Presentation Detail

Molecular Evolution

Liebeskind, Benjamin [1], Hillis, David [2], Zakon, Harold [2].

Sodium Channels and the Origin(s) of Animal Behavior.

Animal behavior arises from an electric neural code. Exquisitely tuned voltage impulses, called action potentials, are propagated along excitable cells, transmitting signals over large distances and coordinating cellular activity. Action potentials are present in all animal phyla, even sponges, which lack a nervous system (Leys1999), but the speed and complexity of the neural code vary greatly over the animal kingdom. The genomics age offers a new window into the origins of the neural code: the molecular evolution of the proteins that underlie action potentials.Voltage impulses in nerve and muscle are carried by ion channel proteins that catalyze the rapid movement of ionic charges across cell membranes. Three properties of ion channels make the action potential possible. First, channels are selective for certain ion species, and the channel's function is determined by the permeating ion. Second, they are activated by changes in membrane voltage - they are "gated" by voltage - and can therefore carry a generative impulse along the excitable cell. Third, they close under appropriate conditions, allowing the process tostart over. The main upstroke of action potentials is carried by sodium-selective channels (Hodgkin and Huxley 1952). These channels let sodium ions flow into the cell, depolarizing it, and carrying the regenerative action potential along neurons. They then close down at the height of the action potential. This self-limiting property helps give the action potential its characteristic "spike." Using comparative genomics and phylogenetics, we have shown that sodium channels evolved long before the evolution of animal multi-cellularity, as evidenced by the presence of sodium channels in the unicellular sister group to animals, the choanoflagellates (Liebeskind 2011). Our study traces the evolutionary history of the key components of sodium channel function: selectivity, voltage activation, and inactivation. We provide evidence for the adaptive significance of sodium channels in the evolution of animal complexity,and in the multiple origins of complex nervous systems. In particular, we find that early sodium channel homologs were likely selective for calcium. Convergent amino acid substitutions in the ion selectivity filter created sodium-selective channels in cnidarians and early bilaterians (Liebeskind 2011, Gur-Barzilai 2012). Because calcium is toxic to cells at high concentration but sodium is not, this may have allowed the expansion of the neural code, perhaps fostering the convergent evolution of nervous system complexity in these two groups.

1 - University of Texas at Austin, Integrative Biology, Patterson Labs 140, 2401 Speedway, Austin, TX, 78712, USA
2 -

Keywords:
Sodium Channel
Molecular Evolution
Choanoflagellate.

Presentation Type: Regular Oral Presentation
Session: 135
Location: Alpine A and B/Snowbird Center
Date: Tuesday, June 25th, 2013
Time: 11:15 AM
Number: 135004
Abstract ID:146
Candidate for Awards:W.D. Hamilton Award for Outstanding Student Presentation