In vivo functional analysis of the cotton bollworm Helicoverpa armigera 24-dehydrocholesterol reductase (HaDHCR24) in phytosterol metabolism.

Insects have to obtain sterols from food due to the inability to synthesize this essential nutrient de novo. For lepidopteran insects, they can convert a variety of phytosterols into cholesterol to meet their growth needs. The final step of the cholesterol biosynthesis is the metabolism of desmosterol catalyzed by 24-dehydrocholesterol reductase (DHCR24). In this study, we identified a DHCR24 homolog in the cotton bollworm Helicoverpa armigera, designated as H. armigera 24-dehydrocholesterol reductase (HaDHCR24)-1. The quantitative expression analyses indicated that HaDHCR24-1 was highly enriched in the midgut where dietary sterol uptake occurs. Compared to the control, the DHCR24-1 mutant larvae generated by clustered regularly interspaced palindromic repeats (CRISPR) / CRISPR-associated nuclease 9 technology accumulated more desmosterol in the gut, while the content of cholesterol was significantly reduced. A similar phenomenon was observed when the DHCR24 inhibitor, amiodarone, was applied to the insects. Moreover, DHCR24-1 played an important role for the usage of ß-sitosterol, a major sterol in plants, in H. armigera, and loss of function of DHCR24-1 resulted in higher mortality on ß-sitosterol. However, the DHCR24 homolog does not necessarily exist in the genomes of all insects. The loss of this gene occurred more frequently in the insects feeding on animals, which further support the role of DHCR24-1 in using phytosterols. This gene may have important potential in developing new strategies to control herbivory pests in Lepidoptera and other insect orders.

Fitness of Nutrition Regulation in a Caterpillar Pest Mythimna separata (Walker): Insights from the Geometric Framework.

In nature, plants can contain variable nutrients depending upon the species, tissue, and developmental stage. Insect herbivores may regulate their nutrient intake behaviorally and physio- logically when encountering different foods. This study examined the nutritional regulation of the oriental armyworm, Mythimna separata, for the first time. In one experiment, we allowed the cater-pillars to choose between two nutritionally balanced but complementary diets. The caterpillars did not randomly consume the paired foods, but instead chose between the nutritionally balanced but complementary diets. This intake behavior was found to change with their developmental stages. Furthermore, the nutrient concentrations in food significantly impacted the insect's performance. In the other experiment, caterpillars were given one of eleven diets that reflected the different nutrient conditions in the field. The results showed that proteins were significantly associated with developmental time and fecundity. For example, by consuming protein-biased food, the caterpillars developed faster and produced more eggs. In contrast, carbohydrates were more strongly linked to lipid accumulation, and caterpillars accumulated more lipids when consuming the carbohydrate-biased food. Moreover, the caterpillars were also found to actively regulate their intake of proteins and carbohydrates based on food quality and to physiologically prepare for subsequent life stages. These findings enhance our understanding of how M. separata feeds and responds to different nutritional environments in the field, which could have implications for managing insect herbivores in agricultural settings.