FGF diffusion is required for directed migration of postembryonic muscle progenitors in <i>C. elegans</i>

Extracellular signaling molecules mediate crucial aspects of cell-cell communication and play essential roles in development and homeostasis. Fibroblast growth factors (FGFs) are a family of secreted signaling proteins that can disperse long distances between cells and are often thought to form concentration gradients that encode spatial information. However, we know relatively little about the spatial distribution of FGFs in vivo, and endogenously tagged FGFs move between cells using different mechanisms in zebrafish and flies. We used FGF-dependent migration of C. elegans muscle progenitors called sex myoblasts (SMs) to elucidate FGF dispersal mechanisms and dissect how FGF guides migrating cells. Live imaging of cell dynamics and endogenously tagged FGF combined with membrane tethering and extracellular trapping approaches revealed that endogenous FGF is diffusible in vivo and extracellular dispersal is required for SM migration. Misexpression demonstrated that FGF is a bona fide chemoattractant that orients SMs during a critical window, while an unidentified, short-range signal acts in concert to position SMs precisely. Our finding that an invertebrate FGF is endogenously diffusible suggests that this may be the ancestral mode for FGF dispersal.