Odd Couples: The Salamander and the Alga

Joe Eaton
Monday August 09, 2010 - 11:46:00 PM
A double handful of spotted salamander.
Jared Benedict
A double handful of spotted salamander.

This is not a Bay Area story, at least not yet. But I couldn’t resist writing about one of the coolest pieces of research I’m read about in a very long time. 

It’s about the spotted sandpiper salamander (Ambystoma maculatum), a terrestrial amphibian that’s fairly common in eastern North America. Females lay their clusters of gelatinous eggs in water. The salamander was known to have a symbiotic relationship with a single-called alga called Oophila (“egg-lover”) amblystomatis that lives in the egg jelly wherein the algae provide oxygen to the salamander embryos and the embryos reciprocate with nitrogen-rich waste that the algae feed on. 

But the symbiosis turns out to be much more intimate than that. Ryan Kerney at Nova Scotia’s Dalhousie University has discovered that the alga is not just present in the jelly. He also found it in the embryonic salamanders’ cells. Algae, like true plants, generate oxygen and carbohydrates through photosynthesis, building food from sunlight. Oophila appears to be photosynthesizing inside its host, and may be passing along some of the resulting nutrients. 

Kerney’s evidence for some kind of internal nutrient transfer involves the mitochondria, structures found in all animal cells that use oxygen and a glucose product to manufacture ATP, which stores chemical energy. He found that the mitochondria in the salamander cells were clustered around the algal cells, possibly absorbing the carbohydrates and oxygen the alga bring in. 

The idea of a vertebrate partially powered by sunlight is pretty extraordinary. Similar kinds of relationships between animals and single-celled algae have been documented for coral and for a handful of flatworm and clam species—much simpler organisms than salamanders. Vertebrate immune systems are vigilant about keeping foreign biological material out of cells. Is the alga somehow bypassing the sentries, or has the salamander turned its immune system off? In either case, how would that work? 

Kerney is now trying to figure out when and how the algae enter the salamander cells. Work by Roger Hangarter at Indiana University suggests that this occurs as the embryos’ nervous systems start to form. It’s not clear from what I’ve read so far, but the inference is that the symbiosis is limited to the salamander embryo’s time in the egg. Adult female spotted sandpipers salamanders have been found to have algae in their oviducts, likely where the eggs pick them up, but apparently not in the rest of their bodies. 

This system seems a step beyond the common relationship between, say, termites and their gut flora. It reminds me a bit of the sharpshooters—the group containing the vineyard-pest glassy-winged sharpshooter—and their symbiotic bacteria, which are housed in special organs in the insect’s body and passed on from mother to offspring. 

The research prompted an interesting response from UC emeritus professor David Wake, a salamander authority. “I wonder if algae could be getting into the germ cells,” Wake told a reporter for Nature. “That would really challenge the dogma. But why not?” Germ cells are sex cells—gametes. Would this mean that algal genes have become part of the salamander’s genome? That would suggest horizontal gene transfer, a rare process in multicellular organisms, had been at work. 

Horizontal gene transfer is a matter of genes being exchanged between two different biological species. Bacteria are notorious for this; people who work with bacteria like to say that the iconic Tree of Life is beginning to look more like a net, or a web, with all kinds of cross-connections between evolutionary lineages. The process is also believed to have occurred between unrelated species of plants; also between a fungus and the order of plants that includes beets and cacti, a fungus and a species of aphid, and an alga and a sea slug. Viruses, which are good at freighting bits of genetic material around, may be involved in the process. 

Even if the spotted salamander hasn’t taken algal genes on board, the phenomenon Kerney describes is remarkable enough. You have to wonder how old this partnership is, and whether it involves other salamander species related to the eastern spotted, including our local western tiger and long-toed salamanders. I assume someone is looking into it.