In 1888, a biologist called Henry Orr was collecting spotted salamander eggs from a small, swampy pool when he noticed that some of them were green. He wrote, “The internal membrane of each egg was coloured a uniform light green by the presence in the membrane of a large number of minute globular green Algae.” Orr decided that the eggs “present a remarkable case of symbiosis.” The salamanders and the algae co-existed in a mutually beneficial relationship.
Orr was right that the two species have formed a partnership, but he was wrong in one crucial regard. He thought that the algae (Oophila amblystomatis) simply hung around next to the salamander embryos in the same egg. They don’t. More than 120 years later, Ryan Kerney from Dalhousie University has found that the algae actually invade the cells of the growing embryo, becoming part of its body.
With algae inside them, the salamanders become solar-powered animals, capable of directly harnessing the energy of the sun in the style of plants.
The spotted salamander isn’t the only animal to form partnerships with algae. The emerald green sea slug steals the genes and photosynthetic factories from a type of algae that it eats. Coral reefs are built upon a partnership between corals – a type of animal – and algae that provide them with energy. Many other animals, from sponges to worms have developed similar alliances. But the spotted salamander is the only back-boned animal (vertebrate) to have done so.
Since Orr’s discovery, several scientists have teased apart the details of this relationship. With algae in their eggs, the salamanders are more likely to hatch, they do so earlier, and they’re bigger and more developed when they emerge. All of this depends on light – the algae need it to photosynthesise and provide nutrients and oxygen to the embryos. If the eggs are kept in darkness, they never accumulate algae. In return for their services, the algae feast upon the salmanaders’ waste; if they are presented with eggs that have no embryos inside them, they hardly grow.
All of this was clear, but Kerney was the first to show where the algae were. He noticed that many of the embryonic cells were themselves green. Using time-lapse videos, he saw that the algae moved inside the embryos when they were around two weeks old. After that point, Kerney managed to sequence algal DNA from embryos, even after taking them out of their egg capsules and thoroughly washing them. Finally, Kerney injected the embryos with glowing molecules that latch onto algal DNA. Sure enough, they produced pinpricks of light throughout the salamander’s body.
When the salamanders are still larvae, and well before they become adults, most of their algae disappear. The adults, after all, have opaque bodies and spend most of their lives underground – conditions that are less than ideal for a light-dependent alga. Kerney thinks that the dying algae could provide one final boon to their hosts, providing them with an extra burst of nutrients.
How the algae get into the egg in the first place is a mystery. They might follow trails of salamander waste to find the right eggs. Once there, they enter through the blastopore – an opening in the embryo that will eventually become its anus. Alternatively, the algae could be heirlooms. In a few females, Kerney detected algal DNA in the oviducts – the tubes connecting their ovaries to the outside world. The results were inconsistent, but at the very least, they raise the possibility that salamander mothers might pass the algae to their spawn.
The spotted salamander’s partnership is unique among vertebrates. Normally, our immune systems would destroy any foreign algae that tried to enter our bodies. The salamander gets away with it for two possible reasons. Firstly, the algae invade before its immune system has fully developed. Secondly, salamanders have strangely inefficient immune systems. This might account for their incredible ability to regenerate lost body parts, but it could also mean that they recognise their own cells in a very different way to other animals. Perhaps this lax self-recognition opened the door for invading algae.
There is a final, and simpler, possibility. No one has found another solar-powered vertebrate because no one has looked hard enough. Now, with Kerney’s discovery doing the rounds, it’s likely that they will.
Reference: Kerney, Kim, Hangarter, Heiss, Bishop & Hall. 2011. Intracellular invasion of green algae in a salamander host. PNAS http://dx.doi.org/10.1073/pnas.1018259108