Unusual sequence features and gene rearrangements of primitive crabs revealed by three complete mitochondrial genomes of Dromiacea

Three complete mitochondrial genomes of primitive crabs, Dynomene pilumnoides (belong to Dromioidea), Homola orientalis and Moloha majora (belong to Homoloidea) were determined, characterized and compared with other brachyuran crabs. Due to the presence of four intergenic noncoding sequences (IGNs), the complete length of 16,475 bp in D. pilumnoides is relatively large among brachyuran mitogenomes. Its longest IGN is 652 bp in size locating between trnL1 and trnQ, which is regarded as putative control region (CR) considering its high similarity with the same superfamily species, Conchoecetes artificiosus and Takedromia cristatipes. Compared with the gene order of putative ancestors of insects and crustaceans, D. pilumnoides mitogenome exhibits rearrangements with positional translocation of three genes (trnQ, CR and trnH). The mitogenomes of H. orientalis and M. majora are 16,084 bp and 15,903 bp in size, respectively. Their gene arrangements are consistent with those reported for most brachyuran species. The lengths of CR in these two Homoloidea crabs are 1242 bp and 1037 bp, respectively. The occurrence of tandem repeat sequences (TRS) in control region is shared by the three Homoloidea crabs (besides Homologenus malayens), but is not found in the other reported brachyuran crabs. Moreover, the repeat units in H. orientalis and H. malayens show the high level of sequence identity with stable secondary structure. The phylogenetic analyses indicate that Dromioidea and Homoloidea form the most basal assemblage within Brachyura, followed by Raninoidea.