Apolipoprotein D in Lepidoptera: Evolution and functional divergence

Immune system modulation & virus transmission during parasitism identified by multi-species transcriptomics of a declining insect biocontrol system

BACKGROUND

The Argentine stem weevil (ASW, Listronotus bonariensis) is a significant pasture pest in Aotearoa New Zealand, primarily controlled by the parasitoid biocontrol agent Microctonus hyperodae. Despite providing effective control of ASW soon after release, M. hyperodae parasitism rates have since declined significantly, with ASW hypothesised to have evolved resistance to its biocontrol agent. While the parasitism arsenal of M. hyperodae has previously been investigated, revealing many venom components and an exogenous novel DNA virus Microctonus hyperodae filamentous virus (MhFV), the effects of said arsenal on gene expression in ASW during parasitism have not been examined. In this study, we performed a multi-species transcriptomic analysis to investigate the biology of ASW parasitism by M. hyperodae, as well as the decline in efficacy of this biocontrol system.

RESULTS

The transcriptomic response of ASW to parasitism by M. hyperodae involves modulation of the weevil's innate immune system, flight muscle components, and lipid and glucose metabolism. The multispecies approach also revealed continued expression of venom components in parasitised ASW, as well as the transmission of MhFV to weevils during parasitism and some interrupted parasitism attempts. Transcriptomics did not detect a clear indication of parasitoid avoidance or other mechanisms to explain biocontrol decline.

CONCLUSIONS

This study has expanded our understanding of interactions between M. hyperodae and ASW in a biocontrol system of critical importance to Aotearoa-New Zealand's agricultural economy. Transmission of MhFV to ASW during successful and interrupted parasitism attempts may link to a premature mortality phenomenon in ASW, hypothesised to be a result of a toxin-antitoxin system. Further research into MhFV and its potential role in ASW premature mortality is required to explore whether manipulation of this viral infection has the potential to increase biocontrol efficacy in future.

The evolution and functional divergence of 10 Apolipoprotein D-like genes in Nilaparvata lugens

Apolipoprotein D (ApoD), a member of the lipocalin superfamily of proteins, is involved in lipid transport and stress resistance. Whereas only a single copy of the ApoD gene is found in humans and some other vertebrates, there are typically several ApoD-like genes in insects. To date, there have been relatively few studies that have examined the evolution and functional differentiation of ApoD-like genes in insects, particularly hemi-metabolous insects. In this study, we identified 10 ApoD-like genes (NlApoD1-10) with distinct spatiotemporal expression patterns in Nilaparvata lugens (BPH), which is an important pest of rice. NlApoD1-10 were found to be distributed on 3 chromosomes in a tandem array of NlApoD1/2, NlApoD3-5, and NlApoD7/8, and show sequence and gene structural divergence in the coding regions, indicating that multiple gene duplication events occurred during evolution. Phylogenetic analysis revealed that NlApoD1-10 can be clustered into 5 clades, with NlApoD3-5 and NlApoD7/8 potentially evolving exclusively in the Delphacidae family. Functional screening using an RNA interference approach revealed that only NlApoD2 was essential for BPH development and survival, whereas NlApoD4/5 are highly expressed in testes, and might play roles in reproduction. Moreover, stress response analysis revealed that NlApoD3-5/9, NlApoD3-5, and NlApoD9 were up-regulated after treatment with lipopolysaccharide, H2 O2 , and ultraviolet-C, respectively, indicating their potential roles in stress resistance.

Comparative Transcriptomic Analysis Reveals Adaptation Mechanisms of Bean Bug Riptortus pedestris to Different Food Resources

The bean bug, Riptortus pedestris (Hemiptera: Heteroptera), poses a significant threat to soybean production, resulting in substantial crop losses. Throughout the soybean cultivation period, these insects probe and suck on various parts of plants, including leaves, pods, and beans. However, the specific mechanisms by which they adapt to different food resources remain unknown. In this study, we conducted gut transcriptomic analyses of R. pedestris fed with soybean leaves, pods, and beans. A total of 798, 690, and 548 differently expressed genes (DEGs) were monitored in G-pod vs. G-leaf (comparison of insect feeding on pods and leaves), G-bean vs. G-leaf (comparison of insect feeding on beans and leaves), and G-pod vs. G-bean (comparison of insect feeding on pods and beans), respectively. When fed on pods and beans, there was a significant increase in the expression of digestive enzymes, particularly cathepsins, serine proteases, and lipases. Conversely, when soybean leaves were consumed, detoxification enzymes, such as ABC transporters and 4-coumarate-CoA ligase, exhibited higher expression. Our findings indicate that R. pedestris dynamically regulates different metabolic pathways to cope with varying food resources, which may contribute to the development of effective strategies for managing this pest.

Genome-wide identification and comparative analysis of lipocalin families in Lepidoptera with an emphasis on Bombyx mori

Lipocalins exhibit functional diversity, including roles in retinol transport, invertebrate cryptic coloration, and stress response. However, genome-wide identification and characterization of lipocalin in the insect lineage have not been thoroughly explored. Here, we found that a lineage-specific expansion of the lipocalin genes in Lepidoptera occurred in large part due to tandem duplication events and several lipocalin genes involving insect coloration were expanded more via tandem duplication in butterflies. A comparative analysis of conserved motifs showed both conservation and divergence of lepidopteran lipocalin family protein structures during evolution. We observe dynamic changes in tissue expression preference of paralogs in Bombyx mori, suggesting differential contribution of paralogs to specific organ functions during evolution. Subcellular localization experiments revealed that lipocalins localize to the cytoplasm, nuclear membrane, or nucleus in BmN cells. Moreover, several lipocalin genes exhibited divergent responses to abiotic and biotic stresses, and 1 lipocalin gene was upregulated by 300 fold in B. mori. These results suggest that lipocalins act as signaling components in defense responses by mediating crosstalk between abiotic and biotic stress responses. This study deepens our understanding of the comprehensive characteristics of lipocalins in insects.

Proteome Analysis of Male Accessory Gland Secretions in the Diamondback Moth, Plutella xylostella (Lepidoptera: Plutellidae)

In insects, male accessory gland proteins (ACPs) are important reproductive proteins secreted by male accessory glands (MAGs) of the internal male reproductive system. During mating, ACPs are transferred along with sperms inside female bodies and have a significant impact on the post-mating physiology changes of the females. Under sexual selection pressures, the ACPs exhibit remarkably rapid and divergent evolution and vary from species to species. The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a major insect pest of cruciferous vegetables worldwide. Mating has a profound impact on the females' behavior and physiology in this species. It is still unclear what the ACPs are in this species. In this study, two different proteomic methods were used to identify ACPs in P. xylostella. The proteins of MAGs were compared immediately before and after mating by using a tandem mass tags (TMT) quantitative proteomic analysis. The proteomes of copulatory bursas (CB) in mated females shortly after mating were also analyzed by the shotgun LC-MS/MS technique. In total, we identified 123 putative secreted ACPs. Comparing P. xylostella with other four insect ACPs, trypsins were the only ACPs detected in all insect species. We also identified some new insect ACPs, including proteins with chitin binding Peritrophin-A domain, PMP-22/ EMP/ MP20/ Claudin tight junction domain-containing protein, netrin-1, type II inositol 1,4,5-trisphosphate 5-phosphatase, two spaetzles, allatostatin-CC, and cuticular protein. This is the first time that ACPs have been identified and analyzed in P. xylostella. Our results have provided an important list of putative secreted ACPs, and have set the stage for further exploration of the functions of these putative proteins in P. xylostella reproduction.

Involvement of apolipoprotein D in desiccation tolerance and adult fecundity of Acyrthosiphon pisum

Apolipoprotein D (ApoD) is a lipocalin superfamily member that plays important roles in the transport of small hydrophobic molecules, lipid metabolism, and stress resistance. Cuticular hydrocarbons are the principal components of the epicuticular lipid layer and play a critical role in water retention against environmental desiccation stress; however, the mechanism underlying the role of ApoD in insect desiccation tolerance has not yet been elucidated. Here, we report the molecular constitution, functional analysis, and phylogenetic relationship of the ApoD gene in Acyrthosiphon pisum (ApApoD). We found that ApApoD was transcribed throughout the life cycle of A. pisum, but was prominently expressed in the embryonic period and abdominal cuticle. In addition, we optimized the dose and silencing duration of RNAi, observing that RNAi against ApApoD significantly reduced the levels of both internal and cuticular hydrocarbons and adult fecundity. Moreover, cuticular hydrocarbon deficiency increased the sensitivity of aphids to desiccation stress and reduced their survival time, while desiccation stress significantly increased ApApoD expression. Together, it is confirmed that ApApoD participates in regulating cuticular hydrocarbon content of aphids under desiccation stress and is crucial for aphid reproduction. Therefore, the ApApoD gene of A. pisum may be a potential target for RNAi-based insect pest control due to its involvement in cuticular hydrocarbon accumulation and reproduction.