Biotechnological tools such as molecular diagnostic methods ; use of vaccines and immunostimulants are gaining popularity for improving the disease resistance in fish and shell fish species world over for viral diseases, avoidance of the pathogen is very important .
2.3 CHROMOSOME ENGINEERING
Chromosome sex manipulation techniques to induce polyploidy (triploidy and tetraploidy ) and uniparental chromosome. Inheritance (gynogenesis and androgenesis ) have been applied extensively in cultured species (pandian and koteeswaran, 1998,lakra and Das 1998). These techniques are important in the improvement of fish breeding as they provide a rapid approach for gonadal sterilization, sex control improvement of hybrid viability and clonation. Vertebrates are diploid, meaning they possess two complete chromosome sets in their somatic cells. Polyploidy individuals possess on or more additional chromosome sets, bringing the total to three in triploids, four in tetraploids and so on. Induced triploids is widely accepted as the most effective method for producing sterile fish for aquaculture and fisheries management.The method used to induce triploids and other types of chromosome set manipulations In fishes and the application of these biotechnologies to aquaculture and fisheries management are well described (purdom, 1983; chowrout 1987; thorgaard, 1983; pandian and koteeswan, 1998).
2.4 Crypo preservation of gametes and gene banking
Crypopreservation is a techniques which involved long- term preservation and storage of biological material at a very temperature usually -1960C , the temperature liquid nitrogen. It is based on the principle that very low temperature tranquilize or immobilize the physiological and biochemical activities of cells, thereby making it possible to keep them viable for very long period. The technology of cryopreservation of fish spermatozoa (milt) has been adopted for animals husbandry. The first success in preserving fish sperm at low temperature was reported by Blaxter (1953 ) who fertilizes herring (clupaherengus ) eggs with frozen thawed semen. The spermatozoa of almost all cultivable fish species has now been cryopreserved (Lakra 1993). Cryopreserved overcome problems of male maturing before females allows selective breeding and stock improvement and enables conservation (Hawey,1996). one of the requirement for that can be used by breeders for evolving new strains. Most of the plant varieties that has been produced are based on the gene collections.
2.5 INTERGENRIC HYBRIDIZATION =It consists of crossing fish belonging to different species. This is done to unite the useful characters of two different species in one of the new forms and to produce a new construction in consequent of the heterosis effect. Result of intergeneric hybridization are;
- i. New construction with new feeding habits for composite fish culture or for water reservoirs.
- ii. Better growth rates and survivals.
- iii. More restful behavior, having better tolerance for reduced levels of oxygen (silvercarp x big head).
- iv. Better quality of meat (common carp x silver carp).
- v. Monosex triploid grass feeder fish (grass carp x big head).
2.6 Artificial Gynogenesis And Hormone Sex Reversion
The main purpose of gynogensis is to produce inbreed individuals having only maternal chromosomes for the further genetics work. For causing gynogenesis artificially, inactivated sperm can be used for fertilization of eggs, with a cold shock treatment after fertilization. For the inactivation of sperm, a cobalt 60 gamma ray source of ultra-violet germicide lamp has to be used. The genogenetic populations are female with diploid chromosome sets similar to the mother. The technique of artificial gynogenesis was demonstrated on the common carp. For producing gynogenetic inbreed male populations, methyl testosterone hormone feeding has to be provided for the sex formation in direction of the male. For the purpose of increasing the inbreeding level, the gynogenesis can be applied from generation to generation. It is important to make a proper selection within the gynogenetic population to avoid the undesirable effect of inbreeding depression.