Project Summary / Abstract Capsaspora owczarzaki is a protozoan that may control a neglected tropical disease and reveal the earliest animal cell-cell signaling mechanisms. However, little is known about this organism at a chemical level. Capsas- pora lives inside Biomphalaria snails (the vectors that transmit schistosome worms that cause schistosomiasis). Capsaspora hunts and kills schistosomes, making it a promising biocontrol agent. However, no one knows which molecules Capsaspora senses to colonize its snail host, nor how it senses its schistosome prey. Furthermore, Capsaspora is one of the closest living relatives of animals, with which it shares signaling and adhesion genes. It exhibits reversible aggregation and chemotaxis, reminiscent of human neural crest cells, immune cells, and metastatic cancer cells. Therefore, it is a phylogenetically relevant model to study how multicellular phenotypes in animals evolved and act in healthy and disease states. However, no one has found which molecules regulate Capsaspora’s adhesion and chemotaxis phenotypes to determine if its mechanisms are conserved with those in animals. For Capsaspora to reach its potential as a biocontrol agent and to reveal insight into the evolution of animal multicellularity, its mechanisms of chemical signaling must be uncovered. The proposed research program will discover the molecular mechanisms that regulate two phenotypes, each of which is relevant both to killing schistosomes and to understanding the evolution of multicellular traits in animals. First, Capsaspora forms aggregates in response to snail serum, which is notable for two reasons: (1) it is the only Capsaspora response to a snail host factor, presumably enabling Capsaspora to recognize its snail host environment, and (2) it is the only regulated cell-cell adhesion process in Capsaspora, which is the only non- animal with complete integrin and cadherin adhesion complexes. Therefore, discovery of the mechanism of se- rum-induced aggregation may reveal both how Capsaspora colonizes its host and whether or not regulated animal cellular adhesion mechanisms were active in animals’ common ancestor. Second, Capsaspora migrates toward schistosome prey, which is notable for two reasons: (1) it is the only known avenue by which Capsaspora senses schistosomes, and (2) it is the only reported chemotaxis behavior in the closest relatives of animals (the holozoans). Therefore, discovery of the chemoattractant and response may reveal how Capsaspora hunts schis- tosomes and whether or not animal chemotaxis mechanisms were active in animals’ common ancestor. In both projects, the cue molecule and the Capsaspora receptor will be identified. Subsequent work will decipher how the signal is transduced. Finally, the significance of these phenotypes for snail colonization and schistosome killing will be tested in vivo, and the evolutionary conservation of these signaling mechanisms with present-day animals will be examined. Discover...