Abstract
The trans-splicing rps12 gene of fern plastomes (plastid genomes) exhibits a unique structure owing to its variations in intragenic exon location and intron content, and thus, it provides an excellent model system for examining the effect of plastid gene structure on rates and patterns of molecular evolution. In this study, 16 complete fern plastome sequences were newly generated via the Illumina HiSeq sequencing platform. We reconstructed the phylogeny of ferns and inferred the patterns and rates of plastid rps12 gene evolution in a phylogenetic context by combining these plastome data with those of previously published fern species. We uncovered the diversity of fern plastome evolution by characterizing the structures of these genomes and obtained a highly supported phylogenetic framework for ferns. Furthermore, our results revealed molecular evolutionary patterns that were completely different from the patterns revealed in previous studies. There were significant differences in the patterns and rates of nucleotide substitutions in both intron-containing and intron-less rps12 alleles. Rate heterogeneity between single-copy (SC) and inverted repeat (IR) exons was evident. Unexpectedly, however, IR exons exhibited significantly higher synonymous substitution rates (dS) than SC exons, a pattern that contrasts the regional effect responsible for decreased rates of nucleotide substitutions in IRs. Our results reveal that structural changes in plastid genes have important effects on evolutionary rates, and we propose possible mechanisms to explain the variations in the nucleotide substitution rates of this unusual gene.