Reconstructing explicit mating schemes in poplar hybrids – a case study in the Populus nigra L. – Populus × canadensis Moench complex

In the plant kingdom, a large percentage of taxa are known to interbreed. If these hybrids are fertile, introgressive gene flow may foster the development of hybrid swarms or even promote gene swamping. Our study focuses on the Eurasian black poplar (Populus nigra L.) which may be threatened by hybridization with the cultivated fertile Euramerican hybrid. Using a combination of taxa specific DNA markers from the chloroplast and the nuclear genome we set up a straightforward and cost efficient method for identification of all possible mating scenarios in the hybrid complex of P. nigra and its cultivar Populus × canadensis Moench. Within a mixed population, we analyzed seed collections from individual trees of both taxa as well as juveniles from natural regeneration for proportions of second-generation hybrids (F2 hybrids) and first generation backcrosses. While F2 hybrids were detected in the seeds only, first generation backcrosses occurred in seeds as well as in juveniles. Due to the meiotic segregation of alleles, a certain amount of such progeny may remain undetected. Based on Mendelian rules, we developed a scheme to adjust the observed proportion of hybrid progeny for these undetected cases. Moreover, the scheme can be used to iteratively add loci necessary to detect poplar hybrids beyond the second hybrid and first generation backcrosses. We questioned whether there is a risk of hybrid swarm formation or swamping of the P. nigra gene pool. We discuss the likelihood of such a scenario and draw conclusions for conservation issues while poplar plantations are increasingly appreciated as renewable resources.