Embryonic liaisons

Fig. 1: Stunning images from Kerney et al. (2011). (A) depicts the egg capsule, and (C) shows the head of the developing organism.
‘Occasionally, researchers stumble across something extraordinary in a system that has been studied for decades.’
Anna Petherick, Nature, commenting on Kerney et al.’s discovery 

On occasion, biology’s surreal complexity can seem completely absurd. A few thoughts about life’s limitless microcosms – with their fragile, living enigmas and infinite struggles – and it’s a little hard not to laugh. I, at least, find the whole thing thoroughly terrifying. But please, no hysteria here. Zootrogle policy. It’s best to keep calm and carry on – even if only for appearance’s sake.

I felt pretty much compelled to start with such a cautionary preamble because today’s snippet from science is about as surreal and absurd as it’s possible to be. It involves salamander embryos and a spectacular level of symbiotic intimacy, and is essentially a wonderful story of evolutionary ingenuity.

If you just happen to come across the embryo of a spotted salamander, the first thing you should probably ask is “but why is it so green?”  The answer, if a spotted salamander authority just happens to be passing, will apparently be “Oophila”. (This will not be particularly helpful, unless you are an authority on algae, in which case you probably wouldn’t have asked the question in the first place.) We are talking about Oophila amblystomatis, an agreeable unicell in a serene and pleasant mutualism with its amphibious collaborator (Ambystoma maculatum). Hence the green veneer. The arrangement is like this: nitrogeneous wastes drift out of the embryo and into the alga, and oxygen drifts out of the alga and into the embryo.

Or, at least, that’s what your helpful salamander expert would have said a couple of years ago. Substantial work on O. amblystomatis can be traced to 1927, when somebody called Lambert Printz dignified it with the present name, but, although its mutualism was recognised decades before Printz, only this year have the remarkable facts surfaced. Incredibly, it has now become clear that O. amblystomatis constitutes the first case of a photosynthetic alga engaging in intracellular symbiosis with a vertebrate.

How does this brilliant embryonic conspiracy develop? In turns out that, before even the mouth’s prototype (the stomodeum) develops, most Oophila cells slip in through the blastopore, while others will actually transit the surfaces of the developing salamander. Plausibly, the myriad microscopic algal blooms observed as endosymbiosis is established are responses to the expulsion of nitrogenous byproducts: the algae are seeking the door. As Nature‘s Anna Petherick points out, ‘if waste is released,… there must also be a way in’.

Uncertainty remains over the adaptive benefits to each organism beyond the basic  material exchanges mentioned above. Intriguingly, mitochondria have been found congregating around Oophila (suggesting that oxygen is indeed playing an important role). But just think: a phototroph in the cells of a vertebrate, miraculously not destroyed by the immune system’s violent tantrums. It’s a tremendous thing to say, and a genuinely exciting insight. Crucial questions remain over the strange evolutionary trajectory such a system must have taken, and Roger Hangarter, a coauthor on the Kerney paper, hints at greater things to come:

‘It raises the possibility that more animal/algae symbioses exist that we are not aware of. Since other salamanders and some frog species have similar algae/egg symbioses, it is possible that some of those will also have the type of endosymbioses we have seen in the spotted salamander.’
Roger Hangarter, cited in Indiana University News, 2011.

Wonderful stuff. It’s this sort of thoroughly surprising phenomenon that makes biology so abidingly fascinating. And ok, I give in – maybe a little bit of euphoric hysteria might be just about excusable…

See the following for more:

Kerney, R., Kim, E., Hangarter, R. P., Heiss, A. A., Bishop, C. D., & Hall, B. K. (2011). Intracellular invasion of green algae in a salamander host. PNAS 108: 6497-6502.
Petherick, A. (2010). A solar salamander. Nature News. doi:10.1038/news.2010.384
IU News Room (2011). Algae that live inside the cells of salamanders are the first known vertebrate endosymbionts. Indiana University.

2 responses to “Embryonic liaisons

  1. Wow how amazing and interesting. Well done; your blog is again beautifully written and leaves me with a touch euphoric hysteria! Always a nice feeling to start a new day with.

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