Curated by RSF Research Staff
Study reveals surprising find in cephalopod species
In the expanded evolutionary synthesis -- an update of Darwinian theory -- it is understood that evolutionary changes can occur via epigenetic mechanisms. That is, changes in gene expression without altering the specific underlying nucleotide sequence of loci in the genome.
One such remarkable method is known as RNA-editing, a post-transcriptional process that allows the diversification of proteomes beyond what is encoded in the genome. Ribonucleic acid (RNA) is the hydroxylated form of DNA (deoxyribonucleic acid), and functions as the intermediary messenger of genomic information, as well as an enzymatic catalyst.
Genetic information, encoded in the DNA base-pair sequence, is transcribed into messenger RNA and the RNA transcripts, newly transcribed from the gene loci of the genome, are modified before being translated into functional and structural proteins. Normally, this alteration of the RNA transcript does not change its faithful correspondence with the genetic information encoded in the DNA. However, in RNA-editing, the nucleotide bases are changed to non-canonical forms, such as converting adenine into inosine, which is then read differently than the original transcript -- producing a novel gene product.
There are a large host of potential RNA alterations that can take place: in our manuscript The Unified Spacememory Network we discuss the 100-plus naturally occurring nucleotides and how they factor into the evolutionary emergence of the first molecular replicators.
Moreover, there are numerous ways in which such epigenetic editing can take place, and it is often mediated by transposons. Transposons are genetic elements that are foreign DNA introduced into host genomes, often via retroviral gene insertion. The human genome contains hundreds of thousands of such inserts that completely shape the transcriptome landscape.
In a recent study, it was found that cephalopod species utilize such mechanisms to a remarkable degree. Scientists discovered that more than 60 per cent of RNA transcripts in the squid brain are re-coded by editing. In most other animals such re-coding events only occur a fraction of the time. Similar high levels of RNA editing were identified in three other “smart” cephalopod species, two octopuses and one cuttlefish -- confirming their reputation as "Earth-bound aliens".
Such epigenetic mechanisms can allow species that utilize it, like humans and cephalopods, to change the neuronal wiring of the brain. The mobile genetic elements direct somatic nuclear recombination, which drives neuronal genetic diversity in the host organism. Such neuronal plasticity, down to the genetic level, could be a central evolutionary driving mechanism giving rise to the relatively large brains observed in humans, cephalopods, and other key species.