Herbivory and fitness components in an introgressive hybrid swarm of Senecio hercynicus and S. ovatus (Compositae, Senecioneae)

• Analysis of a mixed stand of the rare S. hercynicus and its congener S. ovatus.
• Survey of intrinsic (pollen fertility, seed set, germination rate) fitness components.
• Survey of extrinsic fitness component (leaf damage by herbivory).
• Hybrid and backcrossed genotypes do not suffer from a reduced fitness.
• The rare S. hercynicus is at the verge of being completely absorbed.

Hybridization among plant species may have considerable evolutionary consequences, from the formation of new taxa to the extinction of species by genetic swamping. The evolutionary fate of a system of hybridizing species is highly dependent on fitness differences between parental and hybrid genotypes. In an introgressed hybrid swarm of the rare Senecio hercynicus and its widely distributed congener Senecio ovatus (Compositae, Senecioneae) we assessed pollen fertility, seed set, and germination rates, along with leaf damage dynamics due to herbivory throughout the vegetation period of 2006 in order to compare intrinsic and extrinsic fitness components among genotypes. Correlation and linear regression analyses carried out with fitness components as dependent and genetic constitution, phenology, and pyrrolizidine alkaloid (PA) content as independent variables reveal that: (a) intrinsic fitness components (pollen fertility, seed set, germination rate) are not reduced in hybrid and backcross individuals, (b) the extrinsic fitness component of leaf damage caused by herbivores is significantly influenced by genetic constitution (S. hercynicus-like individuals being more severely damaged than hybrid genotypes and S. ovatus-like individuals), by phenology (early flowering individuals exhibiting higher damage than late-flowering ones), and PA concentration (with the astonishing result of hercynicus-like genotypes being increasingly damaged by herbivory with increasing PA concentrations and a reversed trend – decreasing damage with increasing PA content – in the hybrid and the ovatus-like genotypes). When all surveyed fitness parameters are considered in conjunction, the present results nicely demonstrate that hybrid genotypes neither suffer from intrinsic nor from extrinsic fitness reductions and that the rare and early-flowering S. hercynicus is outperformed by hybrids and S. ovatus-like genotypes. The present study, therefore, adds further support to the prediction that S. hercynicus will be genetically swamped by the spreading S. ovatus.