A fructose receptor gene influences development and feed intake in Helicoverpa armigera

The larva and adult of Helicoverpa armigera use differential gustatory receptors to sense sucrose

Almost all herbivorous insects feed on plants and use sucrose as a feeding stimulant, but the molecular basis of their sucrose reception remains unclear. Helicoverpa armigera as a notorious crop pest worldwide mainly feeds on reproductive organs of many plant species in the larval stage, and its adult draws nectar. In this study, we determined that the sucrose sensory neurons located in the contact chemosensilla on larval maxillary galea were 100-1000 times more sensitive to sucrose than those on adult antennae, tarsi, and proboscis. Using the Xenopus expression system, we discovered that Gr10 highly expressed in the larval sensilla was specifically tuned to sucrose, while Gr6 highly expressed in the adult sensilla responded to fucose, sucrose and fructose. Moreover, using CRISPR/Cas9, we revealed that Gr10 was mainly used by larvae to detect lower sucrose, while Gr6 was primarily used by adults to detect higher sucrose and other saccharides, which results in differences in selectivity and sensitivity between larval and adult sugar sensory neurons. Our results demonstrate the sugar receptors in this moth are evolved to adapt toward the larval and adult foods with different types and amounts of sugar, and fill in a gap in sweet taste of animals.

Disruption of Zfh3 abolishes mulberry-specific monophagy in silkworm larvae

Feeding behavior is critical for insect survival and fitness. Most researchers have explored the molecular basis of feeding behaviors by identifying and elucidating the function of olfactory receptors (ORs) and gustatory receptors (GRs). Other types of genes, such as transcription factors, have rarely been investigated, and little is known about their potential roles. The silkworm (Bombyx mori) is a well-studied monophagic insect which primarily feeds on mulberry leaves, but the genetic basis of its monophagy is still not understood. In this report, we focused on a transcription factor encoded by the Zfh3 gene, which is highly expressed in the silkworm central and peripheral nervous systems, including brain, antenna, and maxilla. To investigate its function, Zfh3 was abrogated using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) mutagenesis. Since Zfh3 knockout homozygotes are not viable, we studied feeding behavior in heterozygotes, and found that disruption of Zfh3 affects both gustation and olfaction. Mutant larvae lose preference for mulberry leaves, acquire the ability to consume an expanded range of diets, and exhibit improved adaptation to the M0 artificial diet, which contains no mulberry leaves. These results provide the first demonstration that a transcription factor modulates feeding behaviors in an insect.

Ligand dose-dependent activation of signaling pathways through the gustatory receptor NlGr11 linked to feeding efficacy in Nilaparvata lugens

Insects often face both conditions with sufficient nutrients and conditions of undernutrition in the field. Through gustatory receptors, insects sense nutrients and regulate their physiological functions such as feeding and reproduction. However, it remains unclear whether signaling pathways activated by gustatory receptors depend on the concentration of nutrients and whether the difference in signaling pathways directly affects insects' physiological functions. Herein, we found that a sugar gustatory receptor, NlGr11, from the brown planthopper (BPH), Nilaparvata lugens, activated G protein-coupled signaling and ionotropic pathways when bound to high galactose concentration. BPHs subsequently demonstrated longer feeding times, feeding loads, and higher vitellogenin (NlVg) expression than BPHs exposed to high galactose concentrations, which only activated the ionotropic pathway. For the first time, our findings link plant nutrient conditions, signaling pathways activated by nutrients, and their gustatory receptors, and nutrient dose-dependent feeding efficacy and vitellogenin (Vg) expression in an insect. This will help us to better understand the molecular mechanism for insect feeding strategies on plants at different stages of nutritional conditions.

CRISPR/Cas9-mediated genome editing of gustatory receptor NlugGr23a causes male sterility in the brown planthopper Nilaparvata lugens

Gustatory receptors (Grs) have an essential role in chemical recognition so as to evaluate food quality. Insect Grs also participate in non-gustatory functions, such as olfaction, temperature sensing, and mating. In this study, we knocked out NlugGr23a, a putative fecundity-related Gr, using the CRISPR/Cas9 system in the brown planthopper Nilaparvata lugens, a serious insect pest of rice. Surprisingly, homozygous NlugGr23a mutant (NlugGr23a^-/-) males were sterile but their sperm were motile and morphologically normal. DAPI staining of mutant sperm inseminated eggs showed that most of NlugGr23a^-/- sperm failed to fertilize eggs, even if they were capable of entering into the egg as a result of their arrested development prior to male pronucleus formation. Immunohistochemistry demonstrated the expression of NlugGr23a in testis. Moreover, prior mating by NlugGr23a^-/- males suppressed female fertility. To our knowledge, it is the first report that a chemoreceptor is implicated in male sterility and provides a potential molecular target for genetic pest control alternatives.