Parthenogenesis, Natural parthenogenesis, Artificial parthenogenesis
- 1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai Parthenogenesis Dr. Sonia Bajaj (Head of Department)
- 2. Parthenogenesis Parthenogenesis is the type of asexual reproduction involving the development of female gametes without any fertilization.” This type of reproduction is mostly shown by lower plants, some reptiles, and fishes, who do not possess sex chromosomes. Parthenogenesis is a Greek word that literally translates to "Virgin Origin". Parthenogenesis animals may be incapable of sexual reproduction or capable of switching between the modes of reproduction under certain conditions. It can be naturally occurring between some plants or can be artificially induced in some amphibians or fishes. A normal egg cell forms from the mother containing half the number of chromosomes. The offspring may be a full clone of the mother containing the mother's full genetic material or maybe a half clone with half of the genetic material from the mother. Organisms that can switch between sexual reproduction and parthenogenesis, mostly get triggered for such a form of asexual reproduction under bad weather conditions or under conditions of stress. This is because parthenogenesis does not involve modification of genes or the formation of male gametes who do not give birth further. The formation of female gametes by parthenogenesis is called thelytoky: And the formation of male gametes, which is very rare, is called arrhenotoky.
- 3. Types of Parthenogenesis There are two types of parthenogenesis: 1. Natural Parthenogenesis 2. Artificial Parthenogenesis 1. Natural Parthenogenesis-In some organisms parthenogenesis is a regular and constant process occurring naturally. This is natural parthenogenesis Ex.- Honey bees, wasps, some ants, rotifrs. Natural Parthenogenesis can be of two types 1. Complete Parthenogenesis-A few insects have no males and no sexual phase. Such organisms depend upon self-reproduction. This is known as complete parthenogenesis or obligatory parthenogenesis. Ex-Rotifers
- 4. 2. Incomplete or Cyclic Parthenogenesis- In this, the unfertilized eggs produce males and the diploid eggs produce females. This type of parthenogenesis is called partial or incomplete parthenogenesis. Examples- Bees, wasps, ticks, mites, spiders, etc. Ex-Aphids Aphids have many successive generations of females develop from unfertilized eggs in spring and summer. In late summer both sexes are formed by parthenogenesis. Now the females mate with males and lay fertilized eggs that survive the cold winter season and hatch into females in the next spring to continue parthenogenesis.
- 5. Honey bee: The queen honey bee is inseminated by a male called drone during her nuptial flight . The sperms are stored in pouch that communicate with her genital tract . She lays fertilized eggs by opening the valves to let some sperms to escape. The fertilized eggs developed into females {queen and worker bees}. The unfertilized eggs developed into male bees . Spermatogenesis in drones is peculiar in lacking reduction division.
- 6. The complete or incomplete type of natural parthenogenesis may be of following two types: 1. Haploid or arrhenotokous parthenogenesis; 2. Diploid or thelytokous parthenogenesis. 1. Haploid or arrhenotokous parthenogenesis: In the arrhenotokous parthenogenesis, here female is always diploid ad male always haploid. The female is always produced from fertilized egg while male always develops parthenogenetically from unfertilized egg. Ex. Honey bees and wasps 2. Diploid or thelytokous parthenogenesis: In the diploid parthenogenesis, the young individuals develop from the unfertilized diploid eggs.
- 7. 2. Artificial Parthenogenesis The fertilized eggs might sometimes develop parthenogenetically by various chemical and physical means. This is known as artificial parthenogenesis. Physical Means A rise in temperature induces parthenogenesis in some animal eggs. Parthenogenesis is caused by ultraviolet light. Electrical shocks cause parthenogenesis. When an egg is pricked by a needle, the development occurs parthenogenetically. Chemical Means The chemicals that are responsible for the parthenogenesis of eggs are: Chloroform Urea and Sucrose Strychnine Fat solvents Acids Chlorides
- 8. Mechanism of Parthenogenesis Parthenogenesis can be considered as an incomplete sexual reproduction since the resultant offspring develops from the gametes. Gametes are reproductive or sex cells formed by meiosis. There are two types of cells in individuals- Haploid and diploid cells. Haploid cells contain a single set of chromosomes and diploid cells consist of two complete sets of chromosomes which are double in number than in haploid cells. Similarly, in haploid parthenogenesis, a haploid egg gives rise to an offspring which develops into a haploid adult. Diploid Parthenogenesis may occur in two ways 1. Automixis- It is haploid parthenogenesis in which a haploid cell may either duplicate its chromosome or pair with another haploid cell. In either of the cases, a diploid zygote is formed which develops into a diploid adult. 2. Apomixis-In apomictic parthenogenesis, a parent cell produces two genetically identical diploid egg cells by mitosis. These develop into diploid offspring. Examples. Mostly wasps, bees, crayfish, snakes, sharks, Komodo dragons reproduce by parthenogenesis.
- 9. Significance of Parthenogenesis Parthenogenesis is important for the following reasons: • Parthenogenesis helps in determining the sex of an individual in honey bees, wasps, etc. • It supports the chromosomal theory of inheritance. • Variations from populations are eliminated by parthenogenesis. • It is the simplest, most stable and easy process of reproduction. • Polyploidy in organisms is caused by parthenogenesis. • It helps in the development of advantageous mutant characters. • Non-adaptive combination of genes is controlled. • There are no sterile races.
- 10. References- Modern text book – R.L.Kotpal Jantu Vigyan- S.M. sexsena Jantu Vigyan- Dr.H.N. Baijal