Genetic traces of cultivated hybrid poplars in the offspring of native Populus nigra in Austria.

Hybrid poplars like Populus ×canadensis or balsam poplar (Populus section Tacamahaca) hybrids have been propagated in Europe since the early 18th century. They replaced many stands of native black poplar on the banks of the major rivers. While spontaneous crosses between hybrid and native black poplars were not considered to be common or of importance in nature, it is shown that such crosses have occurred in Austria, as P. deltoides and section Tacamahaca alleles (PCR-amplified chloroplast and nuclear DNA markers) were found in plants morphologically similar to P. nigra. In the localities studied, a realistic estimate of the introgression rate is between 0 and 10% of plants in a given stand. Female hybrid trees produce viable seed. This shows that hybrid poplars can spread their genes by sexual means, which may result in continuing introgression and consequently, a reduction in genetic diversity and fitness of the endangered P. nigra.