Dinophiliformia early neurogenesis suggests the evolution of conservative neural structures across the Annelida phylogenetic tree

Abstract

Despite the increasing data concerning the structure of the adult nervous system in various Lophotrochozoa groups, the early events during the neurogenesis of rare and unique groups need clarification. Annelida are a diverse clade of Lophotrochozoa, and their representatives demonstrate a variety of body plans, lifestyles, and life cycles. Comparative data about the early development are available for Errantia, Sedentaria, Sipuncula, and Palaeoannelida; however, our knowledge of Dinophiliformia is currently scarce. Representatives of Dinophiliformia are small interstitial worms combining unique morphological features of different Lophotrochozoan taxa and expressing paedomorphic traits. We describe in detail the early neurogenesis of two related species: Dimorphilus gyrociliatus and Dinophilus vorticoides, from the appearance of first nerve cells until the formation of an adult body plan. In both species, the first cells were detected at the anterior and posterior regions at the early trochophore stage and demonstrated positive reactions with pan-neuronal marker anti-acetylated tubulin only. Long fibers of early cells grow towards each other and form longitudinal bundles along which differentiating neurons later appear and send their processes. We propose that these early cells serve as pioneer neurons, forming a layout of the adult nervous system. The early anterior cell of D. vorticoides is transient and present during the short embryonic period, while early anterior and posterior cells in D. gyrociliatus are maintained throughout the whole lifespan of the species. During development, the growing processes of early cells form compact brain neuropile, paired ventral and lateral longitudinal bundles; unpaired medial longitudinal bundle; and commissures in the ventral hyposphere. Specific 5-HT- and FMRFa-immunopositive neurons differentiate adjacent to the ventral bundles and brain neuropile in the middle trochophore and late trochophore stages, i.e. after the main structures of the nervous system have already been established. Processes of 5-HT- and FMRFa-positive cells constitute a small proportion of the tubulin-immunopositive brain neuropile, ventral cords, and commissures in all developmental stages. No 5-HT- and FMRFa-positive cells similar to apical sensory cells of other Lophotrochozoa were detected. We conclude that: (i) like in Errantia and Sedentaria, Dinophiliformia neurogenesis starts from the peripheral cells, whose processes prefigure the forming adult nervous system, (ii) Dinophiliformia early cells are negative to 5-HT and FMRFa antibodies like Sedentaria pioneer cells.

Dimorphilus gyrociliatus (Annelida: Dinophiliformia) Dwarf Male Nervous System Represents a Common Pattern for Lophotrochozoa

“…These elements are not 5-HT- or FMRFamide-positive and specific nerve elements appear later in development. 5-HT-like nerve elements appear at the middle trochophore stage as three cells located symmetrically on the left and right sides at the base of the first commissure [ 12 ]. The long basal processes of these cells join the first commissure and the ventral bundles going posteriorly and anteriorly to the brain neuropile.…”

“…The samples were fixed with 4% paraformaldehyde (PFA) in phosphate-buffered saline (PBS, 0.01mM, pH=7.4) at 4 °C overnight. Then, the samples were subjected to immunostaining according to the protocol described in [ 12 ]. Each staining was performed with at least 50–60 male specimens.…”

“…While data on the morphology and development of the nervous system are available for Oweniidae [ 5 , 6 ], Sipunculida [ 7 , 8 , 9 ], Pleistoannelida (Errantia [ 10 ] and Sedentaria [ 11 ]), and even Dinophiliformia ( D. vorticoides both sexes and D. gyrociliatus females [ 12 ]), data on D. gyrociliatus dwarf males are limited.…”

“…Data on the development of the nervous system of female D. vorticoides and D. gyrociliatus were recently described in detail [ 12 ]. The first neurons, which could be detected only with tubulin antibodies, are located at the anterior and posterior ends.…”

“…Then, nerve development continues from both sides. Specific 5-HT and FMRFamide nerve elements appear later in development [ 12 ]. The same developmental pattern from both sides has been described for Errantia [ 10 ] and Sedentaria [ 11 ].…”