Figure 6. Phylogenetic relationship of different SI systems in flowering plants.

Phylogenetic tree showing relationships between the SI families (yellow box) with established S-determinants (green box). Bold text indicates the S-RNase and SLFs (blue box) identified in this study. The S-RNase-based SI systems (red text) all use S-RNase and SLFs/SFB (S-haplotype-specific F-box) /SFBB (S-locus F-box brothers) as the pistil and pollen S-determinants, respectively, and are highly divergent from the SI systems of the Papaveraceae and Brassicaceae. The S-RNases from Rutaceae, Rosaceae, Solanaceae and Plantaginaceae group into three separate branches (Asterids, Eurosids I and II; see also Supplementary Fig. 3); these families are highly diverged (>100 m.y.a.). The most plausible interpretation is that the S-RNases in the core eudicots have a single origin and evolved only once, prior to the divergence of these families >100 m.y.a.; this is more likely than several parallel gains of S-RNase at least three times. The tree is based on APG III⁶² and the divergence time is based on the website of timetree (http://timetree.org/).