Immature symbiotic system between horizontally transmitted green algae and brown hydra

Some strains of brown hydra (Hydra vulgaris) are able to harbor the green algae Chlorococcum in their endodermal epithelial cells as symbionts. However, the relationship between brown hydra and chlorococcum is considered to be incipient symbiosis because most artificially introduced symbionts are not stable and because symbiotic H. vulgaris strains are rare in the wild. In this study, we compared the gene expression levels of the newly established symbiotic hydra (strain 105G), the native symbiotic strain (J7), and their non-symbiotic polyps to determine what changes would occur at the early stage of the evolution of symbiosis. We found that both the 105G and J7 strains showed comparable expression patterns, exhibiting upregulation of lysosomal enzymes and downregulation of genes related to nematocyte development and function. Meanwhile, genes involved in translation and the respiratory chain were upregulated only in strain 105G. Furthermore, treatment with rapamycin, which inhibits translation activity, induced the degeneration of the symbiotic strains (105G and J7). This effect was severe in strain 105G. Our results suggested that evolving the ability to balance the cellular metabolism between the host and the symbiont is a key requirement for adapting to endosymbiosis with chlorococcum.

Automated summary

The study compared the gene expression levels of newly established symbiotic hydra, native symbiotic hydra, and non-symbiotic hydra, revealing upregulation of lysosomal enzymes and downregulation of genes related to nematocyte development in symbiotic strains. Translation and respiratory chain genes were upregulated only in the newly established symbiotic strain. Treatment with rapamycin, inhibiting translation, induced degeneration in both symbiotic strains, particularly severe in the newly established strain. Results suggest that balancing cellular metabolism between host and symbiont is crucial for adapting to endosymbiosis with Chlorococcum.