Lack of Overt Genome Reduction in the Bryostatin-Producing Bryozoan Symbiont Candidatus Endobugula sertula

The uncultured bacterial symbiont Candidatus Endobugula sertula is known to produce cytotoxic compounds called bryo-statins, which protect the larvae of its host, Bugula neritina. The symbiont has never been successfully cultured, and it was thought that its genome might be significantly reduced. Here, we took a shotgun metagenomics and metatranscriptomics approach to assemble and characterize the genome of Ca. Endobugula sertula. We found that it had specific metabolic deficiencies in the biosynthesis of certain amino acids but few other signs of genome degradation, such as small size, abundant pseudo-genes, and low coding density. We also identified homologs to genes associated with insect pathogenesis in other gammaproteobacteria, and these genes may be involved in host-symbiont interactions and vertical transmission. Metatranscriptomics revealed that these genes were highly expressed in a reproductive host, along with bry genes for the biosynthesis of bryo-statins. We identified two new putative bry genes fragmented from the main bry operon, accounting for previously missing enzymatic functions in the pathway. We also determined that a gene previously assigned to the pathway, bryS, is not expressed in reproductive tissue, suggesting that it is not involved in the production of bryostatins. Our findings suggest that Ca. Endobugula sertula may be able to live outside the host if its metabolic deficiencies are alleviated by medium components, which is consistent with recent findings that it may be possible for Ca. Endobugula sertula to be transmitted horizontally.