Genetic and morphological conservatism of Calophaca wolgarica (Fabaceae), a paleoendemic species of the Southern East European Plain

Calophaca wolgarica is a relict paleoendemic species of the Southern East European Plain. In this study, we assessed the degree of morphological and genetic variation in its 24 natural and 2 reintroduced populations. Nonmetric multidimensional scaling (nMDS) on 17 traits was used to evaluate morphological variability. To assess genetic variation, we sequenced 8 chloroplast DNA regions (atpB − rbcL, matK, trnL − trnF, trnS − trnG, rbcL, rps16, rpoC1, ycf1) and ITS1-5.8S-ITS2 nuclear DNA region and performed inter simple sequence repeat (ISSR) analysis as well. We found that there was no interpopulation morphological variation. Sequencing of chloroplast and nuclear DNA regions also showed no differences between populations of the species. Based on the results of Bayesian analysis using ISSR markers in STRUCTURE and NEW HYBRIDS programs, we identified genetic groups, the spatial location of which indicates that during the last glaciations and regressions of Caspian Sea, one ancestral population was preserved in a refugium on the Stavropol Upland. The refugium of another ancestral population covered the south of Volga Upland up to the Ergeni Upland. We argued that the species dispersed from the Tien Shan Mountains to the Volga Upland, bypassing the Caspian Sea from the north, and then spread to the Stavropol Upland. We suggest that the endemism of C. wolgarica in the absence of geographical barriers linked with low ecological heterogeneity of Southern East European Plain could arise not only, or not so much from the gene flow limitation, but from morphological and genetic conservatism associated with species adaptation to a narrow ecological niche. Over millions of years of the species’ existence, its range has probably experienced multiple constrictions and expansions depending on changing climatic conditions associated with glacial and interglacial periods and dynamics of Caspian Sea level.