No abstract

Netrin is a remarkably conserved midline landmark, serving as a chemotactic factor that organizes the bilateral neural architecture in the post-gastrula bilaterian embryos. Netrin signal also guides cell migration in many other neural and non-neural organogenesis events in later developmental stages, but it has never been before found to participate in gastrulationthe earliest cell migration in metazoan embryogenesis. Here, we found that netrin signaling molecules and their receptors are expressed during gastrulation of the leech Helobdella.Intriguigly, Hau-netrin-1 was expressed in the N lineage, the precursor of ventral ectoderm, at the onset of gastrulation. We demonstrated that the N lineage is required for the entrance of mesoderm into the germinal band and that misexpression of Hau-netrin-1 in early gastrulation prevented mesoderm from entering the germinal band. Together, these results suggested that Hau-netrin-1 secreted by the N lineage guides mesoderm migration during germinal band assembly. Furthermore, ectopic expression of Hau-netrin-1 after the completion of germinal band assembly disrupted the epibolic migration of the germinal bands in a later stage of gastrulation. Thus, Hau-netrin-1 is likely involved in two distinct events in sequential stages of leech gastrulation: the assembly of germinal bands in early gastrulation and their epibolic migration in mid-gastrulation. This mode of gastrulation as observed in the leech is apomorphic for clitellate annelids. We postulated that a heterochronic shift of netrin gene expression in the clitellate ancestor might have facilitated the evolutionary emergence of a novel form of gastrulation in this lineage.