Glycoalkaloids of Wild and Cultivated Solanum: Effects on Specialist and Generalist Insect Herbivores

Abstract

Plant domestication by selective breeding may reduce plant chemical defense in favor of growth. However, few studies have simultaneously studied the defensive chemistry of cultivated plants and their wild congeners in connection to herbivore susceptibility. We compared the constitutive glycoalkaloids (GAs) of cultivated potato, Solanum tuberosum, and a wild congener, S. commersonii, by liquid chromatography coupled to mass spectrometry. We also determined the major herbivores present on the two species in field plots, and tested their preference for the plants and their isolated GAs in two-choice bioassays. Solanum commersonii had a different GA profile and higher concentrations than S. tuberosum. In the field, S. tuberosum was mostly attacked by the generalist aphids Myzus persicae and Macrosiphum euphorbiae, and by the specialist flea beetle Epitrix argentinensis. In contrast, the most common herbivore on S. commersonii was the specialist sawfly Tequus sp. Defoliation levels were higher on the wild species, probably due to the chewing feeding behavior of Tequus sp. As seen in the field, M. persicae and E. argentinensis preferred leaf disks of the cultivated plant, while Tequus sp. preferred those of the wild one. Congruently, GAs from S. commersonii were avoided by M. persicae and preferred by Tequus sp. The potato aphid performed well on both species and was not deterred by S. commersonii GAs. These observations suggest that different GA profiles explain the feeding preferences of the different herbivores, and that domestication has altered the defensive capacity of S. tuberosum. However, the wild relative is still subject to severe defoliation by a specialist herbivore that may cue on the GAs.

Glycoalkaloid composition explains variation in slug resistance in Solanum dulcamara

“…By utilizing a metabolomics approach to analyse the underlying chemical mechanisms, we identified four prominent steroidal glycoalkaloids (GAs) showing particularly strong negative correlations with slug feeding preference. This is in line with previous studies reporting toxic or repellent effects of different types of alkaloidal secondary metabolites to gastropods (Aguiar and Wink 2005; Bog et al 2017; Speiser et al 1992; Wink 1984) and insect herbivores (Altesor et al 2014; Hare 1983). …”

Preference–performance in a specialist sawfly on congeneric host plants

“…This preference is in line with our previous studies using contiguously planted field plots of both Solanum species. After 6 weeks of insect surveillance, we found the larvae of T. schrottkyi almost exclusively on S. commersonii (154 out of 155 larvae), even though both Solanum plants were readily available in close proximity (Altesor et al., ). These larvae most likely originated from eggs laid by females that arrived naturally to the field site.…”

“…It may be argued that host preference by the ovipositing females is actually the result of adaptive avoidance of a toxic, non‐suitable host. However, we have previously shown that T. schrottkyi larvae choose to feed on S. tuberosum if GAs from S. commersonii are added to the leaf substrate (Altesor et al., ). The larvae thus display a positive preference for the GAs of their host plant, rather than rejection of GAs of the non‐host plant.…”

“…Our previous studies with local populations of T. schrottkyi have shown that the larvae are found almost exclusively on S. commersonii , even in the presence of S. tuberosum . In choice laboratory bioassays, the larvae preferred to feed on S. commersonii , but they also fed on the cultivated potato (Altesor et al., ). Therefore, S. tuberosum may also be a suitable host, as is the case for other Tequus species, making the wild and cultivated Solanum species a potential system for testing preference–performance correlations.…”

“…We also compared the nutritional value of both Solanum plants, in order to correlate preference and performance with plant chemistry. The assessment of nutritional parameters adds to our previous comparative study of glycolalkaloid contents (Altesor et al, 2014), the main defensive metabolites produced by Solanum species. Specifically, in this study we tested (1) the performance of immature stages of T. schrottkyi on both plants in a non-choice setup, (2) the preference of female T. schrottkyi for laying eggs on S. commersonii when offered a dual choice with S. tuberosum, and (3) differences in the contents of nonstructural proteins, carbohydrates, and water in both Solanum species.…”