A new genetic study of myxozoans, a large group of parasites that infect invertebrate and aquatic vertebrate hosts, confirms that these weird microorganisms are actually ‘highly reduced’ cnidarians (jellyfish, corals, hydrozoans and sea anemones).
Fresh spores of the myxozoan species Kudoa iwatai; each spore is 10 micrometers in width. Image credit: A. Diamant.
“This is a remarkable case of extreme degeneration of an animal body plan. First, we confirmed they’re cnidarians. Now we need to investigate how they got to be that way,” said Dr Paulyn Cartwright of the University of Kansas, senior author of the study, which was published online November 16 in the Proceedings of the National Academy of Sciences.
The study characterizes the genomes and transcriptomes of two distantly related myxozoan species, Kudoa iwatai and Myxobolus cerebralis, and another cnidarian parasite, Polypodium hydriforme.
“The myxozoan genomes were 20 to 40 times smaller than average jellyfish genomes,” Dr Cartwright said.
“It’s one of the smallest animal genomes ever reported. It only has about 20 million base pairs, whereas the average cnidarian has over 300 million. These are tiny little genomes by comparison.”
“Analyses of the myxozoan genomes indicate that the transition to the highly reduced body plan was accompanied by massive reduction in genome size, including depletion of genes considered hallmarks of animal multicellularity,” the researchers said.
Despite its radical phasedown of the modern jellyfish’s body structure and genome over millions of years, myxozoans have retained the essential characteristic of jellyfish – its stinger (nematocyst) – along with the genes needed to make it.
“Because they’re so weird, it’s difficult to imagine they were jellyfish,” Dr Cartwright said.
Myxozoans don’t have a mouth or a gut. They have just a few cells. But then they have this complex structure that looks just like stinging cell of cnidarian.
“Jellyfish tentacles are loaded with them – little firing weapons,” Dr Cartwright said.
“The confirmation that myxozoans are cnidarians would necessitate the re-classification of Myxozoa into the phylum Cnidaria,” she said.
“Moreover, these micro jellyfish could expand understanding of what makes up an animal.”
“Their biology was well-known, but not their evolutionary origins. They’re microscopic, only a few cells measuring 10 to 20 microns. Some people originally thought they were single-celled organisms. But when their DNA was sequenced, researchers started to surmise they were animals – just really weird ones.”
“Indeed, traits scientists understood as vital for animal development are absent in Myxozoa,” Dr Cartwright said.
“Hox genes are one example, which are important to development of all animals, and these lack them,” she explained.
“But Myxozoa is definitely an animal because its evolutionary origin is shared with jellyfish, and we use species’ ancestry to define them. Animals are usually defined as macroscopic multicellular organisms, and this is not that. Myxozoa absolutely redefines what we think of as animal.”
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E. Sally Chang et al. Genomic insights into the evolutionary origin of Myxozoa within Cnidaria. PNAS, published online November 16, 2015; doi: 10.1073/pnas.1511468112
