Varying Herbivore Population Structure Correlates with Lack of Local Adaptation in a Geographic Variable Plant-Herbivore Interaction

Abstract

Local adaptation of parasites to their hosts due to coevolution is a central prediction of many theories in evolutionary biology. However, empirical studies looking for parasite local adaptation show great variation in outcomes, and the reasons for such variation are largely unknown. In a previous study, we showed adaptive differentiation in the arctiid moth Utetheisa ornatrix to its host plant, the pyrrolizidine alkaloid-bearing legume Crotalaria pallida, at the continental scale, but found no differentiation at the regional scale. In the present study, we sampled the same sites to investigate factors that may contribute to the lack of differentiation at the regional scale. We performed field observations that show that specialist and non-specialist polyphagous herbivore incidence varies among populations at both scales. With a series of common-garden experiments we show that some plant traits that may affect herbivory (pyrrolizidine alkaloids and extrafloral nectaries) vary at the regional scale, while other traits (trichomes and nitrogen content) just vary at the continental scale. These results, combined with our previous evidence for plant population differentiation based on larval performance on fresh fruits, suggest that U. ornatrix is subjected to divergent selection even at the regional scale. Finally, with a microsatellite study we investigated population structure of U. ornatrix. We found that population structure is not stable over time: we found population differentiation at the regional scale in the first year of sampling, but not in the second year. Unstable population structure of the herbivore is the most likely cause of the lack of regional adaptation.

Mycorrhizal Symbiosis and Local Adaptation in Aster amellus: A Field Transplant Experiment

“…The absence of local adaptation has also been reported in other plant species, such as Carlina vulgaris [32], Crotalaria pallida [33], Stipa capillata [34] and Helleborus foetidus [35]. Each of these studies provided different explanations for the lack of local adaptation.…”

A free lunch? No cost for acquiring defensive plant pyrrolizidine alkaloids in a specialist arctiid moth (Utetheisa ornatrix)

“…Therefore, although our higher concentration treatment represents around 100 times the average concentration of hosts in the population where the moths used in this experiment were collected (Cogni et al . ), it is likely that there has been selection in the past to use such high concentrations in other hosts.…”

“…By preying on the seeds, U. ornatrix can have a significant impact on the fitness of Crotalaria plants (Cogni et al . ). PAs sequestered by the larvae are maintained in the pupal and adult stages (Eisner & Meinwald ; Conner & Weller ).…”

Evolutionary dynamics of interactions between plants and their enemies: comparison of herbivorous insects and pathogens

“…Several studies examined physical traits, such as leaf color or other morphological characters, but most of them focused on plant chemical defenses against herbivory. A few studies examined plant traits (e.g., extrafloral nectaries) that reduce herbivory by enhancing the local abundance of natural enemies …”