4. Introduction
Biotechnological manipulation of eggs, sperm or zygote by temperature,
pressure or chemical shocks either to alter chromosome set number or to create
fish only a single parent. This is done to crate haploid(n),triploid(3n) or
tetraploid(4n) and to create gynogens or androgens. It occurs through the or
subtraction of haploid set of chromosome present in the second polar body of the
egg by applying some sort of stimuli- heat, cold or pressure shock to the egg at
post fertilization stage.
5. Triploids:
The triploids are created for two reasons; increased growth rate and sterility.
Triploid are created by shocking newly fertilized eggs shortly after fertilization.
Fish eggs do not extrude the second polar body until they are fertilized. Because
of this if a newly fertilized egg is shocked, this shocks prevents the second polar
body from leaving the eggs. Consequently the fertilized egg will contain three
haploid nuclei; one from the egg, one from the sperm, and one from the second
polar body. The three haploid nuclei will fuse to form a triploid zygote nucleus
creating a triploid.
6. Fig: Process of triploid formation
Egg nucleus
sperm
Egg
2 𝑛𝑑
polar body
shock
7. Triploids:
Triploidy can also be produced by crossing between a triploid & diploid. Such
tetraploid parent would produce diploid gametes and diploid will produce a haploid
gamete. In the cross a triploid can readily be produced.
This approach is attractive, as it does not require applying potentially damaging
treatment (heat, cold or pressure shock).
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Tetraploids
Tetraploid have four sets of chromosomes instead
of two. They can be created by shocking a diploid
zygote when it is undergoing first cleavage.
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Gynogens
In gynogenesis the eggs are fertilized most
often with irradiated sperm. Such sperm can
only initiate the process of fertilization but no
gametic or nuclear exchange takes place.
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Androgenesis
The androgens are opposite to gynogens. The
most commonly used method is to fertilize eggs
whose genetic material has been destroyed by
Gamma or X- ray with normal sperm. This
produces haploid androgen. When the haploid
androgenic zygote undergo first cleavage, it is
shocked to prevent cell division, and the two
haploids (N) nuclei fuse to form a diploid (2N)
nucleus.
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Conclusion
Manipulation of chromosome is technically
feasible & it is easy for fish. Although reports
on the induction of polyploidy was made on
aquatic animals in 1940s. the practical
appeal of its production and application in
fish started in 1970s and since then the
literature on the line has been staggering.
So this process should be more developed.
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16.
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Prepared by
Md. Masudur Rahman
B.Sc. In Fisheries(Hons.)
Patuakhali Science and Technology University
Contact: masudranapstu@gmail.com
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