Suppose you’re nearing the end of your life. In a strange twist of fate, you won the lottery. You decide to split the jackpot equally between your two children. While one child uses the money to fund a charity dedicated to fighting poverty, the other one uses it to start the crime syndicate he has always been dreaming of! Such might be the tale of the evolution of the malaria parasite, where an algal plastid stars as the unlikely jackpot.
Both apicomplexa and dinoflagellates inherited a red algal plastid from their forebears. Most dinoflagellates use this plastid for photosynthesis, trapping energy from the sun. Apixomplexans took a shadier path with this plastid when some of them became infectious parasites, like the malaria parasite that kills millions of people every year. They use this plasmid, the apicoplast, not for photosynthesis, but to synthesize fatty acids and other metabolites that are necessary for these parasites to enter host cells. From a human perspective, the apicomplexa are definitely the black sheep in the Alveolate family tree (below).
The genes that lie on the plasmids in dinoflagellates and ampicomplexans reflect their different lifestyles. The dinoflagellates’ plastid only contains ‘photosynthetic’ genes and not much else. The apicoplast has genes that are involved in several different metabolic pathways, such as fatty acid and heme biosynthesis. The overlap between the two plastid genomes is virtually zero, making it difficult for scientists to compare them and come to meaningful conclusions about their ancestry.
Complicating matters, some closer parasitic nephews of the apixomplexa (like Cryptosporidium) have no plastid at all! This either means that the apicomplexan and dinoflagellate ancestor acquired an algal plastid independently from each other OR that photosynthesis / the entire plastid had been lost several times in different lineages. Using the lottery analogy, the question boils down to whether apicomplexans and dinoflagellates both won the lottery, or whether their common ancestor was the lucky winner, passing on its spoils to its common descendants? Since evidence was so sparse, both hypotheses garnered support in the scientific community.
Luckily a new player looking to break the stalemate entered the field recently: Chromera velia. This little bug turns out to be a closer photosynthetic relative of apicomplexa than the dinoflagellates are, and thus has the potential to settle some difficult ancestry issues. A team of scientists from Czechia and Canada sequenced the entire plasmid of C. velia to find out where exactly the malarial plasmid came from.
As you can see in the Venn diagram below, both the plastids of C. Velia and a related algal species (imaginatively known as CCMP3155) contain all the genes that are present on the apixomplexan and dinoflagellate plastid. On their turn, all these genes can be found back on the plastid found in red algae. The simplest way to explain this pattern is to say that all these plastids derived from a single red algal ancestor. It thus seems that the ancestor of apicomplexa and dinoflagellates got lucky in the lottery, and obtained this plastid by endosymbiosis after which the lineages diverged.
From a plastid’s perspective, evolving from a photosynthetic C. velia-like organism into an organism with a parasitic lifestyle seems to have mainly been accompanied by gene loss. The authors provide evidence that many plastid genes had already migrated to the nucleus in the ancestor of dinoflagellates and apicomplexans, but that it continued after the lineages diverged. Perhaps some of these losses and migrations were exaptations for the different walks of life these protists would come to develop.
Many of the protists in this branch of the tree of life, including C. velia, live in symbiotic relationships with coral reefs. Could it be that the apicomplexans started their parasitic lifestyle by infecting coral cells with which they lived in close contact? I find this prospect both fascinating and terrifying. Fascinating because this research started with a desire to know where the malaria parasite came from, and in this quest for knowledge learnt more about its ancestry and evolution in general than we could have hoped. It is also terrifying because we are trying to eradicate a parasite that seems to have been infecting different forms of life on earth for nearly half a billion years.
Is malaria the price we now pay for a symbiotic relationship that existed hundreds of millions years before the first humans opened their eyes for the first time?
Janouskovec, J., Horak, A., Obornik, M., Lukes, J., & Keeling, P. (2010). A common red algal origin of the apicomplexan, dinoflagellate, and heterokont plastids Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1003335107
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