Origin of new chimeric gene or transcript structures. (A) DNA-based (genomic) gene fusion. Partial duplication (and hence fission) of ancestral source genes precedes juxtaposition of partial duplicates and subsequent fusion (presumably mediated by the evolution of novel splicing signals and/or transcription termination/polyadenylation sites). (B) Transcription-mediated gene fusion. Novel transcript structures may arise from intergenic splicing after evolution of novel splicing signals and transcriptional readthrough from the upstream gene. New chimeric mRNAs may sometimes be reversed transcribed to yield new chimeric retrogenes (see also Fig. 1). (Green, blue, red large boxes) Exons, (red right-angled arrows) transcriptional start sites (TSSs), (black connecting lines) constitutive splicing, (dotted lines) splicing of ancestral gene structures, (green lines) intergenic splicing that results in new chimeric transcripts.
