High diversity in neuropeptide immunoreactivity patterns among three closely related species of Dinophilidae (Annelida)

Abstract

Neuropeptides are conserved metazoan signaling molecules, and represent useful markers for comparative investigations on the morphology and function of the nervous system. However, little is known about the variation of neuropeptide expression patterns across closely related species in invertebrate groups other than insects. In this study, we compare the immunoreactivity patterns of 14 neuropeptides in three closely related microscopic dinophilid annelids (Dinophilus gyrociliatus, D. taeniatus and Trilobodrilus axi). The brains of all three species were found to consist of around 700 somata, surrounding a central neuropil with 3-5 ventral and 2-5 dorsal commissures. Neuropeptide immunoreactivity was detected in the brain, the ventral cords, stomatogastric nervous system, and additional nerves. Different neuropeptides are expressed in specific, non-overlapping cells in the brain in all three species. FMRFamide, MLD/pedal peptide, allatotropin, RNamide, excitatory peptide, and FVRIamide showed a broad localization within the brain, while calcitonin, SIFamide, vasotocin, RGWamide, DLamide, FLamide, FVamide, MIP, and serotonin were present in fewer cells in demarcated regions. The different markers did not reveal ganglionic subdivisions or physical compartmentalization in any of these microscopic brains. The non-overlapping expression of different neuropeptides may indicate that the regionalization in these uniform, small brains is realized by individual cells, rather than cell clusters, representing an alternative to the lobular organization observed in several macroscopic annelids. Furthermore, despite the similar gross brain morphology, we found an unexpectedly high variation in the expression patterns of neuropeptides across species. This suggests that neuropeptide expression evolves faster than morphology, representing a possible mechanism for the evolutionary divergence of behaviors.

New data on dinophilid neurogenesis: a variation of a common pattern

“…The prototroch stage matches well with the early trochophore stage since the last is characterized by the presence of the prototroch, the rst ciliary structure to develop in Spiralia [27,28,10,60,48,53,21]. Moreover, recent analysis of gene expression con rmed the homology of the indirect-developing annelid larval prototroch and the ciliary band of adult D. gyrociliatus [ 43,45] developing from the prototroch [44]. The ventral ciliary eld stage matches middle trochophore stage (or just trochophore) [60].…”

“…Later on, these rst processes will be joined by numerous newly differentiated nervous elements (Fig. 9-12); the number of neural bers increases drastically during the development so the central nervous system at the late trochophore stage is very similar to that of juveniles [62,63,75,23,[43][44][45]. Therefore, the rst detected neurons in dinophilids are the rst elements of the central nervous system and certainly are not the typical pioneer neurons.…”

“…Summarizing the data from different lophotrochozoan taxa the evidence comes that the cells of the apical organ almost always have either 5-HT or FMRFamide IR [43, 72-74, 67, 68, 93-95, 51, 55, 97]. Pharmacological treatments and other approaches demonstrate that these neurons play critical role in reception of external cues [30,53,13,44], regulation of developmental rates [87,29], and morphogenetic patterning [56,57,[72][73][74]104].…”

“…The rst works depict dinophilid anatomy, histology, and embryogenesis as early as at the end of XIX -beginning of XX centuries [30,31,27,40,48,76,77,31,64,65]. Further investigations con rm an extraordinary morphology of dinophilids, which combines characteristics of different lophotrochozoan animals such as Polychaeta (segmentation of epithelial structures, though no chaeta), Platyhelminthes (parenchymatous organization, protonephridia), Mollusca (gliding ciliary locomotion), and even trochophore larva (prototroch, ciliary bands, ventral ciliary eld, and protonephridia) [4,5,6,8,35,1,58,8,78,81,82,[44][45][46]. That adult worms have larval elements suggests progenetic origin of dinophilids [98,100,63,36,103,97,60,[97][98][99]101].…”

“…That adult worms have larval elements suggests progenetic origin of dinophilids [98,100,63,36,103,97,60,[97][98][99]101]. The nervous system of Dinophilus attracted particular attention of researches in recent times [100,7,63,64,23,24,[44][45][46]. Nevertheless, the early events of neurogenesis ( rst differentiating neurons, pioneer neurons, the structure of the apical organ) have never been described in details.…”