The brown alga, Cladosiphon okamuranus (Okinawa mozuku), is economically one of the most important edible seaweeds, and is cultivated for market primarily in Okinawa, Japan. C. okamuranus constitutes a significant source of fucoidan, which has various physiological and biological activities. To facilitate studies of seaweed biology, we decoded the draft genome of C. okamuranus S-strain. The genome size of C. okamuranus was estimated as ∼140 Mbp, smaller than genomes of two other brown algae, Ectocarpus siliculosus and Saccharina japonica. Sequencing with ∼100× coverage yielded an assembly of 541 scaffolds with N50 = 416 kbp. Together with transcriptomic data, we estimated that the C. okamuranus genome contains 13,640 protein-coding genes, approximately 94% of which have been confirmed with corresponding mRNAs. Comparisons with the E. siliculosus genome identified a set of C. okamuranus genes that encode enzymes involved in biosynthetic pathways for sulfated fucans and alginate biosynthesis. In addition, we identified C. okamuranus genes for enzymes involved in phlorotannin biosynthesis. The present decoding of the Cladosiphon okamuranus genome provides a platform for future studies of mozuku biology.

Genome evolution and development of unicellular, multinucleate macroalgae (siphonous algae) are poorly known, although various multicellular organisms have been studied extensively. To understand macroalgal developmental evolution, we assembled the ∼26 Mb genome of a siphonous green alga, Caulerpa lentillifera , with high contiguity, containing 9,311 protein-coding genes. Molecular phylogeny using 107 nuclear genes indicates that the diversification of the class Ulvophyceae, including C. lentillifera , occurred before the split of the Chlorophyceae and Trebouxiophyceae. Compared with other green algae, the TALE superclass of homeobox genes, which expanded in land plants, shows a series of lineage-specific duplications in this siphonous macroalga. Plant hormone signalling components were also expanded in a lineage-specific manner. Expanded transport regulators, which show spatially different expression, suggest that the structural patterning strategy of a multinucleate cell depends on diversification of nuclear pore proteins. These results not only imply functional convergence of duplicated genes among green plants, but also provide insight into evolutionary roots of green plants. Based on the present results, we propose cellular and molecular mechanisms involved in the structural differentiation in the siphonous alga.

When the body of P. flava is severed, the animal has the ability to regenerate its missing anterior or posterior as appropriate. We have focused on anterior regeneration when the head and branchial regions are severed from the body of the worm. After transection, the body wall contracts and heals closed in 2 to 3 days. By the third day a small blastema is evident at the point of closure. The blastema grows rapidly and begins the process of differentiating into a head with a proboscis and collar. At 5 days the blastema has increased greatly in size and differentiated into a central bulb, the forming proboscis, and two lateral crescents, the forming collar. Between 5 and 7 days a mouth opens ventral to the differentiating blastema. Over the next few days the lateral crescents extend to encircle the proboscis and mouth, making a fully formed collar. By 10 to 12 days a new head, sized to fit the worm's body, has grown attached to the severed site. At about this time the animal regains apparently normal burrowing behavior. After the head is formed, a second blastema-like area appears between the new head and the old body and a new branchial region is inserted by regeneration from this blastema over the next 2 to 3 weeks. The regenerating tissues are unpigmented and whitish such that in-situ hybridization can be used to study the expression of genes during the formation of new tissues.

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.