Do differences in herbivore resistance contribute to elevational niches of species and hybrids in the central European Senecio nemorensis (Compositae, Senecioneae) syngameon?

Abstract

Elevational gradients with adjacent distribution zones of closely related and hybridising plant species provide a suitable natural set-up for studying evolutionary processes like adaptation and speciation. Transplantation experiments along these gradients are a straightforward strategy to infer fitness differences among genotypes and to pinpoint intrinsic and extrinsic factors that govern elevational stratification. Senecio germanicus, S. ovatus, and S. hercynicus belong to the S. nemorensis syngameon and show differential elevational distribution with mutual hybridisation in their range overlaps. We here report on the results of transplantation experiments with the three species and their reciprocal F1 hybrids along an elevational transect in the Bavarian Forest, SE Germany, in which we measured leaf area loss through mollusc herbivores and through withering caused by climatic influences. We find a strong correlation of the amount of the remaining photosynthetically active leaf area and rhizome dry weight as a proxy for overall viability of plants and their reproductive and propagational success. In the S. hercynicus-S. ovatus system, higher damage values caused by herbivores in the former species and by withering in the latter one, along with intermediate values found for hybrid genotypes, suggests a trajectory following an ́advancing wave modeĺ of introgressive hybridisation. In the S. germanicus-S. ovatus system, the more effective herbivore deterrent strategy of S. ovatus, together with its higher susceptibility to colder conditions should follow a ́mosaic hybrid zone modeĺ and eventually to a patchy distribution of pure parental and hybrid populations governed by the heterogeneity of climatic conditions at lower elevations.