Parasite shows how the jellyfish 'devolved'


Myxozoan spores from Kudoa iwatai, left. Each spore is approximately 10 micrometres in width. The jellyfish Aurelia aurita – moon jelly (right). The bell is approximately 25 centimetres wide or 2,500 times larger than a myxozoan spore.
A. Diamant (left), P. Cartwright (right)

Scientists have uncovered evidence that jellyfish have actually devolved over the centuries to become a microscopic parasite consisting of just a few cells.

The research is published in the Proceedings of the National Academy of Sciences.

Researchers at the University of Kansas sequenced the genome of myxozoans, a diverse group of microscopic parasites that infect invertebrate and vertebrate hosts. They discovered that they are actually "highly reduced" cnidarians – the phylum that includes jellyfish, corals and sea anemones.

"This is a remarkable case of extreme degeneration of an animal body plan," said Paulyn Cartwright, associate professor of ecology and evolutionary biology at KU and principal investigator on the research project. "First, we confirmed they're cnidarians. Now we need to investigate how they got to be that way."

The genome of the parasites were tiny.

"These were 20 to 40 times smaller than average jellyfish genomes," 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."

But Myxozoa has retains the jellyfish's stinging cells along with the genes needed to make it.

"Because they're so weird, 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 stinging cell of cnidarian. Jellyfish tentacles are loaded with them – little firing weapons."

In a similar evolutionary simplification, last month Cosmos reported on research that showed how viruses have streamlined, successfully jettisoning all but a handful of essential genes. See What came first, cells or viruses?

  1. https://cosmosmagazine.com/life-sciences/what-came-first-cells-or-viruses
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