1. Introduction
Aphids, small sap-sucking insects of the order Hemiptera, are notorious agricultural pests known for their ability to cause significant damage to crops through feeding activities and virus transmission [
1,
2]. As pests of economic importance, aphids pose formidable challenges to farmers worldwide, necessitating effective management strategies to mitigate their impact on crop health and yield. Acetamiprid is a neonicotinoid insecticide widely used in aphid management due to its systemic activity and broad spectrum of activity [
3]. However, concerns have been raised regarding the overreliance on acetamiprid and its potential for inducing resistance in target pests [
4].
At the center of the discussion on aphid control is the phenomenon of insecticide resistance, a widespread problem that has become a major obstacle in pest control [
5,
6]. Insecticide resistance arises from a complex interplay of genetic, physiological, and environmental factors, including the rapid evolution of resistance mechanisms within aphid populations. Resistance mechanisms include insensitivity of the target site, metabolic detoxification, and behavioral adaptations, which together reduce the efficacy of chemical insecticides [
7,
8]. Recent studies have documented resistance to acetamiprid in several aphid species [
9,
10,
11,
12,
13]. For example, in one study, acetamiprid resistance in the melon aphid
Aphis gossypii (Glover, 1877) was linked to overexpression of several P450 genes and suppressing their expression through RNA interference (RNAi) dramatically increased sensitivity to acetamiprid [
13]. However, in a laboratory strain of
Myzus persicae (Sulzer, 1776), resistance to acetamiprid was not accompanied by increased activity of P450 enzymes or any other known resistance mechanism [
14], suggesting that different species may develop resistance to the same insecticide through divergent mechanisms.
In recent years, research has increasingly focused on the role of symbiotic bacteria in shaping aphid physiology and mediating responses to insecticides [
15,
16]. Symbiotic bacteria play a pivotal role in the growth and development of aphids [
17]. The ubiquitous presence of the primary symbiont
Buchnera aphidicola in aphids is ensured through vertical transmission via ovaries, maintaining its continuity from one generation to the next.
Buchnera supplies essential amino acids and vitamin B, crucial nutrients often limited in plant phloem [
18,
19,
20]. Another prevalent symbiont,
Wolbachia, affects host reproduction to promote its proliferation and transmission [
21,
22,
23,
24]. Additionally, aphids can harbor secondary or facultative symbionts [
25,
26], transmitted both vertically and horizontally within and between species.
Rickettsiella, a facultative endosymbiont, induces a change in aphid body color from red to green [
27], potentially providing protection from natural enemies.
Arsenophonus nasoniae influences the sex ratio of parasitic wasp offspring by selectively killing male embryos [
28,
29].
Hamiltonella strains, efficiently transmitted to aphid offspring, confer robust protection against dominant parasitoid species [
30]. Li et al. observed that infection with
H. defensa decreased the sensitivity of aphids to insecticides, particularly at low concentrations [
31].
The pea aphid
Acyrthosiphon pisum (Harris, 1776) is known worldwide as an important pest of legumes and pulses [
32,
33], where it causes significant economic losses. Despite the economic impact, there are currently no effective non-chemical methods to control this pest, so farmers rely heavily on chemical insecticides for control. Although resistance of the pea aphid to insecticides has not been extensively documented, a recent study found a high level of phenotypic resistance to several pyrethroids in field-collected strains compared to a susceptible reference strain [
34]. Biochemical and molecular analyzes indicated the involvement of P450 and esterases in resistance: Increased P450 and esterase activities were observed in the resistant strains, and transcriptome profiling identified the P450 gene
CYP6CY12 as highly overexpressed. These results highlight the occurrence of pyrethroid resistance in the pea aphid and the importance of understanding the molecular mechanisms underlying the development of resistance to other insecticides commonly used to control this species, including neonicotinoids.
In this study, we analyzed the transcriptome and metatranscriptome of pea aphids selected with acetamiprid for nine consecutive generations. Our aim was to investigate the constitutive changes in gene expression and microbiome of the acetamiprid-selected strain compared to the non-selected strain to identify possible target mechanisms involved in adaptation. Quantitative real-time PCR (qRT-PCR) was used to further validate the differentially expressed genes. Our results suggest a complex, multifaceted metabolic response to acetamiprid in the RS strain of pea aphid, with increased expression of enzymes involved in the synthesis of the epidermal wax layer components playing a central role in survival mechanisms. However, our analysis of the microbiome of RS suggests the involvement of two secondary symbionts in the adaptation process.
4. Discussion
In the present study, we have obtained a strain RS of pea aphid that exhibits reduced susceptibility to acetamiprid and characterized it by transcriptomic and metatranscriptomic approaches. Most intriguingly, the RS strain, despite being exposed to acetamiprid for nine consecutive generations, exhibited only a twofold increase in LC
50 compared to the unselected SS strain. In fact, documented cases of insecticide resistance in pea aphids are rare. Only one case has been reported so far, against pyrethroids [
34,
50]. This rarity suggests that resistance to most insecticides is low in this species. Furthermore, we found that key players in cellular metabolism and stress response, such as TPI, PHI and P5CS, were downregulated in the RS strain. This suggests that the insect has adopted strategies, such as metabolic reprogramming, that prioritize survival over growth to mitigate the negative effects of acetamiprid. While this adaptive response may improve short-term survival, it may impose a long-term fitness cost on the insect as resources are diverted from other essential biological functions, such as growth. Indeed, we observed differences in the growth cycle between the RS and SS strains: the RS strain required an additional 24 hours to reach the adult stage (data not shown), indicating a possible fitness cost in the absence of acetamiprid.
Both transcriptomic sequencing and qRT-PCR experiments have revealed the upregulation of ACSF2 and FAR, two important enzymes involved in lipid metabolism, in the RS strain. ACSF2 facilitates the conversion of fatty acids into fatty acyl-CoA, a crucial step in fatty acid metabolism. Meanwhile, FAR plays a key role in the conversion of fatty acyl-CoA into fatty alcohols, which are essential for the synthesis of cuticular hydrocarbons (CHCs). CHCs form the protective wax layer found on the surface of many aphids and mealybugs, shielding them from natural enemies and adverse environmental conditions [
51,
52]. The pea aphid is known to have a wax layer covering all parts of its body [
53]. The constitutive upregulation of ACSF2 and FAR in RS due to prolonged exposure to acetamiprid may have increased the production of CHCs and, hence, the wax layer, ultimately reducing acetamiprid penetration. This explanation needs further investigation.
RNAi-mediated suppression of FAR expression in the cotton mealybug,
Phenacoccus solenopsis Tinsley, resulted in reduced CHC levels in the wax layer [
54]. Mealybugs with reduced CHC content exhibited increased mortality when exposed to desiccation and deltamethrin treatments, highlighting the importance of this enzyme for insect adaptation to water loss and insecticide stress. Therefore, we hypothesize that the potential mechanism responsible for reduced susceptibility to acetamiprid in the RS strain involves the thickening of the physical barrier through the wax layer that prevents the penetration of acetamiprid molecules into the aphid's body. This process may involve alterations in wax composition and layer deposition. However, while an increased wax layer may confer resistance to insecticides, it may also impose fitness costs on aphids, as the production of a thicker or more abundant wax layer requires resources and energy that could otherwise be allocated to essential biological processes such as growth.
Interestingly, genes encoding ribosomal proteins showed significant upregulation in RS transcriptome over SS transcriptome. While ribosomal proteins are primarily known for their involvement in protein synthesis within ribosomes, they have been found to have additional functions in various cellular processes. These proteins may interact with other molecules, such as proteins or nucleic acids, to exert these non-canonical functions [
55]. Previously, Yu et al [
56] linked the ribosomal protein S29 to deltamethrin resistance by binding to CYP6N3 -a member of the CYP6 class of cytochrome P450 enzymes involved in metabolic resistance to pyrethroids- and stimulating its degradation by the 26S proteasome. Overexpression of RPS29 reduced cell viability in the presence of deltamethrin. In addition, another study identified the ribosomal protein RpS2 as a potential receptor for the insecticidal protein Vip3Aa from Bacillus thuringiensis [
57]. RNAi-mediated silencing of RpS2 gene expression in both transfected Sf21 cells and in larvae of
Spodoptera litura (Fabricius, 1775) injected with double-stranded RNA resulted in reduced toxicity of the Vip3A protein. Further evidence for the specific upregulation of ribosomal proteins in response to a selective agent was provided by HT-SuperSAGE analysis of a Vip3Aa-selected population of
Heliothis virescens (Fabricius, 1777), which confirmed this phenomenon [
58]. Whether the constitutive overexpression of ribosomal proteins in acetamiprid-selected pea aphids responds to an increased demand for protein synthesis in response to stress or is involved in a specific adaptive mechanism remains to be elucidated.
In our study, we detected two secondary symbionts of pea aphids, namely
H. huttiense and
D. acidovorans, exclusively in RS, while
L. iners was more prevalent in RS compared to SS. The precise impact of these secondary symbionts on the adaptive response of RS to acetamiprid remains uncertain. Some research suggests that symbiotic bacteria actively participate in detoxifying insecticides. For instance,
Serratia oryzae has been implicated in insecticide resistance in
Aedes albopictus (Skuse, 1894), contributing to resistance development by upregulating the expression and activity of metabolic detoxification enzymes in mosquitoes [
59]. In
A. gossypii, the composition of symbiotic bacteria undergoes significant changes after insecticide treatment [
60]. Notably, antibiotic treatment has also been found to increase the sensitivity of
A. gossypii to spirochetes [
61]. However, the interaction between aphids and symbionts is intricate. While aphids may gain benefits from hosting symbionts, negative effects could lead to the reallocation of aphid energy resources [
62]. For instance, infection of the corn leaf aphid,
Rhopalosiphum maidis (Fitch, 1856), with
H. defensa and
Regiella insecticola could have a partially negative effect on aphid growth and development, despite endosymbionts were maintained in aphids over time [
63]. Previously, these two symbionts were shown to protect aphids, with
R. insecticola shielding
A. pisum from the aphid-specific fungal entomopathogen
Zoophthora occidentalis [
64], and
H. defensa reducing aphid susceptibility to insecticides [
31].
In summary, we have shown that acetamiprid-selected pea aphid RS strain exhibits changes in gene expression, the most interesting of which was the constitutive overexpression of genes related to the synthesis of the components of the epidermal wax layer. Overall, the development of an increased waxy layer represents a mechanism by which the pea aphid can mitigate the effects of acetamiprid. This emphasizes the importance of understanding wax biosynthesis and its role in acetamiprid resistance for effective pest management strategies. We have also shown the differential infection of the acetamiprid-selected RS strain with two specific secondary symbionts. Understanding the trade-offs that aphids must make to survive acetamiprid exposure while harboring these secondary symbionts is crucial for developing sustainable pest management strategies that consider both immediate efficacy and long-term ecological impact.
Author Contributions
Conceptualization, D.L. and Y.W.; methodology, Z.C.; software, S.G.; validation, Z.C., X.Z. and Y.Q.; formal analysis, Z.C.; investigation, Z.C.; resources, Y.K.; data curation, Z.C. and C.A.-P; writing—original draft preparation, Z.C.; writing—review and editing, C.A. -P and D.L.; visualization, Z.C. and K.Z.; supervision, D.L., Y.W and C.A. -P; project administration, D.L., Y.W. and C.A. -P; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.