Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China.
BMC Evol Biol. 2010 Dec 28;10:394. doi: 10.1186/1471-2148-10-394.
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.
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.
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.
牡蛎在形态上具有可塑性，因此很难进行分类学和进化研究。长期以来，人们一直怀疑，基于观察到的非凡物种多样性，亚太地区是牡蛎物种形成的中心。为了了解物种多样性及其进化历史，我们从亚洲收集了五个牡蛎物种，并对其完整的线粒体（mt）基因组进行了测序，此外还对两个新发布的亚洲牡蛎（C. iredalei 和 Saccostrea mordax）进行了测序，以进行全面分析。
这六个亚洲牡蛎的 mt 基因组大小在 18226 到 22446 bp 之间，都在同一链上编码了 39 个基因（12 个蛋白、2 个 rRNA 和 25 个 tRNA）。它们的基因组包含 rrnL 基因的分裂和 trnM、trnK 和 trnQ 基因的重复。它们具有相同的基因顺序，与大西洋姊妹物种有多达 9 个 tRNA 变化（6 个转位和 3 个重复），甚至与 S. mordax 在蛋白质编码基因上也有显著差异。系统发育分析表明，这六个亚洲牡蛎物种在 3 到 43 百万年前之间出现，而大西洋物种在 83 百万年前进化而来。
在 43 百万年的时间里，这六个亚洲牡蛎物种的基因顺序完全保守，这在它们的姊妹物种和其他双壳类动物中显著的重排率下是非常不寻常的。这为亚洲的六个牡蛎物种的近期物种形成提供了有力证据。这进一步表明，mt 基因顺序的变化可能不是严格的时间函数，而是受到其他目前尚不清楚的限制。
