Eiichi Shoguchi

Zoantharians (Cnidaria) are globally distributed from shallow reefs to the deep sea, yet their genomic resources remain scarce. Within the zoantharian genus Palythoa, most species harbor photosymbiotic dinoflagellates, whereas some have secondarily lost this symbiosis, allowing genomic changes associated with symbiotic status to be examined. Palythoa is also the original source of palytoxin, one of the most potent marine toxins known, but whether it is produced by the host or by associated microorganisms remains unresolved. Here we generated high-quality genome assemblies of two zooxanthellate Palythoa species and compared five Palythoa genomes spanning both symbiotic states with those of other anthozoans. Across anthozoans, genome size scaled with transposable element content, particularly retrotransposons. The azooxanthellate lineage carries substitutions in conserved domains of the symbiosis-related proteins LePin and NPC2 and shows rapid evolution of the lectin gene CLEC4A, whereas NF-κB and V-type H+-ATPase remain conserved. Palythoa genomes showed no lineage-specific expansion of polyketide synthase (PKS) genes; cnidarian PKS genes were restricted to P. caribaeorum and P. sp. sakurajimensis (clade III). In contrast, 27 bacterial PKS sequences were recovered from metatranscriptomes and a metagenome-assembled genome across species, including an azooxanthellate species, indicating that they derive from associated bacterial rather than Symbiodiniaceae. These genomes provide a comparative framework for zoantharians and reveal genomic changes associated with symbiotic status. We propose that palytoxin originates from associated bacteria rather than from the host, a hypothesis that can be tested by identifying the producing bacteria and their biosynthetic gene clusters.

Photosymbiosis with dinoflagellates of the family Symbiodiniaceae enables giant clams to thrive in oligotrophic coral reef environments. However, mechanisms by which clams utilize algal-derived biomolecules remain largely unexplored. Using newly available genome resources for photosymbiotic bivalves ( Tridacna and Fragum ), we conducted a comparative genomic analysis to identify positively selected genes from these photosymbiotic bivalve lineages, that are potentially involved in symbiotic adaptations. Among candidate genes, we focused on sulfoquinovosidase (SQase), an enzyme that hydrolyzes sulfoquinovose (SQ) from sulfoquinovosyl diacylglycerol (SQDG), a sulfur-containing sulfolipid abundant in photosynthetic membranes. Although SQDG degradation has been characterized in bacteria, the distribution and role of SQase in animals have not been systematically examined. We found that possibly functional SQase homologs are widely distributed among aquatic invertebrates, but are largely absent in terrestrial taxa. In silico predictions showed that most animal SQases possess signal peptides and enter the secretory pathway, with lineage-specific gains and losses of membrane association, suggesting functional diversification. Transcriptomic analyses further demonstrated that SQase is predominantly expressed in digestive organs in diverse taxa, whereas in giant clams, it is also highly expressed in the outer mantle, the tissue primarily involved in harboring symbionts enriched in SQDG. Together, these results suggest that in aquatic invertebrates, SQase functions as a digestive enzyme for algal-derived sulfolipids and that photosymbiotic bivalves have expanded its deployment to tissues specialized for symbiosis. Our findings uncover a previously unrecognized metabolic interface between photosymbiosis and sulfur utilization in aquatic invertebrates, with broader implications for host-symbiont interactions and marine sulfur cycling.

Giant clams are representative bivalves in coral reef ecosystems that host photosynthetic dinoflagellates extracellularly and rely on their photosynthates, functioning as "solar-powered animals." Unlike corals, which harbor intracellular dinoflagellates, molecular mechanisms and evolutionary history underlying this symbiosis remain largely unknown. Here, we integrated chromosome-scale genome assembly, transcriptome profiling, and bleaching experiments involving Tridacna crocea to explore the genetic basis of extracellular symbiosis. Signals associated with sterol transport by Niemann-Pick disease type C2 (NPC2) transporters and carbon-concentrating mechanisms suggest that giant clams share some nutrient exchange strategies with corals. Strikingly, the nitrate transporter NRT2, a "plant-like" gene previously thought to be absent in animals, represents an unexpected evolutionary retention that enables nitrate-based nutrient supply, highlighting a fundamental divergence from coral symbiosis. Together, our findings reveal both conserved and distinct molecular strategies of photosymbiosis in reef-dwelling marine invertebrates and provide insights into evolution and ecological resilience of coral reef ecosystems.

Dinoflagellates are important unicellular eukaryotes found in freshwater and marine ecosystems. The Family Symbiodiniaceae is well-known as symbiotic algae in various marine organisms, including many endosymbiotic species living in cnidarian cells. In contrast, the genus Effrenium is non-symbiotic, exhibiting traits such as resting cyst formation and heterotrophic feeding. Effrenium strains have been used as controls in symbiosis-related comparative studies. The only described species, E. voratum, is cosmopolitan, and draft genomes of three strains have been reported. However, genomic data from strains originating in temperate Japanese regions are lacking. A high-quality draft genome of an additional E. voratum strain will provide valuable resources for molecular biology of symbiosis and comparative genomics on local adaptation in Symbiodiniaceae. We describe the first draft genome (version 1) of E. voratum strain NIES-2908, which was isolated from the coral Hydnophora exesa in the temperate Japanese coastal water of Goto islands, Japan. Using PacBio HiFi sequencing, the genome was assembled to a total size of 1,006 Mbp, comprising 6,051 contigs. The GC content was 50.5%, and repetitive sequences accounted for 37.9% of the genome. Gene prediction identified 54,346 protein coding genes, with 68.4% completeness based on the BUSCO alveolate dataset.

Gene family expansion illustrates a critical aspect of evolutionary adaptation. However, the mechanisms by which gene family expansions emerge and are maintained in the genome remain unclear. Here, we report de novo, nearly telomere-to-telomere (T2T), haplotype-phased genome assemblies of 2 coral species, Acropora tenuis and Acropora digitifera. By comparing haplotypes within a single individual and across species, we identified genomic regions spanning several megabases with highly disordered gene arrangements, termed non-syntenic regions (nSRs). In these nSRs, there are clusters of genes that emerged by lineage-specific gene family expansion. The gene repertoire within nSRs exhibits significant sequence diversity and distinct expression patterns, suggesting functional diversification. We propose that lineage-specific gene family expansion in nSRs occurs through recurrent tandem duplications mediated by non-allelic homologous recombination (NAHR) events, with nSRs serving as reservoirs for a diverse gene repertoire advantageous for survival. The nearly T2T-phased genomes provide new insights into the remarkable flexibility of genome organization and the evolution of gene family expansions.

The red seaweed Asparagopsis taxiformis (Bonnemaisoniaceae, Rhodophyta) produces a bioactive natural product, bromoform, which, when fed to ruminant livestock, can eradicate methane emissions. However, to cultivate enough A. taxiformis to produce a yield that would have a meaningful impact on global greenhouse gas emissions, we need to advance our current understanding of the biology of this seaweed species. Here, we used both a domesticated diploid tetrasporophyte (>1.5 years in culture) and wild samples to establish a high-quality draft nuclear genome for A. taxiformis (lineage 6 based upon phylogenetic analyses using the cox2-3 spacer). The constructed nuclear genome is 142 Mb in size (including 70.67% repeat regions) and was determined to encode for approximately 10,474 protein-coding genes, including those associated with secondary metabolism, photosynthesis, and defence. To obtain information regarding molecular differences between cultured and wild tetrasporophytes, we further explored differential gene expression relating to their different growth environments. Cultured tetrasporophytes, which contained a relatively higher level of bromoform compared to wild tetrasporophytes, demonstrated an enrichment of regulatory factors, such as protein kinases and transcription factors, whereas wild tetrasporophytes were enriched for the expression of defence and stress-related genes. Wild tetrasporophytes also expressed a relatively high level of novel secretory genes encoding proteins with von Willebrand factor A protein domains (named rhodophyte VWAs). Gene expression was further confirmed by proteomic investigation of cultured tetrasporophytes, resulting in the identification of over 400 proteins, including rhodophyte VWAs, and numerous enzymes and phycobiliproteins, which will facilitate future functional characterisation of this species. In summary, as the most comprehensive genomic resource for any Asparagopsis species, this resource for lineage 6 provides a novel avenue for seaweed researchers to interrogate genomic information, which will greatly assist in expediating production of Asparagopsis to meet demand by both aquaculture and agriculture, and to do so with economic and environmental sustainability.

Zoantharia is an order among the Hexacorallia (Anthozoa: Cnidaria), and includes at least 300 species. Previously reported genomes from scleractinian corals and actiniarian sea anemones have illuminated part of the hexacorallian diversification. However, little is known about zoantharian genomes and the early evolution of hexacorals. To explore genome evolution in this group of hexacorals, here, we report de novo genome assemblies of the zoantharians Palythoa mizigama (Pmiz) and Palythoa umbrosa (Pumb), both of which are members of the family Sphenopidae, and uniquely live in comparatively dark coral reef caves without symbiotic Symbiodiniaceae dinoflagellates. Draft genomes generated from ultra-low input PacBio sequencing totaled 373 Mbp and 319 Mbp for Pmiz and Pumb, respectively. Protein-coding genes were predicted in each genome, totaling 30,394 in Pmiz and 24,800 in Pumb, with each set having ∼93% BUSCO completeness. Comparative genomic analyses identified 3,036 conserved gene families, which were found in all analyzed hexacoral genomes. Some of the genes related to toxins, chitin degradation, and prostaglandin biosynthesis were expanded in these two Palythoa genomes and many of which aligned tandemly. Extensive gene family loss was not detected in the Palythoa lineage and five of ten putatively lost gene families likely had neuronal function, suggesting biased gene loss in Palythoa. In conclusion, our comparative analyses demonstrate evolutionary conservation of gene families in the Palythoa lineage from the common ancestor of hexacorals. Restricted loss of gene families may imply that lost neuronal functions were effective for environmental adaptation in these two Palythoa species.

Bacterial–algal interactions strongly influence marine ecosystems. Bacterial communities in cultured dinoflagellates of the family Symbiodiniaceae have been characterized by metagenomics. However, little is known about whole‐genome analysis of marine bacteria associated with these dinoflagellates. We performed in silico analysis of four bacterial genomes from cultures of four dinoflagellates of the genera Symbiodinium , Breviolum , Cladocopium and Durusdinium . Comparative analysis showed that the former three contain the alphaproteobacterial family Parvibaculaceae and that the Durusdinium culture includes the family Sphingomonadaceae. There were no large genomic reductions in the alphaproteobacteria with genome sizes of 2.9–3.9 Mb, implying they are not obligate intracellular bacteria. Genomic annotations of three Parvibaculaceae detected the gene for diacetylchitobiose deacetylase (Dac), which may be involved in the degradation of dinoflagellate cell surfaces. They also had metabolic genes for dissimilatory nitrate reduction to ammonium (DNRA) in the nitrogen (N) cycle and cobalamin (vitamin B ₁₂ ) biosynthetic genes in the salvage pathway. Those three characters were not found in the Sphingomonadaceae genome. Predicted biosynthetic gene clusters for secondary metabolites indicated that the Parvibaculaceae likely produce the same secondary metabolites. Our study suggests that the Parvibaculaceae is a major resident of Symbiodiniaceae cultures with antibiotics.

Genes encoding snake venom toxins have been studied extensively. However, genes involved in the modification and functioning of venom proteins are little known. Protobothrops is a genus of pit vipers, which are venomous and inhabit the Nansei (Southwest) islands of Japan, Taiwan China, Vietnam, Thailand, Myanmar, Nepal, Bhutan, and India. Our previous study decoded the genome of Protobothrops flavoviridis, a species endemic to the Nansei Islands, Japan, and revealed unique evolutionary processes of some venom genes. In this study, we analyzed genes that are highly expressed in venom glands to survey genes for candidate enzymes or chaperone proteins involved in toxin folding and modification. We found that, in addition to genes that encode venom proteins and ribosomal proteins, genes that encode protein disulfide isomerase (PDI) family members (orthologs of human P4HB and PDIA3), Selenoprotein M (SELENOM), and Calreticulin (CALR) are highly expressed in venom glands. Since these enzymes or chaperones are involved in protein modification and potentially possess protein folding functions, we propose that P4HB, SELENOM, CALR, and PDIA3 encode candidate enzymes or chaperones to confer toxic functions upon the venom transcriptome.