Physical and chemical properties of primary defences in Tenebrio molitor

Abstract

Prevention and reaction are the foundation for any defence system. In insects, the primary defences against pathogens and parasites limit invasion; the secondary ones (e.g. immune system) act when the cuticle and other primary defences fail. Because investment in both aspects of defence may be costly, they should be regulated in a plastic or variable way in accordance with the risk of infection. The mealworm beetle Tenebrio molitor L. changes cuticle colour and its resistance to fungal infection when subject to high population density, although such resistance is a result of the primary (cuticle) defences rather than the secondary (immunological) ones. The present study tests the hypothesis that the physical and chemical properties of the primary defences in T. molitor change with cuticular darkness. Beetles expressing black phenotypes (or with darker cuticle) have a thicker cuticle, with four well organized layers (epi-, exo-, endocuticle and formation zone) and more melanin than tan beetles. The cuticle properties investigated in the present study are likely to be the underlying mechanisms of pathogen resistance in black beetles, including the content of carbonylated proteins, which in black beetles was almost half that of tan beetles after exposure to ultraviolet radiation. It is proposed that, in polyphenic insects (such as mealworm beetles), primary and secondary defences are regulated pleiotropically, with the genes responsible for the expression of one defence having a positive effect on others, whereas, in polymorphic insects, there is no such link and so investment in one defence may impair others.

Cuticular colour reflects underlying architecture and is affected by a limiting resource

“…Our results for cuticular thickness are corroborated by a recent study (Silva 370 et al, 2016), which demonstrated that all the cuticle layers, including the endocuticle, of T. 371 molitor cuticle were thicker in blacker adult beetles than in tan stock beetles; the authors 372 examined abdominal sternites, suggesting that our results can be generalised beyond the 373 pronotum. The consistency of both our results and those of Silva et al (2016), despite 374 measurement of different layers also suggest that the exocuticle, measured in our study, is the 375 cuticular structure driving the relationship between colour and thickness. A similar 376 phenomenon has been found in larvae of the greater wax moth, Galleria mellonella 377 (Dubovskiy et al 2013), whereby a melanic (dark) morph had a thicker cuticle than a non-378 19 melanic morph.…”

“…colour). On the other 401 hand, the melanin content of sternites from black T. molitor has been shown to be higher than 402 that of tan beetles (Silva et al, 2016) …”

External Immune Inhibitory Efficiency of External Secretions and Their Metabolic Profiling in Red Palm Weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

“…It is also one of a very few reports to study another primary immune system besides the innate internal immunity, i.e., external immunity, which is overlooked in most studies. In addition to the insect innate barriers posed by the cuticle and internal immune system (Vilcinskas and Götz, 1999;Silva et al, 2016), other factors can limit the effectiveness of pathogens (Pu et al, 2017a). For instance, both ants (Gupta et al, 2015) and bees (Otti et al, 2014) exhibit an array of behavioral and biochemical defenses to prevent pathogenic infection.…”

“…External immune defense involves by definition any heritable trait acting outside an organism improving protection from pathogens, such as bacteria, fungi, viruses, and nematodes, or manipulating the composition of the microbial community in favor of the organism (Otti et al, 2014). This immune defensive system, which includes the cuticle and external secretions, constitutes an initial physical and chemical barrier to pathogens (Gołebiowski et al, 2008;Silva et al, 2016;Pu et al, 2017a).…”

Sustainable farming of the mealworm Tenebrio molitor for the production of food and feed

“…Cuticle darkness in the adult beetle correlates with pathogen resistance [93,151]. Darker beetles have a thicker cuticle and stronger melanin staining than tan beetles [152]. Furthermore, melanin offers protection against UV damage and is involved in wound repair [153].…”