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RESEARCH PRODUCT

Characterisation of phenotypic plasticity in seedling stage of native and invasive populations in Ambrosia artemisiifolia L.

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Increased phenotypic plasticity has often been invoked as an hypothesis to explain the capacity of alien invasive plant species to colonize new environments, because it may enhance the ability of plants to cope with new biotic and abiotic conditions. Following this hypothesis, alien invasive populations should be represented by more plastic genotypes than populations in the native range. In annual species, the seedling stage is of crucial importance for population dynamics but surprisingly very few studies have investigated differences in phenotypic plasticity on seedling traits. Based on populations from the native range (North America) and the invasive range (France) of Ambrosia artemisiifolia, we evaluated whether invasive populations exhibit higher levels of phenotypic plasticity for seedling traits (seed germination and hypocotyl’s growth) than native populations. We suspect that phenotypic plasticity has genetically evolved during the invasion process in Europe, since the original admixture. We also compared the level of phenotypic plasticity between invasive and non-invasive historical populations (i.e. historical populations that have been localised from herbarium specimens and that did not expand following introduction) in Europe in order to test whether evolution of phenotypic plasticity occurred with invasiveness. We measured seed germination patterns and hypocotyl’s growth in 3 native, 3 historical and 3 invasive populations of A. artemisiifolia L. for population differentiation in plasticity to 7 levels of germination temperature. We used the loess smoothing procedure to analyse the complex norms of reaction and to quantify total plasticity to temperature for each population (Simons & Wagner 2007). Then, we compared seed germination and hypocotyl’s growth traits expressed at population and half-sib family levels, in two temperature environments. The continuous norms of reaction for each of the 9 populations of A. artemisiifolia exhibited similar patterns of germination through the temperature gradient and total plasticity level was not significantly different for the 9 populations tested. The 95% confidence intervals of loess predictors overlapped for cold and warm temperatures. Furthermore, there was no evidence for a differentiation in germination for population statuses (native vs. historical vs. invasive). However, above 8°C, germination was significantly different among historical populations although this was not the case among populations of other statuses. Reaction norm differences among populations and half-sib families were observed for germination and hypocotyl’s growth traits. However we did not find any evidence of differentiation due to population status and no significant interaction with the environment. These results suggest that there was no evolution of plasticity after introduction and during invasion process. Besides, we did not find any differentiation in germination and hypocotyl’s growth trait responses for the geographical origin or the age of invasive populations. The populations tested did not exhibit any rapid evolutionary change in seedling traits although the historical invasive populations exhibited higher differentiation in seed germination compared to native and recent invasive populations.