Coordinated and distinct mechanisms of small GTPase and Wnt signaling guide leader cell migration driving organogenesis in C. elegans

Organogenesis of the C. elegans gonad is a postembryonic morphogenesis process driven by long-distance collective cell migration reliant on conserved cell-signaling mechanisms. This process is led by the somatic distal tip cell (DTC), whose migratory path determines gonad shape. Unlike classical leader cells, the DTC lacks leading edge protrusions. Instead, gonadogenesis shares similarities with morphogenesis of branched tubular organs like the mammalian lung and mammary gland. The gonad is a simple model retaining key features of complex tissues – various somatic-support cells, multipotent germ stem cells, and roles for several cell-signaling pathways – making it well suited to uncover novel morphogenesis mechanisms. The gonadal sheath (Sh) cell is another somatic cell type interacting with the germline through an unknown mechanism. Chapter 2 builds on recent work characterizing the position of distalmost Sh1 cells relative to the DTC resulting in a new understanding of gonad anatomy. Here, I validate this model by analyzing endogenous markers of Sh-expressed proteins, confirming this new understanding of Sh1 position and function. In Chapter 3, I show that a canonical protrusive migration regulator, CED-10/Rac GTPase, has a distinct role in non-protrusive DTC migration - maintaining leader cell integrity during mechanically stressful migration. Although CED-10 is known to act downstream of Wnt signaling in DTC directional sensing, I demonstrate that this new role is Wnt-independent. I find that loss of CED-10 activity results in gonad bifurcations due to fragmentation caused by aberrant regulation of adhesion polarity during DTC turning. This chapter identifies a previously unknown role for a Rac GTPase ortholog in smooth-edged leader cell migration. Rab GTPases are conserved regulators of membrane trafficking, essential to cell signaling. Chapter 4 details a screen of all 31 C. elegans Rab GTPases, where we identify roles for 11 in gonad development. I further characterized the most severe hit, implicating somatic gonad-specific RAB-1 in gonad elongation and surprisingly, germline differentiation. This highlights the role of somatic-germline communication beyond the DTC and suggests this signal could rely on RAB-1 for trafficking. This work expands on how interactions between leader, follower and supporting cells together shape morphogenesis, in part through non-classical migration mechanisms.