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Background  Oysters are morphologically plastic and hence difficult subjects for taxonomic and evolutionary studies. It is long been suspected, based on the extraordinary species diversity observed, that Asia Pacific is the epicenter of oyster speciation. To understand the species diversity and its evolutionary history, we collected five  Crassostrea  species from Asia and sequenced their complete mitochondrial (mt) genomes in addition to two newly released Asian oysters ( C. iredalei  and  Saccostrea mordax ) for a comprehensive analysis.    Results  The six Asian  Crassostrea  mt genomes ranged from 18,226 to 22,446 bp in size, and all coded for 39 genes (12 proteins, 2 rRNAs and 25 tRNAs) on the same strand. Their genomes contained a split of the  rrnL  gene and duplication of  trnM ,  trnK  and  trnQ  genes. They shared the same gene order that differed from an Atlantic sister species by as many as nine tRNA changes (6 transpositions and 3 duplications) and even differed significantly from  S. mordax  in protein-coding genes. Phylogenetic analysis indicates that the six Asian  Crassostrea  species emerged between 3 and 43 Myr ago, while the Atlantic species evolved 83 Myr ago.    Conclusions  The complete conservation of gene order in the six Asian  Crassostrea  species over 43 Myr is highly unusual given the remarkable rate of rearrangements in their sister species and other bivalves. It provides strong evidence for the recent speciation of the six  Crassostrea  species in Asia. It further indicates that changes in mt gene order may not be strictly a function of time but subject to other constraints that are presently not well understood.

Unusual conservation of mitochondrial gene order in Crassostreaoysters: evidence for recent speciation in Asia