Calpain Proteases and the Evolving Signaling Network in Insect Embryonic Patterning.

Early embryogenesis relies on a series of maternal factors to coordinate cell division and axial patterning. In insects, maternal Toll receptor and Bone Morphogenetic Protein (BMP) pathway activities drive the spatial allocation of zygotic gene expression territories along the dorsal-ventral (DV) axis. The respective contribution of each pathway varies among insect orders and suggests that Toll was co-opted from an ancestral immune function for insect DV patterning. The Ca⁺² dependent modulatory Calpain proteases also influence insect DV patterning, in addition to their role in regulating mitosis. Calpains alter gene expression patterns in insect species that depend on either Toll or BMPs for the establishment of DV territories, raising the question of when calpains acquired this crucial role. Here we review the function of calpains in early embryogenesis within the context of an evolving insect DV patterning landscape. We discuss the essential components for the transition from BMP-driven to Toll-centered axial patterning and how insect Calpains may have adapted to this changing regulatory network.