NEWCOMB, J.M.**; KATZ, P.S.; Georgia State University, Atlanta; Georgia State University, Atlanta: Patterns of neural evolution in nudibranch molluscs: functional divergence of homologous neurons embedded in a common neural network

We are examining the evolution of central pattern generators (CPGs) by comparing the functions of homologues of two identified neurons, Swim Interneuron 1 (Si1) and the Dorsal Swim Interneurons (DSIs), in nudibranch molluscs that exhibit a variety of locomotor behaviors. Si1 is a member of the swim CPG in Melibe leonina, which swims by flexing its body from side-to-side (i.e. laterally). Depolarization of Si1 can elicit a swim motor pattern (SMP) in a quiescent preparation. Surprisingly, the homologue of Si1 in another lateral-swimming species, Dendronotus iris, was not part of the swim CPG, although it was capable of eliciting an SMP. A similar partial divergence of function was observed with the DSIs and their homologues. The serotonergic DSIs are members of the swim CPG in Tritonia diomedea, which swims by alternately flexing its body in the dorsal and ventral directions. Depolarization of a DSI can elicit an SMP and the DSIs also excite crawling neurons. We have identified homologues of the DSIs in 7 species that do not exhibit dorsal-ventral swimming and have named these neurons Cerebral Serotonergic Posterior (CSP) cells. In contrast to the DSIs, which fire rhythmic bursts of action potentials during a Tritonia SMP, the CSPs in all 7 species were not rhythmically active. However, CSPs in the lateral-swimming Melibe could elicit an SMP and CSPs in two non-swimming species, Tochuina tetraquetra and Triopha catalinae, excited putative crawling neurons. Thus, homologous identified neurons can diverge significantly in certain functions while retaining other common functions. This suggests that these species share a common neural network that has been reconfigured repeatedly.