Fixation of conserved sequences shapes human intron size and influences transposon-insertion dynamics.

The basis for intron expansion in humans is largely unexplored. In this article, we demonstrate that intron expansion has primarily been determined by fixation of multispecies conserved sequences (MCSs) over time. The presence of MCSs has shaped intron features: the insertion of transposable elements (TEs) has been constrained as more MCSs were fixed. Analysis of TE and MCS distribution suggested an unprecedented estimate of information requirements for proper splicing of long introns with indication of sequence constraints extending up to >3 kb downstream 5' splice sites.