Urtica kioviensis, a rare species of stinging nettle threatened by hybridization

Hybridization is a widespread and important force in plant evolution. It can either hinder speciation or result in the formation of new species. Repeated hybridization with backcrossing with one or both of two hybridizing species is referred to as introgressive hybridization and leads to the introgression of genetic variation from one taxon to another or to other taxa. Hybridization can have consequences for rare species if they are in contact with a more abundant relative with incomplete genetic barriers, as it can lead to genetic erosion, population decline and even the extinction of species. Hybridization between two closely related species of the genus Urtica were studied where the rare diploid species U. kioviensis and the widespread species complex U. dioica with two cytotypes (2x and 4x) occurred at six sites in central Europe. Flow cytometric relative genome size estimation, morphometrics and analyses of nuclear and chloroplast DNA markers were used to confirm the hybrid origin of intermediate plants. The results provide proof of both homoploid (2x × 2x) and heteroploid (2x × 4x) hybridization. The detected continuous variation in relative genome size and morphology at the diploid level indicate homoploid hybridization between U. kioviensis and the diploid cytotype of U. dioica; subsequent introgression is possible but not proved with certainty and needs further study. Triploid individuals were also detected, showing differences in relative genome sizes and different positions in morphometric analyses compared to the parental taxa. They also have lower fertility (pollen viability, 68.9%) compared to their parents and diploid hybrids and no introgressive hybridization (back crosses) involving triploids was recorded. Based on the results, it is not possible to unequivocally determine their origin. They may be triploid hybrids between U. kioviensis and tetraploid U. dioica, between diploid and tetraploid cytotypes of U. dioica and between diploid hybrids (U. kioviensis × U. dioica 2x) and tetraploid U. dioica. The frequency of hybridization differs between sites; the highest risk of genetic erosion was recorded at Plačkův les in southern Moravia (Czech Republic), where it might be a threat to U. kioviensis via both genetic and demographic swamping. The results also demonstrate a case where hybridization could pose a risk to small populations of a rare species and highlights the increasing need to protect endangered species of plants.