How habitats shape movement—even for a fungus

You may not think of movement when you think about the lifespan of fungi. The fleshy mushrooms that sprout on a forest path or the mossy mold that engulfs a forgotten block of cheese present more as cryptic plants that reproduce by wafting their reproductive spores around like pollen. But some species of fungi, called chytrids, produce zoospores—independent, mobile, reproductive cells with flagella for swimming around their habitats.

Zoosporic fungi like chytrids are key components of aquatic food webs, acting as prey and breaking down decaying organic matter. A couple of species are infamous pathogens causing chytridiomycosis, an infectious disease decimating amphibians worldwide. Little is known, however, about how their spores’ ability to swim affects their dispersal, biogeographical range, or impact on their ecosystems.

A recent study from the University of Chicago tracked the swimming patterns of zoospores from 12 species of fungi and found two major swimming patterns that are driven by their inner cellular structure. The findings will help researchers understand how structures like flagella that are common across many organisms help those creatures evolve in, adapt to, and move through many different types of environments.

Jasmine Nirody, an Assistant Professor of Organismal Biology and Anatomy at UChicago and senior author of the new study published in Current Biology, studies the relationship between habitat and movements of all kinds of organisms, from fungal spores and tardigrades to reptiles and jumping spiders.

“The thing that draws me to any group of organisms is that they show a broad ecological range. They live in a bunch of different habitats, and they have a kind of morphological and behavioral diversity,” she said.

Zoosporic fungi are just that kind of organism. They come in a wide range of sizes. They live both in marine and freshwater environments, from the tropics to the poles, including the open ocean, lakes, mud—almost any aquatic environment possible. However, regardless of the environment they inhabit or their lifestyle, the zoospores from most species have a single long, flagellar filament, with a few structural tweaks that allow them to travel so widely.

Nirody’s co-authors Luis Javier Galindo and Thomas Richards from the University of Oxford took videos of the zoospores and saw two distinct movement patterns. Some simply swam in circles, while others had what they call a “random walk” pattern—move, stop, change direction, move again.

Originally, they thought the swimming patterns would be grouped by phylum—i.e. by fungi that were closely related to each other. One phylum did have all random walker zoospores, but the other was mixed. After using confocal microscopes to look at each type of zoospore more closely, the researchers saw structural differences: the random walkers had stiff tubulin ribs in their cytoplasm, whereas the circular swimmers did not.

“We realized that this correlation between movement and cellular structure made perfect sense. Those zoospores with a more ‘complex’ tubulin-based cytoskeleton are capable of performing a more ‘complex’ set of movements, like the random walk,” Galindo said.

After treating the different zoospores with drug compounds that depolymerized the tubulin structures, the random walk species changed to a circular pattern. “It was clear by then that the different walking pattern wasn't something that evolved once and then and then evolved away, but it was more related to their structure,” Nirody said.

“These results are exciting from an evolutionary and cell biology point of view,” Galindo said. “However, from a more applicable perspective, understanding the way in which these zoospores move is the first step to develop efficient motility inhibition methods that could be used as a strategy for controlling the spread of pathogenic chytrid species.”

It’s not clear yet what advantage the random walk pattern could lend to a zoospore over swimming in circles, or vice versa. Some of these species are sensitive to light, and Galindo is studying how this might interact with the cytoskeletal structure and influence swimming behavior. For Nirody, the remaining questions fit in the sweet spot of her research program.

“One of the things that we're really excited about is thinking about how different environments have shaped these guys’ motility patterns. What does that mean for the organism to be moving through their environment in circles or in a random pattern? What are the costs and tradeoffs?” she said. “The studies that I get most excited about are the ones that then pose 20 different questions after the fact, and then we have lots of stuff to do later.”