Host lipids regulate multicellular behavior of a predator of a human pathogen.

As symbionts of animals, microbial eukaryotes benefit and harm their hosts in myriad ways. A model microeukaryote () is a symbiont of snails and may prevent transmission of parasitic schistosomes from snails to humans. However, it is unclear which host factors determine s ability to colonize snails. Here, we discovered that forms multicellular aggregates when exposed to snail hemolymph. We identified a molecular cue for aggregation: a hemolymph-derived phosphatidylcholine, which becomes elevated in schistosome-infected snails. Therefore, aggregation may be a response to the physiological state of its host, and it may determine its ability to colonize snails and exclude parasitic schistosomes. Furthermore, is an evolutionary model organism whose aggregation may be ancestral to animals. This discovery, that a prevalent lipid induces multicellularity, suggests that this aggregation phenotype may be ancient. Additionally, the specific lipid will be a useful tool for further aggregation studies.