Dated Phylogeny of Astragalus Section Stereothrix (Fabaceae) and Allied Taxa in the Hypoglottis Clade

SIMPLE SUMMARY: Biological taxonomic research deals with the grouping of organisms into entities that reflect their evolutionary history and relationships. In the species-rich plant genus Astragalus, the systematic grouping of many species changed several times during recent decades, which indicates problems in correctly recognizing relationships based on morphological characteristics. Here, we analyzed the relationships of Astragalus species from Iran and neighboring countries based on DNA sequences from three different loci. We found that species traditionally classified into two different sections of Astragalus occur intermingled in our phylogenetic trees instead of forming clear groups reflecting their taxonomic units. In addition, species thought to be only distantly related to the target species were found in this cluster. From this, we conclude that the currently used circumscription of taxonomic entities for these Astragalus species is false and should be abandoned. The reasons behind the the systematic classification problems of Astragalus include independent, parallel evolution or the loss of characteristics that were assumed to be unique and used to define certain systematic units. Thus, it is necessary to analyze the relationships of many Astragalus species to (i) identify traits useful for taxonomic classification and (ii) to understand the ecological and habitat differences driving their fast speciation. ABSTRACT: The Astragalus subgenus Hypoglottis Bunge, which consists of several sections, is one of the taxonomically most complicated groups in the genus. The Astragalus section Stereothrix Bunge belongs to this subgenus and is a significant element of the Irano-Turanian floristic region. A molecular phylogenetic analysis of this section and its closely related taxa using nuclear ribosomal DNA internal transcribed spacers (ITS) and external transcribed spacer (ETS) regions as well as plastid matK sequences were conducted. Parsimony analyses and Bayesian phylogenetic inference revealed that the section is not monophyletic in its current form, as some taxa belonging to closely related sections such as Hypoglottidei DC. and the Malacothrix Bunge group within the sect. Stereothrix render it paraphyletic. Moreover, species groups belonging to sect. Stereothrix are placed in different clades within the phylogenetic tree of subgenus Hypoglottis, which indicates polyphyly, i.e., multiple independent origins of taxa placed in the sect. Stereothrix. Molecular dating of the group estimated an age of 3.62 (1.73–5.62) My for this assemblage with the major diversification events happening during the last 2 My. Many species groups separated only within the last 0.5 to 1 My. Based on morphological and molecular data, we discuss the phylogenetic relationships of the groups and synonymy of species. In addition, the included taxa of sect. Hypoglottidei are not monophyletic and include species belonging to sects. Hololeuce, Koelziana, Malacothrix, Onobrychoideae, and Ornithodpodium group within the sect. Stereothrix taxa. We conclude that only an analysis including all groups and nearly all species of the sections within the Hypoglottis clade can finally result in an new evolutionary-based system for these taxa.