Additive genetic effects in interacting species jointly determine the outcome of caterpillar herbivory

Patterns of genetic variation and local adaptation of a native herbivore to a lethal invasive plant

Understanding the evolutionary processes that influence fitness is critical to predicting species' responses to selection. Interactions among evolutionary processes including gene flow, drift and the strength of selection can lead to either local adaptation or maladaptation, especially in heterogenous landscapes. Populations experiencing novel environments or resources are ideal for understanding the mechanisms underlying adaptation or maladaptation, specifically in locally co-evolved interactions. We used the interaction between a native herbivore that oviposits on a patchily distributed introduced plant that in turn causes significant mortality to the larvae to test for signatures of local adaptation in areas where the two co-occurred. We used whole-genome sequencing to explore population structure, patterns of gene flow and signatures of local adaptation. We found signatures of local adaptation in response to the introduced plant in the absence of strong population structure with no genetic differentiation and low genetic variation. Additionally, we found localized allele frequency differences within a single population between habitats with and without the lethal plant, highlighting the effects of strong selection. Finally, we identified that selection was acting on larval ability to feed on the plant rather than on females' ability to avoid oviposition, thus uncovering the specific ontogenetic target of selection. Our work highlights the potential for adaptation to occur in a fine-grained landscape in the presence of gene flow and low genetic variation.

Genetic and behavioral differences between above and below ground Culex pipiens bioforms

Efficiency of mosquito-borne disease transmission is dependent upon both the preference and fidelity of mosquitoes as they seek the blood of vertebrate hosts. While mosquitoes select their blood hosts through multi-modal integration of sensory cues, host-seeking is primarily an odor-guided behavior. Differences in mosquito responses to hosts and their odors have been demonstrated to have a genetic component, but the underlying genomic architecture of these responses has yet to be fully resolved. Here, we provide the first characterization of the genomic architecture of host preference in the polymorphic mosquito species, Culex pipiens. The species exists as two morphologically identical bioforms, each with distinct avian and mammalian host preferences. Cx. pipiens females with empirically measured host responses were prepared into reduced representation DNA libraries and sequenced to identify genomic regions associated with host preference. Multiple genomic regions associated with host preference were identified on all 3 Culex chromosomes, and these genomic regions contained clusters of chemosensory genes, as expected based on work in Anopheles gambiae complex mosquitoes and in Aedes aegypti. One odorant receptor and one odorant binding protein gene showed one-to-one orthologous relationships to differentially expressed genes in A. gambiae complex members with divergent host preferences. Overall, our work identifies a distinct set of odorant receptors and odorant binding proteins that may enable Cx. pipiens females to distinguish between their vertebrate blood host species, and opens avenues for future functional studies that could measure the unique contributions of each gene to host preference phenotypes.

Are exotic host plants a life raft or a trap for butterflies?

Many landscapes across the world are dominated by exotic (non-native) plant species. These plants can directly impact native species, including insect herbivores. There are many reported cases of native butterfly species using exotic host plants, and these new interactions have had diverse effects on butterfly populations. In this mini-review, I highlight recent developments in the study of the effects of exotic host plants on butterflies, focusing on two areas that have seen major advances: the genetic basis of host use and the influence of other trophic levels on butterfly-plant interactions. Understanding how these multiple factors interact is a key outstanding question for better predicting if an exotic plant might be a trap or a life raft for a herbivorous insect.

Additive and interactive effects of anthropogenic stressors on an insect herbivore

The pressures of global change acting on wild plants and animals include exposure to environmental toxins, the introduction of non-native species, and climate change. Relatively few studies have been reported in which these three main classes of stressors have been examined simultaneously, allowing for the possibility of synergistic effects in an experimental context. In this study, we exposed caterpillars of the Melissa blue butterfly (Lycaeides melissa) to three concentrations of chlorantraniliprole, under three experimental climates, on a diet of a native or a non-native host plant throughout larval development in a fully factorial experiment. We find that high pesticide exposure and a non-native diet exhibit strong negative effects on caterpillars, resulting in 62% and 42% reduction in survival, respectively, while interactive effects tend to be weaker, ranging from 15% to 22% reduction in survival. Interactive effects have been shown to be strong in other contexts, but do not appear to be universal; however, our study shows that the cumulative effects of stressors acting in isolation (additively) are sufficiently strong to severely reduce survival and by extension population persistence in the wild.

Genome-wide association studies in plant pathosystems: success or failure?

Harnessing natural genetic variation is an established alternative to artificial genetic variation for investigating the molecular dialog between partners in plant pathosystems. Herein, we review the successes of genome-wide association studies (GWAS) in both plants and pathogens. While GWAS in plants confirmed that the genetic architecture of disease resistance is polygenic, dynamic during the infection kinetics, and dependent on the environment, GWAS shortened the time of identification of quantitative trait loci (QTLs) and revealed both complex epistatic networks and a genetic architecture dependent upon the geographical scale. A similar picture emerges from the few GWAS in pathogens. In addition, the ever-increasing number of functionally validated QTLs has revealed new molecular plant defense mechanisms and pathogenicity determinants. Finally, we propose recommendations to better decode the disease triangle.