Title: Sexually Dimorphic Development of the Caenorhabditis elegans Nervous System
Description: Sexual reproduction is an evolutionary innovation that arose 1.2 billion years ago, and in that time, has allowed a rapid diversification of species outpacing that of asexually reproducing organisms. Successful sexual reproduction in animals requires the incredible coordination of complex genetic and behavioral factors; from the most fundamental levels of ensuring correct chromosome segregation and ploidy to the most complex of behavioral mating rituals, any failure can result in a complete loss of evolutionary fitness. In this thesis, I have explored the developmental programs that function to ensure somatic sex determination, sexual differentiation, and mating behaviors in C. elegans. C. elegans is an androdiecious nematode species that has been extensively characterized in regard to the sexual dimorphism of its development, nervous system, and behavioral outputs. Sex determination pathways are widely diverged across phyla, and C. elegans has coopted a Gli family transcription factor to serve as a cell autonomous global regulator of somatic sex determination.

I investigated the expression of this transcription factor, tra-1, with cellular, subcellular, sex-specific, and temporal resolution in both sexes of C. elegans and found that it is dynamically regulated to control sex determination. In contrast to the upstream sex determination pathway, genes that control downstream sexual differentiation in animals display much higher functional conservation, and many of the regulators of sexual differentiation belong to a family of transcription factors known as the DMRT family. Downstream of the tra-1 global regulator, I found that the highly conserved DMRT family gene dmd-4 acts much more specifically in adult hermaphrodites to generate sexual dimorphism at the level of the phasmid sensory neurons PHA and PHB. Furthermore, the sexual dimorphism of DMD-4 is regulated post-translationally by a ubiquitin-binding domain that I also found to be functionally conserved in the human ortholog, Dmrt3. Although these transcription factors both demonstrate the high degree of genetic control that contributes to sex determination and sexual differentiation, I also described male-specific effects of early life stress on sexual dimorphic synaptic connectivity and behavior generated by the phasmid sensory neurons, indicating that sexual differentiation is also plastic to environmental cues encountered during the life of an organism.

This thesis provides insight into how genetic pathways function at multiple levels to give rise to extensive sexual dimorphism in the soma of an animal, both globally and in regard to the development on individual cells, in addition to the ways in which these genetic pathways can be modified by environmental factors and organismal life history.