Some Deviations in Sexually Reproducing Animals: We are accustomed to thinking of animals reproducing sexually or asexually. However, there are several variants on this topic across the animal kingdom. Parthenogenesis is one such method of reproduction.

Parthenogenesis is an asexual mode of reproduction. It is a phenomenon in which a young one is produced from a single parent without the process of fertilisation. Parthenogenesis is used by several vertebrate creatures, including reptiles, amphibians, and fish. Read further to learn in detail about parthenogenesis and some deviations in sexually reproducing animals.

Reproduction in Animals

The biological process by which new offspring (individual organisms) are formed from their parents is known as a reproduction (or procreation). Reproduction is the key to a species’ existence. The known techniques of reproduction may be divided into two categories: 

(1) Asexual (Non-gametic)                          
(2) Sexual (gametic)

Asexual reproduction: Asexual reproduction is the generation of offspring by a single parent without the development and fusing of gametes. A child inherits all of its genes from a single parent. Asexual reproduction is also known as agamogenesis or agamogeny. It’s also known as somatogenic reproduction since it solely includes mitotic cell divisions. Clones are the result of asexual reproduction, which generates identical offspring. 

Sexual reproduction: Sexual reproduction is the union of two individuals’ reproductive cells (typically haploid, or having a single set of unpaired chromosomes) to produce a third (usually diploid, or having a pair of each kind of chromosome) unique offspring. Offspring from sexual reproduction have unique gene combinations. In unstable or uncertain circumstances, this can be an adaptive benefit. 

Sexual reproduction involves four processes :

1. Meiosis results in the formation of unique haploid cells known as gametes. (Gametogenesis)
2. The gametes fuse in pairs, generating diploid cells called zygotes (Fertilization)
3. The formation of embryos through repeated mitotic divisions of zygotes (Embryogenesis)
4. Embryo development into a new person (Development)Sexual reproduction is also called syngenesis.

Syngamy 

Syngamy is the joining of two cells to produce a cell with twice the number of chromosomes. Gametes are the two cells that are fused together, and the resultant cell is a zygote. Syngamy is concerned with the renewal of genetic material. The genotype of the new cells differs from that of the gametes.

Deviations from Syngamy and Sexual Reproduction in Animals

Syngamy has some special types. There are three distinct types of syngamy:

(a) Neoteny: Neoteny is defined as the persistence of a larval or embryonic characteristic in an adult organism.” Retention of larval gills in certain adult salamanders, for example.

(b) Paedogenesis or paedomorphosis: It refers to the “development of gonads and creation of young ones by the larva. Examples include Salamander Axolotl larvae, liver fluke Redia larvae, and gall fly larvae.

(c) Polyembryony: In the early stages of development, the blastomeres generated by zygote division split, each generating a full individual (fasciola liver fluke). Armadillo produces 4-8 young per zygote on a regular basis. Another example is identical twins in humans. 

Other Modes of Asexual Reproduction in Animals

1. Budding: Budding is a type of asexual reproduction that occurs when a portion of a cell or body area outgrows, resulting in the separation of the original organism into two individuals. Some invertebrate species, such as corals and hydras, are prone to budding.

2. Fragmentation: Fragmentation is a type of asexual reproduction or cloning in multicellular or colonial organisms, in which an organism is broken into fragments. Each of these fragments matures into a fully grown adult who is a clone of the original creature. sponges, acoel flatworms, some annelid worms and sea stars are examples of animals that reproduce through fragmentation.

3. Fission: It’s usually found in prokaryotic bacteria and certain multicellular invertebrates. An organism splits into two separate organisms after a period of growth. In some species, such as tapeworms and scyphistome polyps, regular transverse fission is referred to as strobilation.

Parthenogenesis (Virgin birth)

Asexual reproduction in which an egg grows into a full person without being fertilised is known as parthenogenesis. Depending on the procedure and the species, the offspring might be either haploid or diploid. Water fleas, rotifers, aphids, stick insects, certain ants, wasps, and bees are examples of invertebrates that go through parthenogenesis. Parthenogenesis is used by bees to generate haploid males (drones) and diploid females (workers). A queen is formed when an egg is fertilised. The queen bee regulates the type of bee produced by controlling the hive bees’ reproduction. 

Parthenogenesis is also used by several vertebrate creatures, including reptiles, amphibians, and fish. Parthenogenesis has been reported in animal species that were segregated by sex in terrestrial or marine zoos, despite being more frequent in plants. When female Komodo dragons, a bonnethead shark, and a blacktip shark were separated from males, parthenogenetic young were generated.

Types of Parthenogenesis

Parthenogenesis is mainly of two main types :

1. Natural Parthenogenesis

It’s a common occurrence in the lives of certain animals. It can be further classified into three types.

a. Complete (Obligatory) parthenogenesis: There are several insects that do not have a sexual phase or males. They are solely reliant on parthenogenesis for self-reproduction. e.g., rotifers, Typhlina brahmina (small lizard, 15 cm long), Lacerta saxicola-armeniaca (Caucasian Rock Lizard), Cnemidophorus (Whiptail Lizards of America).

b. Incomplete (cyclic) parthenogenesis: Certain insects have two generations, the sexual generation and the parthenogenetic generation, which alternate. Females are produced by diploid eggs, whereas males are produced by unfertilized eggs. Partial, incomplete, or cyclic parthenogenesis is the name given to this form of parthenogenesis. Unfertilized eggs produce male bees (drones) with haploid cells, whereas fertilised eggs produce females (queen bees and worker bees) with diploid cells in honeybees.

Fig: Male bees (Drones) are Produced by Parthenogenesis

c. Paedogenetic parthenogenesis: Larvae deposit eggs, which grow parthenogenetically into a new generation of larvae in certain insects. Paedogenesis refers to parthenogenesis in larvae.

Fig: Parthenogenesis in Aphids

2. Artificial Parthenogenesis

Under specific artificial settings, the eggs that invariably grow into young people via fertilisation may sometimes develop parthenogenetically. Artificial parthenogenesis is the name for this sort of parthenogenesis. Various chemical and physical methods can be used to create artificial parthenogenesis. Artificial stimuli can cause annelids, molluscs, starfish, frogs, hens, rabbits, and other animals’ eggs to develop parthenogenetically. 

Artificial stimuli maybe: 

1. Physical: Temperature, electric shock, pH, UV light, or pricked by needles.
2. Chemical: Chloroform, strychnine, hypertonic and hypotonic seawater K+, Ca++, Na+, Mg++, and other chlorides, Butyric acid, lactic acid, oleic acid, and other fatty acids are examples of acids. Urea and sucrose, as well as fat solvents such as toluene, alcohol, benzene, and acetone.

There are two forms of parthenogenesis based on chromosomal sets. 

1. Arrhenotoky (Haploid parthenogenesis): Arachnids and certain insects, for example, produce haploid males from haploid eggs (honey bees).
2. Thelotoky (Diploid parthenogenesis): The Gall fly is an example of a diploid egg that grows into a diploid person, usually a female, without fertilisation.

Advantages of Parthenogenesis

The advantages of Parthenogenesis are as follows:

1. This reduces the amount of sperm and ova wasted. Adult organisms devoted only to eating and reproduction, such as aphids, are a method of high reproduction.
2. By crossing across, there is no opportunity of separating valuable combinations of genes, and they are transferred as such.
3. The offspring are identical to their parents.
4. Haploid parthenogenesis is a direct demonstration of the chromosomal sex-determination theory.

Disadvantages of Parthenogenesis

The disadvantages of Parthenogenesis are as follows:

1. There is no gene flow from one group to the next. Because environments are inherently unstable, populations with genetic variety are better equipped to adapt to changing situations than those without. 
2. It eliminates the possibility of novel gene combinations, avoiding population selection. It reduces the likelihood of adaptation, which leads to extinction.

Significance of Parthenogenesis

The significance of Parthenogenesis are as follows:

1. In honey bees, wasps, and other insects, parthenogenesis functions as a mechanism of determining sex.
2. The chromosomal hypothesis of heredity is backed up by parthenogenesis.
3. The most basic, stable, and straightforward method of reproduction is parthenogenesis.
4. Population variation is eliminated via parthenogenesis.
5. In some insects, such as aphids, parthenogenesis is the most efficient method of rapid growth.
6. Polyploidy in organisms is brought about via parthenogenesis.
7. The development of beneficial mutant traits is aided by parthenogenesis.
8. The non-adaptive combination of genes that may be induced by a mutation is checked during parthenogenesis.
9. Parthenogenesis eliminates the need for organisms to squander energy during the mating process, allowing them to put that energy to better use elsewhere.

Summary

The biological process by which new “offspring” (individual organisms) are formed from their “parents” is known as reproduction. Asexual reproduction is the generation of offspring by a single parent without the development and fusing of gametes. Scientists have now succeeded in artificially producing multicellular animal clones in the laboratory. Neoteny is defined as the persistence of a larval or embryonic characteristic in an adult organism. In the early stages of development, the blastomeres generated by zygote division split, each generating a full individual (fasciola liver fluke). Parthenogenesis is used by bees to generate haploid males and diploid females.

There are two forms of parthenogenesis based on chromosomal sets: artificial and non-artificial. Artificial stimuli can cause annelids, molluscs, starfish, frogs, hens, rabbits, and other animals’ eggs to develop parthenogenetically. Unfertilized eggs produce male bees (drones) with haploid cells; fertilised eggs produce females (queen bees and worker bees) with diploid cells. In honey bees, wasps, and other insects, parthenogenesis functions as a mechanism of determining sex. Parthenogenesis is a direct demonstration of the chromosomal sex-determination theory. It eliminates the need for organisms to squander energy during the mating process in order to put that energy to better use elsewhere.

FAQs on Some Deviations in Sexually Reproducing Animals

Q.1. Who discovered Parthenogenesis?
Ans: Parthenogenesis was discovered by Bonnet (1745).

Q.2. What is parthenogenesis?
Ans: It is a process of creating a new individual without the use of fertilisation.

Q.3. Which animals reproduce by budding?
Ans: Corals, some sponges, some acoel flatworms, and echinoderm larvae are examples of animals that reproduce through budding.

Q.4. Which animals reproduce by fragmentation?
Ans: Sponges, acoel flatworms, some annelid worms and sea stars are examples of animals that reproduce through fragmentation.

Q.5. Which animals reproduce by regeneration?
Ans: Planarians and Hydra can reproduce by regeneration.

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