Bhattarai, G. P., L. A. Meyerson, J. Anderson, D. Cummings, W. J. Allen and J. T. Cronin. 2017. The biogeography of a plant invasion: Genetic variation and phenotypic plasticity in latitudinal clines for plant-herbivore interaction traits. Ecological Monographs DOI: 10.1002/ecm.1233.


The juxtaposition of plant-species invasions with latitudinal gradients in herbivore pressure is an important yet mostly unexplored issue in invasion biology. Latitudinal clines in defense and palatability to herbivores are expected to exist in native plant species but the evolution of these clines may lag behind for invasive plant species resulting in non-parallel latitudinal clines that may impact invasion success. Using native and invasive genotypes of Phragmites australis collected across a 17° latitudinal range in North America, we performed experiments in replicate northern and southern common gardens to investigate whether these genotypes exhibited different genetically based latitudinal clines in defenses, nutritional condition and palatability to their herbivores (the aphid Hyalopterus pruni and the fall armyworm Spodoptera frugiperda). We also tested whether invasive genotypes are more phenotypically plastic than natives and whether plasticity varies with latitude. Although invasive genotypes did not exhibit higher defense levels (leaf toughness, phenolics, % carbon), they were considerably less palatable to aphids and armyworms than native genotypes. Both native and invasive genotypes exhibited genetic-based latitudinal gradients in a putative defense trait (leaf toughness) and palatability traits (water content and aphid colony growth). Genetic-based latitudinal clines were common among the 12 plant traits studied but were twice as frequent for the invasive than native genotypes. In 36% of the cases where clines were evident, they were non-parallel between the two genotypes. These data suggest that clines in the invasive genotypes of P. australis evolved within the past ~100 years. Moreover, our study showed that the occurrence and direction of latitudinal clines in plant traits were commonly dependent on where the study was conducted (north or south), indicating strong phenotypic plasticity in these genetic-based clines. Finally, traits for invasive genotypes of P. australis were 2.5 times more plastic than traits for native genotypes. Interestingly, plasticity for native but not invasive genotypes was strongly dependent on latitude of origin. Such spatial heterogeneity within and between genotypes with respect to their interactions with herbivores has the potential to generate substantial spatial variability in biotic resistance that can have important implications for the establishment and spread of invasive genotypes and species.

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