Looks like cephalopods have useful, active, and conserved ADAR activity that changes an Adenosine to an Inosine in many RNA transcripts. ADAR (Adenosine Deamin

Looks like cephalopods have useful, active, and conserved ADAR activity that changes an Adenosine to an Inosine in many RNA transcripts. ADAR (Adenosine Deaminase acting on RNA) is a protein that recognizes specific RNA sequences (likely, specific structures) [1]. And upon recognition it switches an RNA's 'A' to an 'I' (a non-canonical base pair (RNA is usually made of A, C, U, & G only)). The ribosome compiles a non-canonical 'I' as if it were a 'G'. So this process effectively intercepts some DNA sequences prior to compilation and swaps some (very particular) A's to G's with respect to coding a protein. This process was found by noticing this very consistent 'mistake' between DNA and and expected sequences in high-throughput analysis. So during the minification of the genetic code prior to compiling, under certain conditions some very specific characters are swapped for out-of-bounds characters that are read-through as an alternative character (if the surrounding code suggests it should). So you can have alternate builds without changing either the source code or the compiler, but instead by turning on/off a bit in the the minifier (expression of ADAR protein). This process happens a bit in humans and flies, but seems to happen a lot in squid - specifically in the code around compiling neuronal proteins. Curiously they show that this editing process slows down the evolution around those neural proteins that utilize this mechanism. Because the ADAR protein recognizes the structure of an entire sequence set in order to perform its A->I swap, if there is any mutation in the entire sequence set, no A->I swap occurs. And if no A->I swap occurs, the neural protein likely doesn't work as well, lowering the fitness in any individual that has sequence drift anywhere in that entire recognized region. In organisms that do not have this A-I shift mechanism there is more evolutionary sequence drift in similar proteins than in Cephalopods which utilize the A->I swap. Super cool! [1] https://en.wikipedia.org/wiki/ADAR1 https://en.wikipedia.org/wiki/ADAR1