Consequences of squash (Cucurbita argyrosperma) domestication for plant defence and herbivore interactions.

MAIN CONCLUSION: Cucurbita argyrosperma domestication affected plant defence by downregulating the cucurbitacin synthesis-associated genes. However, tissue-specific suppression of defences made the cultivars less attractive to co-evolved herbivores Diabrotica balteata and Acalymma spp. Plant domestication reduces the levels of defensive compounds, increasing susceptibility to insects. In squash, the reduction of cucurbitacins has independently occurred several times during domestication. The mechanisms underlying these changes and their consequences for insect herbivores remain unknown. We investigated how Cucurbita argyrosperma domestication has affected plant chemical defence and the interactions with two herbivores, the generalist Diabrotica balteata and the specialist Acalymma spp. Cucurbitacin levels and associated genes in roots and cotyledons in three wild and four domesticated varieties were analysed. Domesticated varieties contained virtually no cucurbitacins in roots and very low amounts in cotyledons. Contrastingly, cucurbitacin synthesis-associated genes were highly expressed in the roots of wild populations. Larvae of both insects strongly preferred to feed on the roots of wild squash, negatively affecting the generalist's performance but not that of the specialist. Our findings illustrate that domestication results in tissue-specific suppression of chemical defence, making cultivars less attractive to co-evolved herbivores. In the case of squash, this may be driven by the unique role of cucurbitacins in stimulating feeding in chrysomelid beetles.

Sequestration of cucurbitacins from cucumber plants by Diabrotica balteata larvae provides little protection against biological control agents.

Cucurbitaceae plants produce cucurbitacins, bitter triterpenoids, to protect themselves against various insects and pathogens. Adult banded cucumber beetles (Diabrotica balteata), a common pest of maize and cucurbits, sequester cucurbitacins, presumably as a defensive mechanism against their natural enemies, which might reduce the efficacy of biological control agents. Whether the larvae also sequester and are protected by cucurbitacins is unclear. We profiled cucurbitacin levels in four varieties of cucumber, Cucumis sativus, and in larvae fed on these varieties. Then, we evaluated larval growth and resistance against common biocontrol organisms including insect predators, entomopathogenic nematodes, fungi and bacteria. We found considerable qualitative and quantitative differences in the cucurbitacin levels of the four cucumber varieties. While two varieties were fully impaired in their production, the other two accumulated high levels of cucurbitacins. We also observed that D. balteata larvae sequester and metabolize cucurbitacins, and although the larvae fed extensively on both belowground and aboveground tissues, the sequestered cucurbitacins were mainly derived from belowground tissues. Cucurbitacins had no detrimental effects on larval performance and, surprisingly, did not provide protection against any of the natural enemies evaluated. Our results show that D. balteata larvae can indeed sequester and transform cucurbitacins, but sequestered cucurbitacins do not impact the biocontrol potential of common natural enemies used in biocontrol. Hence, this plant trait should be conserved in plant breeding programs, as it has been demonstrated in previous studies that it can provide protection against plant pathogens and generalist insects. Supplementary Information: The online version contains supplementary material available at 10.1007/s10340-022-01568-3.

The effect of squash domestication on a belowground tritrophic interaction.

The domestication of plants has commonly resulted in the loss of plant defense metabolites, with important consequences for the plants' interactions with herbivores and their natural enemies. Squash domestication started 10'000 years ago and has led to the loss of cucurbitacins, which are highly toxic triterpenes. The banded cucumber beetle (Diabrotica balteata), a generalist herbivore, is adapted to feed on plants from the Cucurbitaceae and is known to sequester cucurbitacins, supposedly for its own defense. However, the evidence for this is inconclusive. In this study we tested the impact of squash domestication on the chemical protection of D. balteata larvae against a predatory rove beetle (Dalotia coriaria). We found that cucurbitacins do not defend the larvae against this common soil dwelling predator. In fact, D. balteata larvae were less attacked when they fed on cucurbitacin-free roots of domesticated varieties compared to high-cucurbitacin roots of wild plants. This study appears to be the first to look at the consequences of plant domestication on belowground tritrophic interactions. Our results challenge the generalized assumption that sequestered cucurbitacins protect this herbivore against natural enemies, and instead reveals an opposite effect that may be due to a tradeoff between coping with cucurbitacins and avoiding predation.