Genetic Diversity Of Wild Populations Of Plecoglossus Altivelis In China

Ayu(Plecoglossus altivelis) is an amphidromous and annual fish endemic to east Asia, and it has been esteemed as water treasure with highly economic value. In recent years, owing to environmental pollution, inhabitation of hydraulic constructions, destruction of spawning ground by mining and dredging, and unstrained fishing etc, the resources of wild ayu populations in China have decreased rapidly, so we must take effective measures to protect ayu. To effective conserve and sustainable exploit the germplasts necessitate the knowledge of the species'status and genetic variations in wild populations, however the taxonomic status of Chinese ayu is controversious, and their genetic background is lack. In present study, sequences of complete mitochondrial control region and ND2 gene and partial ND4 gene in 7 ayu populations with 116 individuals were sequenced, combined with homologous sequences of Japanese ayu populations, and the taxonomic status, genetic diversity and structure of different geographical populations of Chinese ayu were analyzed, and their evolutionary potentials were evaluated, with the aim to provide the scientific basis for stock enhancement, and sustainable exploitation and selection of germplast resources for breeding and aquaculture. The results were as following:1. The termination associated sequence domain, the central conserved domain and the conserved block domains were identified in the mtDNA control region of Chinese ayu, and the general characters of the functional units including ETAS, CSB-F, CSB-E, CSB-D, CSB1, CSB2 and CSB3 and so on were provided. The core sequences of ETAS have two groups of reverse complementary sequences:TACAT and ATGTA, ATGG and CCAT, and they can make up a stable dual hairpin structure which forms a strong termination signal. Besides, there were repeat sequences in the area of 3'and 5'-terminal control region, and poly(T) was found in the middle of control region.2. In Neighbor-joining tree based on mtDNA ND4 sequences, all haplotypes representing Chinese ayu are embeded within the clade containing all individuals of Japanese P.altivelis altivelis, and the other clade exclusively include individuals of Japanese P.altivelis ryukyuensis. It showed that Chinese ayu belong to P.altivelis altivelis, and might not be a new subspecies.3. the mtDNA control region sequences of 7 wild ayu populations in China were divided into 2 lineages with almost no gene flow(Fst=0.58228, Nm=0.36), and their geographic distributions were conformed to the isolation by distance model. TCS network of haplotypes were also divided into 2 clades, of which one clade were exclusively composed by individuals from Dandong population, and another clade were made up of the remaining 6 populations. Analysis of molecular variance(AMOVA) indicated that 62.40% variations were found among groups, and only 29.43% variations existed within populations. As the following analyses of sequences of ND4 gene and ND2 gene yielded similar results, indicating that there were marked differentiation in genetic structure of Chinese ayu, of which DanDong population were different from the other populations. Consequently 2 Evolutionary Significant Units(ESUs) were determined, and their differentiation were dated back to the Late Pleistocene period (0.022-0.030 Mya ago)。Dandong population, Yangjiaxi River population, Dongzhang reservoir population and DongXing population might be treated as Management Units(MU).4. As far as genetic diversity is considered, due to few variable sites and slow evolution rates, analyses of sequences of mtDNA ND4 and ND2 gene might not be able to completely reflect genetic diversity in Chinese ayu populations. But on the whole, the characteristics of the genetic diversity in wild Chinese ayu populations were the coexist of low level nucleotide diversity and high haplotype diversity, indicating that Chinese ayu populations may have experienced a recent bottleneck or founder effect, as sufficient haplotype diversity could be accumulated in short-term by mutation, but it were not enough to accumulate nucleotide diversity. In Chinese ayu populations the highest genetic diversity were found in Dandong population, and the lowest were in one population from Guangdong Province. As ayu is cold water fish(the most suitable temperature is 15-25℃), and genetic diversity in Japanese ayu populations of P. altivelis altivelis and Dandong population in China were significantly higher than the remaining Chinese ayu populations, it was deduced that Chinese ayu(except Dandong populations) may have originated from north.5. Mismatch distribution,Tajima's D and Fu's Fs neutral tests showed that historical population expansion only occurred in Dandong population, but Chinese ayu as a whole didn't occured population expansion. The population expansion of Dandong population appeared in 6,000-8,000 years ago, indicating that in Holocene period the large-scale transgression might promoted the north China ayu population expansion to a large extant.6. Sequence analyses indicated that all mtDNA regions used in the present study could recover genetic structure well. As far as taxonomic status is concerned, ND4 gene showed higher resolution than the control region, the possible reason may be that frequent back mutations in the control region sequence of Chinese ayu may fuzzy the delimitation between subspecies of ayu. Whether ND2 gene has the same resolution as ND4 gene or not remained to be further studied due to Due to the lack of ND2 gene sequences of Japan ayu. Compared to ND2 and ND4 gene, with much better resolution in revealing genetic variations in wild ayu populations, the control region turned out to be a relatively reliable molecular markers for genetic diversity studies.