Studies On Genetic Background Of The Breeding Populations Of Complete Set Lines In Nile Tilapia (Oreochromis Niloticus)

The Nile tilapia, Oreochromis niloticus (Perciformes, Cichlidae), is aspecies of tropical fish native to Africa. This fish is one of the mainspecies of the world’s tilapia aquaculture industry owing to its rapidgrowth rate, omnivorous and strong disease resistance. In this study, thegenetic variation and combining results of complete set lines in Niletilapia were analyzed to detect the genetic background of them by usingmolecular genetics and quantitative genetics. Those studies are presentedin detail as follows:1. Genetic variation of mitochondrial DNA from complete set linesand reference populations of Nile tilapia (O. niloticus)In this study, genetic diversity and genetic relationships of4complete set lines (GD、 BL、 GNM、 NGF) and4referencepopulations(EGY、USA、GLD、LY) of Nile tilapia were examined usingpartial mitochondrial control region (D-loop) sequence. The resultsshowed that the number of haplotypes of complete set lines were less thanthat of the reference populations. In four complete set lines, the Number of variable sites (S), average number of nucleotide differences (K),haplotype diversity (h) and nucleotide diversity (π) ranged from61to83,13.56to37.26,0.2698to0.5493and0.0207to0.0569, respectively; inthe four reference populations, the four corresponding parameters rangedfrom4to67,0.50to32.95,0.1908to0.8023and0.0008to0.0503,respectively. The results of the analysis of molecular variance (AMOVA)indicated significant genetic divergence (FST=0.4470, P<0.01) andpairwise FSTvalues between populations also demonstrated significantgenetic differentiation (P<0.01) among the eight populations. The clusteranalysis based on Kimura two-parameter model exhibited the two clustersamong them, one cluster included the three populations of complete setlines BL, GD and NGF, and another cluster consisted of the fourreference populations (EGY, USA, LY, GLD) and GNM. However, itwas not well supported by the haplotype network profile in network.2. Genetic variation of complete set lines and reference populationsof Nile tilapia (Oreochromis niloticus) by microsatellite markersThe genetic diversity and genetic differentiation of4complete setlines and4reference populations were amplified by fifteen pairs ofmicrosatellite primers. The results showed that the four complete set linesdisplayed the average allele number (Na) ranging from5.00to5.73, average of effective number of alleles (Ne) of3.07to3.67, averageexpected heterozygosity (HE) with0.6509to0.7126, average observedheterozygosity (HO) with0.5378to0.6230, inbreeding coefficients (FIS)of0.072to0.249and polymorphism information content (PIC) rangingfrom0.5928to0.6529. Whereas the above parameters in four referencepopulations from4.47to5.53,2.50to3.40,0.5707to0.6877,0.4978to0.6822,-0.008to0.184and0.5079to0.6252, respectively. Except theinbreeding coefficients (FIS), the other parameters of LY were the lowest.The pairwise FSTvalues between populations demonstrated significantgenetic differentiation (P<0.01) among the eight populations. Of the15loci,3appeared to be under positive directional selection in the completeset lines, and1appeared to be under positive directional selection in thereference populations. It was concluded that strong selection pressures, inparticular artificial selection, have driven the population geneticdifferentiation. The dendrograms based on UPGMA methods andSTRUCTURE genetic clustering analysis indicated that the two clustersamong them, one cluster included the three populations of complete setlines BL, GNM and NGF, and another cluster consisted of the fourreference populations (EGY, USA, LY, GLD) and GD.3. Preliminary studies on combining effect of the complete set linesin Nile tilapia(Oreochromis niloticus) Heterosis were evaluated for body weight and4morphologicaltraits(total length, standard length, pre-dorsal height and pre-dorsal width)from three complete set lines of Nile tilapia, Genomar(GNM), NewGIFT(NGF) and GIFT(GF), based on diallel cross data at differentgrowth stages. The results indicated that negative heterosis was found inNGF（）×GF（♂）, and the heterosis gradually declined in four differentinvestigated growth stages, whereas obvious heterosis was observed inother hybrids, especially, the hybrids of NGF（）×GNM（♂）which wasfound with the most obvious heterosis in body weight and4morphological traits. Furthermore, the hybrids from GNM with othervariants displayed significant heterosis.