Animals first diverged from their unicellular ancestors in oceans dominated by bacteria, and have lived in close association with bacteria ever since. Interactions with bacteria critically shape diverse aspects of animal biology today, including developmental processes that were long thought to be autonomous. Yet, the multicellularity of animals and the often-complex communities of bacteria with which they are associated make it challenging to characterize the mechanisms underlying many bacterial-animal interactions. Thus, developing experimentally tractable host-microbe model systems will be essential for revealing the molecules and mechanisms by which bacteria influence animal development.

The choanoflagellate Salpingoeca rosetta, one of the closest living relatives of animals, has emerged as an attractive model for studying host-microbe interactions. Like all choanoflagellates, S. rosetta feeds on bacteria; however, we have found that interactions between S. rosetta and bacteria extend beyond those of predator and prey. In fact, two key transitions in the life history of S. rosetta, multicellular “rosette” development and sexual reproduction, are regulated by environmental bacteria.

The experimental tractability of S. rosetta allowed us to characterize the molecules and regulatory logic underpinning the bacterial regulation of rosette development (Chapters 2 and 3). We found that the bacterium Algoriphagus machipongonensis produces three classes of structurally distinct lipids that are interpreted by S. rosetta as activators, synergistic enhancers, and inhibitors of rosette development. Although activating sulfonolipid RIFs (Rosette Inducing Factors) elicited relatively low levels of rosette development, the combined activity of the RIFs and synergizing lysophosphatidylethanolamines (LPEs; which alone had no detectable activity) was sufficient to fully recapitulate the rosette-inducing activity of Algoriphagus bacteria. Moreover, we identified a potent antagonist of the RIFs, IOR-1 (Inhibitor of Rosettes), but found that the synergistic activities of the RIFs and the LPEs overcame the inhibitory activities of IOR-1. We hypothesize that the integration of multiple activating, enhancing, and inhibitory bacterial cues act to ensure that rosette development is not initiated under the wrong environmental conditions.

Until recently, bacteria were not known to influence any life history transition in S. rosetta other than rosette development. We serendipitously discovered that the bacterium Vibrio fischeri produces an “aphrodisiac” that regulates sexual reproduction in S. rosetta (Chapter 4). To our knowledge, the interaction between Vibrio and S. rosetta is the first known example of bacteria regulating mating in a eukaryote. After observing that S. rosetta cells aggregate into large swarms in response to Vibrio bacteria, we demonstrated that swarming, a behavior that had not been previously observed in choanoflagellates, was a prelude to sexual fusion. We next found that Vibrio secreted a chondroitinase aphrodisiac (EroS) that depolymerized chondroitin sulfate, a glycosaminoglycan previously thought to be restricted to animals, in the S. rosetta extracellular matrix. Finally, we determined mating in S. rosetta was triggered by low cell densities of Vibrio bacteria, and picomolar concentrations of EroS (as well as other bacterial chondroitinases), indicating that bacteria could plausibly trigger S. rosetta swarming and mating in the environment. We predict that the presence of chondroitinase-producing bacteria may indicate environmental factors that favor mating in S. rosetta.