What's the News: Spotted salamander embryos, a recent study found, have green algae living inside their cells. While scientists have long known that the two species are symbiotic, each helping the other to survive, the new findings show that the arrangement is, in the researchers' words, "more intimate than previously reported." In fact, it's the first such organism-within-cell partnership---known as endosymbiosis---ever observed in vertebrates. How the Heck:
Spotting a cell within a cell isn't easy. The researchers used fluorescent techniques to spot the algae, since their chlorophyll glows under certain types of light, and RNA probes to measure whether the algae's genetic material---and therefore, the algae cells themselves---were still intact.
Salamanders lay their eggs in ponds, also home to algae of the species Oophila amblystomatis, whose genus means "egg-loving." It may be that that's when the algae burrows into the cells of the salamander embryos. Alternatively, parents might pass the algae on to their offspring.
However it happens, algae takes up residence throughout a salamander embryo early on, when different tissues are still differentiating. Later on, the algae is mostly in cells in the salamander's digestive tract.
What's the Context:
Carrying around an intracellular hitchhiker, or being stuck in someone else's cells, doesn't sound great, but earlier work suggests that each species benefits from the presence of the other. In order to develop normally, salamander embryos need oxygen, which the algae produces. The algae needs lots of nitrogen and a place to stay, and salamander cells meet both criteria.
While this is the first time endosymbiosis has been found in vertebrates, it's been observed in lots of other living things, like the nitrogen-fixing bacteria that live on the roots of some plants.
One prominent theory holds that this kind of arrangement led to the advent of eukaryotes and gave rise to mitochondria and chloroplasts, two self-contained, gene-carrying cellular components found in animal and plant cells, respectively.
Image of chlorophyll-colored salamander embryos courtesy of Roger Hangarter