From cellular biochemistry to systems physiology: New insights into insect lipid metabolism

A First Step Towards Black Soldier Fly Larvae (Diptera: Stratiomyidae) Welfare by Considering Dietary Regimes (Part I)

The insect farming sector is expanding, but knowledge of insect welfare is still limited. This article aims to optimize the dietary regime for "black soldier fly" (Hermetia illucens L., BSF) larvae by applying a holistic view of welfare. Four diets were tested: control (CONTR, commercial laying hen feed), vegetable (VEG), omnivorous (OMN), and carnivorous (MEAT) diet, conducting experiments at a large (2000 larvae) and small scale (100 larvae). Rearing parameters were calculated including the growth rate, substrate reduction, efficiency of conversion of digested food, waste reduction index, and survival rate. Chemical analyses were conducted on BSF larvae and the residual frass. While the MEAT diet appears to be non-well-performing for the larvae, the VEG diet performed comparably to the control diet. Interestingly, the OMN diet demonstrated improved efficiency when evaluating the growth process at both scales. The chemical composition of larvae and frass highlighted the nutritional adequacy of the OMN diet, with the BSF larvae showing adequate protein and lipid content without nutrient catabolism or signs of discomfort. Applying the five freedoms of Brambell's report as a welfare standard for animal rearing and evaluating performance as an indirect indicator of welfare, the OMN diet appears to promote larval welfare in rearing practices.

Microorganism Contribution to Mass-Reared Edible Insects: Opportunities and Challenges

The interest in edible insects' mass rearing has grown considerably in recent years, thereby highlighting the challenges of domesticating new animal species. Insects are being considered for use in the management of organic by-products from the agro-industry, synthetic by-products from the plastics industry including particular detoxification processes. The processes depend on the insect's digestive system which is based on two components: an enzymatic intrinsic cargo to the insect species and another extrinsic cargo provided by the microbial community colonizing-associated with the insect host. Advances have been made in the identification of the origin of the digestive functions observed in the midgut. It is now evident that the community of microorganisms can adapt, improve, and extend the insect's ability to digest and detoxify its food. Nevertheless, edible insect species such as Hermetia illucens and Tenebrio molitor are surprisingly autonomous, and no obligatory symbiosis with a microorganism has yet been uncovered for digestion. Conversely, the intestinal microbiota of a given species can take on different forms, which are largely influenced by the host's environment and diet. This flexibility offers the potential for the development of novel associations between insects and microorganisms, which could result in the creation of synergies that would optimize or expand value chains for agro-industrial by-products, as well as for contaminants.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

Non-classical digestive lipase BmTGL selected by gene amplification reduces the effects of mulberry inhibitor during silkworm domestication

Efficient utilization of dietary lipids is crucial for Bombyx mori, also known as domesticated silkworms. However, the effects of domestication on the genes encoding lipases remain unknown. In this study, we investigated the expression difference of one triacylglycerol lipase (BmTGL) between B.mori and wild (ancestor) silkworm strains (Bombyx mandarina). An immunofluorescence localization analysis showed that BmTGL was present in all parts of the gut and was released into the intestinal lumen. BmTGL expression was significantly enhanced in different domesticated silkworm strains compared to that in the B. mandarina strains. The BmTGL copy numbers in the genomes of the domesticated silkworm strains were 2-to-3 folds that of the B. mandarina strains and accounted for the enhanced expression of BmTGL in the domesticated silkworm strains. The Ser144Asn substitution in the Ser-Asp-His catalytic triads of BmTGL resulted in relatively lower lipase activity and reduced sensitivity to the lipase inhibitor morachalcone A. Moreover, BmTGL overexpression significantly increased the weights of the B. mori silkworms compared to those of the non-transgenic controls. Thus, the selection of BmTGL by gene amplification may be a trade-off between maintaining high enzymatic activity and reducing the effects of mulberry inhibitors during silkworm domestication.

Metagenomics of the midgut microbiome of Rhipicephalus microplus from China

Background

Ticks, which are ectoparasites of animals, may carry multiple pathogens. The cattle tick Rhipicephalus microplus is an important bovine parasite in China. However, the midgut microbiome of R. microplus from China has not been characterized via metagenomic methods.

Methods

Rhipicephalus microplus were collected from cattle in the city of Changsha in Hunan province, China. The DNA of the midgut contents was extracted from fully engorged adult female R. microplus. A DNA library was constructed and sequenced using an Illumina HiSeq sequencing platform. SOAPdenovo software was used to assemble and analyze the clean data. The latent class analysis algorithm applied to system classification by MEGAN software was used to annotate the information on the species’ sequences. DIAMOND software was used to compare unigenes with the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, and functional annotation was carried out based on the results of the comparison.

Results

The dominant phyla in the five samples were Firmicutes, Proteobacteria, and Actinobacteria. Streptococcus, Mycobacterium, Anaplasma, Enterococcus, Shigella, Lactobacillus, Brachyspira, Pseudomonas, Enterobacter, Bacillus, and Lactococcus were the dominant genera in the five samples. The endosymbiotic bacterium Wolbachia was also detected in all of the samples. Mycobacterium malmesburyense, Streptococcus pneumoniae, Anaplasma phagocytophilum, Enterococcus faecium, Shigella sonnei, Enterococcus faecalis, Lactobacillus casei, Brachyspira hampsonii, Pseudomonas syringae, Enterobacter cloacae, and Lactococcus garvieae were the dominant species in the five samples. In addition to these bacterial species, we also detected some eukaryotes, such as Rhizophagus irregularis, Enterospora canceri, Smittium culicis, Zancudomyces culisetae, Trachipleistophora hominis, and viruses such as orf virus, human endogenous retrovirus type W, enzootic nasal tumor virus of goats, bovine retrovirus CH15, and galidia endogenous retrovirus in all of the samples at the species level. The results of the annotated KEGG pathway predictions for the gene functions of the midgut microflora of R. microplus indicated genes involved in lipid and amino acid metabolism, infectious diseases (e.g., Streptococcuspneumonia infection, human granulocytic anaplasmosis, Shigellasonnei infection, Salmonella enterica infection, and pathogenic Escherichia coli infection), and cancer.

Conclusions

Our study revealed that the midgut microbiome of R. microplus is not only composed of a large number of bacteria, but that a portion also comprises eukaryotes and viruses. The data presented here enhance our understanding of this tick’s midgut microbiome and provide fundamental information for the control of ticks and tick-borne diseases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05161-6.