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What are Myxozoans?  To be continued...

Posted by John Jackson on Jan 11, 2011 4:54:07 PM

Myxozoans are a diverse group of microscopic endoparasites - tiny worm-like parasites that live within other organisms, depending on their hosts for nutrition, oxygen and other needs.  Examples of myxozoans featured as NHM species of the day in 2010 were  Buddenbrockia plumatellae  and Tetracapsuloides bryosalmonae.

Scientific study of evolution and evolutionary relationships has in the past often depended on study of the physical forms of organisms (morphology) and comparing particular characteristics, such as body plan, organs, limbs or other features. Similarities and differences in these characters are used to classify the organisms.  However, with some organisms, particularly parasites, evolution can result in the loss of features with an apparently more simple body form.  This makes clarification of relationships difficult.  However, the use of DNA sequences can provide additional information that leads to understanding of evolutionary relationships and a clear evolutionary classification (phylogeny).

Myxozoans have been the focus of much controversy regarding their phylogenetic position. Two dramatically different hypotheses have been put forward for the position of the Myxozoa within Metazoa (all multicelled animals).

The first hypothesis, supported by rDNA sequence data (a specific kind of DNA from the ribosomes of the cell), suggests that Myxozoa is a sister group to Bilateria (all organisms with a single line of symmetry to their body plan, ranging from simple worms to humans, and representing most groups of animals). However, the alternative hypothesis, supported by phylogenomic data (a broader range of DNA) and morphology, suggests that Myxozoan are cnidarian. Cnidarians are an animal group containing sea anemones, coral and jellyfish that have radial symmetry and a very different body plan from the Bilateria.  These different ideas represent evolutionary events that would have occurred hundreds of millions of years ago.

Professor Beth Okamura (NHM department of Zoology) and colleagues, funded by the US National Science Foundation, investigated these conflicting hypotheses with Buddenbrockia and explored the effects of missing data, different statistical methods, and different models on evolutionary classification.  In addition, they identified subsets of the data that most influence the placement of Myxozoa and explored their effects by removing them from the datasets.

The results confirm the existence of two relatively stable placements for myxozoans and demonstrate that conflicting signal exists not only between the two types of data but also within the phylogenomic dataset. These analyses underscore the importance of careful model selection, taxon and data sampling, and in-depth data exploration, when investigating the phylogenetic placement of highly divergent taxa.

In other words, the  available information does not yet allow Myxozoans to be placed  definitely within one or other fundamental group - further development of data, and new scientific techniques will be needed to answer this question, but the work in the paper is important in defining the current limits and uncertainties of this area of science, and suggests ways forward for the future.

Evans, N.M., Holder, M.T., Barbeitos, M.S., Okamura, B. & Cartwright, P. 2010. The phylogenetic position of Myxozoa: Exploring conflicting signals in phylogenomic and ribosomal datasets. Molecular Biology and Evolution 27: 2733-2746. doi:10.1093/molbev/msq159

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