Local adaptation of the pondweed Potamogeton pectinatus to contrasting substrate types mediated by changes in propagule provisioning

1 We studied local adaptation to substrate type within a population of the clonal aquatic macrophyte Potamogeton pectinatus and the role that genotypic variation in propagule-provisioning plays therein. 2 P. pectinatus reproduces mainly by means of subterranean asexual propagules (tubers), whose survival and sprouting success depends on the interaction of factors such as the size of tubers, substrate type and predation risk by Bewick's swans. 3 We studied a population of P. pectinatus in which genotypes producing large tubers predominate at the sandy shore and those producing small tubers at the clay-rich shore. Clonal lines originating from different shores were grown on a sandy and a clay-rich substrate in a common-garden. Plants from all clonal lines were grown from tubers of a comparable size range, but the various clones from within each shore differed in the average size of tubers they are genetically determined to produce. 4 The performance of all clones was much lower on sandy substrate than on clay-rich substrate, indicating that the former is a stressful (nutrient-poor) environment. The reaction norms of morphological traits varied significantly among clones, revealing genetic variation in phenotypic plasticity. However, these differences were not related to our correlates of fitness (total tuber biomass, tuber size and tuber number). We found no evidence of local adaptation independent of genotypic tuber size. Instead, tuber size mediated local adaptation: clones producing larger tubers had a higher fitness in sandy substrate, while clones producing smaller tubers had a higher fitness in clay-rich substrate. 5 Our results imply that diversifying selection for tuber size takes place between the two substrate types and confirms the importance of tuber-size provisioning for local adaptation to substrate heterogeneity.