Wednesday, 3 December 2014

Missing link between fungi and parasites


Zoologists at the University of Basel in Switzerland have now discovered a new parasite species that represents the missing link between fungi and an extreme group of parasites.

Microsporidia are a large group of extreme parasites that invade humans and animals and cost great damage for health care systems and in agriculture; over 1,200 species are known. They live inside their host's cells and have highly specialized features: They are only able to reproduce inside the host's cells, they have the smallest known genome of all organisms with a cell nucleus (eukaryotes) and they possess no mitochondria of their own (the cell's power plant). In addition, they developed a specialized infection apparatus, the polar tube, which they use to insert themselves into the cells of their host. Due to their phenomenal high molecular evolution rate, genome analysis has so far been rather unsuccessful: Their great genomic divergence from all other known organisms further complicates the study of their evolutionary lineage.

The team of zoologists lead by Prof. Dieter Ebert has been studying the evolution of microsporidia for years. When they discovered a new parasite in water fleas a couple of years ago, they classified this undescribed species as a microsporidium, mostly because it possessed the unique harpoon-like infection apparatus (the polar-tube), one of the hallmarks of microsporidia. The analysis of the entire genome had several surprises in store for them: The genome resembles more that of a fungi than a microsporidium and, in addition, also has a mitochondrial genome. The new species, now named Mitosporidium daphniae, thus represents the missing link between fungi and microsporidia.

With the help of scientists in Sweden and the U.S., the Basel researchers rewrote the evolutionary history of microsporidia. First, they showed that the new species derives from the ancestors of all known microsporidians and further, that the microsporidians derive from the most ancient fungi; thus its exact place in the tree of life has finally been found. Further research confirms that the new species does in fact have a microsporidic, intracellular and parasitic lifestyle, but that its genome is rather atypical for a microsporidium. It resembles much more the genome of their fungal ancestors.

The scientists thus conclude that the microsporidia adopted intracellular parasitism first and only later changed their genome significantly. These genetic adaptations include the loss of mitochondria, as well as extreme metabolic and genomic simplification.

For further details, see:

Haag, K.L., James, T.Y., Pombert, J.-F., Larsson, R., Schaer, T.M.M., Refardt, D. & Ebert, D. Evolution of a morphological novelty occurred before genome compaction in a lineage of extreme parasites. Proceedings of the National Academy of Sciences, October 2014 DOI: 10.1073/pnas.1410442111

Posted by Tim Sandle