ИСТИНА

Dinophilidae is a unique group among Polychaeta and its representatives combine morphological traits of different Lophotrochozoan taxa. Moreover, adult dinophilids demonstrate morphological similarity with trochophore larva. The earliest events in Dinophilid neurogenesis will help to shed light on our understanding of the evolution of this group: whether they demonstrate archaic origin or neoteny. We applied standard neuronal and pan-neuronal immunochemical markers (antibodies against serotonin, FMRFamide, and acetylated tubulin) to reveal the first nerve elements during larval development of two relative species Dinophilus gyrociliatus and Dinophilus taeniatus. Contrary to all other Lophotrochozoa investigated, the earliest positive cells demonstrate reaction with acetylated tubulin only. The first nerve cell differentiates in episphere at the anterior pole of the embryo, and its caudally running fibers construct two ventral cords. Later three tubulin-positive cells adjacents to each ventral cord appear in hyposphere. They send fibers anterior and posterior along the processes of the first cell and together construct the anlagen of the nervous system: brain neuropil, paired ventral cords with commissures, and esophageal ring. No specific neuronal markers were detected in all early cells. Serotonin- and FMRFamide-immunoreactive neurons differentiate much later within the brain and along ventral cords in a course of development, and their axons follow the path already established by the processes of the early cells. Such a scenario of early neurogenesis is unique for all Annelids representatives studied so far and allow to suggest the archaic origin of Dinophilids. The reported study was funded by RFBR, project number 19-34-6004