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

Abstract

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.

Quantification of the zygotic barrier between interbreeding taxa using gene flow data

“…This term added the undetectable rest of either 7.25% or 32.64% to the empirically identified found introgression rates (Eq. 3 and Table in Bialozyt et al ). The explicit correction terms are valid for four unlinked markers and the first backcross generations or F 2 hybrids exactly reflecting the parental setting of F 1 hybrids and pure P. nigra in our study landscape.…”

“…In a subsequent study, Bialozyt et al () collected seeds from twelve female trees (six from each taxa) in two consecutive years (2006, 2007) and sowed them on filter paper. From the 3000 sown seeds, 2606 seedlings were harvested and genetically analyzed as described by Rathmacher et al ().…”

“…Accounting for Mendelian rules and iteratively using four unlinked diagnostic markers, cumulative probabilities of hybrid detection sum up to 93.75% in matings between P. nigra and P. × canadensis, and they reach 68.36% in the matings between P. × canadensis hybrids (see Fig. in Bialozyt et al ). While losing the undetectable proportion would introduce a strong bias or underestimation of introgression rates into the model, we worked with a simple mathematically derived correction term.…”

“…In this study, we used the corrected introgression rates from the mother trees analyzed by Bialozyt et al (). Introgression rates were available for eight mother trees for 2006 and for eleven mother trees for 2007 (Table ).…”

The use of habitat and dispersal models in protecting European black poplar ( Populus nigra L.) from genetic introgression in Slovenia

“…Once hybridization has begun, it is in general difficult to stop, especially if hybrids are fertile and produce progeny among themselves and with the parental species (Allendorf et al, 2001). Bialozyt et al (2012) have detected F2 hybrids between P. nigra and P.  canadensis in the seeds, while first generation backcrosses occurred in seeds as well as in juveniles. High levels of introgression from Populus  canadensis can be expected in the situations where isolated female black poplar trees are surrounded by interspecific hybrid males (Vanden Broeck et al, 2004, 2005.…”