A Reference Genome from the Symbiotic Hydrozoan, .

Various species have been employed as model organisms since the 18 century. Introduction of transgenic and knock-down technologies made them ideal experimental systems for studying cellular and molecular mechanisms involved in regeneration, body-axis formation, senescence, symbiosis, and holobiosis. In order to provide an important reference for genetic studies, the genome (species name has been changed to ) was sequenced a decade ago (Chapman , 2010) and the updated genome assembly, Hydra 2.0, was made available by the National Human Genome Research Institute in 2017. While belongs to the non-symbiotic brown hydra lineage, the green hydra, , harbors algal symbionts and belongs to an early diverging clade that separated from the common ancestor of brown and green hydra lineages at least 100 million years ago (Schwentner and Bosch 2015; Khalturin , 2019). While interspecific interactions between and endosymbiotic unicellular green algae of the genus have been a subject of interest for decades, genomic information about green hydras was nonexistent. Here we report a draft 280-Mbp genome assembly for strain A99, with a scaffold N50 of 1.1 Mbp. The genome contains an estimated 21,476 protein-coding genes. Comparative analysis of Pfam domains and orthologous proteins highlights characteristic features of , such as diversification of innate immunity genes that are important for host-symbiont interactions. Thus, the assembly provides an important hydrozoan genome reference that will facilitate symbiosis research and better comparisons of metazoan genome architectures.