Evolutionary assembly of cooperating cell types in an animal chemical defense system
A long-standing challenge in biology is explaining how the functions of multicellular organs emerge from the underlying evolution of cell types. We deconstructed evolution of an organ novelty: a rove beetle gland that secretes a defensive cocktail. We show that gland function was pieced together via assembly of two cell types that manufacture distinct compounds. One cell type forms a chemical reservoir in the beetles abdomen and produces alkane and ester compounds. We demonstrate that this cell type is a hybrid of cuticle cells and ancient pheromone and adipocyte-like cells, and executes its function via a mosaic of enzymes sourced from each parental cell type. The second cell type synthesizes noxious benzoquinones using a chimeric pathway derived from conserved cellular energy and cuticle formation pathways. We present evidence that evolution of each cell type was shaped by coevolution between the two cell types: the benzoquinones produced by the second cell type dissolve in solvents produced by the first, yielding a potent secretion that confers adaptive value onto the gland as a whole. Our findings illustrate how cooperation between cell types can arise, generating new, organ-level behaviors.