Developing salamander embryos in eggs that are green due to symbiotic algae. Image credit: Roger Hangarter

The struggle to live in a salamander

In the symbiotic relationship formed between algae and salamanders, only the algae show stress.

Throughout the natural world, when different species form a close association, it is known as a symbiosis. One species can depend on another for food, defense against predators or even for reproduction. Corals, for example, incorporate single-celled algae into their own cells. The algae photosynthesize, harnessing energy from sunlight to make sugars and other molecules that also feed the coral cells. In return, corals protect the algae from the environment and provide them with the materials they need for photosynthesis. This type of relationship where one organism lives inside another species is called an endosymbiosis.

In animals with a backbone, endosymbioses with a photosynthetic organism are rare. There is only one known example so far, which is between a green alga called Oophila amblystomatis and the spotted salamander, Ambystoma maculatum. The female spotted salamander deposits her eggs in pools of water, and algae enter the eggs, proliferate, and later invade tissues and cells of the developing embryos. However, it is not understood how similar the interaction between the alga and the salamander is to that in coral-algal symbioses, or whether it is rather more similar to a parasitic infection.

John Burns and co-workers now address this question by comparing salamander cells harboring algae to those that lacked algae. A technique called RNA-Seq was used to characterize the changes in gene activity that take place in both organisms during the endosymbiosis. The results show that algae inside salamander cells are stressed and they change the way in which they make energy. Instead of carrying out photosynthesis to produce food for the salamander host — as happens in coral-algal interactions — Oophila amblystomatis is fighting to adapt to its new environment and switches to a less efficient energy producing pathway known as fermentation.

Burns and colleagues found that, in striking contrast to the alga, affected salamander cells do not show signs of stress. Instead several genes that are known to suppress immune responses against foreign invaders are expressed to high levels. This may explain how salamander cells are able to tolerate algae inside them.

The next challenge is to understand how the alga enters salamander cells. The current work identified some potential routes of entry, and follow up studies are now needed to explore those possibilities.

To find out more

Read the eLife research paper on which this eLife digest is based: “Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis” (May 2, 2017).
Read a commentary on this research paper by Steven Ball and Ugo Cenci: “Endosymbiosis: Gasping for air”.
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
This text was reused under the terms of a Creative Commons Attribution 4.0 International License.