Abstract

Plant domestication often reduces plant defenses by selection on chemical and physical defense traits. Thus, it is expected that herbivorous insects perform better on crop varieties than on their wild relatives. However, recent studies show that this pattern is not ubiquitous. We examined how varieties of squash (Cucurbita spp.) domesticated for different purposes (for consumption or as ornamentals), differ in plant defense traits and in their interactions with a leaf and a root herbivore. Two types of defenses were measured: cucurbitacins, which are toxic phytochemicals, and trichomes that are physical barriers for most herbivores. We addressed the following questions: (i) what is the variation in cucurbitacin content and leaf trichome density among varieties? (ii) does purpose of domestication explain differences in defense traits among varieties? and (iii) are herbivore feeding preferences and performance altered by the defense traits of squash varieties? We found great variation in cucurbitacin content among varieties, but not according to their purpose of domestication. Cucurbitacins were found mostly in cotyledons and roots and in very small quantities in the leaves. In contrast, trichome density was greater on the varieties selected for consumption than on the ornamental varieties. The performance of a leaf herbivore (Spodoptera latifascia) and a root herbivore (Diabrotica balteata), was not different among squash varieties. Moreover, in a choice experiment, larvae of the root herbivore preferred to feed on squash varieties with high cucurbitacin content. Whereas, in the field, native leaf herbivores preferred to feed on varieties selected for consumption. Our results contribute to a better understanding on how varietal selection may differentially affect plant defenses. This knowledge could help in the development of crop varieties with enhanced insect resistance.

Highlights

  • Plant domestication is an evolutionary process whereby wild plants are modified to meet human needs (Meyer and Purugganan, 2013; Cornille et al, 2014)

  • We addressed the following questions: (i) what is the level of variation in cucurbitacin content and leaf trichome density among varieties? (ii) does purpose of domestication explain differences in defense traits among varieties? and (iii) are herbivores feeding preferences and performance altered by defense traits of squash varieties? we quantified the content of cucurbitacins in roots, cotyledons and leaves, and density of trichomes in leaves and cotyledons of several squash varieties

  • The relative content of cucurbitacins in cotyledons and roots of the Slg variety was significantly different than Scpo, the other variety selected for seed consumption (Supplementary Figure 2), but similar to the content in the Ott, Fcf, and Hsc and White Fall F1 (Hwf) varieties

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Summary

Introduction

Plant domestication is an evolutionary process whereby wild plants are modified to meet human needs (Meyer and Purugganan, 2013; Cornille et al, 2014). There has been a surge in research examining how these domesticated traits have affected the interactions between crop plants and their associated herbivores Overall, these studies reveal that often a decrease in chemical and physical plant defense results in increased insect performance (Idris and Grafius, 1996; Benrey et al, 1998; Gols et al, 2008). Turcotte et al (2014) compared the performance of a leaf chewing herbivore (Spodoptera exigua) and a phloem-feeding aphid (Myzus persicae) on 29 pairs of crop plants and their wild relatives They found that while domestication reduced the levels of morphological and chemical defenses in most of the examined crops, it did not always result in increased insect performance. The authors argue that the trade-off between lower levels of defense and increased insect performance is supported for the seeds because this is the organ targeted by the domestication process

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