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).