Evolutionary arc

The evolutionary arc from toxic dinoflagellate to marine symbiont to deadly blood parasite is not improbable, given natural nature's inventiveness and opportunism.

Female Anopheles mosquitoes deliver the human-infecting species of Plasmodium as they seek a high-protein blood meal - various other blood-feeding insects including Culex midges provide the same service for Plasmodium species that infect other mammals, birds and reptiles.

Plasmodium takes temporary lodgings in the salivary glands of its intermediate host, and is careful not to kill its vector, so it can be delivered into its vertebrate host's bloodstream. Mosquito and midge larvae live in fresh or brackish water, like some dinoflagellates. Join the dots ...

Chromera is a photosynthetic dinoflagellate, that has adapted to life as intercellular symbiont in colonies of single-celled coral polyps, but as Robert Moore showed, is easily grown in culture, independently of its host.

Coral polyps pass their friendly dinoflagellates on to their daughters at meiosis, but zooxanthellae are facultative symbionts that can also exist as free-living cells in seawater.

At water temperatures above 32 degrees, stressed polyps expel their zooxanthellae back into the water - or the zooxanthellae quit the partnership. The result is coral bleaching. When the crisis passes, polyps reinstate the partnership by recruiting free-living zooxanthellae.

Plasmodium is a non-photosynthetic, intracellular parasite of vertebrate red blood cells that drifts in a fluid with the ancient salt signature of seawater.

The Apicomplexa include familiar waterborne pathogens like Cryptosporidium - which has dispensed altogether with its apicoplast - and Toxoplasma gondii, the urine-borne agent of cat-scratch disease, which can cause severe developmental abnormalities in babies if the mother is infected during pregnancy.

Carter et al observe in the introduction to their paper that the discovery of Chromera provides a powerful model with which to study the evolution of parasitism in Apicomplexa.

"This discovery shows that you can find some very interesting things in the Australian environment - unexpected things," Carter says. "It shows the value of curiosity-driven research.

"If we had told anybody we were going to look for the closest relative of the world's deadliest human parasite in the sea, we might have been laughed out of the room."

Carter believes the Chromera paper is the first taxonomic paper published in Nature for a long time. "Taxonomy has been so undervalued that it has been devastated [as a branch of science]," she says.

"We had some grant money from the Australian Biological Resources Survey, which is the only Australian agency that funds taxonomy at the moment, because the previous Federal Government just wasn't interested. The amount of money ABRS has to distribute is minuscule."

Plasmodium is tricky to grow, and cannot be replicated in vitro because of its multi-stage life cycle. Robert Moore has now demonstrated that its endosymbiont cousin, Chromera, with which it shares basic biosynthetic pathways, is easily grown and replicated in the laboratory.

The authors predict C. velia will be of practical use for high-throughput screening of prospective anti-apicoplast drugs - potentially a big bang for the few bucks invested by ABRS in the project.