We typically think of evolution as a progression from simplicity to complexity. But one organism seems to have thrown the rulebook out the window: a microbial animal that offers a striking example of evolution run "backwards."
That would be myxozoa, a microscopic, twelve-celled parasite whose cousins include decidedly macroscopic jellyfish and corals.
"This is a remarkable case of extreme degeneration of an animal body plan," biologist Paulyn Cartwright of the University of Kansas said in a statement. "Animals are usually defined as macroscopic multicellular organisms, and this is not that. Myxozoa absolutely redefines what we think of as animal."
Found in both freshwater and marine habitats, myxozoa are a diverse clan of nearly 2000 microscopic parasites that infect fish and invertebrates. For a long time, scientists classified them as protists, a sort of catch-all group of microscopic critters with complex sub-cellular structure that includes amoebas and paramecia. But some biologists have questioned that classification, noting that myxozoa contain an elaborate structure (called a polar capsule) that allows them to latch onto hosts. That structure is awfully reminiscent of a jellyfish stinger.
Could there be more to myxozoa than meets the eye? That suspicion was confirmed this week by a genomic analysis published in the Proceedings of the National Academies of Sciences. Turns out, myxozoa aren't some weird group of protist loners — they're cnidarians, the same family that includes jellies, hydra and corals. Their closest relative is Polypodium hydriforme, another cnidarian parasite with a jellyfish-like life stage.
How did myxozoas forgo their complex, mascroscopic heritage to become so small and simple? By shedding a lot of DNA. As their bodies pared down, their genomes shrunk to just 22.5 million bases — a fortieth the size of Polypodium and other jellies.
"It's one of the smallest animal genomes ever reported," Cartwright said. "It only has about 20 million base pairs, whereas the average Cnidarian has over 300 million."
Perhaps unsurprisingly, myxozoas are most deficient in genes related to development, cell differentiation, and cell-to-cell communication — all the bits of code you'd need to become a large, complex organism.
This may be the first known case of simplification from a macro to a microorganism, but Cartwright suspects evolution has performed this trick more than once. In fact, many things we call protists — a group that was cobbled together based on morphology, not genetics — may, in fact, be simple, degenerate animals. "If it can happen once in evolution, it certainly can happen again," she said.
Now who's writing the sci-fi novel about humanity's microscopic descendants?
[Read the full scientific paper at PNAS h/t University of Kansas News]
Top: Myxozoan spores from Kudoa iwatai (left) and the jellyfish Aurelia aurita (moon jelly, right). The jelly is roughly 2500 times larger than a myxozoan spore. Left photo: A. Diamant. Right photo: P, Cartwright