Exon skipping-rich transcriptomes of animals reflect the significance of exon-shuffling in metazoan proteome evolution.

ᅟ: Animals are known to have higher rates of exon skipping than other eukaryotes. In a recent study, Grau-Bové et al. (Genome Biology 19:135, 2018) have used RNA-seq data across 65 eukaryotic species to investigate when and how this high prevalence of exon skipping evolved. They have found that bilaterian Metazoa have significantly increased exon skipping frequencies compared to all other eukaryotic groups and that exon skipping in nearly all animals, including non-bilaterians, is strongly enriched for frame-preserving events. The authors have hypothesized that "the increase of exon skipping rates in animals followed a two-step process. First, exon skipping in early animals became enriched for frame-preserving events. Second, bilaterian ancestors dramatically increased their exon skipping frequencies, likely driven by the interplay between a shift in their genome architectures towards more exon definition and recruitment of frame-preserving exon skipping events to functionally diversify their cell-specific proteomes." Here we offer a different explanation for the higher frequency of frame-preserving exon skipping in Metzoa than in all other eukaryotes. In our view these observations reflect the fact that the majority of multidomain proteins unique to metazoa and indispensable for metazoan type multicellularity were assembled by exon-shuffling from 'symmetrical' modules (i.e. modules flanked by introns of the same phase), whereas this type of protein evolution played a minor role in other groups of eukaryotes, including plants. The higher frequency of 'symmetrical' exons in Metazoan genomes provides an explanation for the enrichment for frame-preserving events since skipping or inclusion of 'symmetrical' modules during alternative splicing does not result in a reading-frame shift. REVIEWERS: This article was reviewed by Manuel Irimia, Ashish Lal and Erez Levanon. The reviewers were nominated by the Editorial Board.