A segmental genomic duplication generates a functional intron

An intron is an extended genomic feature whose function requires multiple constrained positions—donor and acceptor splice sites, a branch point, a polypyrimidine tract and suitable splicing enhancers—that may be distributed over hundreds or thousands of nucleotides. New introns are therefore unlikely to emerge by incremental accumulation of functional sub-elements. Here we demonstrate that a functional intron can be created de novo in a single step by a segmental genomic duplication. This experiment recapitulates in vivo the birth of an intron that arose in the ancestral jawed vertebrate lineage nearly half-a-billion years ago. The appearance of a new intron that splits an exon without disrupting the corresponding peptide sequence is a rare event in vertebrate genomes. Hellsten et al. demonstrate that, under certain circumstances, a functional intron can be produced in a single step by segmental genomic duplication.