MBE Advance Access originally published online on March 7, 2008
Molecular Biology and Evolution 2008 25(6):1129-1137; doi:10.1093/molbev/msn058
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Research Articles |
Two Circular Chromosomes of Unequal Copy Number Make Up the Mitochondrial Genome of the Rotifer Brachionus plicatilis
* Nagasaki Industrial Promotion Foundation, Omura, Japan
Institute for East China Sea Research, Nagasaki University, Nagasaki, Japan
Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA
Faculty of Fisheries, Nagasaki University, Nagasaki, Japan
|| Graduate School of Science and Technology, Nagasaki University, Nagasaki, Japan
E-mail: dmarkwelch{at}mbl.edu
Accepted for publication February 27, 2008.
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.
Key Words: aneuploidly • mitochondrial genome structure • metazoan evolution • Gnathifera • Rotifera • Brachionus plicatilis
Richard Thomas, Associate Editor