Gathering swarm Arielle Woznica

Bacterial secretions put protozoans in the mood for sex. This unexpected aphrodisiac may open the bedroom door for microbiologists to study sexual behaviour in many poorly understood species – and perhaps even in the earliest animals.

Until now, the choanoflagellate protozoan Salpingoeca rosetta had shown little interest in mating in lab cultures. A few years ago, a team led by Nicole King at the University of California, Berkeley, coaxed some into mating, though the process took 11 days and involved only a tiny fraction of the cells.

But when King’s student Arielle Woznica happened to add a common bacterium, Vibrio fischeri, to a culture, she was surprised to see the choanoflagellates form what appeared to be a mating swarm within a few hours.


More careful experiments showed that the choanoflagellates were indeed mating and producing genetically recombined offspring. In collaboration with chemists at Harvard University, the team eventually determined that a protein released by the bacteria was responsible for triggering the swarm. “To our knowledge, it’s the first evidence of bacteria regulating mating,” says King.

Eros protein

The protein, which the researchers have dubbed EroS, is an enzyme that cuts sugar groups off protein molecules on the surface of the choanoflagellates. The bacteria are probably harvesting these sugars for energy, says King.

The choanoflagellates, for their part, may use high concentrations of EroS as a sign that conditions are suitable for sexual reproduction — though why that is so remains unknown.

If other protozoans also use environmental cues to trigger mating, this may explain why so few of them exhibit sexual behaviour in lab cultures. “It may be that they require some signal that we don’t know about,” says Joseph Heitman at Duke University in Durham, North Carolina. If so, many species now thought to be entirely asexual may actually be sexual under the right conditions. “I think this will stimulate a lot of further work.”

The discovery might have a further implication, King speculates. Modern multicellular animals probably evolved from an ancestor very similar to a choanoflagellate, and animal sperm use enzymes similar to EroS to identify and penetrate eggs of the right species.

What began as a mating signal could have been co-opted in animals for an even more important role. “That’s the extreme possibility,” says King.

Journal reference: bioRxiv, DOI: 10.1101/139022