A Trojan horse inside the gates: Alien–native mating interactions in mixed populations of <i>Solidago</i> taxa

The establishment and possible impacts of alien-native hybrids depend on their formation frequency and ability to spread using generative and vegetative reproduction. Here, we investigated the frequency and pathways of hybrid Solidago ×niederederi formation and its subsequent mating interactions with parental species in mixed populations of invasive alien S. canadensis and native S. virgaurea. DAPI flow cytometry was used for progeny screening (991 seedlings) of the 15 plants from two mixed populations to document their mating interactions based on previously shown differences in relative genome size of the studied taxa. Seedlings of presumed hybrid origin (F1 hybrids) were formed at low frequency (up to 3%) by both S. canadensis and S. virgaurea. About 46% of germinating seeds produced by S. ×niederederi plants represented later hybrid generations. Backcrossing of S. ×niederederi with parental species within the mixed populations was thus intense, but its frequency differed in direction: backcrossing was less frequent with S. virgaurea as pollen donor (<8%), while there was a strong asymmetric pattern towards S. ×niederederi backcrossing with S. canadensis as the pollen donor (almost 47%). Furthermore, the formation of tri- and tetrapolyploid seedlings within the progeny of the two hybrid plants was recorded. Hybridisation and introgression appear to be established in mixed populations of S. canadensis, S. virgaurea and S. ×niederederi. These post-invasion processes might later be mirrored in the spreading success of descendants of the invasive species and genetic erosion of the native species. Therefore, their pathways, extent and consequences require further attention.