Asexual reproduction occurs by several methods. The dominant methods that are found among animals for asexual reproduction may be grouped under the following heads :
1. Fission:
This is the common method of reproduction among most protozaons and which is simple division in which the maternal organism divides into approximately two equal daughter parts, each growing until it has acquired the usual size and structure of the full- grown organism.
It, thus, consists in a division of the cytoplasm of the animal preceded by an orderly division of the nucleus which is always mitotic.
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When fission produces two daughter individuals, each about half the size of the parent, it is said to be simple and binary.
This type of binary fission is found in Amboeba and Paramecium. In simplest sporozoans a peculiar type of fission has been observed what is known as multiple fission.
In this multiple fission the nucleus divides several times to produce a temporary syncytium, and then the cytoplasm falls apart round the nuclei.
Multiple fission exhibits a specific condition of the nucleus called polyeuetgid in which several sets of chromosomes appear to be present; when the nucleus divides, these sets of chromosomes separate.
2. Budding or gemmation:
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A few of the multicellular animals (coelehterates and ascidians) produce small buds which grow gradually, ultimately acquiring the form characteristic of the producing organism.
The sponge, such as Scypha, is such an animal which develops small buds near the point of attachment and eaoh bud grows into an entire animal.
Since the buds tend to remain attached to the parent, large colonies of sponges result. In Hydra, however, the bud grows iftto a small animal which breaks away from the parent and becomes an independent new organism.
In compound ascidians buds develop in quite differenet ways. In Clavellina, a hollow median stolon grows out from the ventral side of the abdomen andlon this buds grow.
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Blood vessels and other structure grow into these buds and remain common to the entire colony.
To Botryllus the buds grow on the paired outgrowths of the atrium while in Doliolum at one point. These buds usually detach from the place of their origin and then migrate to another place where they become attached again.
3. Spore formation:
Spore formation is a common form of asexual reproduction which is widely distributed among plants, but there is one class of Protozoa, the Sporozoa, which produce the small asexual reproductive bodies, the spores.
It involves a series of cell divisions giving rise to several small cells called spores which temporarily remain within the confines of the original cell membrane or cell wall of the parent cell.
They are eventually liberated by rupture of the parental membrane or wall and under favourable conditions resume growth.
In general, spores are able to withstand unfavourable environmental conditions such as dryness, extreme heat or cold.
The malaria parasite, Plasmodium, after invading the human red blood cells, produces many spores within each invaded cell.
Also, spores are produced in the body of a mosquito which has sucked the blood of an infected person. These spores may be injected along with mosquito’s saliva when another person is bitten.
4. Fragmentation:
This method of asexual reproduction is common in coelenterates, flatworms and a few oligochaetes.
In this method the body of adult organism breaks apart into two or more pieces (fragments), each of which then grows and reforms the parts it lacks to reconstitute a complete animal.
Sometimes the fragments are roughly equal in size; more often, one is smaller and removes no vital organ from the parent animal.
5. Parthenogenesis:
The development of an egg cell into a new individual without the participation of sperm cell from the opposite sex is called the parthenogenesis.
It is considered to be a form of asexual reproduction since it does not involve the fusion of two gametes.
Parthenogenesis is a naturally occurring phenomenon among insects, crustaceans, rotifers and some platyhemininthes.
In the honeybee, an example of natural parthenogenesis, the males or drones all have their origin from unfertilized eggs and are, therefore, haploid.
The females which are the workers and the queen develop from fertilized eggs and are; therefore, diploid.
The queen is apparently only inseminated once but can store the sperm for as long as five years or more using it for fertilizing eggs, apparently at will, during this time.
Aphids, another example of natural parthenogenesis, lay eggs which hatch without fertilization into other females.
In the late summer or early fall, however, some of the eggs hatch into sexual males and females. Mating must take place between these if they are to produce offspring.
Parthenogenesis is rare in the vertebrates. Mastin (1962); Kallman and Harrington (1964) observed parthenogenesis in a few lizards particularly of the genus (Cnemidophorus.
A breed of white turkeys was found at the experiment station at Beltsville, Maryland which laid eggs which would hatch without fertilization.
When such eggs were incubated a certain small percentage of them would produce embryos and some of them would hatch and grow into adults, turkeys without fathers.
6. Gynogenesis and androgenesis:
Recently Parkes (1960) and Kallman (1962a) recognized gynogenesis as one of the methods of reproduction in a few non-segmented worms, a ptinid beetle and two species of teleost fishes.
In gynogenesis the development of a new individual takes place from the egg which is activated by sper- matozoan but spermatozoan does not contribute any genetic material to the egg.
The resulting embryo carries only maternal chromosomes. The best examples in which gynogenesis are common are Poecilia, a fish and Ptinus, a latro beetle.
Androgenesis is the reverse condition of gynogenesis. When chromosome contribution in the developing egg comes exclusively from the male is called androgenesis.
Androgenesis in animals is known only experimentally, naturally occurring androgenesis has not reported so far.