Identification of the Neuropeptide Precursor Genes Potentially Involved in the Larval Settlement in the Echiuran Worm Urechis unicinctus

Abstract

Background: 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. 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.Results: In this study, a total of 54 neuropeptide precursors (pNPs) were identified in the Urechis unicinctus larva and adult transcriptome databases using local BLAST and de novo prediction, of which 10 pNPs belonging to the ancient eumetazoa, 23 pNPs belonging to the ancient bilaterian, 3 pNPs belonging to the ancient protostome, 10 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 verified successfully 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.Conclusions: This 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.

Ecdysis‐related neuropeptide expression and metamorphosis in a non‐ecdysozoan bilaterian

“…2013; Hou et al. 2020). MIP‐related neuropeptides of the Wamide superfamily are among the most extensively studied across metazoans and their ancestral role in metamorphic hormone signalling is well established (Schoofs and Beets 2013; Williams 2020).…”

“…2013; Hou et al. 2020). The ampullary system, on the other hand, is probably a polyplacophoran‐specific feature of yet unknown function (Friedrich et al.…”

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

“…Outside arthropods, multiple LK paracopies are also known. In the annelid worm Urechis unicinctus, eight paracopies of LKs have been identified [ 56 ], and the largest number of LKs was found in the LK precursor of the marine slug Aplysia californica with 30 ( Figure 3 ) [ 22 ]. In some species, such as Frankliniella, Rhodnius, the bed bug Cimex lectularius , and Aplysia , the prepro-LK can give rise to additional non-LK peptides, resulting in a total of about 60 peptides in Aplysia [ 21 , 22 , 52 , 57 ].…”

“…Although LK has been demonstrated in annelids, such as Urechis unicinctus and Capitella teleta [ 56 , 128 ], there are, as far as we know, no reports on the cellular localization of the peptide. In the parasitic nematode Ascaris suum , LK immunoreactivity was detected in neurons [ 45 ], but no LK precursor gene has been identified in nematodes, and thus it is not clear what endogeneous peptide the antiserum recognized.…”