Non-radiative origin for alpine endemics of Draba (Brassicaceae) in the central mountains of the Japanese Archipelago

Abstract

Diversity of endemic species may result from two evolutionary processes: in-situ diversification (radiation) or phylogenetically independent divergences (non-radiation). To explore the evolutionary history of endemic-rich alpine flora in the Japanese Archipelago, we investigated the phylogenetic relationships of four endemic taxa of Draba (Brassicaceae) from the high mountains of central Honshu: D. kitadakensis, D. sachalinensis var. shinanomontana, D. sakuraii var. nipponica, and D. shiroumana. Using molecular phylogenetic analyses on plastid trnL-F and nuclear ITS sequence of 157 taxa, we found that the four taxa endemic to central Honshu did not form a monophyletic group and diverged from at least two evolutionary independent lineages. Moreover, ancestral area reconstruction further revealed that some of their ancestral species may have originated from different geographical regions. These findings indicate that the endemic Draba in central Honshu diverged through non-radiative evolutionary origin. Our study suggests that the richness of endemic species in the alpine zone of the Japanese Archipelago is associated with multiple sources with high species diversity located in nearby geographical regions.

Keywords: Alpine plants; Beringia; BioGeoBEARS; Biogeography; East-Central Asia; Phylogeography.

Fig. 1

Topological map of a the Japanese Archipelago and b distribution of endemic Draba in central Honshu. Black and white shapes indicate whether samples were collected for the present study or not

Fig. 2

Bayesian tree based on ITS. Three highly supported clades (PP ≥ 0.95) are highlighted. Two topologies, detailing the relationships of endemic taxa from central Honshu and their closely related taxa, are shown. Endemic taxa from central Honshu are indicated with colored text. Posterior probability support values (PP ≥ 0.95) are shown

Fig. 3

Haplotype network of Draba belonging to clade 3 from the Bayesian tree based on ITS. Each tick on the line represents a nucleotide polymorphism, and the circumference of each circle is proportional to the number of taxa with no nucleotide polymorphism. Endemic taxa from central Honshu are indicated with colored text

Fig. 4

Ancestral area reconstruction of the Draba belonged to clade 3 from the Bayesian tree based on ITS. a Distribution map of the seven geographical regions used for reconstruction. b Estimated ancestral areas and likelihoods inferred using BioGeoBEARS under best-fitting model BAYAREALIKE + J. The alphabet beside taxonomic name represents the geographical region used for reconstruction. Endemic taxa from central Honshu are indicated with colored text