Mitochondrial gene order evolution in Mollusca: Inference of the ancestral state from the mtDNA of Chaetopleura apiculata (Polyplacophora, Chaetopleuridae).

The mitochondrial genome architecture of polyplacophorans has been usually regarded as being very ancient in comparison to all mollusks. However, even if some complete chiton mtDNAs have been recently sequenced, thorough studies of their evolution are lacking. To further expand the set of complete chiton mtDNAs and perform such analysis, we sequenced the mitochondrial genome of the Eastern beaded chiton Chaetopleura apiculata (Chaetopleuridae) using next-generation sequencing. With mitochondrial sequences from all available chiton mtDNAs, we also built a phylogeny on which we reconstructed the evolution of gene arrangement in this class. The arrangement of C. apiculata proved to be the most primitive known so far for polyplacophorans. Comparing this gene order to those of other molluscan classes, we found that it most probably is the original gene order of the last common ancestor to all extant Mollusca. The ancient mitochondrial genome organization of C. apiculata is an important information that may help reconstructing the phylogeny of Mollusca and their relationship with other lophotrochozoans.