Evolution of Pharyngeal Feeding Behaviors in Free-Living Soil Nematodes
Chiang, Jing-Tzyh Alan
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To explore using nematodes as a model for studying behavioral evolution, I examined pharyngeal behaviors in free-living soil nematodes related to Caenorhabditis elegans. The nematode pharynx is divided into three regions: corpus, isthmus, and terminal bulb. Pharyngeal behaviors consist of stereotyped patterns of two motions: pumping and peristalses. I observed pharyngeal behaviors in multiple species, and constructed the following evolutionary model. In the ancestor of free-living soil nematodes, the pharynx had corpus pumping, isthmus peristalses, and terminal bulb pumping, each of which occurred independently. In the Rhabditidae family, the anterior isthmus switched to pumping, and anterior isthmus and terminal bulb pumping became coupled to corpus pumping. In the Diplogasteridae family, the terminal bulb switched to peristalses. In the Cephalobidae family, isthmus peristalses and terminal bulb pumping became coupled. And in the Panagrolaimidae family, the posterior isthmus switched to pumping. The above changes in isthmus and TB behaviors suggested corresponding changes in their neuronal regulation. Using laser ablations, I found that M4's function evolved significantly: M4 stimulated posterior isthmus peristalsis (Rhabditidae), isthmus/terminal bulb peristalsis (Diplogasteridae), isthmus peristalsis and terminal bulb pumping (Cephalobidae), and posterior isthmus/terminal bulb pumping (Panagrolaimidae). Yet, increased food activated M4 activity in all families. Thus, M4 appeared to be a general "food sensor" neuron, which was co-opted during evolution to perform different downstream functions. Additionally, M2 stimulated anterior isthmus peristalsis in the Panagrolaimidae. Using Caenorhabditis elegans, I investigated possible molecular/genetic causes for the above changes. Why is the terminal bulb unaffected by M4 in Caenorhabditis elegans and the Rhabditidae? I found that mutating slo-1 activated M4-terminal bulb stimulations, suggesting that alterations in synaptic transmission silenced M4-terminal bulb synapses. Also, why does M2 stimulate peristalses in the Panagrolaimidae, and M4 in the other families? I found ceh-28 important for M4 to stimulate peristalsis in Caenorhabditis elegans, but M2 also had the potential for ceh-28 expression. This suggested a genetic/molecular link between the two neurons, and ceh-28 related mechanisms may determine which neuron stimulates peristalsis. Overall, I characterized how pharyngeal behaviors evolved at the behavioral, neuronal, and genetic levels. These results suggested the utility of nematodes for studying behavioral evolution.