This seems to be a question with not so many good answers. “The origins of genome architecture” (Sinauer, 2007) has a nice chapter on “Genes in pieces” covering
- the spliceosome fisson-fusion model: Did a single ancestral spliceosome diverge into different lineages or is there an endogeneous origin within a single species?
- the introns early-introns late debate: Were introns present in the first prokaryotic cell or not?
Mechanisms about intron gain center about AGGT tetramer duplication that result in new splice sites —AG|gt … ag|GT— with the segment in lower cases being the new intron. (What I found a funny fact, when working for the first time with genomic sequences around 1994 is the “codon ignorance” and “domain ignorance” of introns while on the other hand allowing for alternative splicing). Another mechanism of intron gain may be transposable elements although retrotransposons cannot deliver introns as they are spliced out. More likely are already released introns and ectopic reintegration. Another (not mentioned mechanism) could be random mutations activating cryptic splice sites.
The key question remains Continue reading Why do we possess introns?