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 axe often restricted to ecotones or disturbed habitats. In this study we show that i ntrogressive hybrids between the tetraploid Viola riviniana and the diploid V. reichenbachiana invaded pine forests of the Dubener Heide (central Germ any), an area affected by calcareous pollutants. The spread of these violet : populations was correlated with, the impact of pollution on habitat condi tions. We compared morphology, cytology and random amplified polymorphic DN A (RAPD) bands among six Viola populations from the Dubener 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 occurre d in at least 75% of the plants in one pure violet species but in none of t he other. Seven bands were specific to V. riviniana and 11 bands were speci fic to V. reichenbachiana. Two plants of a V. reichenbachiana population we re identified as hybrids. Of the Viola populations from the Dubener Heide, one was diploid and could be classified as V reichenbachiana by morphology and RAPD markers. However, the majority of the Dubener Heide populations we re tetraploid, and of a more variable morphology than V. riviniana and V. r eichenbachiana, showing different combinations of intermediate characters, characters of the pure species and extreme characters. Despite their overal l genetic similarity to V; riviniana, these plants could be identified as i ntrogressive hybrids between V. riviniana and V reichenbachiana by species- specific RAPD bands. Therefore, we propose that recurrent hybridization and backcrossing resulted in novel genotypes adapted to the changed environmen t of polluted pine forests.