Oophila, the amphibian egg-loving green alga: From symbiotic interactions to evolutionary trajectories

Oophila spp. are the only green algae known to establish symbiotic associations with the eggs of amphibians, including salamanders and frogs. They comprise two phylogenetic lineages, Clades A and B, each exhibiting a distinct degree of host specificity. In the North American spotted salamander Ambystoma maculatum, a symbiotic green alga Oophila amblystomatis has been shown to colonize not only the egg capsules but also the embryonic tissues and cells. Consequently, this system has attracted considerable attention as the only known algal-vertebrate symbiosis in nature. While previous studies have demonstrated the geographic distribution, phylogenetic diversity, and symbiotic lifestyle of Oophila, many fundamental questions remain unanswered: Where does Oophila originate? How does it establish symbiosis within amphibian eggs? How has this symbiosis evolved? Here we summarize the current knowledge and remaining challenges at each stage of the algal-amphibian symbiosis, including the life history and ecology of Oophila, host–symbiont interactions, and the origin and evolution of this unique symbiotic lifestyle.