The consequences of squash domestication on the chemical ecology of plant-insect interactions
Authors:   , Jaccard Charlyne 
Published: January 2024

Since the start of agriculture, crop domestication has induced significant changes in both plants and human societies. Plant domestication has generally resulted in decreased chemical and physical defenses in crop plants compared to their wild ancestors. A reduction in plant defensive traits is often expected to result in increased insect performance. By selecting (modifying the genetics) desirable traits for humans, it may favor the pest insects that we have to face today in agriculture. However, recent studies have shown that the impact of crop domestication on plant-insect interactions is not ubiquitous. Surprisingly, there is still much discussion on the way by which crop domestication has influenced insect pests. In my PhD project, I hypothesize that the outcome of plant-insect interactions may be influenced by the type of organ targeted during domestication, as well as the by purpose of domestication. Squash (genus Cucurbita) is one of the earliest and most important domesticated plants in the Americas, along with maize and the common bean. The genus Cucurbita was domesticated on several occasions leading to five different species throughout the Americas, beginning around 10,000 years ago. Wild forms of squash are very bitter and toxic to humans and other mammals because they contain cucurbitacins (toxic secondary metabolites). Squash was domesticated at different times and for different purposes (including soap, oil, music instrument, food and beverage container). As a result, we have great variation on fruits size, color and shape. The aim of my PhD project was to examine the overall consequences of squash domestication on plant defenses and its impact on plant-insect interactions. To date, no studies with these specific questions were done on this important world-wide crop. To meet the challenge, I had four main axes: 1) Study the impact of purpose of domestication on the squash defenses (trichomes and cucurbitacins) and how it altered the interaction with generalist insects (Spodoptera latifascia that eat aboveground tissue and Diabrotica balteata, whose adults feed on aboveground tissues and larvae feed on belowground tissues). To test that, I had squash varieties from different species selected for consumption (fruit and seeds) and varieties selected for an ornamental purpose. I found that the varietal selection rather than the purpose of domestication explained the differences in cucurbitacin content. However, trichome density was reduced on varieties selected for consumption. The herbivore performance was not negatively affected by high cucurbitacin content nor by trichome density. Instead, the root herbivore D. balteata larvae preferred to feed on the varieties with high levels of cucurbitacins. This study highlighted the idea that D. balteata is adapted to cucurbits and the importance of phylogeny while studying plant domestication. 2) Study the consequences of Cucurbita argyrosperma domestication on plant chemical (cucurbitacins) and physical (trichomes) defenses and extent to which altered defenses impacted the interaction with two beetle species, the generalist Diabrotica balteata and the squash specialist, Acalymma spp. (Coleoptera: Chrysomelidae). These plants and insects have a long co-evolutionary history. To add to the behavioral bioassays in the lab, I tested the susceptibility of Cucurbita argyrosperma wild and domesticated varieties in a common garden during a field season in Mexico. I found that cucurbitacins were selected out of the domesticated varieties. Trichome density was not reduced through domestication. Larvae of both insects preferred to feed on roots of wild squash, but this negatively affected the performance of the generalist. In the field, leaves of wild and domesticated plants were mainly attacked by adults of the specialist. 3) Study the indirect consequence of Cucurbita argyrosperma domestication on the third trophic level. One of the main results from previous axes was the reduction of cucurbitacins in domesticated squash varieties. It was hypothesized that D. balteata sequester this compound for its own defense against natural enemies. Then, deprived of cucurbitacin by eating domesticated squash, larvae should be more attractive to soil predators. To test this hypothesis, I used the rove beetle predator (Atheta coriaria), which is a generalist soil predator with no coevolutionary history with D. balteata or squash. Overall, I found no evidence that cucurbitacins serve as a protection against predators. The assumed lethal and deterrent impacts of sequestered cucurbitacin by the herbivore on the predator were not observed. However, the survival of Diabrotica balteata larvae was lower when fed on wild squash populations. 4) Study how squash domestication changed the expression of cucurbitacin biosynthesis genes. From Cucurbita argyrosperma genome and literature on genes involved in the cucurbitacins pathway in cucumber, I targeted seven candidates for cucurbitacins biosynthetic genes. One candidate for the very first enzyme involved in the cucurbitacin biosynthesis: cucurbitadienol synthase, and six cytochrome P-450 enzyme. I analyzed the expression of those genes in roots, cotyledons and leaves to explain the differences in cucurbitacin concentration among plant tissues and among wild and domesticated squash. Overall, gene expression was higher in roots compared to cotyledons and for some genes, wild squash populations had a higher expression than the domesticated varieties.