Variation in frequency of plastid RNA editing within Adiantum implies rapid evolution in fern plastomes

Premise: Recent studies of plant RNA editing have demonstrated that the number of editing sites can vary widely among large taxonomic groups (orders, families). Yet, very little is known about intrageneric variation in frequency of plant RNA editing, and no study has been conducted in ferns. Methods: We determined plastid RNA-editing counts for two species of Adiantum (Pteridaceae), A. shastense and A. aleuticum, by implementing a pipeline that integrated read-mapping and SNP-calling software to identify RNA-editing sites. We then compared the edits found in A. aleuticum and A. shastense with previously published edits from A. capillus-veneris by generating alignments for each plastid gene. Results: We found direct evidence for 505 plastid RNA-editing sites in A. aleuticum and 509 in A. shastense, compared with 350 sites in A. capillus-veneris. We observed striking variation in the number and location of the RNA-editing sites among the three species, with reverse (U-to-C) editing sites showing a higher degree of conservation than forward (C-to-U) sites. Additionally, sites involving start and stop codons were highly conserved. Conclusions: Variation in the frequency of RNA editing within Adiantum implies that RNA-editing sites can be rapidly gained or lost throughout evolution. However, varying degrees of conservation between both C-to-U and U-to-C sites and sites in start or stop codons, versus other codons, hints at the likely independent origin of both types of edits and a potential selective advantage conferred by RNA editing.