|
Description:
|
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 . |