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Parthenogenesis – Definition, Types

MN Editors

What is parthenogenesis? – Parthenogenesis Definition

  • In most cases, an unfertilized ovum develops into a new individual only after fertilisation, however in other instances, the egg develops without fertilisation.
  • This uncommon style of sexual reproduction in which egg development takes place without fertilisation is known as parthenogenesis (Greek: ; parthenos = virgin; = origin).
  • A parthenogenone or parthenote is a parthenogenetic or parthenogenetic creature.
  • The phenomena of parthenogenesis occurs in numerous animal species, including certain insects (Hymenoptera, Homoptera, and Coleoptera), crustaceans, rotifers, and various vertebrates, such as a number of desert lizards, turkeys, and mammals.
  • Certain situations, such as partial fertilisation, gynogenesis, androgenesis, and merogony, exist between parthenogenesis and fertilisation.
    • In partial parthenogenesis, only a portion of the sperm may fertilise the egg. According to Boveri, the sperm aster is responsible for fertilisation (activation) in the egg of the sea urchin. Only during the two-cell stage do the sperm and egg nuclei unite.
    • Sperm reaches the egg but has no role in development during gynogenesis. For example, Rhabditis aberrans degenerates within the egg without fusing with the egg nucleus.
    • In androgenesis, the egg is activated by the sperm, and development occurs without the egg nucleus’ participation. If the eggs of frogs and toads are treated with radium and subsequently fertilised by normal sperms, for instance, the egg nucleus does not participate in development, but the sperm (paternal) nucleus does.
    • Egg fragments devoid of a nucleus develop in merogony when fertilised by a normal sperm. When sea urchin eggs are tossed into small fragments, the fragments form spheres. Several of these spheres lack nuclei. If these enucleated spheres are fertilised regularly, they can grow into dwarf larvae.
  • Two types of parthenogenesis are possible: 1. parthenogenesis of natural origin; 2. parthenogenesis of artificial origin.

Natural Parthenogenesis 

The regular, continuous, and natural occurrence of parthenogenesis in the life cycles of certain animals is known as natural parthenogenesis. There are two types of natural pathogenesis: complete and incomplete.

(i) Complete parthenogenesis

  • Certain insects lack sexual maturity and males.
  • For self-reproduction, they depend only on parthenogenesis.
  • This sort of parthenogenesis is referred to as compulsory or complete parthenogenesis.
  • Some species of earthworms, badelloid rotifers, grasshoppers, roaches, phasmids, moths, gall flies, fish, salamanders, and lizards possess this trait.

(ii) Incomplete parthenogensis

  • During the life cycle of certain insects, the sexual generation and the parthenogenetic generation alternate.
  • In such instances, the diploid eggs create girls whereas the unfertilized eggs yield males.
  • This type of parthenogenesis is known as incomplete, cyclic, or partial parthenogenesis.
  • Several variations in the alternation of sexual (S) and parthenogenetic (P) generations are observed in cyclic parthenogenesis:
    • Annually, gall flies (e.g., Neuroterus) alternate between one sexual and one parthenogenetic generation (P, S,….P, S,…. P, S).
    • During the summer, the sexual generation of aphids (plant lice), daphnids, and rotifers may appear after numerous generations of parthenogenesis (P, P, P, P, P, P, S,…. P, P, P, P, P, P, S….).
    • Larvae of the gall midge (Miaster) reproduce endlessly by paedogenetic parthenogenesis. In this instance, germ cells within the larvae develop parthenogenetically into parasitic, mother-feeding larvae. These larvae typically reside beneath the bark of decaying wood and feed on fungi. Under optimal conditions, men and females with wings are generated. These stages reproduce sexually and aid in the spread of the species.
    • There is no consistency between parthenogenetic and sexual generations in some populations.

Complete and incomplete type of natural parthenogenesis

The two forms of complete and imperfect natural parthenogenesis are as follows: Haploid or arrhenotokous parthenogenesis; diploid or thelytokous parthenogenesis.

(a) Haploid or arrhenotokous parthenogenesis

  • In arrhenotokous parthenogenesis, haploid eggs are not fertilised by sperm and instead develop into haploid organisms.
  • In these instances, haploid individuals are always males and diploid individuals are always females, for example, Hymenoptera (Bees and Wasps), Homoptera, Coleoptera (Micromalthus debilis), and Thysanoptera (Anthothrips verbasi). Arachnids, such as ticks, mites, and certain spiders (Pediculoids ventricusm). Rotifers, for instance Asplanchne amphora.
  • Thus, the queen is only fertilised once by one or more males (drones). She stores sperm in her seminal receptacles, and when she lays her eggs, she has the option of fertilising them or letting them pass unfertilized.
  • The fertilised eggs mature into females (either fertile queens or sterile workers, depending on the amount of royal jelly the developing young consumes), while the unfertilized eggs evolve into fertile males or drones.

(b) Diploid or thelytokous parthenogenesis

  • Young people develop from unfertilized diploid eggs during diploid parthenogenesis. The progeny of the thelytoky might theoretically be either male or female, but in practise only females are produced.
  • In aphids, for instance, females that emerge in the spring generate many generations of females by diploid parthenogenesis caused by suppression of the first or second polar body.
  • Some females generate sexual males and females at the end of summer by diploid parthenogenesis, with males distinguishing from females in that they lack one sex chromosome.
  • Males create haploid gametes during normal meiosis, which unite to make diploid zygotes that emerge as parthenogenetic females in the spring.
  • In addition, since polyploid forms of thelytoky exist, it is also known as somatic parthenogenesis.
  • Following types of the thelytoky have been recognised:

(i) Ameiotic parthenogenesis

  • Occasionally, the first meiotic or reduction division does not occur during oogenesis, but the second meiotic division happens as usual.
  • These eggs possess a diploid number of chromosomes and are capable of producing new individuals without fertilisation.
  • Apomiotic or ameiotic parthenogenesis occurs in Trichoniscus (Isopoda), Daphnia pulex (Crustacea), Compelona rufum (Mollusca), weevils, and the long-horned grasshopper.

(ii) Meiotic parthenogenesis

  • At certain stages of oogenesis, certain eggs undergo diplosis or chromosome duplication, resulting in the creation of diploid eggs.
  • This process, known as meiotic parthenogenesis, occurs when such eggs mature into diploid individuals.
  • The diplosis of the diploid thelytoky may develop as a result of the following processes:

(a) By autofertilization

  • In some instances, the oocyte divides meiotically to produce an ootid and a secondary polocyte.
  • However, the ootid and secondary polocyte fuse to generate a diploid egg that grows into a new individual, such as Artemia salina (Crustacea) and other creatures.

(b) By restitution

  • Occasionally, karyokinesis in the primary oocyte produces the nucleus of the secondary oocyte and the nucleus of the first polocyte. However, cytokinesis does not follow karyokinesis.
  • The chromosomes of both daughter nuclei are positioned on the equator and undergo a second meiotic division to produce a diploid ootid and polocyte.
  • The dipioid ootid or ovum develops into a parthenogenetic diploid person. This sort of diplosis is known as restitution, as seen in insects of the orders Hymenoptera (Nemertis conescens) and Lepidoptera, for example.

Natural Parthenogenesis in Vertebrates 

  • A few occurrences of natural parthenogenesis in animals have also been observed. According to reports, the species Carassius auratus gibelio consists solely of females.
  • Similarly, males are completely absent from the lizard Lacerta sexicola armeniaca. It states that females are the result of parthenogenesis.
  • 80% of incubated turkey eggs exhibit early cleavage stages. Such parthenogenetic forms have hatched and developed into reproducing adults, which have been determined to be male diploids with ZZ sex chromosomes.
  • Up to sixty percent of hamster eggs spontaneously activate and grow to the two-cell stage in mammals.

Artificial Parthenogenesis 

Under specific controlled settings, the eggs that normally develop into young organisms through fertilisation can occasionally develop parthenogenetically. The term for this sort of parthenogenesis is artificial parthenogenesis. Various physical and chemical stimuli can promote parthenogenesis in the laboratory.

A. Physical means

The following physical causes are responsible for parthenogenesis:

  • Temperature. Temperature variation may cause parthenogenesis in eggs. When an egg is transported from 30o C to 0–10o C, for example, parthenogenesis is induced.
  • Electric shocks are capable of causing parthenogenesis.
  • UV radiation is capable of inducing parthenogenesis.
  • When the eggs are punctured with the fine glass needles, parthenogenesis occurs.

B. Chemical means

It has been discovered that the following substances promote parthenogenesis in normal eggs:

  • Chloroform.
  • Strychnine.
  • The sea contains both hypertonic and hypotonic fluids.
  • Various chlorides of K+, Ca++, Na++, and Mg++.
  • Fatty acids such as butyric acid, lactic acid, and oleic acid.
  • Toulene, ether, alcohol, benzene, and acetone are fatty solvents.
  • Urea and sugar.

Diverse researchers have caused artificial parthenogenesis in the eggs of the majority of echinoderms, mollusks, annelids, amphibians, birds, and mammals using the aforementioned physical and chemical methods.

Which of the following characterizes parthenogenesis?
A. An individual may change its sex during its lifetime
B. Specialized groups of cells may be released and grow into new individuals
C. An egg develops without being fertilized
D. Both members of a mating pair have male and female reproductive organs

Correct option is C)
Parthenogenesis, a reproductive strategy that involves the development of a female gamete (sex cell) without fertilization. An egg produced parthenogenetically may be either haploid (i.e., with one set of dissimilar chromosomes) or diploid (i.e., with a paired set of chromosomes). Parthenogenesis has received considerable attention as a tool for the production of stem cells. The possibility of deriving stem cells from parthenogenetic embryos could eliminate the requirement to produce and destroy viable embryos and may reduce the ethical concerns surrounding stem cell research.

Why is parthenogenesis common in ants?

Ants deposit haploid eggs that develop into males. Ants can generate male ants without mating. In some instances, queens store sperm cells and produce fertilised ova, which result in the development of female ants. Parthenogenesis allows animals to reproduce when environmental conditions prevent sexual reproduction.

Why is parthenogenesis not possible in mammals?

In mammals, a new life begins when sperm and oocyte effectively mate. Parthenogenesis, the process of producing offspring only from unfertilized oocytes, is restricted in mammals due to difficulties caused by genetic imprinting (1, 2).

Can parthenogenesis occurs in rotifers?

Parthenogenesis is a reproductive method involving the formation of a female (rarely a male) gamete (sex cell) in the absence of fertilisation. It is common in lower plants and invertebrate animals (especially rotifers, aphids, ants, wasps, and bees), but uncommon in higher vertebrates.

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