Evolution tunes the excitability of individual neurons

The relationship between the genome and the evolution of the nervous system may differ between an animal like C, elegans with 302 neurons, and mammals with tens of billions of neurons. Here we report that a class of nonconser ved potassium channels highly expanded in C, elegans may play a special rol e in the evolution of its nervous system. The C, elegans genome contains an extended gene family of potassium channels whose members fall into two evo lutionary divergent classes. One class constitutes an ancient conserved "se t" of K+ channels with orthologues in both humans and Drosophila(1,24) and a second larger class made up of rapidly evolving genes unique to C, elegan s.(24) Chief among this second class are novel potassium channels having fo ur transmembrane domains per subunit(6,8,14,20) that function as regulated leak conductances to modulate cell electrical excitability. This inventory of novel potassium channels is far larger in C. elegans than in humans or D rosophila, We found that, unlike conserved channel genes, the majority of t hese genes are expressed in very few cells. We also identified DNA enhancer elements associated with these genes that direct gene expression to indivi dual neurons. We conclude that C, elegans may maintain an exceptionally lar ge inventory of these channels (as well as ligand-gated channels') as an ad aptive mechanism to "fine tune" individual neurons, making the most of its limited circuitry. (C) 2001 IBRO. Published by Elsevier Science Ltd. All ri ghts reserved.