Hybridization in the invasive Fallopia complex and its influence on sexual reproduction and herbivore resistance

Biological invasions are considered to be an important cause of current biodiversity loss and can cause significant ecological problems. Several hypotheses have been proposed to explain why exotic plants become invasive. An important determinant of the success of invasive plants in their adventive range is the efficiency of reproduction. High fecundity and the potential for sexual reproduction, which generates genetic variability but also vigorous clonal growth, are attributes frequently observed among the most successful invasive plants. However, external factors also can influence plant invasions. One commonly accepted mechanism for the invasion success of plants is that plants introduced into a new range experience less herbivory due to the release from natural enemies resulting in higher plant fitness. Furthermore, biological invasions may be promoted by climate change. Altered environmental conditions due to climate change could be more favourable for many invasive species and could increase the reproduction, establishment and spread of exotic plants in the introduced range. Recently, it has been claimed that the ability of exotic plants to establish and rapidly expand in a novel range may also be shaped by evolutionary processes. Interspecific hybridization has been identified as a stimulus for the evolution of increased invasiveness in exotic plant species. Hybridization can increase genetic variation and provide the genetic material on which natural selection can act. Novel gene combinations may result in phenotypes that are stronger competitors, have higher reproduction and survival rates and are more resistant against herbivory and may therefore be favoured by natural selection. Moreover, hybridization may facilitate adaptation to different or altered environmental conditions, for example, due to climate change. Successful hybrid genotypes might be fixed and retained by vegetative propagation and increase the invasion success of exotic plants. Taxa of the genus Fallopia (Polygonaceae), native to eastern Asia, are among the most troublesome invasive species worldwide and are particularly aggressive in Europe and North America. The plants cause significant damage to native ecosystems and are especially a problem along rivers. In Europe, exotic Fallopia plants spread mainly by vegetative propagation. Of the widespread F. japonica var. japonica in Europe only a single genotype with male-sterile flowers has been found, and sexual reproduction in the exotic Fallopia complex is thus restricted mainly to hybridization among the taxa. The naturalized but less frequent species F. sachalinensis can act as pollen donor, resulting in the hybrid F. x bohemica. The hybrid is frequent and considered to have a higher invasion potential and a faster spread than its parental species. A third introduced species is the climbing F. baldschuanica. Hybridization between this species and F. japonica var. japonica results in the hybrid F. x conollyana, which so far has only rarely been reported from Europe. In my dissertation I present four studies on hybridization in the invasive Fallopia complex in Western Europe and its consequences. Specifically, I studied the genetic diversity and hybridization patterns (chapter 2), extent and variation in sexual reproduction (chapter 3), regional variation in seedling establishment and the potential effects of climate change (chapter 4) and the influence of hybridization on herbivore resistance (chapter 5)