3/31/2023 0 Comments Blastoporefusiformis develop first into an enlarged anterior domain that forms larval tissues and the adult head¹². fusiformis but starts after gastrulation in the non-feeding larva with gradual metamorphosis of Capitella teleta and the direct developing embryo of Dimorphilus gyrociliatus. We found that trunk development is deferred to pre-metamorphic stages in the feeding larva of O. We performed chromosome-scale genome sequencing in the annelid Owenia fusiformis with transcriptomic and epigenomic profiling during the life cycles of this and two other annelids. Here we show that temporal shifts (that is, heterochronies) in trunk formation underpin the diversification of larvae and bilaterian life cycles. Indirect development with an intermediate larva exists in all major animal lineages¹, which makes larvae central to most scenarios of animal evolution2–11. Thus, our use of diverse bilaterians in the investigation of LGIC expression and function offers a unique hypothesis on the employment of LGICs in early bilaterian evolution. The loss of certain peripheral cells from Ecdysozoa after they separated from other protostomes likely explains their loss of ASICs, and thus the absence of ASICs from model organisms Drosophila and Caenorhabditis elegans. Our broad examination of ASIC gene expression and biophysical function in each major bilaterian lineage of Xenacoelomorpha, Protostomia, and Deuterostomia suggests that the earliest bilaterian ASICs were probably expressed in the periphery, before being incorporated into the brain as it emerged independently in certain deuterostomes and xenacoelomorphs. Our phylogenetic analysis identified an earlier emergence of ASICs from the overarching DEG/ENaC (degenerin/epithelial sodium channel) superfamily than previously thought and suggests that ASICs were a bilaterian innovation. We therefore questioned bilaterian animals’ employment of acid-sensing ion channels (ASICs), LGICs that mediate fast excitatory responses to decreases in extracellular pH in vertebrate neurons. While a complex, bilaterally symmetrical nervous system is a major innovation of bilaterian animals, the employment of specific LGICs during early bilaterian evolution is poorly understood. Nervous systems are endowed with rapid chemosensation and intercellular signaling by ligand-gated ion channels (LGICs). an: anus at: apical tuft bl: blastocoel bp: blastopore cb: chaetoblast cht: chaetae cs: chaetal sac dl: dorsal levator em: esophageal muscle fg: foregut fgm: foregut circular muscle hg: hindgut mg: midgut mo: mouth mt: metatroch np: nephridia nt: neurotroch pt: prototroch rg: refringent globules rm: retractor muscle tm: membrane between chaetal sac and blastocoel. h–k Soon after, the larva grows and expands the metatroch throughout the hyposphere, while more muscle develops, including circular muscles around the foregut. f, g By 18 hpf, the early mitraria now has a secondary ciliary band, the metatroch, in addition to a complete gut, larval muscles and chaetae. By this stage the blastopore elongates to form the presumptive mouth, but still remains open. d, e A short neurotroch then forms on the posterior side of the blastopore by 13 hpf. a–c Ciliogenesis starts at 11 hpf with the formation of the apical tuft and the prototroch dividing the embryo into an apical episphere and a vegetal hyposphere. Images on the left column are ventral views with anterior facing up, while those on the right are lateral views with anterior facing left, except for (b) which is an apical view. CLSM images of early larval stages from a 11 hpf to k 27 hpf. Early mitraria larvae and the beginning of organogenesis.
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