Phylogeny of Diplazium (Athyriaceae) revisited: Resolving the backbone relationships based on plastid genomes and phylogenetic tree space analysis

Despite progress in resolving the phylogeny of twinsorus ferns (Diplazium) based on multilocus phylogenetic studies, uncertainty remains especially for deep, or backbone relationships among closely related clades, suggesting a classic case of rapid evolutionary radiation. Here, we investigated the deep phylogenetic relationships within Diplazium by sampling all major clades and using 51 plastid genomes (plastomes), of which 38 were newly sequenced with high-throughput sequencing technology, resulting more than 127,000 informative sites. Using parsimony, maximum likelihood and Bayesian analyses of plastome sequences, we largely resolved the backbone of the phylogeny of Diplazium with strong support. However, we also detected phylogenetic incongruence among different datasets and moderately to poorly supported relationships, particularly at several extremely short internal branches. By using phylogenetic tree space and topology-clustering analyses, we provide evidence that conflicting phylogenetic signals can be found across the trees estimated from individual chloroplast protein-coding genes, which may underlie the difficulty of systematics of Diplazium. Furthermore, our phylogenetic estimate offers more resolution over previous multilocus analyses, providing a framework for future taxonomic revisions of sectional classification of Diplazium. Our study demonstrates the advantage of a character-rich plastome dataset, combining the comparison of different phylogenetic methods, for resolving the recalcitrant lineages that have undergone rapid radiation and dramatic changes in evolutionary rates.