Figure 4.

Average lengths of CEs compared to average lengths of internal constitutive exons. Species are sorted by the fractional difference between these two lengths. In organisms where CEs are common (animals and plants) CEs are almost identical in length to constitutive exons, while in species where CEs are rare (fungi and protists) CEs tend to be significantly shorter than constitutive exons. In animals and plants, where ED is common, CEs are spliced by the same process as constitutive exons and these two groups are thus subject to the same length constraints. In organisms that splice primarily by ID, including fungi and protists, the lengths of constitutive exons are not constrained by ED. However, CEs in these organisms are still recognized by ED. Thus, in these species, constitutive exons can grow longer than CEs.