Title: Studies of Caenorhabditis elegans neuronal cell fate
Description: The specification and development of nervous system diversity is a driving question in the field of Neurobiology. The overarching goals of the projects described in this thesis are to describe tools to aid in the description of nervous system development and to show the use of the described tools to study nervous system development in the nematode Caenorhabditis elegans. The first chapter of this thesis describes a complete map of the male C. elegans nervous system using a tool developed in the lab to uniquely label all neurons in the C. elegans nervous system, NeuroPAL. The second chapter of this thesis largely focuses on a well-studied homeobox gene, unc-86, and its role in fate transformations in dopaminergic and GABAergic neuron types. These two seemingly disparate projects are united in their effort to investigate nervous system development and neuronal fate determination. NeuroPAL is a multicolor transgene that uniquely labels all neurons of the C. elegans hermaphrodite nervous system and here I show it can be used to disambiguate all 93 neurons of the male-specific nervous system.

I demonstrate the wide utility of NeuroPAL to visualize and characterize numerous features of the male-specific nervous system, including mapping the expression of gfp-tagged reporter genes and neuron fate analysis. NeuroPAL can be used in combination with any gfp-tagged reporters to unambiguously map the expression of any gene of interest in the male, or hermaphrodite, nervous system. Furthermore, NeuroPAL is used in mutants of several developmental patterning genes to confirm previously described defects in neuronal identity acquisition. Additionally, I show that NeuroPAL can be used to uncover novel neuronal fate losses and identity transformations in these mutants because of the unique labeling of every neuron. Lastly, we show that even though the male-specific neurons are generated throughout all four larval stages, the neurons only terminally differentiate in the fourth and final larval stage, termed ‘just-in-time’ differentiation. In the second part of this thesis, I describe a few examples of mutant analysis of homeobox gene family members and describe their function in the C. elegans nervous system.

I focus largely on a couple potential examples of homeotic fate transformations in mutants of the POU homeobox gene, unc-86. In unc-86 mutants, I describe the ectopic expression of multiple GABAergic terminal identity features in one cell in the head of C. elegans. I raise the hypothesis that this cell may be a transformation of a non-GABAergic ring interneuron, RIH, into that of its GABAergic sister cell, AVL, in unc-86 mutants. While ectopic dopaminergic neurons were previously described in unc-86 mutants, I expand the study to show the ectopic expression of all dopaminergic synthesis and packaging genes. I show support that all non-dopaminergic anterior deirid neurons, ADA, AIZ, FLP, and RMG, lose the expression of some of their wild type terminal fate genes and transform to a fate like that of their dopaminergic sister cell, ADE, as assessed by NeuroPAL expression. Taken together, these studies describe tools and methods for studying nervous system development as well as describe many examples of cell fate transformations.