Figure 1. Phylogenetic relationships between three classes of T2/S-type RNases in land plants.

The unrooted maximum-likelihood phylogram is shown, along with posterior support for some key recovered branches. Tip labels are omitted for clarity, and can be found in Fig. S1. S-RNase sequences from the four families with RNase-based SI are indicated, including two genera from the Rosaceae (Prunus and Malus). The scale shown illustrates the vast divergences, in units of nucleotide substitutions per site. Many pairwise distances within each class exceed one substitution per site, and many distances between classes exceed two substitutions per site. Note the low distances within class II T2/S-RNases, compared with class I, and class III.