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Phylogenomics of Selaginellaceae with special reference to the enigmatic sanguinolenta group

Selaginellaceae has been repeatedly proved as monophyly by previous studies with only one genus being recognized. However, the subgeneric classification has been debated during the recent decades. Furthermore, phylogenetic position of the newly identified sanguinolenta group has not been resolved, varying depending on the datasets and analysis methods. We carried out the phylogenomic analyses of twenty-six species from Selaginellaceae with ten species being newly sequenced and three species representing the sanguinolenta group. Four of the ten newly sequenced plastomes are assembled into the complete molecules, whereas the other six species are only assembled into five to sixteen contigs owing to high numbers of repeats. The phylogenetic framework from our study is basically congruent with the subgeneric classification of Weststrand and Korall (2016b). The position of sanguinolenta group was resolved as the basal clade in subg. Stachygynandrum, which support the position β proposed by Weststrand and Korall (2016a), also supported by the morphological characters of dimorphic vegetative leaves, monomorphic sporophylls and intermixed sporangial arrangements. Both values of dS, dN and GC content in Selaginellaceae plastomes were significantly higher than those of other lycophytes (Isoetaceae and Lycopodiaceae). The correlation analysis showed that the elevated synonymous substitution rate was significantly correlated with the high GC content in Selaginellaceae. Besides, the values of dS and dN differs significantly between branches in the phylogenetic tree of Selaginellaceae. We propose that both high GC content and the extensive RNA editing sites contributed to the elevated substitution rate in Selaginellaceae, and all of these three factors could influence the stability of phylogenetic topology of Selaginellaceae.