Molecular Phylogeny Of Bivalvia (Mollusca) Based On 18S RDNA Sequences And Primary Study On The Relationship Between Nuclear DNA Contents And Evolutionary Degrees

Firstly, molecular phylogeny was analyzed based on 18S rDNA sequences of 16 bivalve species. Secondly, the nuclear DNA contents of 38 bivalve species were quantified using flow cytometry and the variation of genome size was discussed respectively at individual and interspecies level. Finally, the relationship between nuclear DNA contents and evolution was discussed.1,Molecular phylogenetic analysis18S rDNA sequences may be of great value for the study of bivalve evolution. Nearly complete 18S rDNA sequences for 16 bivalve species, representing 4 orders and 10 families were analyzed. Combined with additional sequences of 20 bivalves and 6 gastropods downloaded from NCBI (National Center of Biological Institution), molecular phylogenetic trees were constructed using 4 Polyplacophora species as outgroup. The result resolved by maximum-parsimony, maximum-likelihood and Bayes-induce methods were in congruent with each other, which indicated that all bivalves formed a monophyletic group clustering with its sister-group, Gastropod. Bivalvia could be subdevied into 2 major groups, Heterodonta and Pteriomorphia. The former was monophyletic while the phylogenetic status of Pteriomorphia was ambiguous owning to the controversial position of Ostreidae. Inconsistent with the results from other molecular analysis, the monophyletic Myoida clustered together with Veneroida which confirmed the legitimacy of Myoida's order status. Solenidae occupied the basal position of the monophyletic Heterodonta demonstrating its original position. Every family was resolved monophyletic except Tellinidae and Solecurtidae. The paraphyletic Tellinidae and polyphyletic Solecurtidae mixed together, indicating the close relationship between two families.2,Nuclera DNA contents quantified analysisThe nuclear DNA content of 38 bivalve species were quantified using flow cytometry with two internal reference standards, chicken red blood cells (CRBC) and gill cells of Mytilus edulis. Mean value (picograms, pg) of each species ranged from 2.06±0.03 in Crassostrea gigas to 5.42±0.18 in Mercenaria mercenaria. Almost no difference was detected at individual level (p>0.05), while the interspecies difference was very significant (<0.01) according to Two-Way ANOVA. A high percent of the total variation was obtained at interspecies level (97.34%) with only 0.09% at intraspecies level.The correlations between the DNA content and the total chromosome number, the m/sm number and the number fundamental were not significantly positive, but a weak negative correlation was found between the DNA content and the st/t number.3,Relationship between DNA content and evolutionary degreeDNA contents of bivalves were analyzed statistically with SPSS software at subclass, order, family and species grade respectively. Significant difference was detected between mean values of two subclasses according to the result of T-test. The DNA content of Heterodonta (3.77pg) was distinctly larger than that of Pteriomorphia (3.14pg). It was concluded based on ANOVA that the general trend of DNA content was to change from lower to higher values associated with evolution at higher taxonomical classification, i.e. subclass while there was no correlation between DNA content and evolutionary degree at lower taxonomical classification, i.e. order, family and species.