We know that miRNAs play a role in development but also in responses to stresses. So during evolution, due to global climate changes, others organisms needed to develop tools to rapidly control gene expression. So some genes lost functions but they had shared conserved regions with other genes. Did they evolved into MIR genes because of possessing gene regulatory elements ?
Very interesting question Tohir... Personally I do not think so. Regulatory networks based on RNA are a extremely interesting invention in Nature that could be related with the organism complexity. However looks that the miRNA machinery is derived from an ancestral system designed to defend cells against RNA attacks (from virurses, viroids, etc). More likely, miRNAs are probably derived from viral elements that were inserted in the cell genomes, but of course this is a very polemic item.
Regarding the rapid control of gene expression, miRNAs are not the best systems to "rapidly" control anything. They are designed for a tempered homeostatic regulation of biological systems, since they are able to act over hundreds of mRNAs at the same time. So, they are more like "fine tuners". A single miRNA acting over a single mRNA is only to drive a very modest decrease in the protein output.
Of course these are very personal opinions. I am looking forward further discusions.
Good answer Fransisco. yes, you right that miRNA fine tuner. Another thing is if consider that miRNA came from viral genomes then why miRNA acts against virus during infection or it is known very few viral derived miRNAs which maybe got it from host genome during evolution.
Tohir... that's really a good point... Argonaute proteins, involved in miRNA and RNAi mechanisms are intringly conserved in a lot of organisms. Perhaps eukaryotic cells were smart enough to take advantage of viral infections for their profit... Many questions still opened... If you look at the evolution, the invention of the RNA-based regulatory networks is a quantum leap in terms of complexity.
In my opinion, clusters genes like let-7 cluster, conserved along the phylogeny are not the result of a random virus infection. Probably, different mechanisms are acting in the evolution of microRNAs and the degeneration of ex-genes or ex-pseudogenes could as reasonable motor of evolution as virus infections.
In my opinion, non-coding RNAs are a rapid system of evolution. non-coding sequences evolve faster than coding one allowing the cell to explore intensively the environment and found new solutions that are conserved if they works. It is like a fast and low cost evolvability system that use any piece of sequence to evolve.
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