Principles of Genetics and Breeding

6.1.4 Induced Mutation

The frequency of mutations in fishes can be markedly increased by X-rays and chemical treatment.

• X-irradiation of fish gametes results in the appearance of various genic and chromosomal mutations.
• X-rays induce mutations in the platyfish, and these mutations impair the precision of regulation of action of the gene coding for the development of black pigment cells - melanophores. As a result, these fishes develop a tumour, a premelanoma, resembling that which emerges after the hybridization of the platyfish with the swordtail.
   
• Chemical mutagens, particularly nitrosoethylurea (NEU) are highly effective in inducing mutations in fishes. If spermatozoa or eggs are treated with NEU,embryos with many chromosomal defects are usually found : these defects may be easily observed when mitotic figures are inspected at the blastula stage. These observations lead to the conclusion that the frequency of chromosomal mutations induced by such treatment is very high.
• The frequency of genic mutation was measured in the carp using the S and n scale genes as a model: it was equal to 0.02% - 0.04% after dimethyl sulphate treatment.
• In one experiment with NEU 40 mutations of the n gene were observed among the 11500 fishes examined, the frequency being equal to 0.36%.
• It is quite probable,however, that the mutation induced in this gene ( n ) is a chromosomal aberration(deletion); then the surprisingly high mutation frequency in the latter case is due to the induction of deletions in the region of the gene n and is not a consequence of point (genic) mutations.
• The rate of mutation can be increased dozens or even hundreds of times by chemical mutagen.
• X-irradiation of fish spermatozoa in large doses and their treatment by high doses of chemical mutagen result in gynogenesis, that is the development in the absence of the male parent’s chromosomes.
• In such conditions embryos are haploid, but up to 1% of the embryos turn out to be spontaneous diploids.
• The use of a temperature shock (incubation at lowered temperatures) has made it possible to markedly increase the yield of diploid larvae due to gynogenesis in the loach and in the common carp.
• Cooling of eggs also resulted in an increase in the number of triploids after normal fertilization be mentioned that cytochalazine increases the number of polyploid embryos in salmonids.