FETCHO, J.R.; Cornell University: The comparative biology of spinal motor networks in zebrafish

After decades of work, the spinal motor networks for swimming and escape in fishes and amphibians are among the best understood motor circuits in vertebrates. These circuits in anamniotes have much in common, but linking their cell types to the circuits in amniotic vertebrates such birds and mammals has been difficult. Recently a better understanding of the development of spinal cord is allowing us to link cell types broadly among vertebrates and to infer the likely evolutionary changes in networks. The development of spinal interneurons is directed by transcription factors that serve as markers of cell types across species. One good example is the neurons marked by the transcription factor engrailed-1 in different species. The engrailed-1 positive cells in zebrafish are a homogeneous multifunctional population of glycinergic ascending inhibitory interneurons that are rhythmically active during swimming and that play roles in sensory gating, as well as in shaping motor output through monosynaptic inhibitory connections with motoneurons and interneurons. Engrailed neurons in Xenopus tadpoles are strikingly similar in structure and function to those zebrafish. In mammals, the engrailed cells are morphologically similar to the zebrafish and Xenopus neurons early in life, but later form a functionally more heterogeneous population. Some of the engrailed positive neurons in mammals, such as the long-studied Renshaw cells, play a role that seems to reflect just one of the several roles of individual engrailed cells in zebrafish. The evidence suggests that a primitive multifunctional interneuronal class like that in anamniotes maybe have given rise to several more discrete populations of interneurons in amniotes, each with a more restricted functional role. The unification of cell types across species by transcription factors thus allows simple models such as zebrafish to inform studies of vertebrates more broadly.