Tube-Dwelling Anemone Has Biggest Mitochondrial Genome Ever Sequenced

The banded tube-dwelling anemone (Isarachnanthus nocturnes), a sea creature that resembles a prehistoric flower, now holds the record for the largest mitochondrial genome reported to date: the animal has 80,923 base pairs, compared with humans’ 16,569 base pairs.

Isarachnanthus nocturnes. Image credit: Stampar et al, doi: 10.1038/s41598-019-42621-z.

Mitochondria are double-membraned structures found in multitudes within the cell, outside the nucleus.

They are responsible for energy production, and are sometimes called the ‘cellular powerhouses’ of living beings.

Mitochondrial DNA (mtDNA) is usually circular in shape and contains much less information than nuclear DNA.

Tube-dwelling anemones’ mtDNA is a real head scratcher, from its unexpected arrangement to its previously unimagined magnitude.

“These ancient animals have simple behavior and simple anatomy, and so we’ve thought of them as fairly simple creatures until now,” said Ohio State University’s Professor Meg Daly, senior author of the study.

“But their biology is quite complicated. The genomes of these tube anemones may be more dynamic than those of more-complex and more-recent animals like snails, insects and vertebrates.”

Pachycerianthus magnus. Image credit: Stampar et al, doi: 10.1038/s41598-019-42621-z.

Using advanced supercomputer technology, Professor Daly and colleagues examined two species of tube anemones — Isarachnanthus nocturnes and Pachycerianthus magnus — and found that the first has five linear fragments of mtDNA and the other has eight.

Previously, scientists had found a linear genome in the mitochondria of the jellyfish, but the linear structure combined with the variation in size and number of fragments seen in the tube anemone is unprecedented.

“We think that the typical loop arrangement we find makes sense, because one of the advantages of the mitochondria having a circular genome is that that it replicates easily,” Professor Daly said.

“We’ve thought of this loop-shaped design as something that helps the mitochondria do its job quickly and efficiently.”

“So far, there’s no rhyme or reason to the anemones having this unusual mitochondrial genome.”

“The two species examined in this research represent the largest groups of Ceriantharia,” said first author Dr. Sergio Stampar, a researcher at the Universidade Estadual Paulista in São Paulo, Brazil.

“These results present a considerable ‘photograph’ of the group. Besides the large size of genomes, the most surprising thing that we found is the significant difference between the two species.”

“The work raises many questions, since the presumable change from circular to linear genomes in anemones should not be a simple process. A broad discussion about the evolution of several groups of related animals called Cnidaria — which includes jellyfish, corals and sea-anemones — is imperative.”

“It would be tempting to expect similarities in evolutionary pressures among sea animals that are all old and relatively simple in terms of appearance and function, but this new evidence is calling that into question,” Professor Daly said.

“Maybe they all started at the same place but have been subject to different evolutionary schemes and opportunities.”

The results appear in the journal Scientific Reports.

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Sérgio N. Stampar et al. 2019. Linear Mitochondrial Genome in Anthozoa (Cnidaria): A Case Study in Ceriantharia. Scientific Reports 9, article number: 6094; doi: 10.1038/s41598-019-42621-z