Evolution of pharyngeal behaviors and neuronal functions in free-living soil nematodes

To explore the use of Caenorhabditis elegans and related nematodes for studying behavioral evolution, we conducted a comparative study of pharyngeal behaviors and neuronal regulation in free-living soil nematodes. The pharynx is divided into three parts: corpus, isthmus and terminal bulb, and pharyngeal behaviors consist of stereotyped patterns of two motions: pumping and peristalsis. Based on an outgroup species, Teratocephalus lirellus, the ancestral pattern of pharyngeal behaviors consisted of corpus pumping, isthmus peristalsis and terminal bulb pumping, each occurring independently. Whereas corpus pumping remained largely conserved, isthmus and terminal bulb behaviors evolved extensively from the ancestral pattern in the four major free-living soil nematode families. In the Rhabditidae family, which includes Caenorhabditis elegans, the anterior isthmus switched from peristalsis to pumping, and anterior isthmus and terminal bulb pumping became coupled to corpus pumping. In the Diplogasteridae family, the terminal bulb switched from pumping to peristalsis, and isthmus and terminal bulb became coupled for peristalsis. In the Cephalobidae family, isthmus peristalsis and terminal bulb pumping became coupled. And in the Panagrolaimidae family, the posterior isthmus switched from peristalsis to pumping. Along with these behavioral changes, we also found differences in the neuronal regulation of isthmus and terminal bulb behaviors. M2, a neuron that has no detectable function in C. elegans, stimulated anterior isthmus peristalsis in the Panagrolaimidae. Further, M4 was an important excitatory neuron in each family, but its exact downstream function varied between stimulation of posterior isthmus peristalsis in the Rhabditidae, isthmus/terminal bulb peristalsis in the Diplogasteridae, isthmus peristalsis and terminal bulb pumping in the Cephalobidae, and posterior isthmus/terminal bulb pumping in the Panagrolaimidae. In the Rhabditidae family, although M4 normally has no effect on the terminal bulb, we found that M4 can stimulate the terminal bulb in C. elegans if the Ca2+-activated K+ channel SLO-1 is inactivated. C. elegans slo-1 mutants have generally increased neurotransmission, and in slo-1 mutants we found novel electropharyngeogram signals and increased pumping rates that suggested activation of M4-terminal bulb synapses. Thus, we suggest that the lack of M4-terminal bulb stimulations in C. elegans and the Rhabditidae family evolved by changes in synaptic transmission. Altogether, we found behavioral and neuronal differences in the isthmus and terminal bulb of free-living soil nematodes, and we examined potential underlying mechanisms of one aspect of M4 evolution. Our results suggest the utility of Caenorhabditis elegans and related nematodes for studying behavioral evolution.