Exotic herbivorous insects as selective agents for an exotic perennial herb, Solidago altissima

Background/Question/Methods

Biological invasion is increasingly recognized as an excellent topic for studying adaptive evolution over contemporary time scales. Unraveling the changes in ecological processes after the expansion of invasive species is important for understanding how the demographic success of such species may change. Solidago altissima is a North American perennial that have extensively invaded abandoned fields in Japan. The recent expansion of Corythucha marmorata (lacebug), a herbivorous insect in the native range of S. altissima, provides a promising opportunity to test the potential of the selective impacts of this herbivore on the defensive traits of S. altissima. Because lacebugs were introduced into Japan 13 years ago and have been rapidly expanding their habitat concentrically, the dates of population establishment differ geographically. The aim of this study was to investigate defense levels in natural populations of S. altissima across Japan, and to compare these to geographic differences in the establishment years of lacebugs. We conducted a field survey, common garden experiment, and microsatellite analysis to compared quantitative genetic differentiation (Qct) of traits (e.g., resistance, growth, and reproduction) and neutral molecular differentiation (Fct) among 16 populations with different lacebug establishment years in Japan.

Results/Conclusions

Higher resistance was observed in populations with earlier lacebug establishment dates. No differences were observed in growth traits among populations with lacebugs of different establishment years, whereas the reproductive traits were positively affected by lacebug establishment year. There was a positive relationship between reproductive traits and defensive traits, suggesting that resistant genotypes were more successful in reproduction. Comparisons of Qct and Fct suggest that phenotypic variability among populations with different lacebug establishment years was driven by natural selection rather than by stochastic events. Our findings suggest that the defensive traits of S. altissima have evolved locally over the last decade, in response to selective pressures imposed by the expansion of lacebugs.