Primer and interviews: Diverse connections between primary cilia and Hedgehog signaling.

On the surface, the Hedgehog (Hh) pathway and primary cilia make strange bedfellows. Hh is a dynamic regulator of a myriad of developmental processes, ranging from spinal cord and limb patterning to lung branching morphogenesis. By contrast, immotile primary cilia were long considered ancestral holdovers with no known function. Considering the disparate perceptions of these two phenomena, the relatively recent discovery that there is a symbiotic-like relationship between Hh and cilia was unexpected. This primer covers the basics of primary cilia and Hh signaling, highlighting variations in ways they are connected across species, and also discusses the evolutionary implications of these findings. Roles of cilia in signal transduction are analyzed further in an interview with Søren T. Christensen, PhD, and Andrew S. Peterson, PhD, in the A Conversation With the Experts section.