Skip to main content
Premium Trial:

Request an Annual Quote

Genomic Study Places Microscopic Parasite Group into Jellyfish Lineage

NEW YORK (GenomeWeb) – A new genomic study suggests microscopic sea creatures that live as parasites of vertebrate and invertebrate animals are themselves animals, falling into the same lineage as jellyfish.

In the early, online edition of the Proceedings of the National Academy of Sciences, researchers from the US, Israel, France, and Canada described how they did genome sequencing, transcriptome sequencing, and phylogenetic analyses on two representatives from this so-called myxozoan group — obligate parasites once considered protists due to their extremely small size and simple body plan.

The team assessed data for these Kudoa iwatai and Myxobolus cerebralis myxozoans alongside available sequences for a less-dramatically streamlined, but similarly parasitic, member of the cnidarian lineage called Polypodium hydriforme. Together with a phylogenetic analysis of these and other species, the group found that myxozoans are a sister group to the P. hydriforme species within cnidaria, representing animals with dramatically reduced genomic and physical features.

"[A]nimals are usually defined as macroscopic multicellular organisms, and this is not that," senior author Paulyn Cartwright, an ecology and evolutionary biology researcher at the University of Kansas, said in a statement. "Myxozoa absolutely redefines what we think of as animal."

More than 2,100 microscopic myxozoans have been identified so far, the team noted. But while their degenerate and seemingly simple body plan led to their early classification as protists, more recent molecular marker and/or target gene studies suggested the organisms shared genetic similarities with animals such as P. hydriforme.

Moreover, the researchers explained, the myxozoans are known for having polar capsule structures similar to stinging bodies that cnidarians such as sea anemones and jellyfish use to subdue their prey, adding to the mystery of the myxozoans' place in the tree of life.

To tackle this with more extensive genomic data, Cartwright and her colleagues did Illumina genome and transcriptome sequencing on two myxozoans: K. iwatai and M. cerebralis. The latter species is best known for causing a neurological condition called whirling disease in fish such as rainbow trout. 

Based on the resulting sequences, the team estimated that the complete K. iwatai genome sequence spans some 22.5 million bases and contains roughly 5,533 predicted protein-coding genes, making it one of the most miniscule and compact animal genomes characterized to date.

In contrast, the P. hydriforme genome is believed to contain some 561 million bases and more than 17,000 genes, while the genome of the sea anemone Nematostella vectensis contains 450 million bases and around 18,000 genes.

Due to low coverage of M. cerebralis reads representing highly conserved eukaryotic genes, the group did not attempt to predict the full size of that organism's genome.

When they compared myxozoan transcripts with those found in P. hydriforme and in more classical cnidarian examples such the sea anemone and Hydra magnipapillata, meanwhile, the researchers found that P. hydriforme shared more of its gene repertoire with the hydra and sea anemone than the myxozoans did.

But despite showing losses in genes involved in processes such as cell differentiation, development, signaling, and communication between cells, the myxozoan transcriptomes contained many of the genes and pathways employed by model cnidarian species.

Moreover, a phylogenetic analyses that included data for K. iwatai, M. cerebralis, and P. hydriforme, along with almost two-dozen cnidarians, three more myxozoans, and several other animal or unicellular species, pointed to a myxozoan lineage that sat beside that of P. hydriforme in the cnidarians.

"This is a remarkable case of extreme degeneration of an animal body plan," Cartwright said. "[W]e confirmed they're cnidarians. Now we need to investigate how they got to be that way."