Genetic Diversity Of The Resting-eggs In Two Daphnia Species From Lakes In The Middle And Lower Reaches Of The Yangtze River

Increased human activities and environmental changes may lead to the genetic diversity variations of Cladocerans in water. Daphnia pulex and Daphnia similoides are distributed throughout the world and often regarded as a model organism. The 16 S r DNA, cytochrome c oxidase subunit I(COI), 18 S r DNA and ITS genes were used as molecular marks. The genetic diversity and phylogeny of D. pulex obtained from Lakes in the middle and lower reaches of the Yangtze River were studied.1) The 16 S r DNA, cytochrome c oxidase subunit I(COI)and 18 S r DNA were used as molecular marks. The genetic diversity and phylogeny of D. pulex obtained from 10 water bodies in the middle and lower reaches of the Yangtze River were studied. For 16 S r DNA, COI gene, and 18 S r DNA, the A+T content(65.4%, 58.4%, and 54.6%) was significantly higher than the G+C content(34.6%, 41.6% and 45.4%). This result was consistent with higher A and T contents among invertebrates. Based on the genetic distances of 16 S r DNA and COI gene, the genetic differences of D. pulex from 10 water bodies located in the middle and lower reaches of the Yangtze River in China was minimal(0–0.8% for 16 S r DNA and 0–1.5% for COI gene). However, D. pulex evolved into two branches in the phylogenetic trees, which coincided with its geographical distribution. Mann-Whitney Test showed that the genetic differences of D. pulex between within-lakes and between-lakes were significant(COI gene: Z=-3.172, P=0.002; 16 S r DNA: Z=-3.096, P=0.002; 18 S r DNA: Z=-3.378, P=0.001). Two-sample Kolmogorov-Smirnov Test showed the significant differences in both within-lakes and between-lakes(COI gene: Z=1.789, P=0.003; 16 S r DNA: Z=2.012, P=0.001; 18 S r DNA: Z=2.012, P = 0.001). Analysis of Molecular Variance(AMOVA) test revealed that there were significant differences between two groups(middle reach and lower reach) based on 16 S r DNA and COI gene. Compared with D. pulex from other countries, the average genetic distance of D. pulex obtained from 10 water bodies in the middle and lower reaches of the Yangtze River reached 9.1%–10.5%, thereby indicating that D. pulex may have evolved into different subspecies.2) Based on 16 S r DNA, COI gene and ITS gene, the genetic diversity and phylogeny of Daphnia similoides resting-eggs obtained from the different sediment depths(1 cm-20 cm) of Lake Chaohu were studied. The results of Kimura 2-parameter, maximum likelihood and Bayesian inference analyses showed a highly consistency in the phylogenetic tree of D. similoides resting-eggs. For 16 S r DNA and COI gene, the A+T contents(68.8%, 63.8%) was significantly higher than the G+C contents(31.2%, 36.2%). However, for ITS gene, the A+T content(46.3%) was significantly lower than the the G+C content(53.7%). According to the 16 S r DNA, COI gene and ITS gene sequences analysis, the genetic distances were obviously different(16S r DNA: 0-0.7%; COI gene: 0-2.5%; ITS gene: 0-9.7 %).Through the variation analysis of bases sites of gene sequence of D. similoides resting-eggs located in the different sediment layers of Lake Chaohu, the results showed the regular variation of base sites in three sequences of the 16 S r DNA, COI gene and ITS gene. For 16 S r DNA, there were 5 base sites with obviously regular variation. Base T in D. similoides sequence located in the sediment layer(15 cm-20 cm) were replaced by base A located in the sediment layer(1 cm-15 cm) at No 54 variable site. Moreover, the base at No. 58 site was G in D. similoides gene sequence located in the sediment layer(15 cm-20 cm) while it was losing in the sediment layer(1 cm-15 cm).For COI gene, there were 7 base sites with obviously regular variation. Base C in D. similoides sequence located in the sediment layer(5 cm-20 cm) were replaced by base T located in the sediment layer(1 cm-5 cm) at No. 7 variable site, and base C in D. similoides sequence located in the sediment layer(5 cm-20 cm) were replaced by base G located in the sediment layer(1 cm-5 cm) at No. 527 variable site.For the ITS gene, the variation region was observed between No. 205 and No. 925 base sites, among which 15 base sites showed a obviously regular variation. Base A in D. similoides sequence located in the sediment layer(10 cm-20 cm) were replaced by base G located in the sediment layer(1 cm-10 cm) at No. 541 variable site, and base T in D. similoides sequence located in the sediment layer(10 cm-20 cm) were replaced by base C located in the sediment layer(1 cm-10 cm) at No. 1305 variable site. The analysis of ITS gene sequence suggested that the variation at some base site of D. similoides located in the sediment layers of between 1 cm-10 cm and 10 cm-20 cm showed a regular transition.