Life-cycle transitions connecting larval and juvenile stages in metazoans are orchestrated by neuroendocrine signals including neuropeptides and hormones. In marine invertebrate life cycles, which often consist of planktonic larval and benthic 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 yet understood. Here we show that myoinhibitory peptide (MIP)/allatostatin-B, a pleiotropic neuropeptide widespread among protostomes, regulates larval settlement in the marine annelid Platynereis dumerilii. MIP is expressed in chemosensory-neurosecretory cells in the annelid larval apical organ and signals to its receptor, an orthologue of the Drosophila sex peptide receptor, expressed in neighboring apical organ cells. We demonstrate by morpholino-mediated knockdown that MIP signals via this receptor to trigger settlement. These results reveal a role for a conserved MIP receptor-ligand pair in regulating marine annelid settlement.M etazoan life cycles show great diversity in larval, juvenile, and adult forms, as well as in the timing and ecological context of the transitions between these forms. In many animal species, neuroendocrine signals involving hormones and neuropeptides regulate life cycle transitions (1-3). Environmental cues are often important instructors of the timing of life cycle transitions (4), and can affect behavioral, physiological, or morphological change via neuroendocrine signaling (5).Marine invertebrate larval settlement is a prime example of the strong link between environmental cues and the timing of life-cycle transitions. Marine invertebrate life cycles often consist of a free-swimming (i.e., pelagic) larval stage that settles to the ocean floor and metamorphoses into a bottom-dwelling (i.e., benthic) juvenile (6-8). In many invertebrate larvae, a pelagicbenthic transition is induced by chemical cues from the environment (9, 10). Larval settlement commonly includes the cessation of swimming and the appearance of substrate exploratory behavior, including crawling on or attachment to the substrate (11-14). In diverse ciliated marine larvae (15), the apical organ, an anterior cluster of larval sensory neurons (16) with a strong neurosecretory character (17-20), has been implicated in the detection of cues for the initiation of larval settlement (21). Although molecular markers of the apical organ have been described (22-24), our knowledge of the neuroendocrine mechanisms with which apical organ cells transmit signals to initiate larval settlement behavior is incomplete.Here, we identify a conserved myoinhibitory peptide (MIP)/ allatostatin-B receptor-ligand pair as a regulator of larval settlement behavior in the marine polychaete annelid Platynereis dumerilii. MIPs are pleiotro...

BackgroundThe marine annelid Platynereis dumerilii is emerging as a powerful lophotrochozoan experimental model for evolutionary developmental biology (evo-devo) and neurobiology. Recent studies revealed the presence of conserved neuropeptidergic signaling in Platynereis, including vasotocin/neurophysin, myoinhibitory peptide and opioid peptidergic systems. Despite these advances, comprehensive peptidome resources have yet to be reported.ResultsThe present work describes the neuropeptidome of Platynereis. We established a large transcriptome resource, consisting of stage-specific next-generation sequencing datasets and 77,419 expressed sequence tags. Using this information and a combination of bioinformatic searches and mass spectrometry analyses, we increased the known proneuropeptide (pNP) complement of Platynereis to 98. Based on sequence homology to metazoan pNPs, Platynereis pNPs were grouped into ancient eumetazoan, bilaterian, protostome, lophotrochozoan, and annelid families, and pNPs only found in Platynereis. Compared to the planarian Schmidtea mediterranea, the only other lophotrochozoan with a large-scale pNP resource, Platynereis has a remarkably full complement of conserved pNPs, with 53 pNPs belonging to ancient eumetazoan or bilaterian families. Our comprehensive search strategy, combined with analyses of sequence conservation, also allowed us to define several novel lophotrochozoan and annelid pNP families. The stage-specific transcriptome datasets also allowed us to map changes in pNP expression throughout the Platynereis life cycle.ConclusionThe large repertoire of conserved pNPs in Platynereis highlights the usefulness of annelids in comparative neuroendocrinology. This work establishes a reference dataset for comparative peptidomics in lophotrochozoans and provides the basis for future studies of Platynereis peptidergic signaling.

Cilia-based locomotion is the major form of locomotion for microscopic planktonic organisms in the ocean. Given their negative buoyancy, these organisms must control ciliary activity to maintain an appropriate depth. The neuronal bases of depth regulation in ciliary swimmers are unknown. To gain insights into depth regulation we studied ciliary locomotor control in the planktonic larva of the marine annelid, Platynereis. We found several neuropeptides expressed in distinct sensory neurons that innervate locomotor cilia. Neuropeptides altered ciliary beat frequency and the rate of calcium-evoked ciliary arrests. These changes influenced larval orientation, vertical swimming, and sinking, resulting in upward or downward shifts in the steady-state vertical distribution of larvae. Our findings indicate that Platynereis larvae have depth-regulating peptidergic neurons that directly translate sensory inputs into locomotor output on effector cilia. We propose that the simple circuitry found in these ciliated larvae represents an ancestral state in nervous system evolution.neural circuit | zooplankton | sensory-motor neuron | FMRFamide-related peptides T wo different types of locomotor systems are present in animals, one muscle based and the other cilia based. The neuronal control of muscle-based motor systems is well understood from studies on terrestrial model organisms. In contrast, our knowledge of the neuronal control of ciliary locomotion is limited, even though cilia-driven locomotion is prominent in the majority of animal phyla (1).Ciliary swimming in open water is widespread among the larval stages of marine invertebrates, including sponges, cnidarians, and many protostomes and deuterostomes (2-5). Freely swimming ciliated larvae often spend days to months as part of the zooplankton (1, 6). The primary axis for ciliated plankton is vertical, and body orientation is maintained either by passive (buoyancy) or active (gravitaxis, phototaxis) mechanisms. When cilia beat, larvae swim upward, and when cilia cease beating, the negatively buoyant larvae sink. During swimming, the thrust exerted on the body is proportional to the beating frequency of cilia (7-9). The alternation of active upward swimming and passive sinking, together with swimming speed and sinking rate, is thought to determine vertical distribution in the water (8). Because several environmental parameters, including water temperature, light intensity, and phytoplankton abundance, change with depth, swimming depth will influence the speed of larval development, the magnitude of UV damage, and the success of larval feeding and settlement. To stay at an appropriate depth, planktonic swimmers must therefore sense environmental cues and regulate ciliary beating.The ciliated larvae of the marine annelid Platynereis dumerilii provide an accessible model for the study of ciliary swimming in marine plankton (10). Platynereis can be cultured in the laboratory, and thousands of synchronously developing larvae can be obtained daily year-round (11). Platynereis ha...

Animals use spatial differences in environmental light levels for visual navigation; however, how light inputs are translated into coordinated motor outputs remains poorly understood. Here we reconstruct the neuronal connectome of a four-eye visual circuit in the larva of the annelid Platynereis using serial-section transmission electron microscopy. In this 71-neuron circuit, photoreceptors connect via three layers of interneurons to motorneurons, which innervate trunk muscles. By combining eye ablations with behavioral experiments, we show that the circuit compares light on either side of the body and stimulates body bending upon left-right light imbalance during visual phototaxis. We also identified an interneuron motif that enhances sensitivity to different light intensity contrasts. The Platynereis eye circuit has the hallmarks of a visual system, including spatial light detection and contrast modulation, illustrating how image-forming eyes may have evolved via intermediate stages contrasting only a light and a dark field during a simple visual task.DOI: http://dx.doi.org/10.7554/eLife.02730.001

Neurosecretory centers in animal brains use peptidergic signaling to influence physiology and behavior. Understanding neurosecretory center function requires mapping cell types, synapses, and peptidergic networks. Here we use transmission electron microscopy and gene expression mapping to analyze the synaptic and peptidergic connectome of an entire neurosecretory center. We reconstructed 78 neurosecretory neurons and mapped their synaptic connectivity in the brain of larval Platynereis dumerilii, a marine annelid. These neurons form an anterior neurosecretory center expressing many neuropeptides, including hypothalamic peptide orthologs and their receptors. Analysis of peptide-receptor pairs in spatially mapped single-cell transcriptome data revealed sparsely connected networks linking specific neuronal subsets. We experimentally analyzed one peptide-receptor pair and found that a neuropeptide can couple neurosecretory and synaptic brain signaling. Our study uncovered extensive networks of peptidergic signaling within a neurosecretory center and its connection to the synaptic brain.