New data on dinophilid neurogenesis: a variation of a common pattern
Authors: Fofanova Elizaveta , Mayorova Tatiana , Voronezhskaya Elena 
Published: August 2020

Abstract
BackgroundThe structure and development of the nervous system in Lophotrochozoa species is of the most important questions for comparative neurobiology. During the last decade the number of comprehensive studies on the development of serotonergic and FMRFamidergic systems has been skyrocketing. However, the detailed research of the earliest events of Polychaeta neurogenesis is still sparce. Polychaeta is a huge taxon within Lophotrochozoa. Its representatives are widely used as model systems in developmental and physiological investigations. Dinophilidae is a unique Polychaeta group. Its representatives combine morphological traits of different lophotrochozoan taxa. Moreover, adult dinophilids demonstrate morphological similarity to a trochophore larva. This similarity may be associated with either archaic origin of this group or neoteny. The main goal of our study is to provide a detailed description of the earliest events in Dinophilus neurogenesis. These data might improve our understanding of Polychaeta development and evolution.ResultsWe have studied the earliest events in nervous system development in two relative species D. gyrociliatus and D. taeniatus using immunochemical labelling of serotonin, FMRF-amide related peptides, and acetylated tubulin. We used external ciliation as marker for staging. Both species go through the same developmental stages: prototroch, ventral ciliary field and ciliary bands. In both species the first neurons differenciate revealed by anti alpha-acetylated tubulin antibodies only and show no reaction with 5-HT or FMRFa antibodies. These neurons located at the anterior and posterior parts of the embryo in both species. In D. taeniatus embryons the anterior cell is transient and disappear just after head neuropil is constructed. On the contrary, in D. gyrociliatus embryos the anterior cell is not transient and remains at the same position during the whole life span of the specimen. Caudal cell is present during the whole embryogenesis in both species. Neurites of these early neurons surround the stomadeum and constitute anlagen of paired ventro-lateral longitudinal bundles. During the development the number of neurites increases and they form compact head neuropil, paired ventro-lateral and lateral longitudinal bundles, unpaired medial longitudinal bundle and transverse commissures in ventral hyposphere. Serotonin- and FMRFamide-immunoreactive neurons differentiate adjacent to ventro-lateral bundles and head neuropil, respectively, after the establishment of main structures of the nervous system at the ventral ciliary field and ciliary bands stages. Processes of serotonin-, FMRFamide- immunopositive neurons constitute the small portion of tubulin immunopositive neuropil at all described stages.ConclusionsWe announce a detailed data on the earliest events in D. gyrociliatus and D. taeniatus neurodevelopment based on anti-acetylated tubulin, serotonin, and FMRFamide-like immuno labeling. The first nerve elements demonstrate no 5-HT-IR and no FMRFa-IR, which differs from the most Polychaetes and even Lophotrochozoans, investigated so far. Moreover, these animals do not have a typical apical organ (or perhaps do not have it at all) and the pioneer neurons of D.gyrociliatus are also peculiar in that they join the definitive nervous system unlike other lophotrochozoans where pioneer nerons are transient. Thus, Dinophilus neurogenesis demonstrates a variation of common scheme. The reported study was funded by RFBR, project number 19-3460040.