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Abstract:
Neuropeptides are conserved metazoan signaling molecules, which are either widely or narrowly distributed in the nervous system, and may inform on neural function. Since little is known about their variation among closely related species, we assessed the immunoreactivity patterns of 14 neuropeptides in the nervous systems of three dinophilid annelids (Dinophilus gyrociliatus, D. taeniatus and Trilobodrilus axi) using immunohistochemistry, CLSM, and 3D-reconstruction. We mapped the specific and common neuropeptides, partly found in the transcriptomes of all three species, onto detailed anatomical reconstructions of their microscopic brains and nervous systems. Dinophilus gyrociliatus was immunoreactive for the highest number of neuropeptides (13/14), and has most immunoreactive cells in the brain (210 compared to 98 and 101, respectively). The small size of the brains (650-750 cells) made it possible to map each immunoreactive cell onto a DAPI template, suggesting no overlap in the specific peptides tested for. This indicates that a neuron can be specific to one neuropeptide and possibly one function – even in a system consisting of few cells. Ongoing studies of the 68 neurons of the D. gyrociliatus dwarf male will further elaborate on this finding. The distribution of labelled cells within the brain expressed high interspecific variation, which was consolidated by intraspecific consistency. We show that the specific immunoreactive cells are mostly scattered throughout the brain, thereby not demarcating compartments or functional regions in the brain. Furthermore, the apparent variation in the immunoreactivity patterns among the three closely related species indicates a dynamic evolution of the neuromodulatory landscape.
