At the moment I'm lecturing to our first-year biology class about plants. In my lecture about algae, there were a couple of slides about malaria. You might well ask 'why'; I know my students were thinking that. Well, I did have a reason – & today I found a brief summary paper (Keeling, 2008) on just that very topic.
Malaria is caused by a single-celled organism called Plasmodium. Plasmodium belongs to an unusual group of protists called Apicomplexa: all members of this group are parasites, living within the cells of their hosts. (Many of the symptoms of malaria are caused when the parasites burst out of the red blood cells where they've been reproducing, and seek new cells to infect.)
But the apicomplexans have an even more unusual feature. They contain plastids – and plastids are typically found in plants (chloroplasts are an example), while organisms like Plasmodium seem very un-plantlike. However, scientists have known for a while now that apicomplexans' closest relatives are a group of algae called dinoflagellates. And, being algae & hence photosynthetic, dinoflagellates have plastids too. But one question we haven't been able to answer has been, how did these two distinct groups of organisms get their plastids?
Dinoflagellate plastids are the result of endosymbiosis: at some point an ancestral dinoflagellate engulfed a red alga, but instead of digesting it, retained the alga (or, at least, its plastids) inside its own tissue. (Mitochondria & chloroplasts also have their origins in endosymbiotic events.) Did Plasmodium & its relatives get their plastids (also called apicoplasts) from the same alga, or in a separate event? It's been almost impossible to say, one way or the other, because the apicoplast has lost all the genes associated with photosynthesis, while retaining others, but dinoflagellate plastids contain only photosynthetic genes. So it would be pointless to compare their gene sequences directly. But now a 'missing link' has been found 🙂
Keeling describes this link as a 'small brown ball', living as a symbiont in coral. It's important to our story because, while it is a photosynthetic alga, it's also genetically related to Plasmodium & the other apicomplexans. That is, its photosynthetic genes are related to those of dinoflagellate plastids, and its 'housekeeping' genes are like those of apicoplasts. This tells us that the apicoplast and dinoflagellate plastids share a common evolutionary ancestor – they must have formed in the same endosymbiotic event. So an innocuous brown algal cell has helped scientists to look back into deep time & solve an evolutionary puzzle. As Keeling says, the most ordinary-looking objects can sometimes be the most revealing.
P.J. Keeling (2008) Bridge over troublesome plastids. Nature 451: 896-897
And just in – here's a link to science magazine article on the same alga.