Tube anemones, like Pachycerianthus magnus, are closely related to corals, sea anemones, and jellyfish. © Sérgio N. Stampar
Tube anemones, an ancient lineage of animals closely related to corals, sea anemones, and jellyfish, have a mitochondrial genome that is linear and highly fragmented, in contrast to the single, circular genome of most organisms, according to a study published in Scientific Reports. And, one of the ceriantharians analyzed now holds the record for the largest animal mitochondrial genome reported to date at 80,923 base pairs.
“It blew our minds,” says Mercer R. Brugler, a research associate of the Museum, an associate professor at New York City College of Technology (CUNY), and a member of the international team led by Sérgio N. Stampar from Universidade Estadual Paulista (UNESP) in Brazil that sequenced and characterized the mitochondrial genome of the ceriantharians Isarachnanthus nocturnus and Pachycerianthus magnus. “Now that we understand the architecture of their mitogenome, we can quickly sequence and assemble additional mitogenomes from a diversity of tube anemone species and finally nail down where this group belongs in the cnidarian tree of life.”
With the exception of some preliminary data on Ceriantheopsis americana, this was the first complete mitochondrial genome sequenced for cerianthids. I. nocturnus and P. magnus represent two of the 55 known species of tube anemones, a ~540-600-million-year old lineage predated only by the Placozoa (flat animals), sponges, and ctenophores (comb jellies) in the animal tree of life.
At 80,923 base pairs, which are spread across five linear chromosomes, I. nocturnus earns the record for the largest animal mitogenome sequenced to date. The next largest is the other cerianthid studied, P. magnus, with 77,828 base pairs spread across its eight linear chromosomes. The multipartite linear mitogenome of the calcareous sponge Petrobiona massiliana was the previous record holder at ~77,000 bp.
By contrast in both shape and size, most animals have a single, circular mitogenome and far fewer base pairs—typically in the 16,000 range. The human mitochondrial genome, for example, is circular and comprised of 16,569 bp. Even the tube anemones’ closest relatives, the hexacorals and octocorals (named for their six and eight tentacles, respectively) have a single, circular mitochondrial genomic structure and the largest sequenced to date are ~20,000 bp. A linear structure has been reported in the Medusozoa (which includes the true jellyfish, stalked jellyfish, box jellies, and their kin), but this is the first report of a linear mitogenome within the Anthozoa (flower animals).
The unusual finding raises many more questions than it answers. Why would such an ancient, morphologically simple organism—basically a sac with tentacles—have what appears to be a more dynamic mitogenome than more anatomically complex and more recently evolved animals like snails, insects, or vertebrates? What purpose does their unique mitochondrial architecture serve? Does the structure of the tube anemone mitochondrial genome, which is radically different from their phylogenetic sisters, the Hexacorallia and Octocorallia, add further support that cerianthids represent a third major subclass of Anthozoa?
Only time—and more DNA sequencing—will tell.