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.
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