Ecology, evolution and development of a symbiosis between a green alga and some amphibians

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Symbioses are fascinating biological interactions that occur when two or more species interact in a sustained manner, usually for mutual benefit. This phenomenon is common in nature and often involves smaller autotrophic (self-feeding) symbionts that live on or inside optically transparent or translucent host structures. Two celebrated examples include reef-building corals, which house single celled photosynthetic organisms called dinoflagellates and deep sea vent organisms such as mussels or tube worms, which house sulfur-eating bacteria. Symbiotic associations can be extremely ecologically important: entire ecosystems (e.g., The Great Barrier Reef and Deep Sea Vent Communities) are generated as a result of these symbioses. Despite numerous examples of symbioses between photosynthetic eukaryotes and animals, no such interaction had been observed among vertebrates, the most complex animals on the planet. We discovered that unicellular green algae (Oophila amblystomatis), long known to form an ectosymbiotic association with embryos of the yellow-spotted salamander Ambystoma maculatum actually invade tissues and cells of their embryonic hosts. Many subsequent questions have now been raised and this proposal aims to answer some of them.***Many North American amphibians reproduce by depositing egg masses in ponds and three other amphibians form ectosymbioses with Oophila. Our studies provided preliminary evidence that different lineages of Oophila preferentially form associations with each of the different host egg masses. Therefore our first task is to further investigate host-symbiont specificity because it could be the first step to determining how the endosymbiotic association evolved. The second aspect of our work is to find out where the algae that inhabit egg masses are coming from. The most obvious place is to look for the algae in the pond water, but our original work suggested that mothers are transmitting algae to their eggs, a process called vertical transmission. We intend to test whether algae are acquired from surrounding pond water or from the parents of the embryos and therefore solve one of the key mysteries of this association. Our third aim is to test a hypothesis that the algae, through the provision of oxygen (a byproduct of photosynthesis), are conferring upon the embryos increased tolerance to thermal stress. In a warming climate, it is important to be able to predict organismal responses to thermal stress. By manipulating the amount of algae in the egg capsules, we shall be able to test the role that algae play in protecting embryos from stress due to warming. In summary, we propose to examine several aspects of the ecology, evolution and developmental progression of this fascinating, newly discovered, biological interaction. *** **