To Fuse or Not to Fuse? An Evolutionary View of Self-Recognition Systems

Self-recognition systems preventing chimera formation following somatic fusion between members of the same species have evolved only in certain phyla (e.g., fungi, cnidarians, poriferans, bryozoans, urochordates). We present here some of the biological features common to fungi and colonial marine invertebrates, which may have driven the evolution of such self-recognition systems. We conclude that the evolution of self-recognition mechanisms in fusible organisms is more likely to result from a complex trade-off between selection pressures linked with a gregariousness and sessile way of life. This trade-off also raises the question of how extrinsic interactions between a group of cooperative cells and its direct environment may have driven the evolution towards a form of individuality, via the emergence self-recognition systems in fusible organisms. Gregariousness and sessility being two features intrinsically associated emergence of multicellularity, somatic incompatibility systems could then be considered as one of the first expressions of individuality in early multicellular organisms