Direct Repeats Co-occur with Few Short Dispersed Repeats in Plastid Genome of A Spikemoss, Selaginella vardei (Selaginellaceae, Lycophyta)

Abstract

BackgroundIt is hypothesized that the highly conserved inverted repeat (IR) structure of land plant plastid genomes (plastomes) is beneficial for stabilizing plastome organizations, whereas the mechanism of the occurrence and stability maintenance of the newly reported direct repeats (DR) structure was yet awaiting further exploration. Here we introduced the DR structure of plastome in Selaginella vardei (Selaginellaceae, Lycophyta), trying to elucidate the mechanism of DR occurrence and stability maintenance.\n\nResultsThe plastome of S. vardei is 121,254 bp in length and encodes 76 different genes, of which 62 encode proteins, 10 encode tRNAs and four encode rRNAs. Unexpectedly, the two identical rRNA gene regions (13,893 bp) are arranged into DR, and a ca. 50-kb trnN-trnF inversion spanning one DR copy exists in S. vardei, comparing to the typical IR organization of Isoetes flaccida (Isoetaceae, Lycophyta). We find extremely rare short dispersed repeats (SDRs) in plastome of S. vardei and is confirmed in its closely related species S. indica. The occurrence time of DR in Selaginellaceae is estimated at late Triassic (ca. 215 Ma) based on the phylogenetic framework of land plants.\n\nConclusionsWe propose that the unconventional DR structure, co-occurred with extremely few SDRs, plays key role in maintaining the stability of plastome, and reflects a relic of the environmental upheaval during extinction event. We suggest that the ca. 50-kb inversion resulted in the DR structure, and recombination between DR regions is confirmed to generate multipartite subgenomes and diverse multimers, which shed lights on the diverse structures in plastome of land plants.