In the animal kingdom, metamorphosis is a well-known developmental transition within various taxa (Cnidarians, Echinoderms, Molluscs, Arthropods, Vertebrates, etc.), which is characterized by the switching from a larval stage to an adult form through the induction of morpho-anatomical, physiological, behavioral, and/or ecological changes. Over the last decades, numerous studies have focused on the hormonal control of cellular processes underlying metamorphosis. Recently, another regulatory network has emerged trough the discovery of microRNAs, non-coding RNAs of 19 to 25 nucleotides that are highly conserved among taxa and act by modulating gene expression at the post-transcriptional level. Experiments carried out on model insects highlighted the relevance of microRNAs in several developmental processes during metamorphosis. This review aims to give an overview of the regulatory actions of microRNAs in the programming of cellular and molecular events associated with the metamorphosis of insects and also to provide new insights into the evolutionary history of this taxon.

As in other animals, insects can modulate their odor-guided behaviors, especially sexual behavior, according to environmental and physiological factors such as the individual's nutritional state. This behavioral flexibility results from modifications of the olfactory pathways under the control of hormones. Most studies have focused on the central modulation of the olfactory system and less attention has been paid to the peripheral olfactory system. To understand how nutritional inputs influence the detection of sex pheromones in insects, we turned to the male moth Agrotis ipsilon, for which the behavioral responsiveness to sex pheromones is dependent on diet and reproductive hormones, juvenile hormone (JH) and 20-hydroxyecdysone (20E). We demonstrated that a sugar-rich diet with sodium increases the sensitivity of olfactory receptor neurons to (Z)-7-dodecen-1-yl acetate, the major sex pheromone compound, and the antennal expression of the pheromone binding protein (PBP2) and the pheromone receptor (OR3). Such a diet also induces overexpression of the methoprene-tolerant receptor to JH and underexpression of the ecdysone receptor to 20E in antennae. The diet-induced olfactory responses were maintained by treatment with cucurbitacin B, a 20E antagonist, but were suppressed by the topic application of precocene, a JH biosynthesis inhibitor. These findings reveal that a positive nutritional state enhances sex pheromone detection through JH actions on the peripheral actors of the pheromone system in male moths. More broadly, in insects, our study provides, for the first time, a neuronal and molecular basis of the dietary-dependent endocrine modulation of the peripheral olfactory system.

Nutritional ecology examines the environmental effects on nutritional needs, food intake and foraging behaviors, and the use of nutrients ingested by animals. Adults of many insects' species feed on nectars rich in sugars allowing them to match the nutritional needs necessary for reproduction. Among insects, Lepidoptera are often considered opportunistic foragers that visit a wide variety of available flowers, although with some preferences. While nutritional ecology of diurnal Lepidoptera is beginning to be explored, very little work focuses on nocturnal species because they are complicated to study in the wild. To address this, we used new laboratory approaches to study feeding behaviors (number and duration of visits to artificial flowers, food preferences associated with the texture and odors of the flowers) as well as gustatory detection by antennae (proboscis extension reflex) in the male crop pest moth Agrotis ipsilon. We showed that (i) food responsiveness is age-dependent and increases mainly with sugar quantity and marginally with sugar quality, (ii) diet quality impacts feeding behaviors in the first days of adulthood, and (iii) male moths choose their food through floral cues. Taken together, these data allow to define this species as a generalist forager with a preference for flowers with sugary nectars rich in sucrose, fructose, and glucose. Our results thus provide considerable information on the close links between food sources and nutritional ecology in this species, which is important for guiding future studies on their behavioral ecology, population dynamics, as well as for population monitoring and for regional pest management.

As in other animals, diet is known to influence insect reproduction, and its impact has been intensively investigated in females. In our study, we examined the effects of various diets on male reproductive success in the moth Agrotis ipsilon, a pest of many crops. Our experiments showed an increase in the rates of fertilization and hatching when males fed with various sugars (sucrose, fructose, and glucose) supplemented with sodium. Such results provide valuable initial information on the nutritional ecology of male moths and could serve to the development of nutritional attractants for the management of crop pests.

In animals, sexual maturation coincides with the development of sexual behaviors and reproductive system. These developmental events are influenced by diet and governed by endocrine signals. Here, for the first time in insects, we explored functional links between nutrition and juvenile hormone (JH) in the male reproductive physiology through the insulin signaling pathway (ISP) acting as a transducer of nutritional signals. We turned to the male moth Agrotis ipsilon for which sexual maturation, including accessory sex glands (ASGs) development concomitantly with antennal lobes (ALs) maturation for female sex pheromone processing and display of sexual behavior, is known to be JH- and diet-dependent. Indeed, a diet rich in sugars with sodium was previously shown to accelerate sexual maturation, which was achieved from the third day of adult life. In this study, we demonstrated that such a diet raised i) the expression of JH signaling actors (Methoprene-tolerant, Taiman, and Krüppel homolog 1) in ALs and ASGs, ii) the biosynthesis and circulating levels of JH, and iii) the expression of both insulin receptor (InR) and insulin-like peptides (ILPs) in corpora allata (CAs) and brain respectively. Insulin injection raised JH biosynthesis following increased HMG-CoA reductase expression in CAs; opposite effects were induced in InR-deficient males. Thus, we highlighted that promoting effects of a diet composed of sugars with sodium on male sexual maturation results from an early induction of ISP causing an increase in JH biosynthesis followed by a potentiation of JH actions on the development of ASGs and ALs in A. ipsilon.

Dans le règne animal, la métamorphose est une transition développementale qui a été décrite au sein de divers taxons (Cnidaires, Échinodermes, Mollusques, Arthropodes, Vertébrés...). Elle se caractérise par le passage d’une forme larvaire à une forme adulte et implique des changements morpho-anatomiques, physiologiques, comportementaux et/ou écologiques. Au cours des dernières décennies, de nombreuses études se sont focalisées sur le contrôle hormonal des processus cellulaires mis en jeu au cours de la métamorphose. Récemment, un autre niveau de régulation a été mis en évidence par la découverte des microARN, ARN non codants de 19 à 25 nucléotides hautement conservés entre les taxons et connus pour moduler l’expression génique au niveau post-transcriptionnel. Des travaux menés sur des Insectes modèles ont mis au jour le rôle des microARN dans de nombreuses transitions développementales dont la métamorphose. La présente revue vise à la fois à donner un aperçu sur les actions régulatrices des microARN dans la programmation des évènements cellulaires et moléculaires liés à la métamorphose des Insectes et à apporter un nouveau regard sur l’histoire évolutive de ce taxon.

Animals invest crucial resources in foraging to support development, sustenance, and reproduction. Foraging and feeding behaviors are rhythmically expressed by most insects. Rhythmic behaviors are modified by exogenous factors like temperature and photoperiod, and internal factors such as the physiological status of the individual. However, the interactions between these factors and the circadian clock to pattern feeding behavior remains elusive. As Drosophila, a standard insect model, spends nearly all its life on food, we rather chose to focus on the adults of a non-model insect, Agrotis ipsilon, a nocturnal cosmopolitan crop pest moth having structured feeding activity. Our study aimed to explore the impact of environmental cues on directly measured feeding behavior rhythms. We took advantage of a new experimental set-up, mimicking an artificial flower, allowing us to specifically monitor feeding behavior in a naturalistic setting, e.g., the need to enter a flower to get food. We show that the frequency of flower visits is under the control of the circadian clock in males and females. Feeding behavior occurs only during the scotophase, informed by internal clock status and external photic input, and females start to visit flowers earlier than males. Shorter duration visits predominate as the night progresses. Importantly, food availability reorganizes the microstructure of feeding behavior, revealing its plasticity. Interestingly, males show a constant number of daily visits during the 5 days of adult life whereas females decrease visitations after the third day of adult life. Taken together, our results provide evidence that the rhythmicity of feeding behavior is sexually dimorphic and controlled by photoperiodic conditions through circadian clock-dependent and independent pathways. In addition, the use of the new experimental set-up provides future opportunities to examine the regulatory mechanisms of feeding behavior paving the way to investigate complex relationships between feeding, mating, and sleep-wake rhythms in insects.

In many animals, drastic changes are observed during sexual maturation characterized by the reproductive system development concomitantly to the sexual behavior ontogenesis. These modifications are under the control of internal and external factors such as food. Sexual maturation requires considerable energetic investment, and diet has been shown to affect reproductive activities in many taxonomic groups, especially in insects and vertebrates. By contrast, diet effects on sexual behavior development remain largely unexplored. To elucidate this aspect, we used the male moth Agrotis ipsilon which undergoes sexual maturation occurring between the third and the fifth day postemergence. During this period, males are sensitive to female sex pheromones and a stereotypical sexual behavior characterized by female-oriented flight takes place. In our study, we compared (1) sex pheromone detection by electroantennography recordings and (2) behavioral response in wind tunnel assays between males fed with different diets found in nature. Compared to standard sucrose diet, males fed with sucrose, fructose, and glucose supplemented with sodium (a mineral element necessary for the locomotor activity in several moths) did not respond better to female sex pheromones but clearly exhibited an earlier behavioral response. Thus, such a diet accelerates the development of sex pheromone-mediated oriented flight, probably by facilitating the central processing of sex pheromone information in male A. ipsilon moths. Our results provide new information on the influence of nutritional intake on the ontogenesis of male sexual behavior in animals.

In insects, juvenile hormone (JH) is critical for the orchestration of male reproductive maturation. For instance, in the male moth, Agrotis ipsilon, the behavioral response and the neuronal sensitivity within the primary olfactory centers, the antennal lobes (ALs), to the female-emitted sex pheromone increase with fertility during adulthood and the coordination between these events is governed by JH. However, the molecular basis of JH action in the development of sexual behavior remains largely unknown. Here, we show that the expression of the paralogous JH receptors, Methoprene-tolerant 1 and 2 (Met1, Met2) and of the JH-inducible transcription factor, Krüppel homolog 1 (Kr-h1) within ALs raised from the third day of adult life and this dynamic is correlated with increased behavioral responsiveness to sex pheromone. Met1-, Met2- and Kr-h1-depleted sexually mature males exhibited altered sex pheromone-guided orientation flight. Moreover, injection of JH-II into young males enhanced the behavioral response to sex pheromone with increased AL Met1, Met2 and Kr-h1 mRNA levels. By contrast, JH deficiency suppressed the behavioral response to sex pheromone coupled with reduced AL Met1, Met2 and Kr-h1 mRNA levels in allatectomized old males and these inhibitions were compensated by an injection of JH-II in operated males. Our results demonstrated that JH acts through Met-Kr-h1 signaling pathway operating in ALs, to promote the pheromone information processing and consequently the display of sexual behavior in synchronization with fertility to optimize male reproductive fitness. Thus, this study provides insights into the molecular mechanisms underlying the hormonal regulation of reproductive behavior in insects.

In male moth Agrotis ipsilon, sexual maturation occurs between the third and the fifth day of adult life and is characterized by the development of the reproductive organs such as testes and accessory sex glands. Since sexual maturation requires considerable energy investment, we hypothesized that diet would be an essential regulatory factor in this developmental process. Indeed, the links between the male diet and reproductive physiology have not been described as in females. To test the previous hypothesis, we offered male moths diets corresponding to different flower nectars found in nature, and measured morphological and functional changes in the testes and accessory sex glands. In comparison to a diet composed of sucrose only, males fed with a diet composed of diverse sugars, including glucose, supplemented with sodium led to an earlier increase in the length and the protein content of accessory sex glands, as well as a reduction of the testicular volume accompanied by an acceleration of the sperm bundle transfer from the testes to the duplex. These results show that these specific diets accelerate the maturation of the reproductive system in male moth Agrotis ipsilon.