Molecular phylogeny and divergence times of Astragalus section Hymenostegis: An analysis of a rapidly diversifying species group in Fabaceae

Abstract

The taxa of Astragalus section Hymenostegis are an important element of mountainous and steppe habitats in Southwest Asia. A phylogenetic hypothesis of sect. Hymenostegis has been obtained from nuclear ribosomal DNA internal transcribed spacer (ITS) and plastid ycf1 sequences of up to 303 individuals from 106 species, including all 89 taxa currently assigned to sect. Hymenostegis, 14 species of other Astragalus sections, and two species of Oxytropis and one Biserrula designated as outgroups. Bayesian phylogenetic inference and parsimony analyses reveal that three species from two other closely related sections group within sect. Hymenostegis, making the section paraphyletic. DNA sequence diversity is generally very low among Hymenostegis taxa, which is consistent with recent diversification of the section. We estimate that diversification in sect. Hymenostegis occurred in the middle to late Pleistocene, with many species arising only during the last one million years, when environmental conditions in the mountain regions of Southwest and Central Asia cycled repeatedly between dry and more humid conditions.Astragalus is with about 2500-3000 species in 250 sections the largest genus of flowering plants [1][2][3][4][5][6][7] . It belongs to the legume family (Leguminosae or Fabaceae) that is, after Orchidaceae and Asteraceae, the third largest family of flowering plants 5 consisting of about 730 genera and nearly 19,300 species. Also its subfamily Papilionoideae, with about 478 genera and 13,800 species 5 , is very species rich. Astragalus belongs within this subfamily together with genera like Oxytropis and Colutea to the Astragalean clade of the so-called Inverted Repeat Lacking Clade (IRLC). Its members are all characterized by the loss of one copy of the two 25-kb inverted repeat regions in the chloroplast genome 2,[8][9][10] .Although species richness seems to be a general characteristic of the legumes, this feature is not evenly distributed among the groups within this family. Thus, Sanderson and Wojciechowski 11 found that Astragalus itself is not significantly more speciose compared to its close relatives, but that species richness is a feature of the entire Astragalean clade in comparison to the other groups of the IRLC. High species numbers can originate through constant accumulation of diversity through time or following a punctuated pattern, where bursts of diversification rates alternate with times of stasis 12 . The latter pattern often results from the evolution of a key innovation that provides the possibility to fill a new ecological niche, or from a key opportunity, i.e. the colonization of a new, formerly not inhabited area and speciation therein [12][13][14][15] . For Astragalus and the entire Astragalean clade the reason(s) for the high species numbers are still unclear. In an effort to better understand reasons for and timing of species diversification in Astragalus we here analyze a large section of the genus, where we put some effort into arriving at a complete species sample.

Endemic diversity and distribution of the Iranian vascular flora across phytogeographical regions, biodiversity hotspots and areas of endemism

“…The complex topography and the large elevational range potentially allowed Iranian plants to survive the Quaternary glaciations, as only the high elevations were covered with ice 65 and lowlands could act as refugia for many relict elements such as Parrotia in the Hyrcanian Forests 66 . Additionally, Quaternary climatic fluctuations and associated shifts in habitats and vegetation zones may have triggered species diversification as, for instance, suggested for a group of steppe species within the hyperdiverse genus Astragalus 67 . However, understanding the origin of the biodiversity of the mountains of Iran requires molecular phylogenetic studies of their characteristic mega-genera.…”

Pollen metabarcoding as a tool for tracking long‐distance insect migrations

“…This is probably because the representation of plant species belonging to taxonomically complex and diverse genera is far from complete in reference databases used for taxonomic assignments and could affect the positive identification of some pollen grains. Some examples of such taxa in our data set include Astragalus, with about 2,500-3,000 species, the largest genus of flowering plants (Bagheri, Maassoumi, Rahiminejad, Brassac, & Blattner, 2017;Podlech & Zarre, 2013); Artemisia that covers approximately 600 species (Richardson, Page, Bajgain, Sanderson, & Udall, 2012); and genus Thymus that includes around 400 species (Karaca, Ince, Aydin, Elmasulu, & Turgut, 2015). Most of the species of these genera are native to the Mediterranean region, Northern Africa and Western Asia.…”

“…Most of the species of these genera are native to the Mediterranean region, Northern Africa and Western Asia. Molecular approaches have limitations to identify and define species in some of these complex genera due to various biological phenomena, such as interspecific hybridization and polyploidy, which are often correlated (Soltis & Soltis, 2009) and can contribute significantly to the taxonomic complexity of Thymus (Morales, 2010), Artemisia (Richardson et al, 2012) and Astragalus (Bagheri et al, 2017;Doyle, 2012). In Thymus, genetic polymorphism at the intraspecific level can hinder the positive identification of some species (Karaca et al, 2015).…”

“…In Thymus, genetic polymorphism at the intraspecific level can hinder the positive identification of some species (Karaca et al, 2015). Conversely, DNA sequence diversity is generally very low in some Artemisia species (Koloren, Koloren, & Eker, 2016) and also among species included within several sections of Astragalus (Bagheri et al, 2017), which has been attributed to a rapid radiation (Sanderson & Wojciechowski, 1996).…”

Advances in legume research in the genomics era

“…Recent molecular studies of these groups exemplify the difficulties of resolving phylogenies for rapidly radiating lineages (e.g. Azani et al 2017;Bagheri et al 2017). Where phylogenies based on data from single or a few genes lack resolution, genomic-scale data may provide a solution.…”