Identification and characterization of homologous neurons in sea slugs exhibiting analogous and non-analogous behaviors.
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1
Georgia State University, Neuroscience Institute, United States
Comparing the functions of homologous neurons in neural circuits across related species can provide insight into the evolution of behaviors. Melibe leonina and Dendronotus iris are two closely related nudibranch molluscs that exhibit similar rhythmic left-right (LR) swimming behaviors. However, there are important differences in the neural mechanisms underlying these analogous behaviors (Sakurai et al., Current Biology, 2011). The central pattern generator (CPG) for swimming in Melibe consists of the swim interneurons Si1 and Si2. Homologues of Si1 and Si2 are also found in Dendronotus. In contrast to Melibe, Si1Dend is not a member of the swim CPG; it has only a modulatory influence on the swim. In this study, we identified homologues of Si1 in three other Nudipleura molluscs and examined their role in swimming behavior.
Si1 can be uniquely identified in Melibe and Dendronotus using a suite of characteristics including its location on the dorsal-medial surface of the brain, its distinct axon projection and its immunoreactivity to the neuropeptide FMRFamide. As a potential additional characteristic, we tested if Si1 is GABA-immunoreactive in the species in this study and found this to not be the case. Using the shared morphological and neurochemical characteristics of Si1, we identified a putative homologue in the LR swimming species Flabellina iodinea. We found that similar to Melibe but unlike Dendronotus, the Si1 homologue in Flabellina is rhythmically active during the swim motor program and hyperpolarization of Si1Flab can terminate an ongoing swim. Strong depolarization of Si1Flab can initiate the swim motor pattern. Based on the current data, Si1Flab appears to be more similar to Melibe than Dendronotus.
We also identified the Si1 homologues in the nudibranch Tritonia diomedea and the more distantly related Pleurobranchomorpha Pleurobranchaea californica. These two species swim using dorsal-ventral (DV) flexions. We found that Si1Trit is not part of the DV swim CPG. Si1Trit appears to be inhibited by two known members of the DV swim CPG, the Dorsal Swim Interneurons (DSI) and Cerebral interneuron 2 (C2). Si1Trit to motor neuron synapses also show post-tetanic potentiation (PTP), which can be modulated and knocked down to basal levels by stimulation of the serotonergic DSIs. This is similar to what has been reported for a DV swim CPG neuron, VSI (Sakurai and Katz, J Neuroscience, 2009). Interestingly, the DSIs do not cause short-term potentiation of Si1Trit synapses. DSI-evoked transient potentiation, particularly of C2 synapses, has been shown to be necessary for the animal’s ability to swim.
Thus, homologues of Si1 can be identified across species regardless of the type of swimming behavior (LR or DV). Si1 homologues have different functions in the LR swim CPG of the closely related Melibe and Dendronotus. Similar to Melibe, Si1 is part of the LR swim circuit in Flabellina, suggesting that Si1 may be part of the ancestral LR swim circuit and the circuitry may have diverged in the Dendronotus lineage. These results lay the foundation for further studies on the role of homologous neurons in the evolution of analogous and non-analogous behaviors.
Supported by NSF- IOS-1120950.
Keywords:
Behavior,
evolution,
molluscs,
neural circuits
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster (but consider for student poster award)
Topic:
Evolution
Citation:
Gunaratne
CA and
Katz
PS
(2012). Identification and characterization of homologous neurons in sea slugs exhibiting analogous and non-analogous behaviors..
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00292
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Received:
30 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Ms. Charuni A Gunaratne, Georgia State University, Neuroscience Institute, Atlanta, GA, 30303, United States, cgunaratne@gmail.com