Fig. 6.

Scenarios for the evolution of DYW-type PPR protein editing factors. A single-target editing factor (a) may persist for some evolutionary time after cytidine-to-uridine conversion of its editing target (b) before functional disintegration (c) and ultimate loss (d). The numerous here reported independent losses of editing factor RARE1 among angiosperms (20 times) are examples for the latter case. The here reported retentions of RARE1 or CLB19 despite loss of their editing targets likely reflect state B rather than C given that no pseudogeniziation is recognizable. Editing factors may extend their functionality by acting on additional targets (e), likely allowing initial pseudo-targets to evolve into new editing sites by uridine-to-cytidine conversions. Once an editing factor serves multiple targets its loss depends on C-to-U conversion at all its targets simultaneously. The loss of CRR28 in Cicer or CLB19 in Amaranthus, Tarenaya and Vaccinium are examples. As an alternative to de-functionalization and loss, an editing factor may be functionally reduced to target recognition while the DYW domain is supplemented in trans (f). The here observed cases of CRR28 among Asterales are examples