Since my research focuses on primates, I don’t exactly work with plant microRNAs. But they’re still fascinating enough that I wanted to touch on them. Plant and animal microRNAs are very similar – they’re approximately 22 nucleotides in length, they’re processed from larger hairpin structures, and they function by downregulating messenger RNA. But they have a number of differences because microRNA in plants and animals evolved independently.
Yes, this similar system arose separately in the plant and animal kingdoms. No, this is not proof for God. This is an example of convergent evolution, where the same trait is acquired independently in different lineages. Think of the ability to fly in insects, birds, and bats. The evolution of microRNA is the same, it’s just more molecular instead of having an obvious effect like flight, which is visible to the naked eye.
Why do we think plant and animal microRNA evolved independently? One major piece of evidence is that there are no homologous microRNAs between plants and animals (homologous meaning shared through a common ancestor). This is especially striking when you compare it to microRNAs within animals, a number of which are homologous. There are some animal microRNAs present throughout the whole animal kingdom, from sponge to fruit fly to orangutan, that just don’t exist in plants. Plants have their own set.
Another thing supporting independent evolution is that plants and animals have different processes for generating mature microRNA. In plants, microRNA is fully matured in the nucleus before being shipped out to the cytoplasm for use. In animals, much of the processing takes place out in the cytoplasm. Animals have additional proteins that are involved in processing – I’ll touch on it a little more in my next post. Also, plant and animal microRNA differs in how it targets messenger RNA. In plants, the whole ~22 nucleotide microRNA is involved in complementary base-pairing with the messenger RNA. In animals, only a 7 nucleotide “seed region” of the ~22 nucleotide mature sequence determines which messenger RNA it’s supposed to match up with.
A final piece of evidence is that microRNAs are missing in other forms of life. They’re absent in fungi, placozoans (the most basal animal lineage), and choanoflagellates (the closest living relative to animals). It’s more likely, especially considering the other evidence, that microRNA arose twice independently, rather than microRNA being lost multiple times in the specific lineages that happen to make it look like it arose twice independently. The latter would be getting into “Satan buried the dinosaur bones to make it look like a natural process” territory!
This is post 8 of 49 of Blogathon. Donate to the Secular Student Alliance here.