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

Abstract

BackgroundOysters 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.ResultsThe 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.ConclusionsThe 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.

Artificial interspecific backcrosses between the hybrid of female Crassostrea hongkongensis × male C. gigas and the two parental species

“…As expected, the maternal type of the H/Hg, Hg/H, Hg/G and Hg/Hg groups was identified as C. hongkongensis, while only the G/Hg group was identified as C. gigas. This finding suggests that the backcrossing process strictly obeyed the maternal inheritance discipline, as observed for the F 1 hybrids in the Crassostrea genus (Obata et al, 2008;Ren et al, 2010;Zhang et al, 2012).…”

New insights about the introduction of the Portuguese oyster,Crassostrea angulata, into the North East Atlantic from Asia based on a highly polymorphic mitochondrial region

“…All haplotypes (with the exception of two sequences from Tahaddart) shared high nucleotide sequence similarity (>98%) for the corresponding region of C. angulata mitochondrial genomes (Ren et al 2010(Ren et al , 2016. Some haplotypes observed in the present study (H5, H6, H11, H32 and H33) were identical to the respective region of the C. angulata mitochondrial genomes (Genbank accession numbers KJ855246, KJ855247, KJ855248, KJ855249 and EU672832) (Ren et al 2016).…”

“…However, several phenotypic and genetic differences were also reported between C. angulata and C. gigas (for review see Batista et al 2009). Based on the sequence of the mitochondrial genome the two species were estimated to have diverged about 2.7 million years ago (Ren et al 2010). The close relationship between C. angulata and C. gigas has led several authors to suggest that they have a recent common origin (e.g.…”

Genetic variation and population structure of the Pacific oyster Crassostrea gigas in the northwestern Pacific inferred from mitochondrial COI sequences

“…Crassostrea oysters are of great economic and ecological importance to global fisheries and aquaculture industries [29]. The Pacific oyster, C. gigas, with a spatially wide-range distribution, is native to the coast of East Asia, including China, Korea and Japan.…”