Rare occurrence of reciprocal hybridization in a sympatric population of the Czech stenoendemic Dianthus arenarius subsp. bohemicus and widespread D. carthusianorum.

Hybridization between rare and common plant congeners can pose a serious threat to the rare species through gene swamping, production of hybrid seed at the expense of conspecific seed and/or hybrid competition for abiotic or biotic resources. Assessing the frequency and dynamics of interspecific hybridization is therefore of paramount importance for conservation purposes. Here we investigate, using DNA flow cytometry, multivariate morphometrics and chloroplast DNA sequencing, the frequency and direction of interspecific hybridization between the critically endangered Czech endemic psammophyte Dianthus arenarius subsp. bohemicus and its sympatric congener D. carthusianorum (Caryophyllaceae) in a single population in central Bohemia. Flow cytometry allowed unambiguous identification of both parental species, based on differences in the amounts of nuclear DNA and revealed a few individuals (< 1.1% of the samples analysed) with intermediate genome sizes that corresponded to F1 hybrids. Clear discontinuities in estimated genome sizes and a low variation within recognized taxonomic groups make backcrossing to parental species or introgression unlikely. Interspecific hybrids were considerably less fertile, producing largely aborted pollen grains and no seed. Analysis of chloroplast haplotypes provided evidence for reciprocal hybridization (both species served as maternal and paternal parents). Length of the lowermost pair of cauline leaves, calyx length and petal length (incl. separate lengths of petal claw and petal limb) were taxonomically the most informative characters, allowing reliable identification of both parental species and their hybrids. The results indicate that interspecific hybridization has only a minor effect on the genetic integrity of the endemic D. arenarius subsp. bohemicus in its last remaining natural population. Nonetheless, we recommend periodic monitoring especially as the recent controlled large-scale disturbances (mechanical removal of the vegetation cover) in the locality may promote the establishment of interspecific crosses.