Artificial Rearing System for Praesagittifera naikaiensis (Acoela, Acoelomorpha) <Article>

Praesagittifera naikaiensis is an acoelomorph worm native to the seashore of the Seto Inland Sea. P. naikaiensis harbors a species of prasinophytes, Tetraselmis sp., as a symbiont. The symbiotic algae are acquired horizontally from the environment during the larval stage. In this study, we established a method for the rearing of adult P. naikaiensis as a first step toward using the animal in the study of development, evolution, and the symbiotic mechanism. We kept the animals in aquaria with closed seawater circulations. The bottom of the aquarium was half covered with sand collected from the seashore of the Seto Inland Sea. An LED light was suspended over the aquarium to aid photosynthesis. The system was maintained at a salinity of 32–33 ppt and a temperature of 14°C, on a 14-h light (35–80 μmol quanta m−2s−1)/10-h dark photoperiod. We succeeded in constantly producing sexually mature animals, independent of season. The system developed here will serve as a foundation for future studies on acoel evolution and development.本研究は科学研究費補助金(26924012)の助成を受けたものである

A draft nuclear-genome assembly of the acoel flatworm Praesagittifera naikaiensis

Background:Acoels are primitive bilaterians with very simple soft bodies, in which many organs, including the gut, are not developed. They provide platforms for studying molecular and developmental mechanisms involved in the formation of the basic bilaterian body plan, whole-body regeneration, and symbiosis with photosynthetic microalgae. Because genomic information is essential for future research on acoel biology, we sequenced and assembled the nuclear genome of an acoel, Praesagittifera naikaiensis.Findings:To avoid sequence contamination derived from symbiotic microalgae, DNA was extracted from embryos that were free of algae. More than 290x sequencing coverage was achieved using a combination of Illumina (paired-end and mate-pair libraries) and PacBio sequencing. RNA sequencing and Iso-Seq data from embryos, larvae, and adults were also obtained. First, a preliminary ∼17–kilobase pair (kb) mitochondrial genome was assembled, which was deleted from the nuclear sequence assembly. As a result, a draft nuclear genome assembly was ∼656 Mb in length, with a scaffold N50 of 117 kb and a contig N50 of 57 kb. Although ∼70% of the assembled sequences were likely composed of repetitive sequences that include DNA transposons and retrotransposons, the draft genome was estimated to contain 22,143 protein-coding genes, ∼99% of which were substantiated by corresponding transcripts. We could not find horizontally transferred microalgal genes in the acoel genome. Benchmarking Universal Single-Copy Orthologs analyses indicated that 77% of the conserved single-copy genes were complete. Pfam domain analyses provided a basic set of gene families for transcription factors and signaling molecules.Conclusions:Our present sequencing and assembly of the P. naikaiensis nuclear genome are comparable to those of other metazoan genomes, providing basic information for future studies of genic and genomic attributes of this animal group. Such studies may shed light on the origins and evolution of simple bilaterians

Hemichordate genomes and deuterostome origins

Acorn worms, also known as enteropneust (literally, ‘gut-breathing’) hemichordates, are marine invertebrates that share features with echinoderms and chordates. Together, these three phyla comprise the deuterostomes. Here we report the draft genome sequences of two acorn worms, Saccoglossus kowalevskii and Ptychodera flava. By comparing them with diverse bilaterian genomes, we identify shared traits that were probably inherited from the last common deuterostome ancestor, and then explore evolutionary trajectories leading from this ancestor to hemichordates, echinoderms and chordates. The hemichordate genomes exhibit extensive conserved synteny with amphioxus and other bilaterians, and deeply conserved non-coding sequences that are candidates for conserved gene-regulatory elements. Notably, hemichordates possess a deuterostome-specific genomic cluster of four ordered transcription factor genes, the expression of which is associated with the development of pharyngeal ‘gill’ slits, the foremost morphological innovation of early deuterostomes, and is probably central to their filter-feeding lifestyle. Comparative analysis reveals numerous deuterostome-specific gene novelties, including genes found in deuterostomes and marine microbes, but not other animals. The putative functions of these genes can be linked to physiological, metabolic and developmental specializations of the filter-feeding ancestor

Artifi cial rearing and oviposition of Waminoa sp. (Acoela, Acoelomorpha) : Toward the development of a model system to study animal-algal symbiosis. <Articles>

Waminoa sp. is an acoelomorph worm that infests corals and harbors two species of dinofl agellates, Amphidinium sp. and Symbiodinium sp.. These symbiotic algae are inherited vertically from parent to offspring during oogenesis. In this study, we established a method for artificial breeding of Waminoa sp., as the first step toward using Waminoa sp. in the study of the mechanism underlying animal–algal symbiosis and the evolution of this symbiosis. We developed a system in which an aquarium circulated approximately 80 L of artifi cial seawater between an upper aquarium and a lower sump, each of which was 45 cm × 30 cm × 35 cm in size and contained 40 L of seawater. Water flowed into the upper aquarium from the sump alternately through a pipe outlet. The upper aquarium contained the hard coral Symphyllia valenciennesii, live rocks, and a layer of fine sand. A 150-W metal halide lamp was suspended over the aquarium to aid photosynthesis. The system was maintained at a salinity of 32–33 ppt and a temperature of 23°C, on a 14-h light (17–50 μmol quanta m-2s-1) : 10-h dark photoperiod. We succeeded in producing almost every stage of the life cycle of Waminoa sp., including the zygote, embryo, larva, juvenile, and adult stages, independent of seasons. In addition, we developed an artificial oviposition procedure that involved isolation from coral in the mature stage. The system developed in this study will serve as a foundation for future studies on acoel–algal symbiosis

Hemichordate genomes and deuterostome origins.

Acorn worms, also known as enteropneust (literally, 'gut-breathing') hemichordates, are marine invertebrates that share features with echinoderms and chordates. Together, these three phyla comprise the deuterostomes. Here we report the draft genome sequences of two acorn worms, Saccoglossus kowalevskii and Ptychodera flava. By comparing them with diverse bilaterian genomes, we identify shared traits that were probably inherited from the last common deuterostome ancestor, and then explore evolutionary trajectories leading from this ancestor to hemichordates, echinoderms and chordates. The hemichordate genomes exhibit extensive conserved synteny with amphioxus and other bilaterians, and deeply conserved non-coding sequences that are candidates for conserved gene-regulatory elements. Notably, hemichordates possess a deuterostome-specific genomic cluster of four ordered transcription factor genes, the expression of which is associated with the development of pharyngeal 'gill' slits, the foremost morphological innovation of early deuterostomes, and is probably central to their filter-feeding lifestyle. Comparative analysis reveals numerous deuterostome-specific gene novelties, including genes found in deuterostomes and marine microbes, but not other animals. The putative functions of these genes can be linked to physiological, metabolic and developmental specializations of the filter-feeding ancestor