Genetic manipulation studies in Oreochromis niloticus L.

Abstract

The results of a study aimed at the identification of treatment optima for triploidy induction in recently fertilised Oreochromis niloticus eggs by altering the intensity, duration and timing of application of pressure, heat and cold shocks are reported. Preliminary, but not directly comparable, trials suggested the following treatments to be close to the individual agent optima. Pressure; 8,000 p.s.i. 2 mins, duration applied 9 mins, after fertilisation (a.f.); heat: 41 *C, 3.5 mins, duration applied 5 mins. a.f.; cold: 9 *C. 30 mins, duration applied 7 mins. a-f. In a directly comparable trial in which the eggs of eight different females were separately exposed to the optimum shocks listed above, individual triploid yields were more variable following cold shocks and mean triploid yields were, therefore, higher following pressure and heat shock. The effects of triploidy on sexual development and maturation in female and male fish were examined in successive age groups (4 - 10 months). Significant differences in ovary weight and OSI were found between 3n and 2n females. But 3n males showed normal testis development and some of them could produce aneuploid sperm. The testis weight and OSI of such males were not significantly different compared to 2n. Comparative performance of growth, biochemical composition and endocrine profiles of normal diploids and pressure, heat and cold shocked triploids was investigated. 3n females were not significantly different in growth rate and proximate nutrition parameters compared to the 2n controls but were found to be functionally and endocrinologically sterile. 3n males showed no significant differences in growth or proximate nutrition parameters but were genetically sterile despite showing normal 2n endocrinological profiles and secondary sexual characteristics. Evidence is presented for successful suppression of first cleavage in O. niloticus and thereby the production of mitotic gynogenetics. The optimal parameters for UV irradiation of milt were 300 - 310 pW/cm> for 2 min at 4-C. The optimal pressure shock W.S 9000 p.s.i. for 2 mins at 28 *C at 40 - SO mins a.f. and that for heat was 41 *C for 3.5 mins at 27.5 - 30 mins. a.f. Isozyme analysis of putative mitotic gynogenetic survivors at ADA' locus confirmed homozygosity, therefore, restoration of diploidy occurred by inhibition of first mitosis. Subsequently gene-centromere recombination frequency estimated at six enzyme loci further revealed no recombination between the respective gene and centromere at all the loci. In contrast, recombination frequency in meiotic gynogenetic progeny was 0 - 100* (mean y = 0.41) which suggesting that this probably a reflection of the relative position of the various loci to centromere on their respective chromosomes. Production of heterozygous clones of O. niloticm was successfully carried out by crossbreeding between viable mitotic gynogenetic female and male sibs. At the same time, homozygous clones of the fish was produced by gynogenetic reproduction (retention of 2nd polar body) using optimal pressure/heat shock treatments. The outbred (OCL) and inbred (ICL) nature of two types of clonal lines were checked and identified at ADA' marker locus. A model for the large scale production of such clonal lines is presented. The effect of inbreeding on various phenotypic characters of two types of gynogenetics (meiotic and mitotic) in comparison to full sib controls was investigated. The coefficient of variation values of growth, meristic and all reproductive train were lowest in control, intermediate in meiotic and highest in mitotic gynogenetic group. This study revealed that the expansion of variation in such performance train in both type of gynogenetics was possibly the result of phenotypic expression of unmasked homozygous recessive and deleterious genes due to increasing levels of homozygosity (F value). Mendelian mode of ted body colour inheritance was studied in Egyptian red O. niloOcus and Thai ted tilapia attains. Cross-breeding between ted a ted and ted x wild parents resulted in mostly all red coloured progenies and in some red x wild crosses progenies were also segregated into red and wild types. The F. red hybrids mated infer se and back-crossed to wild type, the progeny phenotype segregated closely into approximating the expected 3 red: 1 wild and 1 red : 1 wild ratios respectively. These results demonstrate that red body colour in two mutant attains of tilapia is controlled by a single autosomal dominant "R" gene. But both the red strains contain differing proportion of heterozygotes (Rr). In order to produce pure breeding strains, it will be important to identify heterozygotes so that the “r" gene can be culled in some way. In this case. Mendelian test-cross technique has been proposed on the light of the present study as a means of probable solution.

The possible implications of above results of genetic manipulation study in the aquaculture of Oreochromis spp. are discussed.