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.
Highlights
Host race formation or local host adaptation in phytophagous insects is considered an early stage of adaptive speciation in sympatry [1,2,3,4,5,6]
Aphids grown on B. campestris attained, in general, larger body sizes (PC1) and relative body length (PC2)
Phenotypic plasticity and gene flow limits natural selection for host specialization promoting the maintenance of genetic variation in host use
Summary
Host race formation or local host adaptation in phytophagous insects is considered an early stage of adaptive speciation in sympatry [1,2,3,4,5,6]. Local adaptation to a host plant might be limited by the herbivore’s genetic diversity, gene flow and/or phenotypic plasticity [3,7]. Phytophagous insects commonly interact with different host species at a local scale. Even at this scale, local adaptation to different host plants species is an expected outcome if selection in each host species is strong enough to prevent the homogenizing effects of gene flow. Genotypes may respond differentially to each host by producing different phenotypes, those with the most favourable phenotype in each host persisting while others go extinct [14,15]. Adaptive genetic differentiation and adaptive phenotypic plasticity are two evolutionary paths to maximize fitness in response to environmental heterogeneity, and these responses are not mutually exclusive [17,18]
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