The impact of temperature on insecticide sensitivity depends on transgenerational effects

The combined effects of insecticides and temperature are increasingly being studied because species are expected to change their responses to insecticides with climate warming. As recently highlighted, the impact of temperature on insecticide sensitivity might be influenced by the environment experienced by the previous generation. However, a pioneering study that showed this transgenerational effect in the mosquito Culex pipiens needs to be confirmed because two other studies did not show similar results. Here, we performed an experiment on the moth Spodoptera littoralis to test this hypothesis. We analysed reaction norms among experimental families to test transgenerational effects, i.e., the variation in the response of families to the combined effects of the insecticide chlorpyrifos and developmental temperature. Reaction norm analyses revealed that the responses of the families to chlorpyrifos and temperature differed for developmental time and larval survival, two key parameters in S. littoralis. Crucially, for larval survival, a family effect influenced the impact of temperature on chlorpyrifos sensitivity. This finding confirms the pioneering study on C. pipiens that showed transgenerational effects on the combined effects of insecticides and temperature. This result also highlights that transgenerational plasticity can be important to consider in ecotoxicology.

Detoxification gene families in Phylloxera: Endogenous functions and roles in response to the environment

Phylloxera, Daktulosphaira vitifoliae, is an agronomic pest that feeds monophagously on grapevine, Vitis spp. host plants. Phylloxera manipulates primary and secondary plant metabolism to establish either leaf or root galls. We manually annotated 198 detoxification genes potentially involved in plant host manipulation, including cytochrome P450 (66 CYPs), carboxylesterase (20 CCEs), glutathione-S-transferase (10 GSTs), uridine diphosphate-glycosyltransferase (35 UGTs) and ABC transporter (67 ABCs) families. Transcriptomic expression patterns of these detoxification genes were analyzed for root and leaf galls. In addition to these transcriptomic analyses, we reanalyzed recent data from L1 and L2-3 stages feeding on tolerant and resistant rootstock. Data from two agricultural pest aphids, the generalist Myzus persicae and the Fabaceae specialist Acyrthosiphon pisum, and from the true bug vector of Chagas disease, Rhodnius prolixus, were used to perform phylogenetic analyses for each detoxification gene family. We found expansions of several gene sub-families in the genome of D. vitifoliae. Phylogenetically close genes were found to be organized in clusters in the same genomic position and orientation suggesting recent successive duplications. These results highlight the roles of the phylloxera detoxification gene repertoire in insect physiology and in adaptation to plant secondary metabolites, and provide gene candidates for further functional analyses.

Combined influences of transgenerational effects, temperature and insecticide on the moth Spodoptera littoralis

Climate warming is expected to impact the response of species to insecticides. Recent studies show that this interaction between insecticides and temperature can depend on other factors. Here, we tested for the influence of transgenerational effects on the Insecticide × Temperature interaction in the crop pest moth Spodoptera littoralis. Specifically, we analysed reaction norms among experimental clutches based on a split-plot design crossing the factors temperature, insecticide and clutch. The study was performed on 2280 larvae reared at four temperatures (23, 25, 27 and 29 °C), and their response to the insecticide deltamethrin (three concentrations and a control group) was tested. Temperature had a global influence with effects on larval survival, duration of development, pupal body mass, and significant reaction norms of the clutches for temperature variations of only 2 °C. In addition to the expected effect of deltamethrin on mortality, the insecticide slightly delayed the development of S. littoralis, and the effects on mortality and development differed among the clutches. Projection models integrating all the observed responses illustrated the additive effects of deltamethrin and temperature on the population multiplication rate. Variation in the response of the clutches showed that transgenerational effects influenced the impact of insecticide and temperature. Although no evidence indicated that the Insecticide × Temperature interaction depended on transgenerational effects, the studies on the dependence of the Insecticide × Temperature interaction on other factors continue to be crucial to confidently predict the combined effects of insecticides and climate warming.

Chromosomal scale assembly of parasitic wasp genome reveals symbiotic virus colonization

Endogenous viruses form an important proportion of eukaryote genomes and a source of novel functions. How large DNA viruses integrated into a genome evolve when they confer a benefit to their host, however, remains unknown. Bracoviruses are essential for the parasitism success of parasitoid wasps, into whose genomes they integrated ~103 million years ago. Here we show, from the assembly of a parasitoid wasp genome at a chromosomal scale, that bracovirus genes colonized all ten chromosomes of Cotesia congregata. Most form clusters of genes involved in particle production or parasitism success. Genomic comparison with another wasp, Microplitis demolitor, revealed that these clusters were already established ~53 mya and thus belong to remarkably stable genomic structures, the architectures of which are evolutionary constrained. Transcriptomic analyses highlight temporal synchronization of viral gene expression without resulting in immune gene induction, suggesting that no conflicts remain between ancient symbiotic partners when benefits to them converge.

Effects of low concentrations of deltamethrin are dependent on developmental stages and sexes in the pest moth Spodoptera littoralis

Effects of low concentrations of pesticides, with no or moderate mortality of targeted species, are poorly studied even though these low concentrations are common under natural conditions. Studying their effects is critical because they can induce positive hormetic responses, possibly leading to greater pest multiplication and promoting the evolution of pest resistance. Here, we investigated the responses of the pest moth Spodoptera littoralis to low concentrations of deltamethrin, and tested for variation in effects of the pesticide between developmental stages and sexes. Indeed, we show that a given concentration of deltamethrin has different effects between stages, and even between sexes. Two experimental concentrations led to very high mortality early in S. littoralis development (4th larval instar), but only to low mortality rates in adults. Moreover, our highest experimental concentration had only detrimental effects in adult females, but improved the reproductive success of adult males. Model projections showed that the lethality from treatments at the 4th larval instar was the predominant effect. Because of the high multiplication rate of S. littoralis, it was also found that treatments with very similar effects on larval mortality can lead to either population extinction or rapid pest resurgence.

A maternal effect influences sensitivity to chlorpyrifos pesticide in the pest moth Spodoptera littoralis

Transgenerational effects on sensitivity to pesticides are poorly studied. This study investigated the transgenerational influences of maternal body mass in the major pest moth Spodoptera littoralis, with a focus on sensitivity to chlorpyrifos pesticide. In 147 clutches of a laboratory strain of S. littoralis, we compared larval mortality between control larvae and larvae treated with chlorpyrifos. Because of the classic positive relationships between offspring size and maternal size and between offspring size and offspring quality, sensitivity to chlorpyrifos was predicted to be lower in larvae of larger mothers. Surprisingly, we found the opposite result, with higher pesticide toxicity in larvae of larger mothers. This result is partly explained by the lack of a relationship between larval mass and larval sensitivity to chlorpyrifos. This means that another offspring characteristic linked to maternal size should have affected larval sensitivity to chlorpyrifos. More generally, knowledge of the effects of the traits and ecological environments of mothers on offspring sensitivity to pesticides remains limited. Ecotoxicologists should pay more attention to such maternal effects on sensitivity to pesticides, both in pests and non-target species.

The genome sequence of the grape phylloxera provides insights into the evolution, adaptation, and invasion routes of an iconic pest

BACKGROUND

Although native to North America, the invasion of the aphid-like grape phylloxera Daktulosphaira vitifoliae across the globe altered the course of grape cultivation. For the past 150 years, viticulture relied on grafting-resistant North American Vitis species as rootstocks, thereby limiting genetic stocks tolerant to other stressors such as pathogens and climate change. Limited understanding of the insect genetics resulted in successive outbreaks across the globe when rootstocks failed. Here we report the 294-Mb genome of D. vitifoliae as a basic tool to understand host plant manipulation, nutritional endosymbiosis, and enhance global viticulture.

RESULTS

Using a combination of genome, RNA, and population resequencing, we found grape phylloxera showed high duplication rates since its common ancestor with aphids, but similarity in most metabolic genes, despite lacking obligate nutritional symbioses and feeding from parenchyma. Similarly, no enrichment occurred in development genes in relation to viviparity. However, phylloxera evolved > 2700 unique genes that resemble putative effectors and are active during feeding. Population sequencing revealed the global invasion began from the upper Mississippi River in North America, spread to Europe and from there to the rest of the world.

CONCLUSIONS

The grape phylloxera genome reveals genetic architecture relative to the evolution of nutritional endosymbiosis, viviparity, and herbivory. The extraordinary expansion in effector genes also suggests novel adaptations to plant feeding and how insects induce complex plant phenotypes, for instance galls. Finally, our understanding of the origin of this invasive species and its genome provide genetics resources to alleviate rootstock bottlenecks restricting the advancement of viticulture.

The Drosophila odorant-binding protein 28a is involved in the detection of the floral odour ß-ionone

Odorant-binding proteins (OBPs) are small soluble proteins that are thought to transport hydrophobic odorants across the aqueous sensillar lymph to olfactory receptors. A recent study revealed that OBP28a, one of the most abundant Drosophila OBPs, is not required for odorant transport, but acts in buffering rapid odour variation in the odorant environment. To further unravel and decipher its functional role, we expressed recombinant OBP28a and characterized its binding specificity. Using a fluorescent binding assay, we found that OBP28a binds a restricted number of floral-like chemicals, including ß-ionone, with an affinity in the micromolar range. We solved the X-ray crystal structure of OBP28a, which showed extensive conformation changes upon ligand binding. Mutant flies genetically deleted for the OBP28a gene showed altered responses to ß-ionone at a given concentration range, supporting its essential role in the detection of specific compounds present in the natural environment of the fly.

Glutathione-S-Transferases in the Olfactory Organ of the Noctuid Moth Spodoptera littoralis, Diversity and Conservation of Chemosensory Clades

Glutathione-S-transferases (GSTs) are conjugating enzymes involved in the detoxification of a wide range of xenobiotic compounds. The expression of GSTs as well as their activities have been also highlighted in the olfactory organs of several species, including insects, where they could play a role in the signal termination and in odorant clearance. Using a transcriptomic approach, we identified 33 putative GSTs expressed in the antennae of the cotton leafworm Spodoptera littoralis. We established their expression patterns and revealed four olfactory-enriched genes in adults. In order to investigate the evolution of antennal GST repertoires in moths, we re-annotated antennal transcripts corresponding to GSTs in two moth and one coleopteran species. We performed a large phylogenetic analysis that revealed an unsuspected structural—and potentially functional—diversity of GSTs within the olfactory organ of insects. This led us to identify a conserved clade containing most of the already identified antennal-specific and antennal-enriched GSTs from moths. In addition, for all the sequences from this clade, we were able to identify a signal peptide, which is an unusual structural feature for GSTs. Taken together, these data highlight the diversity and evolution of GSTs in the olfactory organ of a pest species and more generally in the olfactory system of moths, and also the conservation of putative extracellular members across multiple insect orders.

Expression and modulation of neuroligin and neurexin in the olfactory organ of the cotton leaf worm Spodoptera littoralis

Carboxylesterases are enzymes widely distributed within living organisms. In insects, they have been mainly involved in dietary metabolism and detoxification function. Interestingly, several members of this family called carboxylesterase-like adhesion molecules (CLAMs) have lost their catalytic properties and are mainly involved in neuro/developmental functions. CLAMs include gliotactins, neurotactins, glutactins, and neuroligins. The latter have for binding partner the neurexin. In insects, the function of these proteins has been mainly studied in Drosophila central nervous system or neuromuscular junction. Some studies suggested a role of neuroligins and neurexin in sensory processing but CLAM expression within sensory systems has not been investigated. Here, we reported the identification of 5 putative CLAMs expressed in the olfactory system of the model pest insect Spodoptera littoralis. One neuroligin, Slnlg4-yll and its putative binding partner neurexin SlnrxI were the most expressed in the antennae and were surprisingly associated with olfactory sensilla. In addition, both transcripts were upregulated in male antennae after mating, known to modulate the sensitivity of the peripheral olfactory system in S. littoralis, suggesting that these molecules could be involved in sensory plasticity.

An antennal carboxylesterase from Drosophila melanogaster, esterase 6, is a candidate odorant-degrading enzyme toward food odorants

Reception of odorant molecules within insect olfactory organs involves several sequential steps, including their transport through the sensillar lymph, interaction with the respective sensory receptors, and subsequent inactivation. Odorant-degrading enzymes (ODEs) putatively play a role in signal dynamics by rapid degradation of odorants in the vicinity of the receptors, but this hypothesis is mainly supported by in vitro results. We have recently shown that an extracellular carboxylesterase, esterase-6 (EST-6), is involved in the physiological and behavioral dynamics of the response of Drosophila melanogaster to its volatile pheromone ester, cis-vaccenyl acetate. However, as the expression pattern of the Est-6 gene in the antennae is not restricted to the pheromone responding sensilla, we tested here if EST-6 could play a broader function in the antennae. We found that recombinant EST-6 is able to efficiently hydrolyse several volatile esters that would be emitted by its natural food in vitro. Electrophysiological comparisons of mutant Est-6 null flies and a control strain (on the same genetic background) showed that the dynamics of the antennal response to these compounds is influenced by EST-6, with the antennae of the null mutants showing prolonged activity in response to them. Antennal responses to the strongest odorant, pentyl acetate, were then studied in more detail, showing that the repolarization dynamics were modified even at low doses but without modification of the detection threshold. Behavioral choice experiments with pentyl acetate also showed differences between genotypes; attraction to this compound was observed at a lower dose among the null than control flies. As EST-6 is able to degrade various bioactive odorants emitted by food and plays a role in the response to these compounds, we hypothesize a role as an ODE for this enzyme toward food volatiles.

Molecular cloning and cellular expression of crustacean PC2-like prohormone convertase

PC2 prohormone convertases are enzymes involved in the proteolytic maturation of neuropeptide precursors. In the present work, a cDNA encoding a PC2-like enzyme (OrlPC2) was cloned from crayfish eyestalk ganglia (medulla terminalis) containing the X-organ, a major neuroendocrine center. The predicted 634 amino acid preproprotein exhibits highest sequence identity, especially in the catalytic domain, with PC2s from arthropods and nematodes, and less with mollusc and vertebrate enzymes. It was demonstrated by in situ hybridization on crayfish medulla terminalis sections that OrlPC2 is expressed in a large number of neuron perikarya, including those producing the well known crustacean hyperglycemic hormone.

Molting cycle-dependent expression of CYP4C15, a cytochrome P450 enzyme putatively involved in ecdysteroidogenesis in the crayfish, Orconectes limosus

A cytochrome P450 enzyme cDNA (CYP4C15) has been previously cloned from a cDNA library of crayfish steroidogenic glands (Y-organs). The conceptual translation of the CYP4C15 cDNA sequence was analyzed for regions of putative high antigenicity and a mixture of two synthetic peptides was chosen for the production of a specific polyclonal antibody. Western blot analysis on Y-organ subcellular fractions indicated an endoplasmic reticulum location of CYP4C15, in agreement with the structural feature of the predicted protein, i.e. the presence of a hydrophobic N-terminal segment. The protein is only expressed in Y-organs, thus showing a similar distribution to the corresponding mRNA. From this tissue specific expression, it has been postulated that CYP4C15 would play a role in ecdysteroid biosynthesis rather than detoxification and the variations of its expression during a molt cycle were carefully examined. CYP4C15 is not detectable in intermolt animals, expression levels are maximal during early premolt and decrease during late premolt. The results are discussed in relation to the variations of hemolymphatic ecdysteroid titers and steroidogenic capacities of the Y-organs during the molt cycle.

Cloning of a novel cytochrome P450 (CYP4C15) differentially expressed in the steroidogenic glands of an arthropod

Biosynthesis of ecdysteroids, arthropod steroid molting hormones, proceeds from dietary cholesterol through a complex and still incompletely elucidated pathway. Most of the known steps are catalyzed by cytochrome P450 enzymes (CYPs) but none of their genes has yet been identified. We have established a cDNA library of crayfish steroidogenic glands (Y organs). A full length CYP-cDNA was characterized containing a 1539 bp open reading frame encoding a predicted protein of 513 amino acid residues. This novel CYP was assigned to the CYP4 family and designated CYP4C15. Northern blots demonstrated predominant expression of this gene in the active molting glands, suggesting a role in ecdysteroid biosynthesis rather than detoxification.