Phyletic association of RNA ligases RNA ligase proteins are detectable within all major lines of descent. Considering the different ligation mechanisms and classes of RNA ligase polypeptides, it seems, however, that several species abandoned one or the other mechanism of RNA ligation (Figs. 5B, 6 ). Most representatives of the Basidiomycota and Spermatophyta lineages might have lost RtcB/HSPC117 homologues during the course of evolution. However, there seem to be rare instances (such as the basidiomycote Piriformospora indica, see supplemental information for details) where an RtcB/HSPC117 protein has been retained [52, 102, 103, 128] (Figs. 5B, 6) . None of the 5 0 -3 0 RNA ligases known to date could be allocated to several higher eukaryotes, such as e.g., humans (Fig. 5B) . Nevertheless, this picture might change upon the identification of a currently unknown type of 5 0 -3 0 RNA ligase. The detection of 5 0 -3 0 RNA ligase activity in humans [86] together with the recent discovery of the B. floridae 5 0 -3 0 RNA ligase BfRNL [87] argue in favor of this view.