How a Killer Parasite Evolved from Pond Scum

881 How a Killer Parasite Evolved from Pond Scum
The corkscrew-shaped Helicosporidium is a parasite that has its origins in green alga / Drion Boucias, University of Florida
A genomic study of a corkscrew-shaped parasite living in the guts of insects shows how it originated from algae -- just like another notorious killer, the malaria parasite. 
The transition to a parasitic lifestyle completely dependent on a host usually comes with genomic reduction -- why keep genes that help you function and take care of yourself when you’re living off someone else at their expense? The most extreme of these transitions is from free-living algae making their own food (autotrophs) to obligate parasites. 
Helicosporidium parasiticum -- which kills juvenile blackflies, caterpillars, beetles and mosquitoes -- was first described about a century ago, but we still don't know much about their origin. When a team led by Patrick Keeling from the University of British Columbia sequenced the genome of Helicosporidium, they found that the parasitic protist evolved from green algae. But surprisingly, the parasite kept most of its ancestral functions. Compared with the closely related green algae, Coccomyxa subellipsoidea and Chlorella variabilis, the parasite’s genome was hardly reduced at all.
There is, however, one major exception: It preserved virtually all of its genes except those needed for harvesting light and photosynthesis, which it doesn’t need as a parasite. “It’s as if photosynthesis has been surgically removed from its genome,” Keeling says in a news release
The researchers have previously shown that the malaria pathogen, Plasmodium, shares a common evolutionary lineage with the algae known for those toxic red tides. But unlike Helicosporidium, which lost nearly nothing, malaria reduced its genome dramatically and became dependent on its host for nutrients. “Both malaria and Helicosporidium started out as alga and ended up as intracellular parasites preying on animals, but they have done it in very different ways,” Keeling says.
By comparing how parasites evolve at the molecular level in these two distantly related lineages, the researchers hope to better understand their methods of infection. Maybe it could help control the population of pest-insect hosts.
The work was published in PLOS Genetics this week. 
[Via UBC]
Image: Drion Boucias, University of Florida


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