Sequencing Of Bryozoan Mitochondrial Genomes With Analyses Of The Phylogeny Of Bryozoa

The Bryozoa is a group of small, sessile, filter-feeding, aquatic, colonial animals. As important marine invertebrates, bryozoans have important ecological significance; they also have important application in the isolation of bioactive components. However, molecular phylogenetic studies of this group are relatively few. There still many controversy concerning the phylogenetic position of this group. The phylogenetic position of Bryozoa within Metazoa is not determined yet; the relationships of Bryozoa with Brachiopoda, Phoronida and Entoprocta are in dispute. There are still no consensuses on the phylogeny of Bryozoa.In this study, we have amplified the complete mitochondrial genomes of two bryozoans Tubulipora flabellaris (Tubuliporida) and Watersipora subtorquata (Cheilostomida) with Long-PCR technique, then DNA library construction and primer-walking strategies are used to obtain the complete sequences. Their complete mitochondrial genomes have many distinct features: 1. The complete mitochondrial genomes of T. flabellaris and W. subtorquata are 13,763 bp and 14,144 bp in lenth respectively, relatively small compared with mtDNAs of other metazoans; 2. Their largest non-coding retion are also small, of 230 bp and 100 bp in length, respectively; 3. Both of the genomes contain 36 genes, including 12 protein-coding genes, 2 ribosomal RNAs and 22 transfer RNAs. In constrast to the typical animal mitochondrial genomes, both of them are lack of the atp8 gene; 4. Gene arrangement comparisons indicate that complete mitochondrial genomes of T. flabellaris and W. subtorquata both have unique gene order when compared with other metazoans. The longest identical gene blocks (excluding tRNAs) shared with other metazoan mtDNAs are of 4 and 3 genes, respectively. The four known bryozoan complete mitochondrial genomes also have very different gene arrangements, indicates that bryozoan mitochondrial genomes have experienced drastic rearrangements.To investigate the phylogenetic relationship of Bryozoa, phylogenetic analyses based on amino acid sequences of 11 protein coding genes (excluding atp6 and atp8) from 26 metazoan complete mitochondrial genomes were carried out utilizing ML and Bayesian strategies, respectively. Our analyses support the monopoly of Lophotrochozoa. Lophophorate is within the group of Lophotrochozoa. The results indicate close relationship between Chaetognatha and Bryozoa, however, more evidence are needed to clarify the relationship between the two groups. Lophophorate appeared to be polyphyletic according to our analyses, however, neither analysis support close relationship between Branchiopod and Phoronid.