11 It was in first year biology that I came across what is, in my opinion, the true diva in the world of algal research: Chlamydomonas. It’s a microbial superstar and the most highly studied alga in the world, being used as a model for investigating everything from photosynthesis to flagellar motion to the origin of plants. It was my enthusiastic first-year biology professor, Denis Maxwell, an algal connoisseur and Chlamydomonas researcher extraordinaire, who first introduced me to “Chlamy.” In a lecture strewn with excitement and hyperbole, Dr. Maxwell projected the spectacular Chlamy on a giant screen and proceeded to list all of the wondrous features of this puny alga. “It even has an eye,” screamed Denis, “which guides it towards the light. And look at those two lovely flagella, my goodness! Those babies propel it to the sweet spot—not too bright, not too dark, that’s the way Chlamy likes it.” All I could think during that lecture was, “Chill out, Dr. Maxwell. It’s just an alga.” So I went back to my residence room and did some online reading to figure out what could possibly make my professor so excited. I found that Dr. Maxwell’s enthusiasm was more than justified: not only does Chlamy have a wide range of basic research applications and a completely sequenced genome, but it also has many practical uses, including biofuel production and bioremediation. Impressive, but Chlamydomonas was too flashy for my tastes. The high-impact papers and competitive research environment were a turn off for me—I wanted plants and algae contain three different types of genome: a nuclear genome, which is big and stored in the head to find a genus that was more mysterious. So I dropped the diva and office of the cell (the nucleus), a mitochondrial genome, went for its sister—the sister lineage of Chlamydomonas, to be precise, which is generally small and found in the power house which is called Polytomella. of the cell (the mitochondrion), and a chloroplast In many ways, Chlamy and Polytomella are alike: both are single-celled swimmers that thrive in fresh water and have multiple fla- genome, which is medium sized and—you guessed it— housed in the chloroplast, where photosynthesis takes gella (although Polytomella has four, which is two more than Chlamy). place. Never has there been a concrete example of a There is, however, one really big difference between them: Polytomella isn’t green. Polytomella lacks chlorophyll and cannot derive energy from plant or alga that doesn’t have all three of these genomes, that is, until Polytomella came along. sunlight—it’s an evolutionary burnout. When I first saw a high-definition image of Polytomella, I was underwhelmed, it just looked like Chlamydomonas, but with a couple of extra tails and no colour. But A recent study led by the Smith Lab at Western University showed that Polytomella has discarded its entire chloroplast genome! Crazy, eh? Let me try to explain how this happened. Polytomella descends from a photosynthetic ancestor, but at some point in the distant past it lost the ability to perform photosynthesis—probably from a deleterious mutation—and therefore survives by absorbing small, energy-rich molecules from its environment as opposed to converting sunlight into sugar. Since losing photosynthesis, Polytomella’s photosynthetic organelle, the chloroplast, has been “put out to pasture.” It’s lost all of the brightly colored chlorophylls, complex inner-membrane structures, and light-harvesting infrastructure found in photosynthetic chloroplasts. Thus, it is not surprising that the genome within the Polytomella chloroplast has also withered away. That said, the Polytomella chloroplast still performs some crucial functions apart from photosynthesis, like synthesizing fats. Other non-photosynthetic plants and algae have also lost large sections of their chloroplast genome, but Polytomella is the first organism to have taken this process to the ultimate extreme—genome loss. Eventually, Polytomella may lose its chloroplast outright, but for this to happen, it will have to find a way to outsource remaining chloroplast functions (like fatty acid biosynthesis) to other organelles in the cell. So that’s how I got hooked on algae—from my first experience with the stunning Volvox, to a short-lived affair with the high-flying Chlamy, and to a lasting love for the eccentric Polytomella. I hope that I piqued your interest in green algae, and sold you on Polytomella, which has been hiding in the shadow of Chlamydomonas for too long. Not that Polytomella would care—it doesn’t crave the limelight or the sunlight.
CHLAMY
POLYTOMELLA