Characterizing nrDNA ITS1, 5.8S and ITS2 secondary structures and their phylogenetic utility in the legume tribe Hedysareae with special reference to Hedysarum

This is the first study to systematically evaluate rRNA secondary structures of Hedysareae with an emphasis on Hedysarum. ITS2 and 5.8S regions of the genus shared a common secondary structure with a four-fingered central loop, whereas ITS1 possessed five distinct structures. The secondary structural features of the two regions provided advantageous data for clades, species groups, and closely related species. Hemi-CBCs were mostly observed in the reconstruction of species groups, and Nsts, mostly between closely related species. The investigations showed that ITS1 varied more than ITS2 in length, GC content, and most of the diversity indices within the tribe. Maximum likelihood analyses of the synchronized sequence-structure tree of ITS1 were performed. The accuracy and phylogenetic signals of ITS1 were higher than ITS2. The similar GC content, and no CBC, in both spacers, fortified the close relationship of CEGO and H. sections Stracheya and Hedysarum clades in the synchronized sequence-structure tree topology of ITS1. In both regions, no inter-generic CBCs were detected inside the CEGO clade and the inter-sectional level of Hedysarum. But, in the ITS2 region, a CBC was detected between H. section Multicaulia, and Taverniera versus H. sections Hedysarum, and Stracheya. The lowest inter-sectional genetic distance and structural features were found between H. sect. Hedysarum and H. sect. Stracheya clades in the ITS2 region.