Immature symbiotic system between horizontally transmitted green algae and brown hydra

Abstract

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.

Symbiont specificity differs among green hydra strains

“…Interestingly, the expression patterns of the genes related to translation, respiratory chain and electron transport changed in host organisms of other endosymbiotic systems in the presence or absence of symbionts, although the direction of regulation is inconsistent. For example, genes involved in translation and the respiratory chain were upregulated only in the newly established symbiotic brown hydra ( Hydra vulgaris 105G) but not in native symbiotic brown hydra ( H. vulgaris J7) [ 40 ]. Acropora tenuis infected with S. trenchii (clade D) showed downregulation of genes involved in translation and upregulation of genes involved in electron transport compared with apo-symbiotic corals [ 39 ].…”

“…Our previous study suggested that a balanced cellular metabolic system would be a key factor for stabilizing and adapting to symbiosis at the early stage of symbiosis evolution in the symbiotic brown hydra, H. vulgaris [ 40 ]. Interestingly, the K10exc polyps showed drastic changes in the expression pattern of genes involved in translation and the respiratory chain, like a newly established symbiotic brown hydra ( H. vulgaris 105G), suggesting a cellular metabolic system in the K10exc cells would be unbalanced ( table 3 ).…”