Introns are derived from transposons
Introns and transposons exhibit many similar features, but the connections between them have yet to be firmly established. Group I introns have commonalities with DNA transposons, while group II introns share many features with retrotransposons. Here, we report the results of an analysis of 214 introns (including group I, group II, group III, twintrons, spliceosomal, and archaeal introns) from members of seven major taxa (within Eukarya, Bacteria, and Archaea) that all have direct repeats at or near both exon/intron borders, indicating that they were inserted via transposition events. Border sequence analysis indicates that after splicing, most mature transcripts would be functionally compromised because they do not restore the DNA sequence information before intron insertion. Transposons and introns thus appear to be members of a diverse assemblage of parasitic mobile genetic elements that secondarily may benefit their host cell and have expanded greatly in eukaryotes from their presumed prokaryotic ancestors. Author SummaryIntrons are found in all domains of life. While they are limited in prokaryotes, they have greatly expanded in number and diversity in eukaryotes. We found direct repeat sequences at or near both exon/intron borders for all 214 introns analyzed among eukaryotes, bacteria, and archaea. We infer that all introns were inserted into genes via transposon-like mechanisms and are members of a large family of mobile genetic elements.