Reaction norm variation in a rare plant, Senecio integrifolius (Asteraceae)

Abstract

This study examines the pattern of reaction norm variation and the extent to which genetic constraints could prevent the evolution of broader ecological tolerance in Senecio integrifolius, a rare and declining plant in Sweden. Following the establishment of biparental progenies, we planted seedlings from each full-sib family in two greenhouse environments: full daylight and 50 per cent shade. A 2-year demographic study indicated that full-sib genotypes representing a large population at a site with a fine-scale mosaic habitat (meadow with clumps of shrubs) had a significantly greater overall performance across the environments and a lower sensitivity to a drastic reduction in light intensity than genotypes representing a small and patchily distributed population at a site without spatial variation (meadow exposed to full sun), confirming the supposition of more broadly adapted genotypes in a large population subject to selection across a wider range of micro-habitats. There was a significant average effect of genotype for all traits but we failed to detect within-population variation in phenotypic response (G × E) for direct components of fitness and conclude that there is little potential for specialization to particular light regimes. Across-environment correlations using full-sib means were small and positive and there was no change in the ranking of population means across the two environments, implying a lack of strong trade-offs within the reaction norm. However, the changes in ecological amplitude may not be of sufficient magnitude to permit long-term survival of remaining populations, as implied by the rapid decline of S. integrifolius in Sweden.