A novel cascade allows Metarhizium robertsii to distinguish cuticle and hemocoel microenvironments during infection of insects

Pathogenic fungi precisely respond to dynamic microenvironments during infection, but the underlying mechanisms are not well understood. The insect pathogenic fungus Metarhizium robertsii is a representative fungus in which to study broad themes of fungal pathogenicity as it resembles some major plant and mammalian pathogenic fungi in its pathogenesis. Here we report on a novel cascade that regulates response of M. robertsii to 2 distinct microenvironments during its pathogenesis. On the insect cuticle, the transcription factor COH2 activates expression of cuticle penetration genes. In the hemocoel, the protein COH1 is expressed due to the reduction in epigenetic repression conferred by the histone deacetylase HDAC1 and the histone 3 acetyltransferase HAT1. COH1 interacts with COH2 to reduce COH2 stability, and this down-regulates cuticle penetration genes and up-regulates genes for hemocoel colonization. Our work significantly advances the insights into fungal pathogenicity in insects.

The Role of Cuticular Hydrocarbons in Forensic Entomology: A Mini Review

The cuticular hydrocarbons (CHCs) are expressed on insect cuticle and provide protection from desiccation and abrasion. The CHC profiles are species-specific and vary in each insect. Forensic entomologists have utilized varying expressions of CHCs to determine the age as well as to delimit insects. This review focuses on the role of CHCs in species identification and aging in the field of forensic entomology.

Biomechanics of insect cuticle: an interdisciplinary experimental challenge

The cuticle exoskeleton plays a key role in facilitating the evolutionary success of insects. Since the mid of the last century, many different biomechanical properties of exoskeletons have been investigated, always utilizing the most sophisticated scientific methods available at the time. So far, information on the biomechanical properties of cuticle seems to be as diverse as the methods used to measure them. As a consequence, insect cuticle is often considered to exhibit the most complex and diverse biomechanical properties of any biological material. However, it remains unclear which role the respective measurement methods and sample treatments used in previous studies play in supporting this claim. This review provides a broad overview of examination techniques used to study biomechanical properties of insect exoskeletons and discusses their respective advantages and disadvantages in describing the properties of a complex material such as cuticle. Our meta-analysis of the present data confirms significant effects of the respective measurement methods, sample treatments and body parts on the obtained mechanical properties. Based on our findings, we highlight research gaps and point out important factors which should be taken into account in future studies on insect cuticle.

How a sticky fluid facilitates prey retention in a carnivorous pitcher plant (Nepenthes rafflesiana)

Nepenthes pitcher plants live in nutrient-poor soils and produce large pitfall traps to obtain additional nutrients from animal prey. Previous research has shown that the digestive secretion in N. rafflesiana is a sticky viscoelastic fluid that is much more effective at retaining insects than water, even after significant dilution. Although the physical properties of the fluid are important for its retentive function, it is unclear how the fluid interacts with insect cuticle and how its sticky nature affects struggling insects. In this study, we investigated the mechanisms behind the efficient prey retention in N. rafflesiana pitcher fluid. By measuring the attractive forces exerted on insect body parts moving in and out of test fluids, we show that it costs insects significantly more energy to separate from pitcher fluid than from water. Moreover, both the maximum force and the energy required for retraction increase after the first contact with the pitcher fluid. We found that insects sink more easily into pitcher fluid than water and, accordingly, the surface tension of N. rafflesiana pitcher fluid was significantly lower than that of water (60.2 vs. 72.3 mN/m). By analysing the pitcher fluid dewetting behaviour, we demonstrate that it strongly resists dewetting from all surfaces tested, leaving behind residual films and filaments that can facilitate re-wetting. This inhibition of dewetting may be a further consequence of the fluid's viscoelastic nature and likely represents a key mechanism underlying prey retention in Nepenthes pitcher plants.

Colour-assisted variation in elytral ICP-OES-based ionomics in an aposematic beetle

Very little is known about how the elemental composition (ionome) of an insect cuticle varies as a result of different colouration. Using inductively-coupled plasma optical emission spectrometry (ICP-OES), we established ionomic profiles in microsamples of two adjacent regions of an insect cuticle with a contrasting colour pattern, namely, the black and orange regions of the elytra of the aposematic burying beetle Nicrophorus vespillo. The analysis revealed 53 elements (ranging in atomic weight from Na to Bi) occurring above the detection limit. The frequency of detectability of individual elements varied strongly, and only ten elements (Ba, Cu, Fe, K, Mg, Mn, P, Rb, Sb and Zn) were present in concentrations exceeding the detection limit in all the samples. The sum of concentrations of all elements in the orange regions of the elytra was 9% lower than in the black ones. The opposite distribution was displayed by the rare earth elements (REEs), the sum of which was 17% lower in the black elytral regions than in the orange ones. The concentrations of six elements were significantly higher in the black than in the orange regions: Al (by 97%), Cu (41%), Mn (14%), Na (46%), Se (97%) and W (47%). The concentrations of essential elements measured in both the black and orange regions exhibited very considerable variance: Ca (σ2 = 1834; 1882, respectively), K (145; 82) P (97; 76), Na (84; 53), Mg (24; 26) and Ba (9; 13). This, in part, could be attributed to individual differences, e.g. those resulting from the consumption of animal carcasses of different quality/chemical composition, but interference between elements and the consequent lowering of measurement quality are also possible. We highlight the fact that deeper insight into the basic relationship between insect colouration and variation in elemental composition requires micro-sampling of the homogeneous layers of an exoskeleton.