The tubular anemone has the largest animal mitochondrial genome ever sequenced

The anemone tube Isarachnanthus nocturnus It is only 15 cm long but has the largest mitochondrial genome of all animals sequenced to date, with 80,923 base pairs. The mitochondrial human genome (mitogenome), for example, comprises 16,569 base pairs.

Tubular anemones (Ceriantharia) are the subject of an article recently published in Scientific reports describing the results of a study conducted by Sérgio Nascimento Stampar, professor at the Faculty of Sciences and Letters of the State University of São Paulo (FCL-UNESP) in Assis Brazil.

The study was funded by FAPESP through a regular grant for the project "Evolution and Diversity of Ceriantharia (Cnidaria") and through its program of international collaborative researchers in São Paulo (SPRINT) within the framework of the project. a cooperation agreement with the University of North Carolina at Charlotte in the United States.

Mitogenome is simpler than the nuclear genome, which in the case of I. nocturnus has not yet been sequenced, explained Stampar. The human nuclear genome comprises for example about 3 billion base pairs. Another discovery reported in the paper is that I. nocturnus and Pachycerianthus magnus (another species studied by the Stampar group, 77,828 base pairs) have linear genomes like those of medusae (Medusozoa), while the others species of their class (Anthozoa) and in fact have circular genomes.

I. nocturnus is found in the Atlantic from the coast of Argentine Patagonia to the east coast of the United States. P. magnus lives in the marine environment around the island of Taiwan in Asia. Both inhabit waters of a maximum depth of 15 m.

"The mitogenome of I. nocturnus is almost five times larger than the human mitogenome," said Stampar. "We tend to think that our molecule is more complex, but in fact our genome has been more" filtered "during our evolution, so keeping this giant genome is probably more expensive in terms of energy expenditure."

The shape of the mitogenomes in these two species of tube anemones and the gene sequences they contain were more surprising than their size.

Because they are closely related species, their gene sequences should be similar, but I. nocturnus has five chromosomes, while P. magnus has eight chromosomes and each has a different composition in terms of genes. This type of variation had previously been found only in the medusozoa, sponges and some crustaceans.

"Humans and bonefish species are more similar than these two tube anemones in terms of the structure of their mitochondrial DNA," said Stampar.

Coast of São Paulo and South China Sea

To achieve these results, researchers captured specimens in São Sebastião, located on the coast of the state of São Paulo in Brazil, and off Taiwan in the South China Sea. Small pieces of animal tentacles were used to sequence their mitogenomes.

The genomes of both species up to date in the databases were incomplete because of the difficulty of sequencing them. Once the study was completed, the researchers published the genomes by offering them to GenBan, a database maintained in the United States by the National Institutes of Health (NIH) National Center for Biotechnology Information (NCBI).

Another obstacle to sequencing was the difficulty of collecting these animals because of their elusive behavior. In response to any potential threat, a tubular anemone hides in the long, tough tube that distinguishes it from true sea anemones, making any capture impossible.

"You have to dig a hole around him, sometimes up to one meter deep, and stop the part of the tube buried in the sand, all of which must be done underwater while carrying diving equipment. hides in the buried part of the tube and you simply can not get it, "said Stampar.

With support from FAPESP's SPRINT program, Stampar and Marymegan Daly, a research colleague at Ohio State University in the United States, have partnered with Adam Reitzel and Jason Macrander of UNC Charlotte. Macrander, a postdoctoral fellow at Reitzel, is a professor at Florida Southern College.

Reitzel and Macrander specialize in the use of bioinformatics to filter genomic data and assemble millions of small fragments of mitochondrial DNA in a single sequence. They used this technique to obtain complete mitochondrial genomes for both species.

"In this technique, you're sequencing bits of the genome and linking them in a circle, the problem is that it only works with circular genomes." Failing to find a piece to close the circle, we realized that the genome had to to be linear, as for Medusozoa, "said Stampar.

This discovery paves the way for a possible reclassification of cnidarian species (hydras, jellyfish, polyps, corals and sea anemones). The tube anemones studied appear to be a distinct group of corals and sea anemones and have some similarities to jellyfish.

However, more data will be needed before a definitive conclusion can be reached. The necessary data could come from sequencing the nuclear genomes of these species, which Stampar and his group intend to complete by the end of 2019.