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Micrograph of Salpingoeca rosetta
Salpingoeca rosetta

The researchers were simply seeking bacteria that would stimulate Salpingoeca rosetta to form the rosette-shaped colonies that give them their name. What they found was a bacterium that motivated S. rosetta to have sex.

“The last thing anyone expected was an aphrodisiac,” says Jon Clardy, the Hsien Wu and Daisy Yen Wu Professor of Biological Chemistry and Molecular Pharmacology at HMS.

The discovery, published August 31 online in Cell, is the first known instance of bacteria-induced mating in eukaryotes, organisms with membrane-enclosed nuclei and other organelles. The finding may provide insight into broader questions of how bacteria influence the development and behavior of all eukaryotes, including plants and animals.

“An appreciation of the importance of symbioses with bacteria, such as the microbiome, is growing,” says J. P. Gerdt, a research fellow in the Clardy lab and co-first author of the study, “but I had never heard of anything like a bacterium that induces sexual fusion.”

The study also revealed that the primitive S. rosetta, which are single-cell saltwater dwellers that are the closest living relatives of animals, can make chondroitin, a molecule scientists thought arose much later in the evolutionary tree.

For the current work, Nicole King of the University of California, Berkeley, who shares senior authorship with Clardy on the study, added the bioluminescent marine bacterium Vibrio fischeri to dishes of S. rosetta, hoping to trigger rosette formation. But no rosettes formed. Instead, the tiny

S. rosetta flagellates propelled themselves into swarms of up to 50 cells each—and began to mate.

What caused this to occur? Tests in Clardy’s lab narrowed the candidates to a single protein not previously known to science. The researchers dubbed it EroS, short for Extracellular regulator of Sex.

Further experiments revealed that EroS is a chondroitinase—an enzyme that breaks down chondroitin. When the researchers peered deeper, they found that S. rosetta makes chondroitin sulfate, a chain of sugars best known for forming part of the structure of cartilage. The serendipitous discovery changes the evolutionary timeline of chondroitin development.

The researchers think their work could illuminate the evolutionary history of bacterial symbioses and help them gain a deeper understanding of how humans are affected by bacteria today.  

Image: Courtesy of Ariella Woznica, Nicole King lab, University of California, Berkeley 

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