For most of us, microbes mean only one thing: disease. Disease-causing microbes are actually the extreme minority of the most abundant form of life on Earth. But because of their immediate and direct importance to our health, they are much better studied than the rest of the microbial world. Still, new discoveries about the basic biology and evolution of some of the most infamous pathogens can throw us a curveball.
My lab works on the evolution of apicomplexans, a diverse group of parasites that can only survive by infecting and living inside animal cells. For every animal scientists have studied, there’s an apicomplexan that infects it. A number of common and serious human pathogens, most famously the parasite that causes malaria, Plasmodium, are apicomplexans.
Even after 100 years of work, malaria researchers overlooked one bizarre feature of the parasite, a cellular compartment called a plastid, better known as a chloroplast. The plastid is the part of plant and algal cells where photosynthesis takes place. They’re highly-simplified relicts of photosynthetic bacteria that took up residence in an ancient ancestor of plants and algae. Today, plastids have become genetically integrated with their hosts and are an essential part of plant and algae cells.
But how did a parasite like malaria, which lives in the blood of humans, come to possess a plastid? They certainly don’t do photosynthesis, so why do they have plastids? To an evolutionary biologist interested in endosymbiosis — cells living within other cells — it’s a hard question to resist. But it’s also a challenging question to study. Apicomplexan plastids are more than a little weird, and both their genomes and structures are severely simplified from their original form, making them hard to compare to those of plants and algae.
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