The Myxozoa count as some of the most enigmatic organisms known. There’s around 1350 species of them, all tissue and cellular parasites. The majority infect aquatic and marine fish, while some use platyhelminths, annelids, reptiles, amphibians (e.g. Hartigan et al., 2011) or moles as primary hosts. One freshwater bryozoan parasite whose affinity has been debated since 1850, Buddenbrockia plumatella, has now also been placed as a myxozoan (Monteiro et al., 2002). The problem with it is that it looks completely different from other myozoans: it’s a wormish animal with muscles, while the rest of the myxozoans are sporeish.
The reason for the myxozoans being so mysterious is that, like all parasites, they’re extremely reduced. It was only in the past few decades that they’ve been confirmed as being animals – this is due to their having differentiated cells and desmosome-like cellular contacts, both of these features being metazoan apomorphies; it was also confirmed by molecular evidence by Smothers et al. (1994). Prior to these discoveries, they were considered a separate group of protozoan (e.g. Cavalier-Smith, 1993).
Their position within the animals is still in doubt though. Morphologically, the polar capsules (see description of life cycle, later) are very reminiscent of cnidarian nematocysts (Lom & de Puytorac, 1965), leading to them being placed as reduced, parasitic cnidarians. This has been supported by molecular analyses (Holland et al., 2011) and total-evidence analyses combining morphology and molecules (Siddall et al., 1995), but rejected by other molecular work (e.g. Anderson et al., 1998), some of which places them at the root of the bilaterians (Evans et al., 2010), a placement especially supported by the confirmation of Buddenbrockia as a myxozoan (at the base of the Bilateria, but not as far reduced as the other myxozoans since it retains muscles). A problem in these molecular analyses, both supportive and rejective, is the long branch of the myxozoan line causing long-branch attraction, a very deceptive artefact in molecular analyses, ultimately caused by improper taxon sampling. So basically, there is no consensus yet; Cavalier-Smith et al. (1996) even suggested they be placed as a separate animalian subkingdom!
It was previously thought that there were, systematically, two groups of Myxozoa: the Actinosporea and the Myxosporea. It’s now known that this is nothing more than two stages of the life cycle of one species. I’ll describe this life cycle using Myxobolus cerebralis, a trout parasite, as a generalised example for all Myxozoa. The only major differences with other species is the primary and secondary hosts’ identities.
M. cerebralis infects the trout in the actinospore stage, using the triactinomyxon spore. These have 3 spaces inside, forming a stem with three projections (hence the tri- prefix). The projections are used as anchors to hang onto the fish’s epidermis (or its intestinal tract, if ingested during feeding). Inside the stem is the multinucleated sporoplasm with inner cells.
When it’s anchored, the sporoplasm is injected into the tissue, letting out the inner cells. These are what infect the host’s cells and cause them to divide. The new cells form pairs consisting of an enveloping cell and an inner cell.
These pairs go into the extracellular space to infect other cells, eventually taking over an entire tissue (this can include the nervous system). At this point, sporogony happens (the formation of new spores). Cell divisions take place, resulting in multicellular myxospores. They have a sporoplasm with two nuclei, 2 polar capsules forming 2 spiral filaments (which bear a very strong resemblance to the cnidas of cnidarian nematocysts), and the whole thing is enclosed in a bivalve-like envelope formed by two cells.
These myxospores are eaten by annelids (in this example: Tubifex). The polar filaments break out (much like a cnida does), anchoring the spore on the annelid’s intestinal epithelium. The sporoplasm is let out into the matrix between the epithelial cells and schizogony takes place, whereby mononucleated cells form. These spread around the epithelium. Some of them fuse together to form dinucleated cells which then split into four-nucleated cells.
These cells then split into four mononucleated cells called the pansporocysts: 2 are vegetative and 2 are generative cells. These each undergo 3 mitoses and 1 meiosis to form 16 haploid gametocytes and 16 polar bodies. Those gametocytes originating from the vegetative cells fuse with those originating from the generative cells, forming 8 zygotes. The polar bodies fuse similarly, and they become the cells enveloping the zygotes.
These are the cells that undergo sporogony to form the triactinomyxon spores. Go back 5 paragraphs and repeat cycle.
Anderson CL, Canning EU & Okamura B. 1998. A triploblast origin for Myxozoa? Nature 392, 346-347.
Cavalier-Smith T. 1993. Kingdom protozoa and its 18 phyla. Microbiology and Molceular Biology Reviews 57, 953-994.
Cavalier-Smith T, Allsopp MTEP, Chao EE, Boury-Esnault N & Vacelet J. 1996. Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence. Canadian Journal of Zoology 74, 2031-2045.
Lom J & de Puytorac R. 1965. Studies on the myxosporidian ultrastructure and polar capsule development. Protistologica 1, 53-65.
Monteiro AS, Okamura B & Holland PWH. 2002. Orphan Worm Finds a Home: Buddenbrockia is a Myxozoan. Molecular Biology & Evolution 19, 968-971.
Evans NM, Holder MT, Barbeitos MS, Okamura B & Cartwright P. 2010. The Phylogenetic Position of Myxozoa: Exploring Conflicting Signals in Phylogenomic and Ribosomal Data Sets. Molecular Biology and Evolution 27, 2733-2746.
Holland JW, Okamura B, Hartikainen H & Secombes CJ. 2011. A novel minicollagen gene links cnidarians and myxozoans. Proc. R. Soc. B 278, 546-553.
Siddall ME, Martin DS, Bridge D, Desser SS & Cone DK. 1995. The Demise of a Phylum of Protists: Phylogeny of Myxozoa and Other Parasitic Cnidaria. The Journal of Parasitology 81, 961-967.
Smother JF, von Dohlen CD, Smith Jr LH & Spall RD. 1994. Molecular evidence that the myxozoan protists are metazoans. Science 265, 1719-1721.
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Evans, N., Holder, M., Barbeitos, M., Okamura, B., & Cartwright, P. (2010). The Phylogenetic Position of Myxozoa: Exploring Conflicting Signals in Phylogenomic and Ribosomal Data Sets Molecular Biology and Evolution, 27 (12), 2733-2746 DOI: 10.1093/molbev/msq159