Paul Katz, PhD
Professor and Director
Department of Neuroscience
University of Massachusetts, Amherst
January 8, 2019
Phylogenetic and Individual Variability of Neural Circuits Underlying Swimming Behaviors in Sea Slugs
Studies on the neural basis of behavior tend to focus on certain well-known model species. While this has led to a broad body of knowledge that can be extrapolated from, it does not enable a study of how behaviors, and the neural circuits involved, evolved. To understand the how circuits may differ due to evolutionary change, a comparison between related species is necessary. Dr. Katz discussed the work of his lab on several species of sea slug, which have easily identifiable and simple nervous systems. They have found that while neural circuits governing swimming behavior are similar between species, details of the circuits differ. These differences highlight the role evolution can play in how neural circuits form.
Darwin puzzled over the origin of behavior. However, Darwin had little knowledge about how nervous systems produce behavior. Modern neuroscience has made great strides in understanding the neural basis of behavior but has focused on a limited number of species. To understand how behaviors and neural circuits evolved, it is necessary to examine more closely related species to determine similarities and differences.
We have been studying the neural basis of swimming behaviors in several species of nudipleura mollusks. These sea slugs have brains with a relatively small number of neurons, many of which are individually identifiable. Furthermore, the same neurons can be identified across species, allowing neural circuits to be compared.
We have studied two distinct forms of swimming that each evolved independently several times. We found convergent evolution in the use of particular neurons and even in the expression of orthologous serotonin receptors. However, details of the circuitry differ. We have also found examples of divergence of the neural circuitry while the homologous behavior is conserved. This suggests that behavior and neural circuitry represent separable levels of hierarchical organization that can have independent evolutionary histories.
The basis for selection is individual variability. Although we have not examined heritable differences between individuals, we have found that there are substantial differences in the strengths of synapses that do not affect the behavior of the animal unless the system is challenged with a lesion. This suggests a mechanism for neural circuit drift that could preserve the behavior while the underlying circuit shifts. We have also found that substantial changes in the functional circuit can occur rapidly (within a day) due to variation in the expression of serotonin receptors.
In summary, it is essential to compare species in order to understand which components of the circuit are essential and which are subject to evolutionary change.