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Different roles for homologous interneurons in species exhibiting similar rhythmic behaviors.


Sakurai A, Newcomb JM, Lillvis JL, Katz PS. Different roles for homologous

interneurons in species exhibiting similar rhythmic behaviors. Curr Biol. 2011

Jun 21;21(12):1036-43. Epub 2011 May 27. PubMed PMID: 21620707.


It is generally assumed that two closely-related species would use similar neural mechanisms to produce similar behavior.  That was shown not to be the case in sea slug swimming in this paper by Akira Sakurai, Jim Newcomb, Josh Lillvis, and Paul Katz.  They found that although two nudibranch species called Melibe leonina and Dendronotus iris contain homologous neurons, they were using those neurons differently to produce a very similar swimming behavior. 


Sea slugs have very large neurons that can be identified from individual to individual within a species by anatomical, neurochemical, and electrophysiological properties.  Those same properties can be used to identify neurons across species. 


It was shown that in Melibe, two neurons, called simply swim interneurons 1 and 2 (Si1 and Si2) form a neural circuit called a Central Pattern Generator (CPG) that generates a rhythmic pattern of neuronal discharges that drives the alternating left-right body flexions that characterize the swim.  Here, it was found that in Dendronotus, Si2 was part of the swim CPG, but Si1 was not.  The paper characterizes the synaptic connectivity of these neurons and shows how each neuron affects the motor output of the CPG.


This work, which was highlighted in the Journal of Experimental Biology ( has implications for understanding the neural basis of other behaviors; it serves as a cautionary tale by showing that similar behavior does not predict the neural mechanism.  The Katz lab is currently examining other aspects of these two circuits and comparing them to neural circuits in other nudibranchs.  Work in the lab is funded by grants from the National Science Foundation.