Viral RNA Trans-Splicing: Chance Event or Product of Evolution?

One year after the discovery of exons and introns in the adenoviral hexon gene by the teams of Roberts and Sharp [1,2], Walther Gilbert suggested that the employment of different exons of a single gene could lead to the generation of various mRNA isoforms [3], a process that is today known as alternative splicing. Alternative splicing implies a number of distinct mechanisms including exon skipping, intron retention, mutually exclusive exons, alternative splice site selection as well as alternative promoter usage and alternative polyadenylation. Constitutive and alternative splicing isn’t mutually exclusive which solves the former problem that for a steady evolutionary process, essential genes don’t need to duplicate before one version can mutate to a new gene function. Alternative splicing generates new functions by creating new exons within existing genes and can relax the negative selection pressure against evolutionary changes in functional genes. Thus, alternative splicing might open nearly neutral paths for evolutionary changes.