Madison’s serene lakes are home to a deadly, vampire-like hunter. Though sessile, it wields tentacle-like structures that first poison and immobilize prey, then act as straws to suck them dry.
This microscopic monster is the predatory ciliate protozoa Podophyra collini. While many single-celled organisms retain their original structures and proportions as they grow, P. collini follows a different strategy. It can increase or decrease its number of appendages (tentacle-like cilia) to suit its environment. In lean times, P. collini sports fewer, longer tentacles that probe the surrounding water in search of prey. When prey is plentiful, it rapidly produces dozens of new tentacles to maximize prey capture.
“Normally, when we think about how biological structures scale, we expect different features to change proportionally: a larger structure should have corresponding increases across all dimensions. But our observations reveal that P. collini follows a different scaling logic. Changes are biased toward increasing tentacle number rather than tentacle length. This allows the organism to efficiently adapt its predatory strategy to changing environments,” says Zhejing “Maggie” Xu, a graduate student in the Coyle Lab.
While P. collini’s predatory strategy may occur at the microscopic scale, figuring out how, why, and when single celled organisms adjust their tentacle number may have cost biochemistry professor Scott Coyle and Xu a few nights of sleep. In their recent publication in Current Biology, the researchers describe the underlying logic dictating P. collini’s morphological changes. Their findings may also hint at new ways that cellular engineers like Coyle can harness protozoan strategies for therapeutic treatments.
Coyle and Xu first watched P. collini under a microscope after a trip to Lake Mendota along UW–Madison’s campus. Their interest was piqued by the morphological differences among individuals. Some, Xu noted, had few long tentacles, while others had many short tentacles.
“Studying cells that look fundamentally different from our own and trying to figure out how they achieve that is an interesting way to explore possibilities in cellular engineering,” says Coyle. “Understanding how this organizing logic works can be used to understand natural systems, but it could also potentially be used to engineer similar kinds of behaviors in other cells if we understand how the cell organizes around these morphological changes.”
And P. collini fits that bill. Although mammalian cells can differentiate into different cell types during development, our morphology remains largely fixed. We don’t, for example, grow an extra set of arms to help balance a baby, a diaper bag, and a trunkful of groceries.
Coyle, Xu, and other researchers scanned hundreds of thousands of individual P. collini cells to determine the number and spatial distribution of their tentacles under different environmental conditions, including varying the amount of available prey. Their findings link tentacle length and number to food availability. When food was abundant, the protozoan hunter produced more, shorter tentacles to capture nearby prey. When food was scarce, it relied on fewer, longer tentacles to search its broader environment.
With this confirmation that P. collini undergoes morphological changes in response to environmental pressures, Coyle and Xu sought to understand the molecular changes underlying these transformations. In collaboration with the Weeks Lab, also in the Biochemistry Department, they analyzed changes to RNA and protein abundances found in cells with different morphologies to identify molecular signatures associated with morphological changes. From these experimental observations, Xu also built a model that can accurately predict P. collini’s morphological adaptation in response to varying resource levels.
“There is a lot of basic research to be done on this organism because the only literature you can find is back in the 1970s and 1980s. I really feel like I’m communicating through time, talking to people who did research 50 years ago. And then, someday, the next person will be communicating with my research when we have new technology that allows them to learn things I couldn’t ask about,” Xu says. “I think we’re in a protozoan renaissance, and we’re starting to appreciate how we can use these single, eukaryotic cells to understand basic cell behavior.”
Coyle describes more about the researchers’ biochemical investigation of P. collini‘s unique predatory strategy in this video for Current Biology:
Written by Renata Solan.