Abstract
BackgroundIn marine invertebrate life cycles, which often consist of planktonic larval and benthonic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition. Settlement is regulated by a specialized sensory–neurosecretory system, the larval apical organ. The neuroendocrine mechanisms through which the apical organ transduces environmental cues into behavioral responses during settlement are not fully understood yet.ResultsIn this study, a total of 54 neuropeptide precursors (pNPs) were identified in the Urechis unicinctus larva and adult transcriptome databases using local BLAST and NpSearch prediction, of which 10 pNPs belonging to the ancient eumetazoa, 24 pNPs belonging to the ancient bilaterian, 3 pNPs belonging to the ancient protostome, 9 pNPs exclusive in lophotrochozoa, 3 pNPs exclusive in annelid, and 5 pNPs only found in U. unicinctus. Furthermore, four pNPs (MIP, FRWamide, FxFamide and FILamide) which may be associated with the settlement and metamorphosis of U. unicinctus larvae were analysed by qRT-PCR. Whole-mount in situ hybridization results showed that all the four pNPs were expressed in the region of the apical organ of the larva, and the positive signals were also detected in the ciliary band and abdomen chaetae. We speculated that these pNPs may regulate the movement of larval cilia and chaeta by sensing external attachment signals.ConclusionsThis study represents the first comprehensive identification of neuropeptides in Echiura, and would contribute to a complete understanding on the roles of various neuropeptides in larval settlement of most marine benthonic invertebrates.
Highlights
In marine invertebrate life cycles, which often consist of planktonic larval and benthonic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition
49 Neuropeptide precursors (pNPs) had been reported in other species, and the remaining 5 pNPs were first identified in U. unicinctus and we named them FxFamide, FILamide, FW, FRWamide and ASYY according to their conserved amino acid residues
In this study, we identified 54 pNP genes in U. unicinctus larvae and adult transcriptome databases based on BLAST and NpSearch prediction, and suggested that the neuropeptide system of U. unicinctus is very close to that of annelids according to their phylogenetic distribution
Summary
In marine invertebrate life cycles, which often consist of planktonic larval and benthonic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition. The larval settlement is the key event for their development and survival, which commonly includes the cessation of swimming and the appearance of substrate exploratory behavior [3,4,5,6]. This is a complex process determined by the interaction of biotic and abiotic factors at different temporal and spatial scale [7, 8]. Researchers found that neuropeptides expressed in chemosensory–neurosecretory cells of the apical organ can innervate ciliary bands, and suggested that they may play a role in the regulation of larval locomotion [11,12,13,14], which contribute to the larval settlement behavior [9]
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