LAUSR

Simple Summary Ascidians, known as the closest invertebrate relative to the vertebrate group, have a biphasic life cycle including the larval and sessile adult stages with strong adaptability to diverse environments. The nervous system of ascidians plays an essential role in its adaption to the external environment, but the molecular mechanisms underlying this process still need to be further clarified. The Class C G protein-coupled receptors (GPCRs) are a group of cell surface sensors for neurotransmitters and external chemicals, playing important functions in neurotransmission. We systematically characterized the putative Class C GPCRs in an important invasive ascidian species Styela clava and then analyzed their expression levels during different developmental stages and distribution in swimming larvae and multiple tissues of the adults. Our study suggests that S. clava Class C GPCRs potentially function as important molecules during neurotransmission related to physiological and morphogenetic changes in larvae and adults. Abstract In this study, we performed the genome-wide domain analysis and sequence alignment on the genome of Styela clava, and obtained a repertoire of 204 putative GPCRs, which exhibited a highly reduced gene number compared to vertebrates and cephalochordates. In this repertoire, six Class C GPCRs, including four metabotropic glutamate receptors (Sc-GRMs), one calcium-sensing receptor (Sc-CaSR), and one gamma-aminobutyric acid (GABA) type B receptor 2-like (Sc-GABABR2-like) were identified, with the absence of type 1 taste and vomeronasal receptors. All the Sc-GRMs and Sc-CaSR contained the typical “Venus flytrap” and cysteine-rich domains required for ligand binding and subsequent propagation of conformational changes. In swimming larvae, Sc-grm3 and Sc-casr were mainly expressed at the junction of the sensory vesicle and tail nerve cord while the transcripts of Sc-grm4, Sc-grm7a, and Sc-grm7b appeared at the anterior trunk, which suggested their important functions in neurotransmission. The high expression of these Class C receptors at tail-regression and metamorphic juvenile stages hinted at their potential involvement in regulating metamorphosis. In adults, the transcripts were highly expressed in several peripheral tissues, raising the possibility that S. clava Class C GPCRs might function as neurotransmission modulators peripherally after metamorphosis. Our study systematically characterized the ancestral chordate Class C GPCRs to provide insights into the origin and evolution of these receptors in chordates and their roles in regulating physiological and morphogenetic changes relevant to the development and environmental adaption.