Abstract
Background In Europe, Populus nigra generally grows sympatric with the exotic related taxon Populus x canadensis. Natural hybridization followed by introgression and genetic swamping is often mentioned as a potential threat for the conservation of P. nigra. However, as evidence for natural hybridization with P. x canadensis is scarce, the significance of this threat is unclear. Methods After first determining the genotypic diversity within 209 trees of P. nigra collected in western Flanders (northwest Belgium), we assessed natural hybridization events between P. nigra females and P. x canadensis males with diagnostic microsatellite markers. We also studied the possible paternity of the frequently planted P. nigra cv. Italica. Furthermore, we compared the potential seed set of P. x canadensis and P. nigra by counting the number of unfertilized ovules per ovary and the number of ovaries per pistillate inflorescences. Results and conclusions Only two different multilocus microsatellite genotypes were detected within the 209 black poplar trees sampled in western Flanders. In 57 % of the seedlings from the open pollinated black poplar trees, at least one species-specific allele of P. deltoides was detected. P. nigra cv. Italica could not be excluded as a father of one seedling. The potential for seed set and the number of ovules per ovary was significantly higher for P. x canadensis compared to P. nigra. Conclusions The extreme low number of P. nigra genotypes in western Flanders suggests a strong human influence on the species’ genetic diversity. The findings further indicate that natural hybridization events between female P. nigra and male P. x canadensis frequently occur in our study area. Furthermore, P. x canadensis has a higher seed set potential compared to P. nigra. Restoration projects of P. nigra in the valley of the IJzer river should initially aim reforestation measures with a mixture of different black poplar genotypes from neighbouring regions.
Highlights
Natural hybridization of a rare species with a more common taxon may result in the extinction of the former (Ellstrand 2003)
Twenty-six [out of 235 (11%)] adult trees were identified as hybrids. They were likely F1-hybrids because they were heterozygous at both loci Pmgc14 and Wpms09 showing one allele diagnostic for P. deltoides and another specific for P. nigra
As clones of P. deltoides are not planted in Belgium, alleles of P. deltoides in P. nigra offspring could only originate from natural hybridization with P. × canadensis males
Summary
Natural hybridization of a rare species with a more common taxon may result in the extinction of the former (Ellstrand 2003). Populus nigra L., the European black poplar, is considered a locally endangered native tree species in Europe. × canadensis are frequently planted for wood production. They are generally first-generation (F1) hybrids resulting from a cross between a female tree of the North American P. deltoides Bartr. Second-generation hybrids resulting from a backcross of a F1-hybrid to P. nigra are sometimes planted in commercial plantations. × canadensis are listed in the Belgian List of Approved Basic Material for the Production of Forest Reproductive Material. From this list, only a few genets are frequently planted and many of them are highly related (fullsibs).
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