Splicing in the eukaryotic ancestor: form, function and dysfunction

Comparative genomics has begun to unravel the evolutionary history of transcript splicing in eukaryotes. The last common ancestor of modern eukaryotes is now known to have had at least moderately intron-dense genes and two complex spliceosomes. For other splicing-related phenomena the evolutionary history is less clear. We suggest that frequent mis-splicing is likely to be ancestral to eukaryotes, whereas trans-splicing and operon splicing are likely to be more recent. The origins of regulated splicing, alternative splicing and splicing of untranslated transcript regions are less certain. The data discussed underscore the significant genomic complexity of early eukaryotes, and should help to frame future questions about the origins of eukaryotic genome structure.