Analysis Of Genetic Diversity Of Selected Stocks Of Litopenaeus Vannamei

Pacific white shrimp, Litopenaeus vannamei, the world’s highest yield shrimp species,has become one of the main aquaculture species in China. This broodstocks and seeds ofthe shrimp had a huge market demand every year. However, China need much capital tointroduce alien species from aboard due to market monopoly of the germplasm, which wascontroled by the United States. In our country, independent selective breeding project ofL.vannamei has initiated and four new species has been developted. Some new lines wereeither in search or spreaded. Compared with the foreign introduced lines, however, newlines bred in China has no priority because of the lack of germplasm resource. Therefore,establishing base population with high genetic diversity, and breeding new species adapt tonative culturing inviroment has become the inevitable trend in China. During a breedingprogram, the genetic background of base population must be fully recognized, which wereessential to take full advantage of principle of heterosis and to accelerate the breedingprocess. We had analyzed the genetic diversity and genetic variation of seven selectedstocks of Litopenaeus vannamei used in China based on SSR and AFLP markers. Inaddition, six SSR DNA markers were used to assess the genetic diversity and geneticdifference within two import populations and its four F1populations produced by diallelcross. The research can not only provide basic data for germplasm improvement but alsogive reference for the utilization of heterosis heterosis of L.vannamei. The main resultswere as follows:1. Seven microsatellite loci was used to analyze the genetic diversity and geneticdifferences of four imported populations (KONAB, SISMAM, OI and CHAI) and threedomestic cultured population (“ZhongXing No.1”(ZX),“ZhongKe No.1”(ZK) andGuangdong Ocean University Genetic Breeding Center (GDOU) in China). The numberof alleles per locus ranged from5to23, with an average of13.867, The mean effectivenumber of alleles at all stocks ranged from2.215to4.804. The mean observed andexpected heterozygosities at all stocks ranged from0.372to0.631and0.553to0.751,respectively. This indicates that a high level of genetic variation had been preserved duringthe selective breeding of seven Pacific White Shrimp stocks in China. Geneticpolymorphism of domestic populations compared with imported populations was slightly higher, and particularly highest in ZK population, but the lowest in CHAI. The fixedcoefficient DH-Wranged from-0.092to0.364(mean DH-W=0.123). AMOVA analysisrevealed that78.45%of genetic variations were within populations, and21.55%of thevariation from the population, and all FSTcomparisons were highly significant (overall FST=0.215, P<0.01). This indicated big differences between stocks. Pairwise multilocus FSTtests, estimates of genetic distance, and phylogenetic analyses showed that were significantdifferences among all stocks.2. The amplified fragment length polymorphism (AFLP) was employed to analyzepopulation genetic diversity and genetic distance between four introduced populations andthree domestic breeding populations. A total of105fragments of100~500bp wereidentified from210individuals by seven AFLP primer combinations. And90of105fragments were polymorphic, and the proportion of polymorphic loci was85.71%.Thepolymorphic loci within populations varied from54.17%to65.63%, with an average of60.53%, and the mean heterozygosities ranged from0.164to0.236, with an average of0.202. AMOVA analysis revealed that51.79%of genetic variations were betweenpopulations, the estimated ΦSTvalue over all polymorphic loci across the sevenpopulations was0.518(P<0.0010), indicating a strong population structure. It is roughlyconsistent with the results of microsatellite marker.3. The genetic diversity and genetic differention between two selected stocks ofPacific white shrimp and four F1population produced by diallel cross progeny wereinvestigated using six SSR markers. six SSR loci were detected in25alleles, The meannumbers of alleles per locus ranged from3to6, with an average of4.328. the averageeffective number of alleles (Ne) was2.675. The mean observed and expectedheterozygosity ranged from0.468to0.609, and from0.465to0.698, respectively. Thepolymorphism information content (PIC) value in all groups ranged from0.468to0.646.FSTvalues between pairs of populations ranged from0.080to0.421. And AMOVAanalysis showed that the genetic differences within populations was significant (P <0.01).In short, The heterozygosity level of parental populations were lower than F1hybridpopulations, but slightly higher than F1self-breeding populations. In addition, The highestgenetic diversity that existed in SC and CS indicated this hybrid had the high potential forfurther breeding. There was obvious divergence between SIS and CHAI populations ofL.vannamei.