Suga Koushirou, Mark Welch David B, Tanaka Yukari, Sakakura Yoshitaka, Hagiwara Atsushi
Nagasaki Industrial Promotion Foundation, Omura, Japan.
Mol Biol Evol. 2008 Jun;25(6):1129-37. doi: 10.1093/molbev/msn058. Epub 2008 Mar 7.
The monogonont rotifer Brachionus plicatilis is an emerging model system for a diverse array of questions in limnological ecosystem dynamics, the evolution of sexual recombination, cryptic speciation, and the phylogeny of basal metazoans. We sequenced the complete mitochondrial genome of B. plicatilis sensu strictu NH1L and found that it is composed of 2 circular chromosomes, designated mtDNA-I (11,153 bp) and mtDNA-II (12,672 bp). Hybridization to DNA isolated from mitochondria demonstrated that mtDNA-I is present at 4 times the copy number of mtDNA-II. The only nucleotide similarity between the 2 chromosomes is a 4.9-kbp region of 99.5% identity including a transfer RNA (tRNA) gene and an extensive noncoding region that contains putative D-loop and control sequence. The mtDNA-I chromosome encodes 4 proteins (ATP6, COB, NAD1, and NAD2), 13 tRNAs, and the large and small subunit ribosomal RNAs; mtDNA-II encodes 8 proteins (COX1-3, NAD3-6, and NAD4L) and 9 tRNAs. Gene order is not conserved between B. plicatilis and its closest relative with a sequenced mitochondrial genome, the acanthocephalan Leptorhynchoides thecatus, or other sequenced mitochondrial genomes. Polymerase chain reaction assays and Southern hybridization to DNA from 18 strains of Brachionus suggest that the 2-chromosome structure has been stable for millions of years. The novel organization of the B. plicatilis mitochondrial genome into 2 nearly equal chromosomes of 4-fold different copy number may provide insight into the evolution of metazoan mitochondria and the phylogenetics of rotifers and other basal animal phyla.
单巢轮虫褶皱臂尾轮虫是一个新兴的模型系统，可用于研究湖泊生态系统动力学、有性重组的进化、隐秘物种形成以及基础后生动物系统发育等一系列不同的问题。我们对褶皱臂尾轮虫严格意义上的NH1L株系的完整线粒体基因组进行了测序，发现它由2条环状染色体组成，分别命名为mtDNA-I（11,153 bp）和mtDNA-II（12,672 bp）。与从线粒体分离的DNA进行杂交表明，mtDNA-I的拷贝数是mtDNA-II的4倍。这两条染色体之间唯一的核苷酸相似性是一个4.9-kbp的区域，其同一性为99.5%，包括一个转移RNA（tRNA）基因和一个包含推定D环和控制序列的广泛非编码区域。mtDNA-I染色体编码4种蛋白质（ATP6、COB、NAD1和NAD2）、13种tRNA以及大亚基和小亚基核糖体RNA；mtDNA-II染色体编码8种蛋白质（COX1 - 3、NAD3 - 6和NAD4L）和9种tRNA。褶皱臂尾轮虫与其线粒体基因组已测序的最亲近亲属棘头虫Leptorhynchoides thecatus或其他已测序的线粒体基因组之间的基因顺序并不保守。聚合酶链反应分析和对18个臂尾轮虫菌株DNA的Southern杂交表明，这种双染色体结构已经稳定存在数百万年了。褶皱臂尾轮虫线粒体基因组的这种新组织形式，即由2条几乎相等但拷贝数相差4倍的染色体组成，可能为后生动物线粒体的进化以及轮虫和其他基础动物门的系统发育学提供见解。
