What You Eat Matters: Nutrient Inputs Alter the Metabolism and Neuropeptide Expression in Egyptian Cotton Leaf Worm, Spodoptera littoralis (Lepidoptera: Noctuidae)

Abstract

Lipids and carbohydrates are the two primary energy sources for both animals and insects. Energy homeostasis is under strict control by the neuroendocrine system, and disruption of energy homeostasis leads to the development of various disorders, such as obesity, diabetes, fatty liver syndrome, and cardiac dysfunction. One critical factor in this respect is feeding habits and diet composition. Insects are good models to study the physiological and biochemical background of the effect of diet on energy homeostasis and related disorders; however, most studies are based on a single model species, Drosophila melanogaster. In the current study, we examined the effects of four different diets, high fat (HFD), high sugar (HSD), calcium-rich (CRD), and a plant-based (PBD) on energy homeostasis in younger (third instar) and older (fifth instar) larvae of the Egyptian cotton leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) in comparison to a regular artificial bean diet. Both HSD and HFD led to weight gain, while CRD had the opposite effect and PBD had no effect in fifth instar larvae and pupae. The pattern was the same for HSD and CRD in third instar larvae while a reduction in weight was detected with HFD and PBD. Larval development was shortest with the HSD, while HFD, CRD, and PBD led to retardation compared to the control. Triglyceride (TG) levels were higher with HFD, HSD, and PBD, with larger lipid droplet sizes, while CRD led to a reduction of TG levels and lipid droplet size. Trehalose levels were highest with HSD, while CRD led to a reduction at third instar larvae, and HFD and PBD had no effect. Fifth instar larvae had similar levels of trehalose with all diets. There was no difference in the expression of the genes encoding neuropeptides SpoliAKH and SpoliILP1-2 with different diets in third instar larvae, while all three genes were expressed primarily with HSD, and SpolisNPF was primarily expressed with HFD in fifth instar larvae. In summary, different diet treatments alter the development of insects, and energy and metabolic pathways through the regulation of peptide hormones.

Keywords: AKH; calcium; high-fat; high-sugar; insulin; lipid; sNPF; trehalose.

FIGURE 1

Effects of different diet treatments on the larval and pupal weights, and development of Spodoptera littoralis. Body weights at third (A), fifth instar (B) larvae, and pupa (C), and larval development durations (D) are shown. Statistical significances in the entire sample set were determined by using one-way ANOVA, followed by a Tukey HSD test (groups associated with different letters are significantly different from each other, P < 0.05). A t-test was used for binomial comparisons (diet treatment vs control) (*p < 0.05, **p < 0.01, and ***p < 0.001 indicate the statistical differences between the particular diet and the control). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet.

FIGURE 2

Effects of different diet treatments on the triglyceride levels of Spodoptera littoralis. Triglyceride levels at third (A) and fifth instar (B) larval bodies are shown as mg in 100 mg body weight. Statistical significances in the entire sample set were determined by using one-way ANOVA, followed by a Tukey HSD test (groups associated with different letters are significantly different among each other, p < 0.05). A t-test was used for binomial comparisons (diet treatment vs control) (*p < 0.05, **p < 0.005, ***p < 0.001 indicate the statistical differences between the particular diet and the control). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet.

FIGURE 3

Effects of different diet treatments on the trehalose levels of Spodoptera littoralis. Trehalose levels at third (A) and fifth instar (B) larval bodies are shown as mg in 1 g body weight. Statistical significances in the entire sample set were determined by using one-way ANOVA, followed by a Tukey HSD test (groups associated with different letters are significantly different among each other, p < 0.05). A t-test was used for binomial comparisons (diet treatment vs control) (***p < 0.001 indicate the statistical differences between the particular diet and the control). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet.

FIGURE 4

Effects of different diet treatments on the fat body lipid droplets of Spodoptera littoralis. The depict lipid droplets using confocal microscopy in fat body tissue from third or fifth instar larvae (A), and lipid droplet sizes at third (B) and fifth instar larvae (C) are shown. Lipid droplets stained with Nile red dye in red, nuclei stained with Hoechst in blue. Statistical significances in the entire sample set were determined by using one-way ANOVA, followed by a Tukey HSD test (groups associated with different letters are significantly different among each other, p < 0.05). A t-test was used for binomial comparisons (diet treatment vs control) (*p < 0.05, ***p < 0.001 indicate the statistical differences between the particular diet and the control). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet. Scale bar, 20 μm in panel (A).

FIGURE 5

Effects of different diet treatments on the expression of Spodoptera littoralis adipokinetic hormone (SpoliAKH), insulin-like peptide 1 (SpoliILP1), and insulin-like peptide 2 (SpoliILP2) at third instar larvae by Real-time PCR. Spodoptera littoralis actin gene was used as the internal reference. The statistical significance of transcript abundance differences among groups was determined through ANOVA, followed by the Tukey HSD test (groups associated with different letters are significantly different from each other, p < 0.05). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet.

FIGURE 6

Effects of different diet treatments on the expression of Spodoptera littoralis adipokinetic hormone (SpoliAKH), insulin-like peptide 1 (SpoliILP1), insulin-like peptide 2 (SpoliILP2), and short neuropeptide F (SpolisNPF) at fifth instar larvae by Real-time PCR. Spodoptera littoralis actin gene was used as the internal reference. The statistical significance of transcript abundance differences among groups was determined through ANOVA, followed by the Tukey HSD test (groups associated with different letters are significantly different from each other, p < 0.05). Values are expressed as means ± standard error. Control, Regular artificial bean diet; HSD, High sugar diet; HFD, High fat diet; CRD, Calcium-rich diet; PBD, Plant-based diet.