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Publications of the Week

Multiple Independent Origins of Apicomplexan-Like Parasites

By September 9, 2019No Comments

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This week we profile a recent publication in Current Biology from Varsha
(pictured, right) in the laboratory of Dr. Patrick Keeling (left) at UBC.

Can you provide a brief overview of your lab’s current research focus?

The lab generally focuses on major evolutionary transitions and how symbiosis, defined as two organisms living in close association with one another, has impacted them. We mostly study protists, which are microbes, but complex ones with their genome contained in a nucleus like our own (as opposed to bacteria, which have much simpler cells). Protists are very diverse and important in ecology and evolution, but relatively few people study them because they can be a lot harder to work with than other microbes. We are interested in how symbiotic relationships between protists and bacteria may have originated, how it affected the evolution of the two partners, what it did to their genomes, and what new functions might have arisen from the partnership. For example, a lot of the biochemistry in our cells for turning sugars into energy come from a symbiotic bacteria taken up and integrated into our cells as “mitochondria” billions of years ago.

Another example is the origin and early evolution of apicomplexan parasites. Apicomplexans are an important group of obligate intracellular parasites that include the causative agents of human diseases like malaria and toxoplasmosis. They are also fascinating for an evolutionary biologist because, despite only being able to grow and divide by infecting and getting right inside the cells of their animal host, they turn out to have evolved from a photosynthetic alga and still have a tiny relict plastid (or chloroplast), which is the compartment where photosynthesis happens in plants and algae. The plastid itself also arose by symbiosis long ago, so we have two interesting symbiosis questions in the same cell – how did they become parasites? And what happens to the plastid when its main function in photosynthesis is lost? A lot of the answers to these questions were foggy because we lacked data from the easiest-diverging members of the apicomplexan lineage, which are called gregarines. These are very diverse and abundant, but they infect invertebrate animals and not vertebrates like us, so people don’t pay much attention to them. But from an evolutionary standpoint they could be a goldmine, so we have been interested in them for the last 20 years or so.

What is the significance of the findings in this publication?

People had looked at the genome of one gregarine in the past and found it had no plastid, but this is a huge and diverse group so there is no reason to believe this represents all gregarines very well (it would be like doing some biology on a mouse and assuming you know everything about mammals). We looked at both terrestrial gregarines and marine ones and found that the terrestrial ones have indeed lost their plastid, but the marine ones still have it. And some have lost the functions we previously thought explained its existence in other apicomplexans, so it opens up some interesting new questions. But what was weirdest of all, and totally unpredictable, is that two of the species we studied turned out to not really be apicomplexans at all. Their infection looks like other apicomplexans, but in the tree they branched close to but outside the group. One was even closely related to a photosynthetic algae, so we have a great opportunity to look at how photosynthesis was lost multiple times, and how parasitism arose multiple times.

What are the next steps for this research?

Although we looked at six diverse parasites, this is still only scratching the surface and the unpredictable results go to show how a little old-fashioned ‘exploration’ is still really important. So our first goal is to try to cover more of the diversity and see if we can reconstruct how all these complex changes evolved. We also want to find the plastids – we can “see” them in the genome, but it would be great to see them in the cell too! Which means we need to get the parasites again and show which compartment the important genes and proteins are functioning in.

This work was funded by:

This work was supported by grants and fellowships from CIHR, NSERC, and the Tula Foundation. It was also a collaboration with colleagues in St. Kitts and Nevis, and Iceland.

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