The C. elegans heterochronic pathway controls the timing of NAB/EGR-mediated terminal differentiation and the onset of adulthood

Most animals pass through a series of juvenile stages on their way from embryo to adult. These stages represent periods of time in which stage-specific developmental processes occur. At the end of development, the organism transitions from a juvenile state to a mature adult. The failure to undergo a specific developmental stage, or the failure to transition to an adult can lead to specific developmental deficits or the retention of juvenile characteristics throughout the remainder of the organism's life. This thesis focuses on the study of the mechanisms controlling the larval-to-adult transition in C. elegans. In Chapter two, I perform a screen to identify mutants that are defective in a specific aspect of the larval-to-adult transition, exit from the molting cycle. From this screen I identify new alleles of the previously identified gene mab-10. I then clone mab-10, showing that it encodes the only C. elegans NAB family transcriptional cofactor. I go on to show that MAB10 acts with LIN-29 to control the larval-to-adult transition and that LIN-29 shares homology with mammalian EGR proteins. These observations highlight the similarity between the mechanisms controlling the onset of adulthood in C. elegans and mammals as LIN-29 is required for the larval-to-adult transition in C elegans and EGRI is required for the onset of puberty in mice. Chapter three is a further analysis of both mab-10 and lin-29, focusing on their expression patterns and roles in the terminal differentiation of tissues outside of the hypoderm. We use a mab-10 translational reporter to show that MAB-10 accumulates precociously in precocious heterochronic mutants and that mab-10 transcription does not require lin-29, suggesting that the heterochronic pathway controls the timing of mab-10 transcription in parallel to lin-29. We use single molecule fluorescence in situ hybridization to establish the wild-type transcription patterns of mab-10 and lin-29 and show that both undergo dynamic changes in expression throughout development. We show that mab-10 is required for terminal differentiation in at least one other tissue outside of the hypoderm and that mab-10 prevents seam cell overproliferation by antagonizing the activities of the C. elegans Runx and CBF-Beta homologs rnt-1 and bro-1. Thesis Advisor: H. Robert Horvitz Title: Professor of Biology Acknowledgments There are many people I would like to thank for helping me to complete the work presented here. First, I would like to thank Ezequiel Alvaarez-Savedra, Daniel Denning, and Nicolas Paquin for reading drafts of each of my thesis chapters. Secondly, I would like to thank Na An, Beth Castor, Rita Droste, Elissa Murphy, Erika Hartwieg, and Victoria Hatch for technical support including strain maintenance, electron microscopy, and DNA sequencing. I would like to thank all of the current members of the Horvitz lab, including several that have moved on to exciting new ventures. I would especially like to thank my roommates: Brendan Galvin, Takashi Hirose, Dengke Ma, and Ala Berdichevsky for creating an excellent working environment and for many fun and interesting conversations. It was Brendan that originally suggested mab-10 to me as a potential project and for that I am extremely grateful. I would also like to thank the many members of the Anna's Taqueria crew: Melissa Harrison, Johanna Varner, Mike Hurwitz, Dan Omura, Nicolas Paquin, and Adam Saffer. Our lunchtime trek was always a welcome distraction and often a time for great discussion. I would like to thank the members of my thesis committee who provided me with helpful advice over the years. The members of my committee were Chris Kaiser, Jianzhu Chen, and Dennis Kim. I would also like to thank Hazel Sive, Troy Littleton, and Victor Ambros for joining my committee in the eleventh hour. I would like to thank my advisor Bob Horvitz for creating the type of open environment that was exactly what I was looking for in my graduate studies. He provided the freedom for me to explore my own ideas to success and failure, but was always there to help me see my way through to the end. Finally, I would like to thank my wife, Heather, and my family for their support. Without Heather to keep my mind focused on the task at hand, none of this would have been possible.