PLOS Biology: Where Do Introns Come From?

Where Do #Introns Come From? A suggestion: exons with premature stops; has implications for #pseudogene formation

We have proposed a novel hypothesis for the origin of spliceosomal
introns, invoking endogenous production within translatable sequences
(at least in the case of protein-coding genes), facilitated by the
activity of cellular surveillance mechanisms. Despite the mutational
hazard associated with intron presence and proliferation [136], we
argue that, at least initially, introns might represent a favorable
life line for an allele that has acquired an ORF-disrupting mutation.
In this sense, in-frame stop codons need not be dead ends, as often
believed, but rather sequences that occasionally facilitate the
evolution of eukaryotic gene structure, possibly favoring not only
intronization, but also processes such as exonization (following a PTC
loss [137]). Further experimental validation of our hypothesis would
not only support the idea that intron birth/death rates depend on both
the population-genetic [136] and the intracellular environment, but
also shed light on a surprising aspect of the evolution of eukaryotic
gene structure, i.e., the ongoing, stochastic process of mutual
conversion between exons and introns within genes.

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