Ultrasensitive detection by maxillary palp neurons allows non-host recognition without consumption of harmful allelochemicals

Mechanisation applied to entomological production cannot ignore insect reactivity: a case study on Bombyx mori in the context of the 'Serinnovation' project

In December 2017 the Venetian Region (local Authority), financed the creation of the Operational Group (OG) 'Serinnovation', within the framework of the Rural Development Plan supported by the European Community. The OG aims at coordinating and spreading innovation in sericulture through mechanisation of processes and centralisation of some rearing steps, the use of waste as by-products and traceability to promote local productions. The project acts on perceived quality by increasing the added value, through production cost efficiency, and on the recovery of the waste material for further applications (circular economy). The final target was to develop a niche-process to obtain traceable 'Made-in-Italy' silk for the luxury market and non-textile applications. A first strategy to increase the efficiency of the process was to build an automatic leaf cutting machine to prepare the feed for the first three instars of the silkworm (Bombyx mori Linnaeus). This new machine - based on a patent - was validated through several tests and compared with the cutting system previously used. The study was completed by a bioassay of production and survival rate associated with the introduction of this innovation. The results showed that labour saving is in the order of 10% compared to a semi-manual process, the leaf quality is not affected, survival of larvae and silk production are not significantly different from the control. This methodology is proposed as a study case for other similar mechanisation processes in entomological production, as the impact of innovations on insect physiology should be carefully considered.

Molecular Functions and Physiological Roles of Gustatory Receptors of the Silkworm Bombyx mori

Complete elucidation of members of the gustatory receptor (Gr) family in lepidopteran insects began in the silkworm Bombyx mori. Grs of lepidopteran insects were initially classified into four subfamilies based on the results of phylogenetic studies and analyses of a few ligands. However, with further ligand analysis, it has become clear that plant secondary metabolites are important targets not only for Grs in the bitter subfamily but also for the Drosophila melanogaster Gr43a orthologue subfamily and Grs in the sugar subfamily. Gene knockout experiments showed that B. mori Gr6 (BmGr6) and BmGr9 are involved in the recognition of the feeding-promoting compounds chlorogenic acid and isoquercetin in mulberry leaves by the maxillary palps, suggesting that these Grs are responsible for palpation-dependent host recognition without biting. On the other hand, BmGr expression was also confirmed in nonsensory organs. Midgut enteroendocrine cells that produce specific neuropeptides were shown to express specific BmGrs, suggesting that BmGrs are involved in the induction of endocrine secretion in response to changes in the midgut contents. Furthermore, gene knockout experiments indicated that BmGr6 is indeed involved in the secretion of myosuppressin. On the other hand, BmGr9 was shown to induce signal transduction that is not derived from the intracellular signaling cascade mediated by G proteins but from the fructose-regulated cation channel of BmGr9 itself. Cryogenic electron microscopy revealed the mechanism by which the ion channel of the BmGr9 homotetramer opens upon binding of fructose to the ligand-binding pocket. Research on BmGrs has contributed greatly to our understanding of the functions and roles of Grs in insects.

Molecular basis of host plant recognition by silkworm larvae

Herbivorous insects can identify their host plants by sensing plant secondary metabolites as chemical cues. We previously reported the two-factor host acceptance system of the silkworm Bombyx mori larvae. The chemosensory neurons in the maxillary palp (MP) of the larvae detect mulberry secondary metabolites, chlorogenic acid (CGA), and isoquercetin (ISQ), with ultrahigh sensitivity, for host plant recognition and feeding initiation. Nevertheless, the molecular basis for the ultrasensitive sensing of these compounds remains unknown. In this study, we demonstrated that two gustatory receptors (Grs), BmGr6 and BmGr9, are responsible for sensing the mulberry compounds with attomolar sensitivity for host plant recognition by silkworm larvae. Calcium imaging assay using cultured cells expressing the silkworm putative sugar receptors (BmGr4-10) revealed that BmGr6 and BmGr9 serve as receptors for CGA and ISQ with attomolar sensitivity in human embryonic kidney 293T cells. CRISPR/Cas9-mediated knockout (KO) of BmGr6 and BmGr9 resulted in a low probability of making a test bite of the mulberry leaves, suggesting that they lost the ability to recognize host leaves. Electrophysiological recordings showed that the loss of host recognition ability in the Gr-KO strains was due to a drastic decrease in MP sensitivity toward ISQ in BmGr6-KO larvae and toward CGA and ISQ in BmGr9-KO larvae. Our findings have revealed that the two Grs, previously considered to be sugar receptors, are molecules responsible for detecting plant phenolics in host plant recognition.

Plasticity in Chemical Host Plant Recognition in Herbivorous Insects and Its Implication for Pest Control

Chemical communication is very important in herbivorous insects, with many species being important agricultural pests. They often use olfactory cues to find their host plants at a distance and evaluate their suitability upon contact with non-volatile cues. Responses to such cues are modulated through interactions between various stimuli of biotic and abiotic origin. In addition, the response to the same stimulus can vary as a function of, for example, previous experience, age, mating state, sex, and morph. Here we summarize recent advances in the understanding of plant localization and recognition in herbivorous insects with a focus on the interplay between long- and short-range signals in a complex environment. We then describe recent findings illustrating different types of plasticity in insect plant choice behavior and the underlying neuronal mechanisms at different levels of the chemosensory pathway. In the context of strong efforts to replace synthetic insecticides with alternative pest control methods, understanding combined effects between long- and close-range chemical cues in herbivore-plant interactions and their complex environment in host choice are crucial to develop effective plant protection methods. Furthermore, plasticity of behavioral and neuronal responses to chemical cues needs to be taken into account to develop effective sustainable pest insect control through behavioral manipulation.