Deformed Mediated Larval Incisor Lobe Development Causes Differing Feeding Behavior between Oriental Armyworm and Fall Armyworm

Abstract

Mandibular incisor lobes are important for insect feeding behavior, living habits and niche. However, the molecular regulation of insect incisor lobe development remains unknown. In this study, we found that two maize pests, oriental armyworm Mythimna separata and fall armyworm Spodoptera frugiperda, have different feeding patterns in maize, which are closely associated with the different development patterns of their incisor lobes. Different from first to sixth instar S. frugiperda, which feed on leaf tissues and whorls with sharp incisor lobes, older instars of M. separata feed from leaf margins with no incisor lobes. Hox gene Deformed (Dfd) is important for head appendages, but its function in incisor lobe development is not clear. Here, Dfds were identified from two armyworm species, and both were expressed highly in heads and eggs. Interestingly, the expression levels of MsDfd were relatively high in larval mandibles and decreased dramatically from fourth-instar mandibles in M. separata. Knockdown of MsDfd resulted in malformed mandibles with no incisor lobe in M. separata, making the larvae unable to perform window-feeding. However, RNAi of SfDfd did not affect the mandibles and window-feeding pattern of S. frugiperda, indicating the different roles of Dfd in these two species. Moreover, the mortality of new first instar M. separata increased after feeding dsMsDfd but did not for S. frugiperda feeding dsSfDfd. These findings revealed that Dfd mediated the larval mandibular incisor lobe morphology, affecting its feeding pattern in M. separata, broadening the knowledge of Dfd functions in insect mandibles and feeding behavior.

Expressions and functions of RR-1 cuticular protein genes in the integument of Mythimna separata

“…The colony has been maintained in the laboratory for more than ten generations and then used for the experiment. The larvae were fed with fresh corn leaves until third instar, which was fed with artificial diet thereafter as in our previous studies (Huang et al 2021, Zhao et al 2022). Adults were fed with 10% honey water and cultured in cages covered with gauze.…”

Effect of short neuropeptide F signaling on larval feeding in Mythimna separata

“…Knockdown of the chitinase-encoding genes MsChi1 and MsChi2 resulted in retarded growth, increased mortality, and reduced body weight of the larvae (Ganbaatar et al, 2017;Bao et al, 2021). The inhibition of MsDfd expression via RNAi led to malformed mandibles with no incisor lobe, making the larvae unable to perform window feeding (Zhao et al, 2022). Ovarian development and egg-laying behavior of females were affected by injection with dsRNA of the ecdysis triggering hormone gene (ETH) or its receptor (ETHR) (Yang et al, 2023).…”

“…The inhibition of MsDfd expression via RNAi led to malformed mandibles with no incisor lobe, making the larvae unable to perform window feeding (Zhao et al. , 2022). Ovarian development and egg‐laying behavior of females were affected by injection with dsRNA of the ecdysis triggering hormone gene ( ETH ) or its receptor ( ETHR ) (Yang et al.…”