Evolution of linear mitochondrial genomes in medusozoan cnidarians

Genome Biol Evol. 2012;4(1):1-12. doi: 10.1093/gbe/evr123. Epub 2011 Nov 22.


In nearly all animals, mitochondrial DNA (mtDNA) consists of a single circular molecule that encodes several subunits of the protein complexes involved in oxidative phosphorylation as well as part of the machinery for their expression. By contrast, mtDNA in species belonging to Medusozoa (one of the two major lineages in the phylum Cnidaria) comprises one to several linear molecules. Many questions remain on the ubiquity of linear mtDNA in medusozoans and the mechanisms responsible for its evolution, replication, and transcription. To address some of these questions, we determined the sequences of nearly complete linear mtDNA from 24 species representing all four medusozoan classes: Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa. All newly determined medusozoan mitochondrial genomes harbor the 17 genes typical for cnidarians and map as linear molecules with a high degree of gene order conservation relative to the anthozoans. In addition, two open reading frames (ORFs), polB and ORF314, are identified in cubozoan, schyphozoan, staurozoan, and trachyline hydrozoan mtDNA. polB belongs to the B-type DNA polymerase gene family, while the product of ORF314 may act as a terminal protein that binds telomeres. We posit that these two ORFs are remnants of a linear plasmid that invaded the mitochondrial genomes of the last common ancestor of Medusozoa and are responsible for its linearity. Hydroidolinan hydrozoans have lost the two ORFs and instead have duplicated cox1 at each end of their mitochondrial chromosome(s). Fragmentation of mtDNA occurred independently in Cubozoa and Hydridae (Hydrozoa, Hydroidolina). Our broad sampling allows us to reconstruct the evolutionary history of linear mtDNA in medusozoans.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Base Sequence
  • Cell Lineage / genetics
  • Cnidaria / genetics*
  • Cubozoa / genetics
  • DNA Fragmentation
  • DNA Replication / genetics
  • DNA, Mitochondrial / genetics*
  • DNA-Directed DNA Polymerase / genetics
  • Evolution, Molecular*
  • Gene Order / genetics
  • Genome, Mitochondrial / genetics*
  • Hydrozoa / genetics
  • Mitochondria / genetics
  • Molecular Sequence Annotation / methods
  • Molecular Sequence Data
  • Nucleic Acid Conformation
  • Open Reading Frames / genetics
  • Phylogeny
  • Recombination, Genetic / genetics
  • Scyphozoa / genetics
  • Transcription, Genetic / genetics


  • DNA, Mitochondrial
  • DNA-Directed DNA Polymerase