Figure 1.

The introns-late theory explains the evolution of spliceosomal introns. The endosymbiosis of a group II intron-rich α-proteobacteria into the archaeal intronless host was followed by the invasion of the host genome by mobile group II introns. The resulting discontinuous genes provoked a strong selective pressure toward evolving intron removal. This included the degradation of group II intron RNA into mRNA introns and small nucleolar RNAs (snRNAs). Separation of the inefficient splicing reaction from translation was achieved by developing a nuclear envelope.