Small nuclear RNAs can regulate gene expression at multiple steps and in many ways. While this section has focused on ncRNAs in eukaryotes, small ncRNAs are also used by bacteria as a defense system, called the CRISPR-Cas9 system, against viruses that infect them. The use of ncRNAs thus evolved long ago, but we don’t yet know how bacterial ncRNAs are related to those of eukaryotes.
What about the evolutionary significance of small eukaryotic ncRNAs? In general, extra levels of gene regulation might allow evolution of a higher degree of complexity of form. The versatility of micro RNA regulation has therefore led some biologists to hypothesize that an increase in the number of different miRNAs specified by the genome of a given species has allowed morphological complexity to increase over evolutionary time. While this hypothesis is still being evaluated, it is logical to expand the discussion to include all small ncRNAs. Exciting new techniques for rapidly sequencing genomes have allowed biologists to begin asking how many genes for ncRNAs are present in the genome of any given species. A survey of different species supports the notion that small interfering RNAs evolved first, followed by miRNAs and later piwi-interacting RNAs, which are found only in animals. And while there are hundreds of types of miRNAs, there appear to be 60,000 or so types of piRNAs, allowing the potential for very sophisticated gene regulation by piRNAs.
Given the extensive functions of ncRNAs, it is not surprising that many of the ncRNAs characterized thus far play important roles in embryonic development. Embryonic development is perhaps the ultimate example of precisely regulated gene expression.
Urry, Lisa A.. Campbell Biology (p. 379). Pearson Education. Kindle Edition.