Who's who in multicellularity. Self-nonself recognition in multicellular organisms that form chimeras. (a) Cryphonectria parasitica, the causal agent of chestnut blight. Hyphal fusion is restricted to strains that match at all vegetative incompatibility loci [71, 72]. Incompatible reactions result in localized cell death and the formation of a barrage zone. Of the six pairings shown, only two (bottom right) are compatible. (b) In the marine tunicate Botryllus schlosseri, fusion or rejection is controlled by a highly polymorphic locus containing multiple immunoglobulin domains (FuHC) . A single population can contain hundreds of different alleles. (c) In the social amoeba Dictyostelium discoideum, thousands of cells aggregate (far left) and subsequently develop into a fruiting body (far right). Cells of genetically different individuals can partially separate during multicellular development . C. parasitica photo by Kent Loeffler, courtesy of the Department of Plant Pathology and Plant-Microbe Biology at Cornell University. B. schlosseri photo courtesy of Tony de Tomaso, University of California, Santa Barbara. D. discoideum, courtesy of Gerda Saxer, Rice University.