The Evolution of Sexually Dimorphic Morphogenesis - Sexual dimorphisms are common in nature. Although the processes that regulate them vary between taxa, a group of genes known as DM domain transcription factors (DMRTs) play a certain role in the development of male-specific traits. How these factors are linked to morphogenetic effectors, however, is unknown. Also, how the robustness of morphogenesis is achieved is also not well understood. Finally, it is unclear what parts of morphogenetic regulation, and its machinery are constrained versus plastic. As a model system to address these knowledge gaps, I use the four tail tip cells of Caenorhabditis elegans. During the last larval stage (L4), in males only, these cells undergo a morphogenetic process, Tail Tip Morphogenesis (TTM). The DMRT transcription factor (TF) DMD-3 is required and sufficient for TTM in C. elegans. Another DMRT TF; MAB-3, known to contribute to TTM robustness. Also, TTM has evolved repeatedly in nematodes, providing an opportunity to determine how conserved or evolvable TTM and its regulatory architecture is. My overall goal is to delineate what genes are transcriptionally controlled by DMD-3 and MAB-3 (directly and indirectly), to determine the extent of MAB-3's redundancy with DMD-3 (i.e. overlap in gene targets), and to determine what parts of this transcriptional control are constrained vs. plastic (via interspecific comparisons). To meet this goal, I will pursue the following specific aims: Aim 1 To delineate how DMD-3 transcriptionally controls TTM and how MAB-3 contributes to TTM robustness, I will compare transcriptomes of single tail tips throughout TTM from wild-type males vs. hermaphrodites (to identify male-specifically expressed genes) and dmd-3(-), mab-3(-), and dmd-3(-);mab-3(-) double mutants (to identify genes transcriptionally controlled by these TFs). By comparing my data to whole-worm DMD-3 ChIP-seq data previously obtained in the lab, I will parse which of these genes are likely direct vs. indirect targets of DMD-3 in TTM. Comparing all targets of DMD-3 to those of MAB-3, I will test how much of MAB-3’s contribution to robustness is due to overlapping vs. non-overlapping control. Aim 2 is to determine how plastic or constrained is the regulation of TTM across different nematode species by using the same tail-tip-specific RNA-seq approach on species in which TTM repeatedly evolved. This will identify which parts of the transcriptome have been conserved with respect to their expression profiles. For example, I will test the hot-spot hypothesis that DMD-3 (or a paralog) was repeatedly recruited when TTM evolved independently. This aim will help to identify which parts of this morphogenetic regulation are evolvable and which are conserved. The expected outcome is an understanding of how transcriptional regulation by DMD-3 and MAB-3 is linked to morphogenetic effectors, what these effectors are, how the redundant role of MAB-3 contributes to robustness, and what parts of the transcriptional regulation are conserved versus evolvable. Identifying key genes conserved in TTM is likely to contribute to understanding about general morphogenetic processes like wound healing, regeneration, and cancer metastasis.
