The role of 20-hydroxyecdysone signaling in Drosophila pupal metabolism

A developmental checkpoint directs metabolic remodelling as a strategy against starvation in Drosophila

Steroid hormones are crucial regulators of life-stage transitions during development in animals. However, the molecular mechanisms by which developmental transition through these stages is coupled with optimal metabolic homeostasis remains poorly understood. Here, we demonstrate through mathematical modelling and experimental validation that ecdysteroid-induced metabolic remodelling from resource consumption to conservation can be a successful life-history strategy to maximize fitness in Drosophila larvae in a fluctuating environment. Specifically, the ecdysteroid-inducible protein ImpL2 protects against hydrolysis of circulating trehalose following pupal commitment in larvae. Stored glycogen and triglycerides in the fat body are also conserved, even under fasting conditions. Moreover, pupal commitment dictates reduced energy expenditure upon starvation to maintain available resources, thus negotiating trade-offs in resource allocation at the physiological and behavioural levels. The optimal stage-specific metabolic shift elucidated by our predictive and empirical approaches reveals that Drosophila has developed a highly controlled system for ensuring robust development that may be conserved among higher-order organisms in response to intrinsic and extrinsic cues.

Effects of benzothiazole on survival for reduced reproduction and development in Tribolium castaneum Herbst (Coleoptera: Tenebrionidae)

BACKGROUND

The red flour beetle, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), is an important stored-product pest that is distributed worldwide and has developed resistance to many insecticides. Identifying novel and effective alternative insecticides is important for the control of T. castaneum. The volatile compound benzothiazole has been identified as having great acute toxic activity against T. castaneum. However, a comprehensive evaluation of a new insecticide should include both direct toxic effects and sublethal effects. The aim of this study was therefore to evaluate the effects of benzothiazole on the development and reproduction of T. castaneum.

RESULTS

Exposure of fourth-instar larvae to lethal and sublethal concentrations of benzothiazole (LC₁₀ , LC₃₀ and LC₅₀ ) significantly decreased pupation rates, food intake and growth rates in T. castaneum. Larval duration was significantly reduced by approximately 1 day in the LC₃₀ and LC₅₀ treatment groups. The LC₅₀ benzothiazole caused a significant decrease in the weight of pupae and adults, fecundity and egg hatchability. Increased and decreased nutrient (carbohydrate and lipid) contents were observed in surviving larvae and pupae, respectively. The LC₃₀ and LC₅₀ treatments caused the down-regulation of five growth-positive regulated genes (PI3K, AKT, CyclinE, S6K1 and S6K2) and the up-regulation of two growth-negative regulated genes (4EBP and FOXO).

CONCLUSION

Benzothiazole presented adverse effects on the development and reproduction of T. castaneum, further supporting benzothiazole as a highly active compound in stored-product protection. © 2020 Society of Chemical Industry.

Comparative Transcriptome Analysis Provides Novel Insight into Morphologic and Metabolic Changes in the Fat Body during Silkworm Metamorphosis

The fat body plays key roles in energy storage and utilization as well as biosynthetic and metabolic activities in insects. During metamorphosis from larva to pupa, the fat body undergoes dramatic changes in morphology and metabolic processes. However, the genetic basis underlying these changes has not been completely understood. In this study, the authors performed a time-course transcriptome analysis of the fat body during silkworm metamorphosis using RNA-sequencing. A total of 5217 differentially expressed genes (DEGs) were identified in the fat body at different developmental time points. DEGs involved in lipid synthesis and degradation were highly expressed at the third day of the last larval instar and during the prepupal-pupal transition, respectively. DEGs involved in the ecdysone signaling and bone morphogenetic protein (BMP) signaling pathways that modulate organ development exhibited a high expression level during the fat body remodeling process from prepupa to pupa. Intriguingly, the RNA interference-mediated knockdown of either decapentaplegic (Dpp) or protein 60A (Gbb), two DEGs involved in the BMP signaling pathway, inhibited fat body dissociation but promoted lipid mobilization, suggesting that the BMP signaling pathway not only is required for fat body remodeling, but also moderately inhibits lipid mobilization to ensure an appropriate lipid supply during the pupal-adult transition. In conclusion, the comparative transcriptome analysis provides novel insight into morphologic and metabolic changes in the fat body during silkworm metamorphosis.

FoxO mediates the timing of pupation through regulating ecdysteroid biosynthesis in the red flour beetle, Tribolium castaneum

The steroid hormone 20-hydroxyecdysone (20E), the major developmental hormone in insects, controls all the developmental transitions including ecdysis and metamorphosis. In our study with last larval stages of the red flour beetle, Tribolium castaneum, dsRNA-mediated gene silencing of Forkhead box protein O (FoxO) resulted in reduced food intake and larval mass and this agreed with a reduction in the expression of insulin signaling-related genes (insulin-like peptides 2, 3, 4, and chico). Interestingly, we also observed a significant delay in the moment of the pupation and these FoxO-silenced larvae then turned brown at the middle pupal stage followed by death. The observed delay of pupation concurred with a significant delay in 20E titer in dsFoxO-injected larvae and this in turn agreed with a significant delay in expression of prothoracicotropic hormone (ptth) that is a gene stimulating ecdysteroid biosynthesis, and of spook (spo) that is one of the early Halloween genes involved in ecdysteroid biosynthesis. In addition, there was also a delayed expression of the ecdysteroid response gene hormone receptor 3 (HR3). In an attempt to rescue the effects by dsFoxO, injection of 20E into T. castaneum larvae stimulated the expression of HR3 and induced one extra larval-larval molt, confirming the responsiveness for ecdysteroid signaling in dsFoxO-injected larvae. The observations of this project suggest that FoxO is a player in the timing of pupation via the regulating of ecdysteroid biosynthesis, together with the regulation of both insulin signaling and nutrition.