Think of an animal – any animal you’d like. Unless you’re a big fan of sponges or jelleyfish, or a protistologist perhaps, you likely thought of an animal that belongs to the Bilateria. This group includes all animals that show bilateral symmetry, so every single insect, vertebrate and mollusk belongs to this group. Like in any large family, some members don’t seem to fit in, like the radial starfish and sea urchins. But on closer inspections, these species are bilaterally symmetrical in their larval stages. The most recent common ancestor of all Bilateria is expected to have lived somewhere between 600 and 550 million years ago. This creature probably had many of the complex features seen in bilaterals today, like a centralized nervous system, internal organs and a blood circulation system. But where do we have to look if want to reconstruct the traits of this ancient ancestor? As this ‘Urbilaterian’ probably didn’t leave any fossils behind, scientists have to look in other places to find their evidence. In a Nature paper published last month, a team of researchers used the conserved expression of mircoRNAs to piece together some information about this great-great grandmother of animals.
MicroRNAs are short strands of RNA that have the ability to silence genes, by binding to the mRNA molecules in a complementary fashion. Think of a microRNA in this way: some genes that play a central role in neuronal cells, are useless for cells that make up the gut. To preven the neuronal genes from being translated in the gut, the cells silence them by expresssing a single set of microRNAs that bind the neuronal gene transcripts. Since this happens in all tissues, it’s safe to say that microRNAs play a central role in the establishment and maintenance of diverse tissue types. The evolution of Bilateria coincides with the evolution of many complex tissue types in this group of animals, so microRNAs have the potential to be a great source of evidence for the evolution of this large group of animals.
Foteini Christodoulou and colleagues analyzed the localization of a whole bunch of microRNAs in a wide variety of species, and infer some characteristics of the ‘Urbilaterian’. The team performed these analyses in the all star team of animals above. The only species that does not belong to the Bilateria is the sea anemone Nematostella you see in the bottom left. The top two creatures are marine worms (annelids, to be specific), which are suspected to have retained many ancestral bilaterian features. In the bottom right we find the sea urchin, Strongylocentrotus, which hides its bilaterism very well, as described in the introduction. You can see how these organisms are evolutionary related in the phylogenetic tree below.
The paper contains several interesting experiments and conclusions, based on localized microRNA expression. Some microRNAs appear to have evolved early on in bilaterian evolution. In the larval forms of both worms and the sea urchin, miR-29, miR-34 and miR-92 were only expressed in cilia cells that are used for locomotion. This set of microRNAs is really old, as the most recent common ancestor of sea urchins and annelids lived just after the split between bilaterals and other animals (in fact, sea urchins and annelids span the protostome / deutorostome divide). In another example, the team analyzed the localization of miR-9 and miR-9*. In mice, both microRNAs are found in the olfactory brain centres that are responsible for processing smells. In one of the worms, miR-9 and miR-9* localized to the base of of a pair of antenna. One of the functions of these antenna is to sense chemicals in the environment of the worm. So in species as distantly related as mice and annelids, this microRNA pair localizes to specialized cells involved in the processing of chemical information! This feature has been retained over the past 500 million years.. pretty cool stuff!
The team found microRNAs that were more or less specific for almost any tissue: gut, muscles, central nervous system, sensory organs, you name it! Aside from generating some beautiful images (see above), this also means that microRNAs played a very important role in the establishment of a wide variety tissues. The fact that some of these localizations have been conserved over hundreds of millions of years only highlights in their importance. In that sense, microRNAs seem to deserve a place alongside HOX genes as robust and conserved signals of animal evolution. Identifying more microRNAs, their localization and their gains and losses in evolution will allow researchers to gain deeper insight into why animals are the animals that they are today.
After all – I’m still an animal.
Christodoulou, F., Raible, F., Tomer, R., Simakov, O., Trachana, K., Klaus, S., Snyman, H., Hannon, G., Bork, P., & Arendt, D. (2010). Ancient animal microRNAs and the evolution of tissue identity Nature, 463 (7284), 1084-1088 DOI: 10.1038/nature08744
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