Table 1.

Intron gains identified with a purportedly known mechanistic origin

Proposed mechanism	In vivodemonstration	Indirect evidence	Number of events unambiguously identified	Specie(s) event identified in
Intron Transposition	No	Yes [25,44]	14	Mycosphaerella, Oikopleura
Transposon Insertion	No	Yes [23,25,54,55]	35	Oryza, Drosophila, Oikopleura, Zea
Tandem Genomic Duplication	Yes [56]	Yes [18,57]	188	Arabidopsis, Oryza, Caenorhabditis, Drosophila, mouse, human, Cryptococcus
Intron Gain during DSBR	No	Yes [22,23,27,58,59]	5	Drosophila, Daphnia, Aspergillus, Bigelowiella, human
Intron Transfer	No	Yes [22,31,44]	3	Mycosphaerella, Aspergillus, Chironomus,
Insertion of a Group II Intron	Likely does not occur [60]	No	0
Intronization	No	Yes [41,42,61,62]	29	Cryptococcus, Caenorhabditis

Only intron gains with novel splice sites (i.e. not simple duplications of pre-existing introns) were included. Only novel introns with EST support were selected from [57]. For a more detailed treatment of events, see the MIGL database located at http://cpath.him.pitt.edu/intron/index.php (manuscript in preparation).