AN OVERVIEW OF FISH GENOMIC MANIPULATION AND PRODUCTION TECHNIQUES *Nwangwu, D.C., Yisa, M & Yahaya M Fish Biotechnology & Genetic Improvement Laboratory National Institute for Freshwater Fisheries Research P.M.B. 6006, New-Bussa, Niger State. *Correspondence – cdnwangwu@gmail.com , +2348035957410 Abstract Genomic studies and genetic improvement programmes for fish can contribute to the productivity both in quantitative and qualitative terms by enhancing traits of major importance such as growth rate – size/weight to harvest time or a specified period, age at sexual maturity, disease resistance and (or) survival rate, tolerance to water temperature and other water attributes, flesh quality and excellent feed conversion or efficiency. The several biotechnological techniques highlighted are being readily applied to farmed aquatic species in an effort to overcome different production challenges. If incorporated into breeding programs they will help in commercial production of farmed aquatic species. Keywords: hybridization, chromosome set and sex manipulations, polyploidy, cryopreservation, transgenesis Introduction The genome of an organism can be defined as its entire DNA content as a whole. Many aquaculture production systems especially in developing countries are largely based on the use of genetically unimproved species and strains. As much as knowledge and experience is gathered in such production systems, it becomes more imperative to make use of genetically more productive stock. (Pullin, RSV et al 1999) In other to utilize the resources more effectively, refinements in the production system and improvement of the stock used must progress hand in hand. Several strategies are readily applied to manipulate the genome of fishes and shell fish to enhance production viz selective breeding, hybridization and crossbreeding, chromosome set and sex manipulations, cryopreservation and transgenic technologies Selective Breeding Traditional selective breeding is the oldest and most commonly used technique for farmed aquatic species. It is practiced by selecting the best individuals in a population with high traits of economic importance for parenting purpose to produce novel races and varieties according to the scheme of quantitative genetics and Mendellian inheritance through phenotypic assumptions. The traits of economic importance in aquaculture are generally quantitative and governed by a large number of genes. Although selection for growth rate has generally been associated with positively correlated responses (e.g. increase in survival or disease resistance) there are examples of long term selections resulting in decreased bacterial resistance, possibly due to genetic correlations changes or inbreeding. (C.Greg Lutz 2001) Selection does not necessarily create new genes but rather changes the gene frequencies (at the loci affecting the trait in the next generation) thus the frequencies of alleles with favorable effects on the phenotype under selection are increased and the frequency of less favorable alleles decreased. Assuming no change in environmental conditions, the average phenotypic value (or trait under selection) of progenies of selected parents is increased. This phenotypic increase/difference is often called response to selection or genetic gain which is the average superiority of progenies of selected parents compared to the generation before them as in the traditional selective breeding. Recently, the advent of molecular genetics has opened possibilities for direct selection of animals and aquatic species on genotype or alternatively, selection based on linkage association between genetic markers and quantitative trait loci (QTL) of an important production trait. This type of selection method is called marker assisted selection (MAS) Hybridization and Crossbreeding Hybrids are the progenies of parents from different lines, strains (intraspecific) or species (interspecific) Hybrids can also be produced through crossing of distinct but closely related species (intergeneric) or by backcrossing of hybrids (when fertile) with one or both of their parental species (introgressive) to maintain genetic traits from the parental breed. The major objective for which breeders cross lines is to exploit non-additive genetic variance in order to obtain “hybrid vigor” (heterosis) resulting from heterozygosity at many loci. Hybridization harnesses short term genetic gains as heterosis is always attained in the first progeny. In most cases it is not necessary to maintain the hybrid strains because they can be produced at anytime as long as the parent strains exits and further hybridization may not necessarily produce better hybrid vigor. The mean phenotypic value of hybrid is often greater than the value of either parental line so hybrids tend to show faster growth, etc. Hybridization between species can also result in offspring that are sterile or have diminished reproductive capacity due to problems with chromosome alignment during meiosis thus hybridization can be used to produce single sex group of fishes as in monosex production. To date much of the breeding work in aquaculture have