Fig. 5.—

Frequencies of variable and invariant sites at different positions of the terminator stems. A, external vs. internal positions of the stem; B, inner vs. outer position of the stem; C, right vs. left arm of the outer position of the stem. Cyan, the category with an excess of substitutions; red, the complementary category. In B and C, AU, GC, and GU ancestral base pairs are pooled together. Tables contain the number of interacting columns where both nucleotides were invariant (“invariant”), or one of the nucleotides was variable (“variable”), in each of the two compared categories, with two-tailed Fisher's test P-value corresponding to each table.