Researchers at Rensselaer Polytechnic Institute (RPI) have discovered a microscopic organism that can transform into a cannibalistic "supergiant" that drastically changes size, shape, and behavior, and abandons filter-feeding to hunt and consume their genetically identical relatives. The work, published on the cover of the Proceedings of the National Academy of Sciences (PNAS), demonstrates how single-celled organisms are capable of complex, regulated development, which scientists have largely only studied in multicellular animals. 

A cell that hunts its own kind 

The organism, Euplotes gigatrox, is a new species of ciliate collected from Caribbean Island of Curaçao inside of a seawater filtration system. In clonal populations of these organisms where every cell shares the same DNA, a small number of cells can spontaneously develop into supergiants more than twice the length of normal cells, with a broader body shape and a larger mouth. Rather than filter-feeding on bacteria as normal cells do, supergiants become raptorial predators, running over smaller clonal relatives to capture and swallow them whole, at a rate of roughly one prey every ten minutes. 

"This is a single cell doing something we usually associate with the development of animals," said Ben T. Larson, Ph.D., assistant professor in the Department of Biological Sciences at RPI and lead corresponding author of the study. "It expands our picture of what single-celled organisms are capable of, and gives us a new system for asking questions about how cells control their form and function.” 

The behavioral shift runs deeper than feeding alone. Normal cells walk across surfaces and swim gracefully along helical trajectories in fluid. Supergiants only walk, moving in circular paths suited to hunting surface-crawling prey, and tumble clumsily rather than swim when displaced from a surface. 

"Supergiant formation represents a tradeoff," Larson said. "These cells become better hunters but worse swimmers, shifting their trophic niche from feeding on bacteria to exploiting a completely different type of prey." 

A regulated developmental stage 

To investigate the molecular basis of the transformation, the team sequenced single-cell transcriptomes from normal cells, supergiants, and cells that had recently reverted from the supergiant state. The results showed that supergiants are a transcriptionally distinct developmental stage, with widespread differences in gene expression including cell cycle regulation, protein production, and membrane organization. 

Cells that revert from the supergiant state also carry a distinct molecular signature, one that appears to temporarily suppress the pathways driving transformation. Populations started from recently reverted cells produced new supergiants more slowly and at lower overall frequency than populations started from normal cells, regardless of external conditions. 

A new model for studying cell differentiation 

Supergiant formation tends to occur as populations transition from rapid growth to stationary phase, particularly when small prey is not too abundant, and they only persist while small prey remains scarce and large prey (normal cells) are present. Supergiants never exceed about five percent of the population, consistent with a bet-hedging strategy in which a small fraction of cells shifts to exploit a different resource. 

The findings provide a new framework for studying development in unicellular organisms, which must carry out all the functions of both a cell and an entire organism within a single membrane. 

"Most of what we know about development comes from animals," Larson said.  “We now have a system where we can study those same fundamental questions, as analogous developmental processes play out in a single-celled organism on a completely different branch of the tree of life."