Ancient brains revealed by microRNA?

Detlev Arendt's lab at EMBL looks at the evolution of the central nervous system in Bilateria. A brand new letter in Nature, Ancient animal microRNAs and the evolution of tissue identity, explores the link between the birth of ancient microRNAs and body plan evolution.

In the press release (read it on ScienceDaily), as is not uncommon, the conclusion of the work is taken just a little bit further than what the study warrants, I think:

Animals from different branches of the evolutionary tree -- different lineages -- possess specific microRNAs that evolved only in their lineage. But they also have microRNAs in common: ones which they inherited from their last common ancestor, and which have been conserved throughout animal evolution.

(...)

The scientists reasoned that if an ancient microRNA is found in a specific part of the brain in one species and in a very similar location in another species, then this brain part probably already existed in the last common ancestor of those species. Thus, they were able to glean a glimpse of the past, an idea of some of the traits of the last common ancestor of worms and humans.

MicroRNA are short RNA nucleotide sequences regulating gene expression mainly by supressing it. So they say that because one specific microRNA is found working in a specific part of the worm's brain, and this same microRNA also works in the same part of the human brain, then the common ancestor also had this same brain part. Saying that this microRNA has been conserved since the last common ancestor - something that the conclusion hinges on - is a bit of a mouthful. It could be true, but it is an assumption that could be contested.

From their abstract in Nature:

We reason that any specific localization shared between protostomes and deuterostomes (the two major superphyla of bilaterian animals) should probably reflect an ancient specificity of that microRNA in their last common ancestor.

This last common ancestor lived sometime during the Cambrian (say 555 million years ago), and we are supposed to infer that a 22 long microRNA could not have changed its role on regulating genes expressed in the brain?

Somehow I am not convinced that it would be impossible, or even unlikely, that microRNA and other regulatory factors change affinity frequently enough to render such inference dubious. In the paper they say this.

At the present state of analysis, we can only speculate about the tissue-specific ancient roles of these microRNAs, by extrapolating from observations in other animal models. [Emphasis added.]

And that's a fine thing to say. But going from this to "Last Ancestor Humans Shared With Worms Had Sophisticated Brain, microRNAs Show" is a little over the top.


a–d, Apical view of 48 h.p.f. Platynereis with expression in dorso-medial brain (green, acTub; red, microRNAs). e, Apical view of 72 h.p.f. Platynereis brain section showing colocalization of microRNA (red) with serotonin (cyan) (blue, DAPI, green, acTub). f, g, Apical views of 5 d.p.f. Platynereis brain. h, 3D reconstruction of 5 d.p.f. apical brain with superficial expression at the base of antennae.

Reference
Christodoulou F, Raible F, Tomer R, Simakov O, Trachana K, Klaus S, Snyman H, Hannon GJ, Bork P, & Arendt D (2010). Ancient animal microRNAs and the evolution of tissue identity. Nature PMID: 20118916