Spread of violets in polluted pine forests: morphological and molecular evidence for the ecological importance of interspecific hybridization

Hybridization between plant species occurs frequently but hybrids are often restricted to ecotones or disturbed habitats. In this study we show that introgressive hybrids between the tetraploidViola rivinianaand the diploidV. reichenbachianainvaded pine forests of the Dübener Heide (central Germany), an area affected by calcareous pollutants. The spread of these violet populations was correlated with the impact of pollution on habitat conditions. We compared morphology, cytology and random amplified polymorphic DNA (RAPD) bands among sixViolapopulations from the Dübener Heide and three populations of each pure species. RAPD analysis using 12 primers revealed 141 scorable bands. We considered bands as species specific if they occurred in at least 75% of the plants in one pure violet species but in none of the other. Seven bands were specific toV. rivinianaand 11 bands were specific toV. reichenbachiana. Two plants of aV. reichenbachianapopulation were identified as hybrids. Of theViolapopulations from the Dübener Heide, one was diploid and could be classified asV. reichenbachianaby morphology and RAPD markers. However, the majority of the Dübener Heide populations were tetraploid, and of a more variable morphology thanV. rivinianaandV. reichenbachiana, showing different combinations of intermediate characters, characters of the pure species and extreme characters. Despite their overall genetic similarity toV. riviniana, these plants could be identified as introgressive hybrids betweenV. rivinianaandV. reichenbachianaby species‐specific RAPD bands. Therefore, we propose that recurrent hybridization and backcrossing resulted in novel genotypes adapted to the changed environment of polluted pine forests.