Birth and Evolution of Human Exons

Abstract

Exons can be independently gained or created in genes by exon shuffling, duplication and mutation of intronic sequences, especially repetitive elements as Alu. Such new exons are more prone to be alternatively spliced, included in transcripts in low frequencies and evolve faster than ancient exons. Alternative splicing is an important component of the evolution of new exons since it allows new gene parts to be slowly added to existing genes without compromising the original protein function. Eventually, new exons may be integrated to the original gene and evolve new functions.

Keywords: exon birth; exon evolution; alternative splicing; exonization of Alu elements

Figure 1.

A general pathway for the birth of a new exon from intronic sequences. Solid lines indicate ‘evolutionary pathways’ and dashed lines ‘splicing pathways’. Stars represent mutations in the DNA. (a) Original (ancient) splicing pathway. (b) An intronic segment (mainly repetitive sequences as Alu) undergoes mutations (stars) that create weak splice sites (and perhaps required ESEs). (c) The segment may be incorporated in a few mRNA transcripts (alternatively spliced), since it bears weak splicing signals. (d) The newly born exon is free to evolve, because it does not compromise the original gene function, thus the observed higher evolutionary rates. (e) Eventually, the new exon might undergo mutations (stars) that create proper splicing regulatory elements (ESEs) or improve splice sites, increasing its inclusion frequency (it might even become constitutively spliced). The high evolutionary rate might also allow the exon to evolve a new function. Owing to selective pressure on both the RNA and DNA levels, evolutionary rate tends to slow down. New exons can remain alternative, providing diversity to the genome.

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Further Reading

Chamary JV, Parmley JL and Hurst LD (2006) Hearing silence: non‐neutral evolution at synonymous sites in mammals. Nature Review Genetics 7: 98–108.

Xing Y and Lee C (2006) Alternative splicing and RNA selection pressure – evolutionary consequences for eukaryotic genomes. Nature Review Genetics 7: 499–509.

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How to Cite close
Vibranovski, Maria Dulcetti, Sakabe, Noboru Jo, and Long, Manyuan(Dec 2007) Birth and Evolution of Human Exons. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020777]