Allatotropin, leucokinin and AKH in honey bees and other Hymenoptera

A fluorescently-tagged tick kinin neuropeptide triggers peristalsis and labels tick midgut muscles

Ticks are blood-feeding arthropods that require heme for their successful reproduction. During feeding they also acquire pathogens that are subsequently transmitted to humans, wildlife and/or livestock. Understanding the regulation of tick midgut is important for blood meal digestion, heme and nutrient absorption processes and for aspects of pathogen biology in the host. We previously demonstrated the activity of tick kinins on the cognate G protein-coupled receptor. Herein we uncovered the physiological role of the kinin receptor in the tick midgut. A fluorescently-labeled kinin peptide with the endogenous kinin 8 sequence (TMR-RK8), identical in the ticks Rhipicephalus microplus and R. sanguineus, activated and labeled the recombinant R. microplus receptor expressed in CHO-K1 cells. When applied to the live midgut the TMR-RK8 labeled the kinin receptor in muscles while the labeled peptide with the scrambled-sequence of kinin 8 (TMR-Scrambled) did not. The unlabeled kinin 8 peptide competed TMR-RK8, decreasing confocal microscopy signal intensity, indicating TMR-RK8 specificity to muscles. TMR-RK8 was active, inducing significant midgut peristalsis that was video-recorded and evaluated with video tracking software. The TMR-Scrambled peptide used as a negative control did not elicit peristalsis. The myotropic function of kinins in eliciting tick midgut peristalsis was established.

Regulation of forager honey bee appetite independent of the glucose-insulin signaling pathway

Introduction

To maintain energetic homeostasis the energetic state of the individual needs to communicate with appetite regulatory mechanisms on a regular basis. Although hunger levels indicated by the energetic state and appetite levels, the desire for food intake, tend to be correlated, and on their own are well studied, how the two cross-talk and regulate one another is less known. Insects, in contrast to vertebrates, tend to have trehalose as the primary sugar found in the hemolymph, which could possibly serve as an alternative monitor of the energetic state in comparison to the glucose-insulin signaling pathway, found in vertebrates.

Methods

We investigate how manipulating hemolymph sugar levels alter the biogenic amines in the honey bee brain, appetite levels, and insulin like peptide gene expression, across three age classes, to determine how the energetic state of the honey bee might be connected to appetite regulation.

Results

We found that only in the forager bees, with a lowering of hemolymph trehalose levels, there was an increase in octopamine and a decrease in tyramine levels in the honey bee brain that corresponded with increased appetite levels, while there was no significant changes in Insulin Like Peptide-1 or 2 gene expression.

Discussion

Our findings suggest that hemolymph trehalose levels aid in regulating appetite levels, in forager bees, via octopamine and tyramine, and this regulation appears to be functioning independent of the glucose insulin signaling pathway. Whether this potentially more direct and rapid appetite regulatory pathway can be generalized to other insects, which also undergo energy demanding activities, remains to be investigated.

Response to comment on "Food Wanting is Mediated by Transient Activation of Dopaminergic Signaling in the Honeybee Brain"

In a technical comment, Barron et al. (1) criticized the work of Huang et al. (2) putting the accent on the quantification of dopamine levels via high-performance liquid chromatography (HPLC), yet also including data interpretation through alternative hypotheses aimed at invalidating the original ones proposed by Huang et al. We thank the authors of this technical comment, which allows us to clarify technical aspects of our work that may have been unclear, and for promoting discussion around the conclusions of our work. Below we provide answers to the points raised in their comment.

The Adipokinetic Peptides of Hemiptera: Structure, Function, and Evolutionary Trends

The Hemiptera comprise the most species-rich order of the hemimetabolous insects. Members of a number of superfamilies, most notably especially the more basal ones such as white flies, psyllids and aphids, belong to the most destructive agricultural insects known worldwide. At the other end of the phylogenetic tree are hemipterans that are notorious medical pests (e.g. kissing bugs). Most of the hemipteran species are good flyers, and lipid oxidation plays a pivotal role to power the contraction of flight muscles and, in aquatic water bugs, also deliver the ATP for the extensive swimming action of the leg muscles. Mobilization of stored lipids (mostly triacylglycerols in the fat body) to circulating diacylglycerols in the hemolymph is regulated by a set of small neuropeptides, the adipokinetic hormones (AKHs). We searched the literature and publicly available databases of transcriptomes and genomes to present here AKH sequences from 191 hemipteran species. Only few of these peptides were sequenced via Edman degradation or mass spectrometry, and even fewer were characterized with molecular biology methods; thus, the majority of the AKHs we have identified by bioinformatics are merely predicted sequences at this stage. Nonetheless, a total of 42 AKH primary sequences are assigned to Hemiptera. About 50% of these structures occur also in other insect orders, while the remaining 50% are currently unique for Hemiptera. We find 9 novel AKHs not shown to be synthesized before in any insect. Most of the hemipteran AKHs are octapeptides (28) but there is an impressive number of decapeptides (12) compared to other speciose orders such as Diptera and Lepidoptera. We attempt to construct a hypothetical molecular peptide evolution of hemipteran AKHs and find quite a bit of overlapping with current phylogenetic ideas of the Hemiptera. Lastly, we discuss the possibility to use the sequence of the aphid AKH as lead peptide for the research into a peptide mimetic fulfilling criteria of a green insecticide.

Adipokinetic hormone (AKH), energy budget and their effect on feeding and gustatory processes of foraging honey bees

The adipokinetic hormone (AKH) of insects is considered an equivalent of the mammalian hormone glucagon as it induces fast mobilization of carbohydrates and lipids from the fat body upon starvation. Yet, in foraging honey bees, which lack fat body storage for carbohydrates, it was suggested that AKH may have lost its original function. Here we manipulated the energy budget of bee foragers to determine the effect of AKH on appetitive responses. As AKH participates in a cascade leading to acceptance of unpalatable substances in starved Drosophila, we also assessed its effect on foragers presented with sucrose solution spiked with salicin. Starved and partially-fed bees were topically exposed with different doses of AKH to determine if this hormone modifies food ingestion and sucrose responsiveness. We found a significant effect of the energy budget (i.e. starved vs. partially-fed) on the decision to ingest or respond to both pure sucrose solution and sucrose solution spiked with salicin, but no effect of AKH per se. These results are consistent with a loss of function of AKH in honey bee foragers, in accordance with a social life that implies storing energy resources in the hive, in amounts that exceed individual needs.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Isolation functional characterization of allatotropin receptor from the cotton bollworm, Helicoverpa armigera

Insect allatotropin (AT) plays multi-functions including regulation of juvenile hormone synthesis, growth, development and reproduction. In the present study, the full-length cDNA encoding the AT receptor was cloned from the brain of Helicoverpa armigera (Helar-ATR). The ORF of Helar-ATR exhibited the characteristic seven transmembrane domains of the G protein-coupled receptor (GPCR) and was close to the ATR of Manduca sexta in the phylogenetic tree. The Helar-ATR expressed in vertebrate cell lines can be activated by Helar-AT and each Helar-ATL in a dose-responsive manner, in the following order: Helar-ATLI > Helar-ATLII > Helar-AT > Helar-ATLIII. Helar-ATLI and Helar-ATLII represented the functional ligands to Helar-ATR in vitro, while Helar-AT and Helar-ATLIII behaved as partial agonists. The in vitro functional analysis suggested that the Helar-ATR signal was mainly coupled with elevated levels of Ca²⁺ and independent of cAMP levels. Helar-ATR mRNA in larvae showed the highest level in the brain, followed by the thorax ganglion, abdomen ganglion, fat body and midgut. Helar-ATR mRNA levels in the complex of the brain-thoracic-abdomen ganglion on the 2nd day of the larval stage and during later pupal stages were observed to be relatively higher than in the wandering and early pupal stages.

GPCR annotation, G proteins, and transcriptomics of fire ant (Solenopsis invicta) queen and worker brain: An improved view of signaling in an invasive superorganism

Knowledge of G protein-coupled receptors (GPCRs) and their signaling modalities is crucial to advancing insect endocrinology, specifically in highly successful invasive social insects, such as the red imported fire ant, Solenopsis invicta Buren. In the first published draft genome of S. invicta, emphasis was placed on the annotation of olfactory receptors, and only the number of predicted GPCR genes was reported. Without an organized and curated resource for GPCRs, it will be difficult to test hypotheses on the endocrine role of neuropeptide hormones, or the function of neurotransmitters and neuromodulators. Therefore, we mined the S. invicta genome for GPCRs and found 324 predicted transcripts encoded by 125 predicted loci and improved the annotation of 55 of these loci. Among them are sixteen GPCRs that are currently annotated as "uncharacterized proteins". Further, the phylogenetic analysis of class A neuropeptide receptors presented here and the comparative listing of GPCRs in the hymenopterans S. invicta, Apis mellifera (both eusocial), Nasonia vitripennis (solitary), and the solitary model dipteran Drosophila melanogaster will facilitate comparative endocrinological studies related to social insect evolution and diversity. We compiled the 24 G protein transcripts predicted (15 α, 7 β, and 2 γ) from 12 G protein genes (5 α, 5 β, and 2 γ). Reproductive division of labor is extreme in this ant species, therefore, we compared GPCR and G protein gene expression among worker, mated queen and alate virgin queen ant brain transcriptomes. Transcripts for ten GPCRs and two G proteins were differentially expressed between queen and worker brains. The differentially expressed GPCRs are candidate receptors to explore hypotheses on division of labor in this species.

Coleoptera genome and transcriptome sequences reveal numerous differences in neuropeptide signaling between species

Background

Insect neuropeptides are interesting for the potential their receptors hold as plausible targets for a novel generation of pesticides. Neuropeptide genes have been identified in a number of different species belonging to a variety of insects. Results suggest significant neuropeptide variation between different orders, but much less is known of neuropeptidome variability within an insect order. I therefore compared the neuropeptidomes of a number of Coleoptera.

Methodology

Publicly available genome sequences, transcriptomes and the original sequence data in the form of short sequence read archives were analyzed for the presence or absence of genes coding neuropeptides as well as some neuropeptide receptors in seventeen beetle species.

Results

Significant differences exist between the Coleoptera analyzed here, while many neuropeptides that were previously characterized from Tribolium castaneum appear very similar in all species, some are not and others are lacking in one or more species. On the other hand, leucokinin, which was presumed to be universally absent from Coleoptera, is still present in non-Polyphaga beetles.

Conclusion

The variability in neuropeptidome composition between species from the same insect order may be as large as the one that exists between species from different orders.

Appetite is correlated with octopamine and hemolymph sugar levels in forager honeybees

Insects have rapidly changing energy demands, so they primarily rely on hemolymph and other carbohydrates to carry out life activities. However, how gustatory responsiveness and hemolymph sugar levels coordinate with one another to maintain energetic homeostasis in insects remains largely unknown for the highly social honeybee that goes through large physiological and behavioral changes. The potential role of biogenic amines and neuropeptides in the connection between the regulation of appetite and fluctuating sugar levels in the hemolymph, due to starvation, as the bee ages, was investigated. The largest appetite increase due to the starvation treatment was within the forager age class and this corresponded with an increase in octopamine levels in the brain along with a decline in hemolymph sugar levels. Adipokinetic hormone (AKH) was found in very small quantities in the brain and there were no significant changes in response to starvation treatment. Our findings suggest that the particularly dynamic levels of hemolymph sugar levels may serve as a monitor of the forager honeybee energetic state. Therefore, there may be a pathway in forager bees via octopamine responsible for their precise precipitous regulation of appetite, but to determine cause and effect relationships further investigation is needed.

In silico prediction of neuropeptides in Hymenoptera parasitoid wasps

Parasitoid wasps of the order Hymenoptera, the most diverse groups of animals, are important natural enemies of arthropod hosts in natural ecosystems and can be used in biological control. To date, only one neuropeptidome of a parasitoid wasp, Nasonia vitripennis, has been identified. This study aimed to identify more neuropeptides of parasitoid wasps, by using a well-established workflow that was previously adopted for predicting insect neuropeptide sequences. Based on publicly accessible databases, totally 517 neuropeptide precursors from 24 parasitoid wasp species were identified; these included five neuropeptides (CNMamide, FMRFamide-like, ITG-like, ion transport peptide-like and orcokinin B) that were identified for the first time in parasitoid wasps, to our knowledge. Next, these neuropeptides from parasitoid wasps were compared with those from other insect species. Phylogenetic analysis suggested the divergence of AST-CCC within Hymenoptera. Further, the encoding patterns of CAPA/PK family genes were found to be different between Hymenoptera species and other insect species. Some neuropeptides that were not found in some parasitoid superfamilies (e.g., sulfakinin), or considerably divergent between different parasitoid superfamilies (e.g., sNPF) might be related to distinct physiological processes in the parasitoid life. Information of neuropeptide sequences in parasitoid wasps can be useful for better understanding the phylogenetic relationships of Hymenoptera and further elucidating the physiological functions of neuropeptide signaling systems in parasitoid wasps.

Stress response in honeybees is associated with changes in task-related physiology and energetic metabolism

In a rapidly changing environment, honeybee colonies are increasingly exposed to diverse sources of stress (e.g., new parasites, pesticides, climate warming), which represent a challenge to individual and social homeostasis. However, bee physiological responses to stress remain poorly understood. We therefore exposed bees specialised in different tasks (nurses, guards and foragers) to ancient (immune and heat stress) or historically more recent sources of stress (pesticides), and we determined changes in the expression of genes linked to behavioural maturation (vitellogenin - vg and juvenile hormone esterase - jhe) as well as in energetic metabolism (glycogen level, expression level of the receptor to the adipokinetic hormone - akhr, and endothermic performance). While acute exposure to sublethal doses of two pesticides did not affect vg and jhe expression, immune and heat challenges caused a decrease and increase in both genes, respectively, suggesting that bees had responded to ecologically relevant stressors. Since vg and jhe are expressed to a higher level in nurses than in foragers, it is reasonable to assume that an immune challenge stimulated behavioural maturation to decrease potential contamination risk and that a heat challenge promoted a nurse profile for brood thermoregulation. All behavioural castes responded in the same way. Though endothermic performances did not change upon stress exposure, the akhr level dropped in immune and heat-challenged individuals. Similarly, the abdomen glycogen level tended to decline in immune-challenged bees. Altogether, these results suggest that bee responses are stress specific and adaptive but that they tend to entail a reduction of energetic metabolism that needs to be studied on a longer timescale.

Gene Expression Dynamics in Major Endocrine Regulatory Pathways along the Transition from Solitary to Social Life in a Bumblebee, Bombus terrestris

Understanding the social evolution leading to insect eusociality requires, among other, a detailed insight into endocrine regulatory mechanisms that have been co-opted from solitary ancestors to play new roles in the complex life histories of eusocial species. Bumblebees represent well-suited models of a relatively primitive social organization standing on the mid-way to highly advanced eusociality and their queens undergo both, a solitary and a social phase, separated by winter diapause. In the present paper, we characterize the gene expression levels of major endocrine regulatory pathways across tissues, sexes, and life-stages of the buff-tailed bumblebee, Bombus terrestris, with special emphasis on critical stages of the queen's transition from solitary to social life. We focused on fundamental genes of three pathways: (1) Forkhead box protein O and insulin/insulin-like signaling, (2) Juvenile hormone (JH) signaling, and (3) Adipokinetic hormone signaling. Virgin queens were distinguished by higher expression of forkhead box protein O and downregulated insulin-like peptides and JH signaling, indicated by low expression of methyl farnesoate epoxidase (MFE) and transcription factor Krüppel homolog 1 (Kr-h1). Diapausing queens showed the expected downregulation of JH signaling in terms of low MFE and vitellogenin (Vg) expressions, but an unexpectedly high expression of Kr-h1. By contrast, reproducing queens revealed an upregulation of MFE and Vg together with insulin signaling. Surprisingly, the insulin growth factor 1 (IGF-1) turned out to be a queen-specific hormone. Workers exhibited an expression pattern of MFE and Vg similar to that of reproducing queens. Males were characterized by high Kr-h1 expression and low Vg level. The tissue comparison unveiled an unexpected resemblance between the fat body and hypopharyngeal glands across all investigated genes, sexes, and life stages.

Similarities between decapod and insect neuropeptidomes

Background. Neuropeptides are important regulators of physiological processes and behavior. Although they tend to be generally well conserved, recent results using trancriptome sequencing on decapod crustaceans give the impression of significant differences between species, raising the question whether such differences are real or artefacts. Methods. The BLAST+ program was used to find short reads coding neuropeptides and neurohormons in publicly available short read archives. Such reads were then used to find similar reads in the same archives, and the DNA assembly program Trinity was employed to construct contigs encoding the neuropeptide precursors as completely as possible. Results. The seven decapod species analyzed in this fashion, the crabs Eriocheir sinensis, Carcinus maenas and Scylla paramamosain, the shrimp Litopenaeus vannamei, the lobster Homarus americanus, the fresh water prawn Macrobrachium rosenbergii and the crayfish Procambarus clarkii had remarkably similar neuropeptidomes. Although some neuropeptide precursors could not be assembled, in many cases individual reads pertaining to the missing precursors show unambiguously that these neuropeptides are present in these species. In other cases, the tissues that express those neuropeptides were not used in the construction of the cDNA libraries. One novel neuropeptide was identified: elongated PDH (pigment dispersing hormone), a variation on PDH that has a two-amino-acid insertion in its core sequence. Hyrg is another peptide that is ubiquitously present in decapods and is likely a novel neuropeptide precursor. Discussion. Many insect species have lost one or more neuropeptide genes, but apart from elongated PDH and hyrg all other decapod neuropeptides are present in at least some insect species, and allatotropin is the only insect neuropeptide missing from decapods. This strong similarity between insect and decapod neuropeptidomes makes it possible to predict the receptors for decapod neuropeptides that have been deorphanized in insects. This includes the androgenic insulin-like peptide that seems to be homologous to drosophila insulin-like peptide 8.

Nosema ceranae alters a highly conserved hormonal stress pathway in honeybees

Nosema ceranae, an emerging pathogen of the western honeybee (Apis mellifera), is implicated in recent pollinator losses and causes severe energetic stress. However, whether precocious foraging and accelerated behavioural maturation in infected bees are caused by the infection itself or via indirect energetic stress remains unknown. Using a combination of nutritional and infection treatments, we investigated how starvation and infection alters the regulation of adipokinetic hormone (AKH) and octopamine, two highly conserved physiological pathways that respond to energetic stress by mobilizing fat stores and increasing search activity for food. Although there was no response from AKH when bees were experimentally infected with N. ceranae or starved, supporting the notion that honeybees have lost this pathway, there were significant regulatory changes in the octopamine pathway. Significantly, we found no evidence of acute energetic stress being the only cause of symptoms associated with N. ceranae infection. Therefore, the parasite itself appears to alter regulatory components along a highly conserved physiological pathway in an infection-specific manner. This indicates that pathogen-induced behavioural alteration of chronically infected bees should not just be viewed as a coincidental short-term by-product of pathogenesis (acute energetic stress) and may be a result of a generalist manipulation strategy to obtain energy for reproduction.

The pleiotropic allatoregulatory neuropeptides and their receptors: A mini-review

Juvenile hormones (JH) are highly pleiotropic insect hormones essential for post-embryonic development. The circulating JH titer in the hemolymph of insects is influenced by enzymatic degradation, binding to JH carrier proteins, uptake and storage in target organs, but evidently also by rates of production at its site of synthesis, the corpora allata (CA). The multiple processes in which JH is involved alongside the critical significance of JH in insect development emphasize the importance for elucidating the control of JH production. Production of JH in CA cells is regulated by different factors: by neurotransmitters, such as dopamine and glutamate, but also by allatoregulatory neuropeptides originating from the brain and axonally transported to the CA where they bind to their G protein-coupled receptors (GPCRs). Different classes of allatoregulatory peptides exist which have other functions aside from acting as influencers of JH production. These pleiotropic neuropeptides regulate different processes in different insect orders. In this mini-review, we will give an overview of allatotropins and allatostatins, and their recently characterized GPCRs with a view to better understand their modes of action and different action sites.

Molecular cloning and characterization of the allatotropin precursor and receptor in the desert locust, Schistocerca gregaria

Allatotropins (ATs) are pleiotropic neuropeptides initially isolated from the tobacco hornworm, Manduca sexta. In 2008, the first receptor for AT-like peptides (ATR) was characterized in Bombyx mori. Since then, ATRs have also been characterized in M. sexta, Tribolium castaneum, Aedes aegypti and Bombus terrestris. These receptors show sequence similarity to vertebrate orexin (ORX) receptors. When generating an EST-database of the desert locust (Schistocerca gregaria) central nervous system, we found cDNA sequences encoding the Schgr-AT precursor and a fragment of its putative receptor. This receptor cDNA has now been completed and functionally expressed in mammalian cell lines. Activation of this receptor, designated as Schgr-ATR, by Schgr-AT caused an increase in intracellular calcium ions, as well as cyclic AMP (cAMP), with an EC₅₀ value in the nanomolar range. In addition, the transcript distribution of both the Schgr-AT precursor and Schgr-ATR was investigated by means of quantitative real-time PCR. Moreover, we found more evidence for the myotropic and allatostimulatory actions of Schgr-AT in the desert locust. These data are discussed and situated in a broader context by comparison with literature data on AT and ATR in insects.

Connecting nutrient sensing and the endocrine control of metabolic allocation in insects

It is clear that specific hormones control an organism's energy use and regulate the differential allocations of energy to activity, growth and maintenance of specific tissues, and reproduction. Appropriate metabolic allocations require an assessment of the nutrient state of the animal, and nutrient sensing must be tied to appropriate signals in order to coordinate the repertoire of behaviors and physiologies accompanying a particular metabolic investment. Here, we review the known and speculated connections between nutrient sensing and the endocrine control of energy allocation in insects. Insects, being speciose and diverse in life history strategies, offer a unique perspective into the general architecture of the signaling mechanisms of energetic allocation and also into unique elements that correlate with specific life histories.

Characterisation of a functional allatotropin receptor in the bumblebee, Bombus terrestris (Hymenoptera, Apidae)

Allatotropins (ATs) are multifunctional neuropeptides initially isolated from the tobacco hornworm, Manduca sexta, where they were found to stimulate juvenile hormone synthesis and release from the corpora allata. ATs have been found in a wide range of insects, but appear to be absent in Drosophila. The first AT receptor (ATR) was characterised in 2008 in the lepidopteran Bombyx mori. Since then ATRs have been characterised in Coleoptera and Diptera and in 2012, an AT precursor gene was identified in hymenopteran species. ATRs show large sequence and structural similarity to vertebrate orexin receptors (OXR). Also, AT in insects and orexin in vertebrates show some overlap in functions, including modulation of feeding behaviour and reproduction. The goal of this study was to identify a functional ATR in a hymenopteran species. We used ATRs (insect sequences) and OXRs (vertebrate sequences) to search the genome of the bumblebee, Bombus terrestris. Two receptors (XP_003402490 and XP_003394933) with resemblance to ATRs and OXRs were found. Phylogenetic analysis provided the first indication that XP_003402490 was more closely related to ATRs than XP_003394933. We investigated the transcript level distribution of both receptors and the AT precursor gene by means of quantitative real-time reverse transcriptase PCR. XP_003402490 displayed a tissue distribution comparable with ATRs in other species, with high transcript levels in the male accessory glands. After pharmacological characterisation, it appeared that XP_003402490 is indeed a functional ATR. Activation of the receptor causes an increase in intracellular calcium and cyclic AMP levels with an EC50 value in the low nanomolar to picomolar range. XP_003394933 remains an orphan receptor.

Neuropeptidergic regulation of reproduction in insects

Successful animal reproduction depends on multiple physiological and behavioral processes that take place in a timely and orderly manner in both mating partners. It is not only necessary that all relevant processes are well coordinated, they also need to be adjusted to external factors of abiotic and biotic nature (e.g. population density, mating partner availability). Therefore, it is not surprising that several hormonal factors play a crucial role in the regulation of animal reproductive physiology. In insects (the largest class of animals on planet Earth), lipophilic hormones, such as ecdysteroids and juvenile hormones, as well as several neuropeptides take part in this complex regulation. While some peptides can affect reproduction via an indirect action (e.g. by influencing secretion of juvenile hormone), others exert their regulatory activity by directly targeting the reproductive system. In addition to insect peptides with proven activities, several others were suggested to also play a role in the regulation of reproductive physiology. Because of the long evolutionary history of many insect orders, it is not always clear to what extent functional data obtained in a given species can be extrapolated to other insect taxa. In this paper, we will review the current knowledge concerning the neuropeptidergic regulation of insect reproduction and situate it in a more general physiological context.

More than two decades of research on insect neuropeptide GPCRs: an overview

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insect's life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.