Figure 7.

Diagram showing evolutionary changes of base pairs, and their frequency/occurrence in ITS2 of the Ulvales. All possible evolutionary changes between canonical (C-G, A-U) and 'wobble' (G-U) base pairs in RNA molecules, and their frequency of occurrence in the entire ITS2 are illustrated. Base pairs are given in 5'-3' orientation, referring to their placement in a helix. Arrows indicate the evolutionary direction of substitutions. ITS2 changes, which were found in the Ulvales are indicated by bold arrows, accompanied by frequencies (encircled numbers), whereas changes that were not existent in the analyzed taxa (frequency 0) are shown as thin arrows. CBCs are shown in blue, hCBCs are in red. Obviously, hCBCs (especially the G-C ⇔ G-U type) occurred more frequently than CBCs. From the frequency of base pairs (number in black boxes) and its percentage (number below black boxes) it is evident that there was a strong selection towards the GC/CG category. Note that the given frequencies of base pairs are confined to extant taxa. The illustration of Helix 1 with pair 8/11 highlighted refers to the hypothesis that a CBC of the U-A ⇔ C-G type may have evolved via two consecutive hCBC steps (see Results). Note, however, that the taxa shown are unrelated, indicated by the simplified trees, which provide no support for this hypothesis.