropods. • Crustaceans , insects and arachnids {spider, mites} • some vertebrates {rock lizards and birds}. • Snails, Sharks, Amphibians
- 5. Types of parthenogenesis I. Natural parthenogenesis: 1 Complete { obligate } parthenogenesis 2 Incomplete {cyclic} parthenogenesis • 3 Paedogenetic parthenogenesis II. Artificial parthenogenesis.
- 6. I. Natural Parthenogenesis In certain animals the parthenogenesis occurs regularly, constantly and naturally in their life cycles and is known as the natural parthenogenesis. • The natural parthenogenesis may be of two types, viz., complete or incomplete
- 7. Complete Parthenogenesis: • Certain insects have no sexual phase and no males. They depend exclusively on the parthenogenesis for the self-reproduction. • This type of parthenogenesis is known as the complete parthenogenesis or obligatory parthenogenesis. • Examples : Rotifers, Caucasian Rock Lizard.
- 8. Incomplete parthenogensis: • The life cycle of certain insects includes two generations, the sexual generation and parthenogenetic generation, both of which alternate to each other. • In such cases, the diploid eggs produce females and the un-fertilised eggs produce males. • This type of parthenogenesis is known as the partial or incomplete or cyclic parthenogenesis. • Examples : Aphids, Honeybees, Rotifers, Birds.
- 10. The complete or incomplete type of natural parthenogenesis may be of following two types: • 1. Haploid or arrhenotokous parthenogenesis. • 2. Diploid or thelytokous parthenogenesis.
- 11. • Haploid or arrhenotokous parthenogenesis: In the arrhenotokous parthenogenesis the haploid eggs are not fertilised by the sperms and develop into the haploid individuals. Eg, Honeybees • Diploid or thelytokous parthenogenesis: In the diploid parthenogenesis, the young individuals develop from the un-fertilised diploid eggs.
- 12. Types: (i) Ameiotic Parthenogenesis: • Sometimes during the oogenesis, first meiotic or reduction division does not occur but second meiotic division occurs as usual. • Such eggs contain diploid number of chromosomes and develop into new individuals without the fertilisation. • This type of parthenogenesis is known as apomictic or ameiotic parthenogenesis and occurs in weevils and long-horned grasshoppers.
- 14. ii) Meiotic Parthenogenesis: • Certain eggs develop by the usual process of oogenesis but at certain stages diplosis or doubling of chromosome number and production of diploid eggs occur. • Such eggs develop into the diploid individuals and this phenomenon is known as the meiotic parthenogenesis.
- 15. Paedogenetic parthenogenisis: • Larvae of some insects lays eggs which develop parthenogenetically into new generation of larvae. Eg , Dipterous insects.
- 16. Artificial Parthenogenesis • The eggs which always develop into the young individuals by the fertilisation sometimes may develop parthenogenetically under certain artificial conditions. • The artificial parthenogenesis may be induced by various chemical and physical means.
- 17. Significance 1. The parthenogenesis serves as the means for the determination of sex in the honey bees, wasps, etc. 2. The parthenogenesis is the most simple, stable and easy process of reproduction. 3. The parthenogenesis eliminates the variation from the populations. 4. The parthenogenesis is the best way of high rate of multiplication in certain insects, e.g., aphids.
- 18. 5. The parthenogenesis causes the polyploidy in the organisms. 6. The parthenogenesis encourages advantageous mutant characters. 7. No need for the organisms to waste their energy in the process of mating.
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