Testing Local Host Adaptation and Phenotypic Plasticity in a Herbivore When Alternative Related Host Plants Occur Sympatrically

Abstract

Host race formation in phytophagous insects can be an early stage of adaptive speciation. However, the evolution of phenotypic plasticity in host use is another possible outcome. Using a reciprocal transplant experiment we tested the hypothesis of local adaptation in the aphid Brevicoryne brassicae. Aphid genotypes derived from two sympatric host plants, Brassica oleracea and B. campestris, were assessed in order to measure the extent of phenotypic plasticity in morphological and life history traits in relation to the host plants. We obtained an index of phenotypic plasticity for each genotype. Morphological variation of aphids was summarized by principal components analysis. Significant effects of recipient host on morphological variation and life history traits (establishment, age at first reproduction, number of nymphs, and intrinsic growth rate) were detected. We did not detected genotype × host plant interaction; in general the genotypes developed better on B. campestris, independent of the host plant species from which they were collected. Therefore, there was no evidence to suggest local adaptation. Regarding plasticity, significant differences among genotypes in the index of plasticity were detected. Furthermore, significant selection on PC1 (general aphid body size) on B. campestris, and on PC1 and PC2 (body length relative to body size) on B. oleracea was detected. The elevation of the reaction norm of PC1 and the slope of the reaction norm for PC2 (i.e., plasticity) were under directional selection. Thus, host plant species constitute distinct selective environments for B. brassicae. Aphid genotypes expressed different phenotypes in response to the host plant with low or nil fitness costs. Phenotypic plasticity and gene flow limits natural selection for host specialization promoting the maintenance of genetic variation in host exploitation.

Life-history trait plasticity and its relationships with plant adaptation and insect fitness: a case study on the aphid Sitobion avenae

“…All significant correlations between plasticity and fitness were negative, suggesting that the cost of maintaining plasticity in S. avenae could be high. This is in contrast to a study on another aphid, Brevicoryne brassicae L., where no significant relationships between trait plasticity and fitness were identified (meaning no cost of plasticity) 32 . Our results also indicate that higher levels of phenotypic plasticity for S. avenae clones could have lower adaptive value.…”

Testing specialization hypothesis on a stress gradient

“…genetic differentiation and local adaptation of life histories) occurs even over very short distances (Linhart & Grant ; Prati & Schmid ). However, there are also contradicting studies that have found that gene flow prevented specialization (Anderson & Geber ; Ruiz‐Montoya & Núñez‐Farfán ). Overall, specialization (and local adaptation) over small scales makes it very likely that specialization is a major force driving adaptation in stressful habitats such as mine sites.…”

Development and verification of SNP arrays to monitor hybridization between two host-associated strains of knotweed psyllid, Aphalara itadori

“…From an evolutionary perspective, strains likely represent an early point in the process of speciation (i.e., before genetic isolation [Dres and Mallet, 2002]) and can be influenced by factors such as geneflow, genetic diversity, and phenotypic plasticity (Ruiz-Montoya and Nunez-Farfan, 2013). How frequently these strains hybridize, or what effect hybridization has on the sustainability of biological control services is unclear as relatively few studies have examined the effects of post-introduction hybridization.…”