Relative DNA content differences reliably identify Solidago ×niederederi, a hybrid between native and invasive alien species

Hybridization between native and alien congeners may pose a serious threat to biodiversity and negatively affect native flora. Here we study Solidago ×niederederi, which originated and became established in Europe as a result of a cross between the alien S. canadensis and native S. virgaurea. The recent increase in the number of records of S. ×niederederi in Europe has highlighted the need to monitor its occurrence, spread and behaviour. In the present study, we tested the effectiveness of flow cytometry for detecting hybrid plants of S. ×niederederi. Sequences of the ITS region of nrDNA and the rpS15-ycf1 spacer of cpDNA were used to confirm the hybrid origin of analysed plants and to identify the maternal species. Our study included 60 single-species populations of S. canadensis, S. gigantea and S. virgaurea, and 16 mixed populations with the presence of hybrid S. ×niederederi sampled from six countries in central Europe and adjacent areas. All individuals of S. canadensis, S. ×niederederi and S. virgaurea investigated were diploid (2n~2x~18) but differed in their relative DNA content values. The DNA content of S. ×niederederi was intermediate between S. canadensis and S. virgaurea with no overlaps, with the differences between the species being statistically significant. Therefore, we conclude that flow cytometry is a reliable and efficient method for detailed screening for hybrids within mixed Solidago populations and for identifying non-flowering or morphologically ambiguous Solidago plants. Since both parental species varied only negligibly in their DNA content, it may also be applicable across a broader geographic scale. Genetic, flow cytometric and distributional data suggest that the hybrids are to a large extent early generation (likely F1) hybrids as very few cases of supposed introgressants were also inferred. The results from chloroplast rpS15-ycf1 spacer showed that hybridization has occurred in both directions.