Project B1 (PIs Hentschel, Fraune, Roth) uses cross-host comparisons to study the molecular and cellular mechanisms that underlie the host differentiation between – and response to – native and nonnative microbes in marine metazoans. The project will also evaluate the routes of transmission and their influence on bacterial colonization in the next generation in the model systems sponge, Nematostella, and teleost fish. The project is joined by the new PI Roth, who enriches it with her unique focus on microbiota transmission during male pregnancy in pipefish, enabling a comparison of immune responses between vertebrates and invertebrates.