Cytogeography of alien <i>Symphyotrichum</i> species in the Pannonian region

Abstract In Europe, only a few North American Symphyotrichum species are naturalized or cultivated, and due to artificial or spontaneous hybridization, their identification is difficult. Some taxa, such as Symphyotrichum lanceolatum, exhibit invasive behaviour. Others, such as Symphyotrichum novi-belgii, are regularly identified incorrectly. Knowledge of their cytogeography will allow for more reliable identification of Symphyotrichum species and hybrids in Europe. A study of the karyological diversity of Symphyotrichum asters was conducted in localities along the Danube and its tributaries in the Pannonian basin and in the surrounding mountains. Species and cytological diversity of asters over the course of three centuries were compared from ploidy estimations of both herbarium specimens and living plants. The most represented taxa in the studied area were S. lanceolatum, S. novi-belgii, Symphyotrichum × salignum, and Symphyotrichum × versicolor. The karyological diversity was investigated using flow cytometry. The ploidy levels were determined by chromosome counting. In addition, some morphological characters and environmental factors were examined for potential connections to genome size. Among contemporary Symphyotrichum populations, mainly hexaploid (S. × versicolor), octoploid taxa (S. lanceolatum), and frequent morphologically variable heptaploid hybrids were found. Based on the genome size estimated from the herbarium specimens, it was shown that the karyological and species diversity was more diverse in the study area in the past. Since genome size is statistically significantly correlated with some environmental factors (altitude, mean annual air temperature, and annual precipitation totals), changing climate may be responsible for the change in species composition. Although regions of Europe may differ in the species composition of Symphyotrichum asters, the estimated genome size for different ploidy levels allows reliable identification of all naturalized taxa if it is combined with the morphological characteristics of inflorescences.