That Stinging Sensation: Modularity and the Origin of the Stinging Cell.

All cells arise through division of other cells; thus, cells with new functions (novel cell types) must evolve from ancestral cells with a different function. How, then, do novel cell types arise? Each cell is a composite of many functions that, together, confer a cell's phenotype. A single cell might have both the capacity to sense the environment and to secrete a specialized product. Allowing those two phenotypic modules to vary independently enables the diversification of groups of cells that either secrete the same product in response to diverse environmental cues or that secrete different products in response to the same cue. In this review, I summarize the shared and unique phenotypic modules that define two types of secretory cells in cnidarians (corals, jellyfish, and their kin): mechanosensory neurons and cnidocytes (stinging cells). I then propose a series of discrete changes that could have driven the origin of a cnidocyte from an ancestral cell that looked much like a modern mechanosensory neuron. I argue that modeling cell type diversification in this way-by gain, loss, and modification of existing phenotype modules-is useful for interpreting patterns of shared gene expression across related cell types and for predicting how new cell types could arise.