Abstract
Both direct repeats (DR) and inverted repeats (IR) are documented in the published plastomes of four Selaginella species indicating the unusual and diverse plastome structure in the family Selaginellaceae. In this study, we newly sequenced complete plastomes of seven species from five main lineages of Selaginellaceae and also re-sequenced three species (S. tamariscina, S. uncinata and S. moellendorffii) to explore the evolutionary trajectory of Selaginellaceae plastomes. Our results showed that the plastomes of Selaginellaceae vary remarkably in size, gene contents, gene order and GC contents. Notably, both DR and IR structure existed in the plastomes of Selaginellaceae with DR structure being an early diverged character. The occurrence of DR structure was right after the Permian-Triassic (P-T) extinction (ca. 246 Ma) and remained in most subgenera of Selaginellaceae, whereas IR structure only reoccurred in the most derived subg. Heterostachys (ca. 23 Ma). The presence of a pair of large repeats psbK-trnQ, together with DR/IR region in S. bisulcata, S. pennata, S. uncinata, and S. hainanensis, could frequently mediate diverse homologous recombination and create approximately equal stoichiometric isomers (IR/DR-coexisting) and subgenomes. High proportion of repeats is presumably responsible for the dynamic IR/DR-coexisting plastomes, which possess a lower synonymous substitution rate (dS) compared with DR-possessing plastomes. We propose that the occurrence of DR structure, together with few repeats, is possibly selected to adapt to the environmental upheaval during the P-T crisis and the IR/DR-coexisting plastomes also reached an equilibrium in plastome organization through highly efficient homologous recombination to maintain stability.