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Abstract
Host specialization is considered a primary driver of the enormous diversity of herbivorous insects. Trade-offs in host use are hypothesized to promote this specialization, but they have mostly been studied in generalist herbivores. We conducted a multi-generation selection experiment to examine the adaptation of the specialist seed-feeding bug, Lygaeus equestris, to three novel host plants (Helianthus annuus, Verbascum thapsus and Centaurea phrygia) and to test whether trade-offs promote specialization. During the selection experiment, body size of L. equestris increased more on the novel host plant H. annuus compared to the primary host plant, Vincetoxicum hirundinaria, but this effect was not observed in other fitness related traits. In addition to selection, genetic drift caused variation among the experimental herbivore populations in their ability to exploit the host plants. Microsatellite data indicated that the level of within-population genetic variation decreased and population differentiation increased more in the selection line feeding on H. annuus compared to V. hirundinaria. We found a negative correlation between genetic differentiation and heterozygosity at the end of the experiment, suggesting that differentiation was significantly affected by genetic drift. We did not find fitness trade-offs between L. equestris feeding on the four hosts. Thus, trade-offs do not seem to promote specialization in L. equestris. Our results suggest that this insect herbivore is not likely to adapt to a novel host species in a time-scale of 20 generations despite sufficient genetic variation and that genetic drift disrupted the response to selection.
Highlights
Despite the obvious advantages of polyphagy with regard to food availability, most herbivorous insects feed only on a limited set of available host plants [1,2]
To investigate whether L. equestris adapted to a novel host plant (H. annuus) during the selection experiment, we examined the fitness of L. equestris on V. hirundinaria and H. annuus before and after selection
Our results suggest that despite the ability to occasionally feed on a variety of plant species, L. equestris is limited in its ability to adapt to novel hosts as we found evidence for adaptation in only one of the four fitness traits for one novel host plant after 20 generations of selection
Summary
Despite the obvious advantages of polyphagy with regard to food availability, most herbivorous insects feed only on a limited set of available host plants [1,2]. Genetic drift and adaptation to novel host empirical tests for the causes and mechanisms of this host specialization (i.e. trade-offs [3]) have been disproportionately conducted with generalist herbivores [4,5,6,7,8] there are some studies examining trade-offs in specialist [9] as well as oligophagous herbivores [10,11,12] This bias may have lead to an overestimation of the ability of herbivores to switch hosts and may limit our understanding of the process of host specialization. Examining specialization and the ability to adapt to novel host plants in species that are more specialized allows novel insights into the process of hostplant specialization and speciation and may help us understand why specialization is common in herbivorous insects
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