Whole-head recording of chemosensory activity in the marine annelidPlatynereis dumerilii

Abstract

Chemical detection is key to various behaviours in both marine and terrestrial animals. Marine species, though highly diverse, have been underrepresented so far in studies on chemosensory systems, and our knowledge mostly concerns the detection of airborne cues. A broader comparative approach is therefore desirable. Marine annelid worms with their rich behavioural repertoire represent attractive models for chemosensory studies. Here, we study the marine worm Platynereis dumerilii to provide the first comprehensive study of head chemosensory organ physiology in an annelid. By combining microfluidics and calcium imaging, we record neuronal activity in the entire head of early juveniles upon chemical stimulation. We find that Platynereis uses four types of organs to detect stimuli such as alcohols, esters, amino acids and sugars. Antennae, but not nuchal organs or palps as generally hypothesised in annelids, are the main chemosensory organs. We report chemically-evoked activity in possible downstream brain regions including the mushroom bodies, which are anatomically and molecularly similar to insect mushroom bodies. We conclude that chemosensation is a major sensory modality for marine annelids, and propose early Platynereis juveniles as a model to study annelid chemosensory systems.Chemical signals are central to animal behaviour, including feeding, predation, courtship and mating, aggregation, defence, habitat selection, communication [1]. Adapting to variable habitats and changing chemical landscapes, animals have evolved a broad variety of chemosensory organs. Investigations of chemosensory systems in mammals, insects and nematodes have provided insights into the molecular and cellular basis of how chemical information is encoded into neuronal activity [2][3][4]. While similar circuit architectures can be found in distant species at some steps of information processing, this appears to be no general rule [5,6]. Genomic studies have revealed that receptor proteins are highly diverse in the animal kingdom [7], and can be entirely different between distant species -vertebrates and insects for example use distinct types of receptors [8,9]. Hence, a broader comparative approach will facilitate the elucidation of both general operating principles and evolutionary origins of animal chemosensation. Despite studies in fish and crustaceans [10,11], and to a lesser extent in molluscs [12,13], our current understanding of animal chemosensation still mainly concerns terrestrial and airborne cues. Marine animals thus deserve more attention.Marine annelids, traditionally referred to as 'polychaetes', represent an attractive group for chemosensory studies. These worms, represented by more than 10.000 species, are typically freeliving, burrow in the marine sediment, or build tubes. They are known to respond to chemical signals in reproduction, feeding, aggression, avoidance, aggregation, environment probing, larval settlement and metamorphosis [14]. Marine annelids are suited for electrophysiological [15][16][17][18] as well as...

Whole-body integration of gene expression and single-cell morphology

“…Other transcription factors were expressed in one of the three sensory ganglia only (e.g., hox1 in CG, pax258 in AG, and pax6 in PG), re-flecting their divergent fates. In addition, the sensory ganglia jointly expressed the receptors pkd1 and pkd2, the Trp channels trpV4 and trpV5, and the ionotropic glutamate receptor iGluR (Figure S4), consistent with neurons in all three ganglia responding to glutamate (Chartier et al, 2018) and indicating that they also detect mechanical stimuli (Bezares-Calderó n et al, 2018). Next, we inspected the circumpalpal GN (CpG) and found that they represented epithelial sheaths surrounding the PG with 26% bipolar sensory neurons.…”

“…With our new resource, this can now be extended to integrate connectomics and transcriptomics for an entire animal and for different developmental stages. Additionally, the multimodal capacity of the PlatyBrowser will allow the incorporation of physiological data for identified cell types using tools such as calcium imaging (Chartier et al, 2018) and CRISPR-Cas9 (Bezares-Calderó n et al, 2018). Importantly, the experimental techniques used for data collection (electron microscopy, in situ hybridization, and single-cell RNA sequencing) can be readily applied to non-canonical laboratory species.…”

“…For a more detailed analysis of how gene expression relates to tissue boundaries, we focused on the animal's head (Figure 5D)-the region with the highest gene density in the expression atlas (Figure S3A) and a more refined subdivision in the UMAP (Figure 5A). The 6-dpf brain is largely differentiated (Chartier et al, 2018) with discernible anatomical subunits called ganglionic nuclei (GN) (Engelhardt et al, 1982) that we manually segmented from the EM volume (Figure 5D, upper row). We then noted that the 6 head clusters closely matched the anatomically defined GN (Figure 5D).…”

“…The detail in the EM volume and cell segmentation allowed us to classify and group the cells in the animal's head in a complete and unbiased manner by using ultrastructural landmarks such as connective tissue and basal membranes (as compared to previous efforts recognizing brain anatomy mostly based on immunohistochemistry (Chartier et al, 2018;Tomer et al, 2010)).…”