Evolution of tribe Arabideae
Among the 18 genera and 545 species within tribe Arabideae (Walden, German, et al., 2020), 63% are neopolyploids, and its mean net diversification rate is more than three times higher than the family mean (Huang et al., 2020), highlighting the evolutionary dynamics of this lineage. Research of the past two decades resulted in a well-resolved phylogenetic tree that demonstrates paraphyly of the genusArabis and established a new systematic concept (Karl & Koch, 2013). Multiple radiations in various clades involved a split between lowland annual and montane/alpine perennial sister species, in which increased speciation rates occurred frequently.
The center of origin of tribe Arabideae is the Irano-Turanian region (Figure A), which ranges from the eastern Mediterranean and Saharo-Arabian regions to the Tian Shan and Pamir Mountains (for a detailed review on this floristic region: Manafzadeh, Staedler, & Conti, 2017). From there, the different clades independently colonized the temperate and alpine mountain regions of the world (Karl & Koch, 2013). Generally, numerous morphological trait characteristics have evolved convergently and are often linked, e.g. in annuals vs.perennials. In contrast to perennial species, annuals exhibit a complete selfing syndrome, have wider and lowland distribution ranges, did not undergo subsequent radiations, and exhibit lower genetic variation that might, among others, result from the selfing syndrome (Karl & Koch, 2013, 2014).
Systematic and taxonomic considerations of genus Arabis
The type species of genus Arabis is represented by the Linnean holotype of A. alpina (Species Plantarum 2: 664, 1753). However, if future taxonomic work aims for a monophyletic genus Arabis,then most of the species contained in the genus at present will have to be transferred into new genera. The resulting monophyletic set of species would contain the type A. alpina together with approximately 20 additional species that are: (i) A. alpina and its closest relatives (ca. 10-12 species), (ii) its sister clade including A. nordmanniana (A. nordmanniana clade; 5 species), and (iii) the A. auriculata clade (3 species; Karl & Koch, 2013; Kiefer et al., 2017). Among those closest relatives ofA. alpina are the red or pink flowering taxa A. purpurea ,A. cypria and A. aubrietoides. Moreover, A. deflexa , A. ionocalyx, A. caucasica and A. tianchanica(Kyrgyzstan) are found within the same phylogenetically unresolved clade. If all of those were included into A. alpina to establish monophyletic entities, subspecies concepts would have to be applied (Karl, Kiefer, Ansell, & Koch, 2012).
Arabis caucasica has also been introduced in synonymy as A. alpina ssp. caucasica (additional synonyms are A. albida ,A. billardieri ; Koch, German, Kiefer, & Franzke, 2018). Although taxonomically accepted, there is no supporting genetic, geographic or morphometric information. The same applies to A. alpina ssp.brevifolia , which has been described from Eastern Mediterranean areas (Greuter & Raus, 1983), but also without any further complementary details. Consequently, there is no reliable and convincing infraspecific taxonomic system for A. alpina to date, and further taxonomic work is needed to correctly relate morphological variation with genetic make-up and biogeographic patterns.
The entire species assemblage likely originated in the Pleistocene (Karl & Koch, 2013), and the three remaining species or taxa (A. montbretiana , A. nova ssp. iberica , A. kennedyae ) build up a well-defined sister clade of exclusively annuals (Karl & Koch, 2013) that provide an excellent source for comparative evolutionary research (Kiefer et al., 2017, 2019). This sister clade split in the Late Pliocene or at the onset of early Pleistocene glaciation/deglaciation cycles (Karl & Koch, 2013).
FIGURE A HERE
FIGURE A (A) Different phylogenetic positioning of tribe Arabideae within Brassicaceae core groups depending on the marker set used, indicating a biological phenomenon rather than an analytical artifact (following Walden, Nguyen, et al., 2020). (B) Summary of present-day phylogenetic knowledge on clade relationships in Arabideae (species number given). Clade definition follows Karl & Koch (2013) and Kiefer et al. (2017). (C) Ancestral areas of the clade includingArabis alpina and its sister clades, highlighting the ”Irano-Turanian region” as a source of origin. The timeline indicates stem and crown group ages (including confidence intervals; Karl & Koch, 2013).
TABLE 1 Conserved and diverse roles of genes functionally characterized in Arabis alpina (Aa) compared to Arabidopsis thaliana (At).