Impact of Domestication on Floral Traits and Rewards: A Comparison Between Wild and Domesticated Squash (Cucurbita)

Abstract

Plant domestication primarily targets traits of direct human interest, such as fruit and seed characteristics; however, its indirect effects on other traits, including floral morphology and rewards (nectar and pollen), remain less understood. In this study, we investigated how domestication has influenced floral traits and rewards in domesticated and wild species of the genus Cucurbita. We compared three domesticated and three wild Cucurbita species in an experimental plot. We measured floral morphological traits, nectar volume, sugar (fructose, glucose, and sucrose), and amino acid concentrations in staminate and pistillate flowers. In addition, we evaluated pollen production and size, as well as protein and lipid concentrations, and the protein: lipid ratio in staminate Cucurbita flowers. Our results show that domesticated Cucurbita species exhibit larger floral morphological traits in both pistillate and staminate flowers compared to their wild relatives. While nectar volume increased in domesticated species, sugar and amino acid concentrations remained unchanged. In contrast, domestication had no significant effect on pollen traits, including production, size, and protein and lipid content. These findings highlight that domestication differentially affects floral traits: while floral morphology is significantly altered, most of the traits of floral rewards remain largely unaffected. This conservation may arise from the recent evolutionary history of these species and their close coevolutionary relationship with Eucera bees, emphasizing pollinator nutritional needs over artificial selection. These results underscore the complex interplay between domestication, resource allocation, and plant-pollinator interactions in shaping floral traits.

Keywords: artificial selection; flower morphology; nectar; phenotypic variation; pollen.

FIGURE 1

Sketch of morphological traits measured in pistillate and staminate Cucurbita flowers, taken from Glasser et al. (2023). A) Corolla diameter (CD) of Cucurbita flowers, B) lateral view of a staminate flower, C) traits that form the androecium of staminate flowers, and D) traits that form the gynoecium of pistillate flowers.

FIGURE 2

Principal component analysis of floral traits by conditions (wild & domesticated) and by species for pistillate (A, C) and for staminate flowers (B, D), respectively. The wild species are CF ( C. foetidissima ), CPF ( C. pepo subsp. fraterna), and CAS ( C. argyrosperma subsp. sororia). The domesticated species are CPP ( C. pepo subsp. pepo), CAA ( C. argyrosperma subsp. argyrosperma), and CM ( C. moschata ).

FIGURE 3

Nectar volume (μL) by Cucurbita species of (A) pistillate and (B) staminate flowers. Different letters indicate differences between species. Abbreviations as in Figure 2.

FIGURE 4

Sugar nectar concentrations (fructose, glucose, sucrose, and total sugar concentration) grouped by conditions (wild and domesticated) and by floral sex.

FIGURE 5

PCA of amino acid concentrations by floral sex, combining all Cucurbita species in this study.

FIGURE 6

Pollen traits (pollen production and size, protein and lipid concentrations, and the protein: lipid ratio) grouped by conditions: wild and domesticated. Different letters indicate differences between conditions.