Tiny parasites myxozoa are jellyfish cousins, study finds

A 200-year-old zoological mystery about the origins of a group of tiny fish parasites is as close as ever to being solved, thanks to research with a Canadian connection, and the answer is potentially astonishing.

The parasites, from a group known as myxozoa, consist of just a handful of cells each, but in a paper published in November, scientists say the genetic evidence is conclusive that they are in fact invertebrate animals — a cousin of jellyfish.

"Animals are usually defined as macroscopic multicellular organisms, and this is not that. Myxozoa absolutely redefines what we think of as animal," said Paulyn Cartwright, a professor of evolutionary biology at the University of Kansas and the paper's lead researcher, in a statement.

Scientists found genetic clues in the 1990s that the 1,300-plus species of myxozoa evolved  into their microscopic form from much larger organisms in a group known as the cnidaria, which includes jellyfish, hydroids, corals and sea anemones. But that view was contested, part of a centuries-old debate about the proper taxonomic classification of the myxozoa. 

So Cartwright and her colleagues, including personnel from Tel Aviv University in Israel and Hervé Philippe, the Canada research chair in evolutionary genomics at the University of Montreal, conducted the biggest-ever analysis of myxozoa genetic material.

What they conclude in their paper, published in the journal Proceedings of the National Academy of Sciences, is that the myxozoa underwent an "extreme evolutionary transition" in which they shed about 95 per cent of their genome and experienced a "dramatic reduction in body plan."  As a result, the myxozoa have among the smallest genomes in the animal kingdom — just 20 million or so DNA base pairs, compared to three billion base pairs in humans.

One feature that myxozoa do have, however, is a nematocyst — a venomous structure that jellyfish use to capture prey or to sting humans. Myxozoa use it to penetrate into their hosts.

The parasites can infect various kinds of fish, including salmon, trout and char, and can cause illnesses including whirling disease, which leads to deformations and neurological problems that make the salmon swim in circles.

No mouth or gut

"It's difficult to imagine they were jellyfish," Cartwright said. "They 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 the stinging cells of cnidarians: "Jellyfish tentacles are loaded with them — little firing weapons."

Mark Siddall, curator of invertebrate zoology and a professor at the American Museum of Natural History in New York City, said it's hard to imagine that a myxozoan could have evolved from jellyfish because of the extreme size difference, but the data is now "definitive."

"It's like the difference between you or I and Mt. Everest, in terms of volume," he said in an interview.

Siddall co-wrote a 1995 paper, based on work he did as a doctoral student at the University of Toronto, that presented some of the first DNA evidence that myxozoa in fact belong to the jellyfish group. Those results came from a small number of genes, though, and left the matter open to debate.

"What the group in Kansas pulled off is exactly the sort of thing I wish I could've done in the first place, but the technology wasn't there at the time," he said.

"It was based on thousands of genes, across a broad swath of both the myxozoa and the cnidaria and even in fact other invertebrate animals. It was comprehensive, and as far as I'm concerned, it's definitive."

Now that the myxozoa origin mystery has been all but put to rest, Cartwright said the next step is to figure out why they evolved like that. 

"We confirmed they're cnidarians," she said. "Now we need to investigate how they got to be that way."