Back to the Roots

(November 11th, 2014) History can be complicated, especially if it’s the evolutionary history of intracellular parasites. Studying a newly discovered parasite of water fleas, Swiss scientists found an answer to one of the notorious “what came first” riddles.

Microsporidia are a large and economically important group of spore-forming unicellular parasites. They infect humans as well as various animals like insects and fish, making themselves at home in their host’s gut. Even though, they don’t kill their landlords, they cause a range of conditions, such as reduced fertility and weight loss. Microsporidia are a large group, with more than 1,200 species known to date. However, the group has so far remained hidden deep within the roots of life’s evolutionary tree, as researchers have no idea of the evolutionary history of this important and enigmatic group. Rumour has it that they are an “early-diverging branch of the fungal tree”.

Now, a recent finding by Basel University researchers, in collaboration with colleagues from Brazil, the US and Sweden, sheds new light on the evolution of Microsporidia. Star of the new research is a strange family member found in the hindgut of water fleas, Daphnia (pictured).

What’s so strange about this parasite? It clearly opted for a microsporidic, intracellular and parasitic lifestyle and even boasts the signature of a highly specialised infection apparatus, unique to the Microsporidia (the polar tube); its genome, however, tells a different story. Genomic analyses of this new species, named Mitosporidium, revealed two surprises: its DNA is more related to the DNA of fungi than to anything else and they host mitochondrial DNA, unlike their Microsporidia relatives that lack this organelle. Mitosporidium, thus, has kept some of the original, ancestral features of this phylum. Based on their results, the researchers conclude that Microsporidia’s odd morphologies were there before they went fully parasitic and shut down their cellular power houses.

“Our results are not only a milestone for the research on Microsporidia but they are also of great interest to the study of parasite-specific adaptations in evolution in general,” says lead author, Dieter Ebert, from the Department of Environmental Science at the University of Basel. The team is now sequencing the genome of other species related to Mitosporidium to gain a better understanding of how this new lineage of parasites evolved. Their studies are under way and have already produced some results. Ebert reveals: “One recent discovery in this group is that there is evidence for horizontal gene transfer. And indeed we were able to confirm this for a newly sequenced species. The paper is submitted,” he adds.

The team is currently also doing experimental work on host-parasites interactions, focusing on how they co-evolve. In addition, they are searching for and mapping genes involved in host resistance and parasite virulence. But it doesn’t end here, as the team is focusing their attention on sex. “We are interested in understanding the evolution of sex in this group of parasites and what the consequences are of genetic recombination of the parasite for the parasite and its host,” Ebert says.

Karl Gruber

Photo: Ebert group

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