Genetic introgression of hybrid <i>Rhododendron</i> x <i>intermedium</i> Tausch is habitat mediated: Evidences from south-eastern Alps (Italy)

<p>Hybridization between <i>Rhododendron ferrugineum</i> L. and <i>R. hirsutum</i> L. in south-eastern Alps was examined in order to (i) evaluate the breeding direction and the extent of backcrossing between hybrids and the parental species, (ii) define which processes facilitate speciation and maintain species identities and (iii) clarify the role of rock geochemistry in hybridization events. Individuals of three hybrid populations were analysed by morphological and molecular markers. The internal transcribed spacer and <i>trnH–psbA</i> distinguished the parental species and F<sub>1</sub> hybrids while only the simple sequence repeat markers recognized genotype classes: F, H, F<sub>1</sub>, F<sub>2</sub>, BxF (backcross to <i>R. ferrugineum</i>) and BxH (backcross to <i>R. hirsutum</i>). Combining morphological and molecular data, we found that the tested populations had complex genetic structure: the F<sub>1</sub> individuals produce F<sub>2</sub> hybrids and backcross to parental species. Due to <i>R. hirsutum</i> phenology, most backcrossing events were with this parental species (asymmetric hybridization). Geochemical analyses indicate that alkaline soil conditions linked to calcareous dolomitic rocks promoted the genetic assimilation of <i>R. hirsutum.</i> In addition, <i>R.</i> x <i>intermedium</i> shows a higher edaphic adaptation than <i>R. hirsutum</i> as it can be found on a wide range of calcareous-dolomitic rocks as well as on weakly acidic soils of natural or anthropogenic origin.</p>