Principles of Genetics and Breeding

8.1.1.2 Shock treatment

The term ploidy induction is multiplication of the chromosome set by manipulating events during embryonic development, such as insemination, release of polar body and first-cell division.

• Chromosome sets can be manipulated by the application of thermal or temperature (cold and heat), pressure or chemical shock.
• Shocking of a freshly inseminated fish egg causes depolymerization of tubulin polymers that form microtubules, which are essential for the formation of the spindle apparatus.
• Therefore, shock treatment results in the inhibition of spindle formation and aster movement.
• When applied shortly before first cleavage, the shock treatment inhibit cytokinesis and cause zygotes to undergo two genomic replications with only one cytoplasmic division.
• This is necessary for diploidizing gynogenetic and androgenetic offspring and in inducing triploidyand tetraploidy.

1. Thermal Shock

• Cold shocks are usually applied near 0°C for cold water species such as salmonids and somewhat at higher temperature (8-12° C) for warm water species like Common carp, Tilapia and Indian major carps.
• Heat shock is applied at a higher temperature (around 26-28°C) in the case of cold water than in warm water fishes (39-42°C).

2. Pressure shock

This method is simple to administer.

• The pressure range varies between 7000 to 9000 pascals (Psi).
• The hydrostatic pressure is applied by a specialised instrument (French Cell Press) designed by mechanical engineering method.
• Pressure shock is supposed to have less side effect than the thermal shock.

3. Chemical shock

• Colchicine and cytochalasin-B have the potential to disrupt cell division and induce ploidy induction.
• Anaesthetics such as nitrous oxide and Freon 22 have also been tried to induce triploidy. But the results are inconsistent and unsatisfactory.