Netrin expressed by the ventral ectoderm lineage guides mesoderm migration in epibolic gastrulation of the leech

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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 has never been found to participate in gastrulation - the earliest cell migration in metazoan embryogenesis. Here, we found that the netrin signaling molecules and their receptors are expressed during gastrulation of the leech Helobdella. Intriguingly, Hau-netrin-1 was expressed in the N lineage, which gives rise in part to the ventral midline of 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. Given that the leech netrin is expressed in the precursor cells of the ventral midline during gastrulation, we propose that a heterochronic change from the midline netrin expression had taken place in the evolution of a novel mode of gastrulation in the directly developing leech embryos.