Due to their adaptive immune systems, it's been long assumed that vertebrates are unable to share cellular space with another organism in a mutualistic relationship. But within various tissues and cell types of embryonic spotted salamanders live smaller algal cells, which may somehow benefit the vertebrate, according to new linkurl:research;http://www.pnas.org/content/early/2011/03/29/1018259108 published today (April 4) in Proceedings of the National Academy of Sciences.
Spotted salamander, Ambystoma maculatum, eggs with developing embryos. The algae in the yolk sacs is visible to the naked eye. Image: Courtesy of Ryan Kerney
"It's really a tremendous finding... that gives us a new angle on algal symbiosis," said linkurl:Angela Douglas,;http://www.angeladouglaslab.com/ an insect physiologist at Cornell University who was not involved in this research. While there are many examples of invertebrates that harbor intracellular mutualists or pathogens, vertebrates were considered an exception, able to host parasites alone, she said. "This really doesn't make sense: how can a parasite persist but not a friend?"
Spotted salamanders, common in the northeast United States, lay their eggs in low-oxygen vernal pools where they undergo metamorphosis into their adult forms. In more than a century of study, scientists have witnessed algae growing on the outside of the egg cells, and found that clutches without the algae tend to have lower survival. But they have always assumed that this was an exclusively external mutualism.
linkurl:Ryan Kerney;http://web.mac.com/ryankerney/Site/Welcome.html of Dalhousie University was observing live tissue under a fluorescent microscope to understand how the algae penetrated the egg sac to reach the cell surface when he noticed specks of fluorescence inside the embryonic tissue. Upon closer examination, the team confirmed that those specks were algal cells and that they were inside the cells of the salamander embryos. Not only did the algae live on the surface of developing salamander embryos, but they invaded their cells as well. (See slideshow below for images.)
"What we've shown is that there is an intracellular stage of this symbiosis which was previously thought to be outside the body entirely," said Kerney. "Nobody knew that this association was at all this intimate."
What exactly the algae are doing inside the salamander embryos is still unclear, however, noted linkurl:Sidney Pierce,;http://biology.usf.edu/ib/faculty/spierce/ a cellular physiologist at the University of South Florida who was not involved in the research. Previously work has suggested that salamander eggs and embryos benefit from increased oxygen produced by the algae in their low oxygen environment, but the studies have been inconclusive. And the algae may benefit from the embryos' nitrogenous waste, but again, more research is needed to confirm the idea. "Without really knowing whether the algae is providing any advantage to the embryo or not, you just don't know what to make of it," Pierce said.
Furthermore, the algal cells don't seem to stick around for long. As the embryos developed into larvae, Kerney and his team saw the algae's fluorescence disappear, possibly as a result of an attack by the salamander's adaptive immune system. But Kerney also found algal DNA in adult salamander cells, raising the possibility that the algal cells go dormant in the cytoplasm for transmission from parent to offspring, though he notes that the evidence is far from conclusive.
The finding of algal cells living inside salamander cells thus raises more questions than it answers. "The authors have done a spectacular job, but they haven't solved everything," said Douglas. "There are definitely mysteries remaining. This is almost like the beginning."
R. Kerney et al., "Intracellular invasion of green algae in a salamander host," Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1018259108, 2011. **__Related stories:__*** linkurl:Astute algae, conned corals;http://www.the-scientist.com/blog/display/55554/ [31st March 2009]*linkurl:Evolutionary Teamwork;http://www.the-scientist.com/2004/5/10/33/1/ [10th May 2004]
**__Related F1000 Evaluations:__** *linkurl:A secondary symbiosis in progress?;http://f1000.com/1028724?key=86h2q03896ftmnq N. Okamoto and I. Inouye, Science, 310(5746):287, 2005. Evaluated by Debashish Bhattacharya (Univ of Iowa) and William Martin (Univ of Dusseldorf). *linkurl:50 million years of genomic stasis in endosymbiotic bacteria;http://f1000.com/1008145?key=v9k0m3mnjrhm4tn I. Tamas et al., Science, 296(5577):2376-9, 2002. Evaluated by David Stern (Princeton Univ), Christoph Dehio (Univ of Basel), and Tamar Barkay (Rutgers Univ).