Algal Architecture What studying Chlamydomonas reinhardtii can teach us about cellular structure and movement By Ana Wang Much of our understanding of human biology comes from observing and studying other organisms. It can be difficult to imagine that a unicellular alga that has features resembling both animals and plants; that can live in soil, lakes, and snow; and that can grow in the light or dark, is being used to better understand our own biology. It’s easy to overlook Chlamydomonas reinhardtii (literally - it’s tiny!), but by studying both what we have in common with this alga and what makes it unique, scientists shed new light on the range of what is biologically possible and new ways of discovering it. Let’s start with what we have in common. Chlamydomonas (chlamy, for short) has two prominent antennae-like structures that protrude from the cell body, called cilia. Cilia are present on all eukaryotic cells, and they are essential for cell sensing and cell movement. Defects in cilia lead to a range of human diseases, and scientists have been studying chlamy’s cilia since the 1900s as a means to better understand how they are formed, how they function, and what happens when things go awry. These studies, as well as those done in the so-called “model organisms”, all suggested that cilia are primarily composed of and regulated by microtubules. Microtubules are one of the main components of the cytoskeleton, the network of proteins that provides structure and shape to cells. Due to the near-universality of ciliary composition and function amongst eukaryotes, information gained from studying chlamy’s cilia and structural proteins can be extrapolated and applied to all eukaryotic cell types. But if all eukaryotes have cytoskeletons and cilia, then you might be wondering what makes chlamy so special? I n the 1990s, researchers discovered that chlamy has two different genes for actin, an exceedingly rare occurrence in a single-celled organism. Actin is one of the other major components of the cytoskeleton, but despite being critical to many cell functions, scientists did not think that it had a role in cilia, which was the domain of microtubules. This is where the unique biology of chlamy comes in: One of chlamy’s actin genes looks almost exactly like the human gene, but the other is wildly divergent. Surprisingly, this second gene can compensate almost entirely for conventional actin! The presence of these two actin genes allowed scientists to tease apart actin’s many roles in ciliary function in ways that had never been accessible before, revealing that actin is indeed a key player in growth of cilia. These results are transforming the way cell biologists think about a protein that has been studied for many decades. Excitingly, they are also shaking up our understanding of the cilia, as roles for actin in ciliary function in other species, including humans, are now coming to light. The parts of chlamy that make it unique are just as important as the parts of it that are conserved. Diverse organisms are not only useful for serving as platforms to discover new biological mechanisms and principles - they are also key to uncovering new insights into established fields. Even if a newly discovered protein or function is not present in humans, as is the case with the divergent form of actin in chlamy, such discoveries can provide opportunities for biologists to explore unique, creative solutions to evolutionary problems. Chlamy, though small in size, plays a big part in helping us understand the fundamentals of actin and cytoskeleton biology.