Latitudinal gradients in the defense characteristics in an invasive plant, Phragmites australis, in North America

Background/Question/Methods

Recent research with invasive species has revealed that contemporary evolution plays an important role in the successful invasion of new habitats. For plants, theory and empirical studies suggest that latitudinal clines should evolve with regard to palatability and defense traits. The expectation is that palatability should increase and defense traits should decrease with increasing latitude. However, this theory has never been tested with a species involved in continent wide invasion.

Phragmites australis represents a model system to examine the evolution of latitudinal gradients because a Eurasian haplotype has recently (within the past 150 years) spread throughout North America and overlaps broadly in distribution with various native genotypes. In this study, we examined whether the Eurasian genotype of P. australis exhibits latitudinal gradients in its introduced range with regard to traits associated with its interaction with the aphid, Hyalopterus pruni. We collected P. australis rhizomes from different latitudes in the eastern United States (Florida to Rhode Island) and grew them in common garden environments in Kingston, RI and Baton Rouge, LA.  After growing them in the common environments for at least one year, plant height, one measure of herbivore defense (leaf toughness) and plant palatability (aphid colony growth rate) were evaluated.

Results/Conclusions

Plant and leaf size varied greatly among the populations collected from different latitudes. Northern populations of the same haplotype produced shorter plants with smaller leaves than southern populations. Leaf toughness decreased gradually with increasing latitudes. Southern plants produced leaves twice as tough as the northern plants. Aphid colony growth rate followed a similar trend with significantly greater growth rate in northern plants compared to southern plants. These results suggest that latitudinal gradients associated with P. australis herbivory have evolved within the past 150 years in the grass’s introduced range. It also supports conventional theory regarding diversity gradients in that plant populations in the south are less palatable and better defended than populations in the north. Future studies are planned to explore whether similar gradient exists for the native genotypes of P. australis as well as the mechanisms responsible for the evolution of these gradients.