Non-Coding RNAs in Lepidoptera

Abstract

In the last few years, the amount of genomic sequence data has grown exponentially. A large number of non-coding RNAs (ncRNAs) have been identified from bacteria to humans. ncRNAs are various and multi-faced; they can regulate gene expression through chromosomal, transcriptional, post-transcriptional, and translational levels and then participate in the whole process of development in different organisms. ncRNAs have been identified in the 1980s in Lepidoptera; they can play roles in growth, metamorphosis, metabolism, sex determination, reproduction, and immune response of insects. Now, the use of ncRNAs in pest control of Lepidoptera is also in process. This chapter will review the recent advance of ncRNAs in Lepidoptera and prospect the future studies of ncRNAs in insects.

1. Introduction

Non-coding RNAs (ncRNAs) are a large class of RNAs with no protein products, including transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA), small nucleolar RNA (snoRNA), microRNA (miRNA), PIWI-interacting RNA (piRNA), endogenous small interfering RNA (siRNA), circular RNA (circRNA), long non-coding RNA (lncRNA), protein functional effector small ncRNA (pfeRNA) and other ncRNAs with unknown functions [1, 2]. ncRNAs have been identified in bacteria and humans and are multi-faced. They can regulate gene expression through chromosomal, transcriptional, post-transcriptional, and translational levels and then participate in the whole process of development. A large number of ncRNAs has been identified since the 1980s in Lepidoptera. More and more ncRNAs have been found to play roles in the growth, metabolism, sex determination, reproduction, and immune response of insects. Now, the use of ncRNAs in pest control of Lepidoptera is also in process. This chapter will review the recent advance of ncRNAs in Lepidopteran, and the prospect for future studies of ncRNAs in insects.

2. A brief history of non-coding RNAs

In 1958, the “Central Dogma” was first proposed by Francis Crick and published in Nature in 1970, which described that the genetic information was generally transferred from DNA to RNA and then from RNA to protein, named transcription and translation, respectively [3]. For a long time, the main research was focused on messenger RNA (mRNA), which worked as the “regular army” in the RNA Legion. But in the RNA corps, there is another “non-regular army” that function as “Jack of all trades”, named non-coding RNAs (ncRNAs).

In the 1950s, the ribosome RNA (rRNA) and transport RNA (tRNA) were first discovered as ncRNAs, followed by small nucleus RNA (snRNA) (1980s), small nucleolar RNA (snoRNA) (1990s), microRNA (miRNA) (1990s), long non-coding RNA (lncRNA) (1990s), circular RNA (circRNA) (1990s), and piRNA (2000s). With the completion of the Human Genome Project (HGP) in 2003 [4], the project Encyclopedia of DNA Elements (ENCODE) was launched to investigate all the functional elements in the human genome [5]. From then on, a new world of RNA was emerging, a variety of ncRNAs were identified through bacterial, viral, plant, insect, mammalian, etc. At the same time, scientists were surprised to find that less than 2% of genome sequence was translated into proteins in humans, and the remaining 98% were considered “garbage” and “noise”. But now we know that at least 80% of the remaining “junk” sequences of the human genome are functional and 93% of the human genome has the ability to transcribe into ncRNAs [6, 7].

3. Differential class of ncRNAs in Lepidotera

3.1 Small nuclear RNA (snRNA)

Small nuclear RNA (snRNA) is a conserved class of ncRNAs in the nucleus, with a length of approximately 100–215 nucleotides in mammals. Generally, snRNAs contain seven categories due to their abundant U content, numbered U1–U7. U3 snRNA is in the nucleolus and the other six categories are present in the non-nucleolar region. Except for U6, which is transcribed by RNA polymerase III, the other snRNAs are transcribed by RNA polymerase II. snRNAs always combine with protein to form the small nuclear heterogeneous ribonucleoprotein particle particle (SnRNP), to play a conserved role in the alternative splicing of mRNAs [8, 9, 10, 11]. High-throughput sequencing studies indicate that alternative precursor messenger RNA (pre-mRNA) splicing occurs in >95 to 100% of human genes and in nearly 63% of mouse genes [12, 13]. Five snRNAs U1, U2, U4/U6, and U5 recruit the proteins, forming snRNP complexes to recognize introns to form a catalytically active spliceosome and remove the intron from a pre-mRNA containing two exons [14, 15]. Alternative splicing plays roles in the sex determination of fruit flies and in learning, memory, and neurotransmission of mammalian [16]. Mutations occur in splice sites, RNA regulatory silencers or enhancers, or genes that encode splicing factors resulting in disease. For example, the first example of human disease mutations affecting splicing were the β-globin thalassemia mutations and mutations in the SMN-2 gene, which give rise to spinal muscular atrophy [17, 18, 19, 20]. The splicing factor hnRNPA1 plays a role in the pre-mRNA splicing of SMN-2 and pyruvate kinase in cancer [21, 22, 23, 24].

The function of snRNAs is conserved and fundamental across different organisms. Recent research in silkworm Bombyx mori showed that depletion of U1 snRNP exhibited abnormal cellular phenotypes with enlarged cell nucleus, scant cytoplasm, and enlarged nuclei. Genes involved in metabolic pathways, biosynthesis of secondary metabolites, and steroid hormone biosynthesis were significantly affected, which led to either delayed or stationary first instar larval development [25].

3.2 Small nucleolar RNAs (snoRNAs)

Small nucleolar RNAs (snoRNAs) are small RNAs found in the nucleolus with lengths of 60–300 nucleotides. snoRNAs are responsible for posttranscriptional modification of rRNAs [26]. In previous research, snoRNAs were categorized as C/D box snoRNAs and H/ACA box snoRNAs, regulating the 2′-O-ribose methylation and pseudouridylation of rRNAs, respectively [26]. However, recent studies showed a large number of snoRNAs without specific targets or determined cell functions, named orphan snoRNAs [27]. Studies showed that besides rRNA modification, snoRNA had other novel functions: (1) alternative splicing, RNA editing, or 3′-end processing of pre-mRNA [28, 29, 30, 31, 32]. In humans, SNORD27 not only guides the 2′-O methylation of A27 in 18S rRNA but also regulates the alternative splicing of the transcription factor E2F7 pre-mRNA [31]. (2) Serving as the precursors of microRNAs [33] or end of long-non-coding RNAs [34, 35]. High-throughput sequencing has revealed that more than half of all snoRNAs were processed to generate smaller fragments [36], named snoRNA-derived RNAs (sdRNAs). sdRNAs can be divided into several categories based upon their origin and length, H/ACA box snoRNAs generated sdRNAs mainly derived from the 3′end with a length of 20–24 nt, the C/D box snoRNAs generated sdRNAs derived from the 5′end and produce two fragments, one greater than 26 nt and another 17–19 nt in length [37, 38], sdRNAs are considered as a novel source of miRNAs.

Dysregulation of snoRNA causes disease and cancer progression [39]. In humans, snoRNA h5sn2 is downregulated in meningiomas compared with normal brain tissue, and 5S snoRNA and SNORA18L5 were linked to the function of the p53 tumor suppressor [40, 41]. Some snoRNAs were ascribed to cancer-associated signaling pathways, such as PI3K/AKT, which plays a pivotal role in cell differentiation, proliferation, and survival [42]. As more and more cancer-related snoRNAs were identified, they were used as potential candidates for cancer biomarkers [43]. Such as in angioimmunoblastic T-cell lymphoma, the upregulation of HBI-239 is used as a prognostic marker [44]. In colorectal cancer, the overexpression of SNORA21 is related to poor patient survival, which can be used as a prognostic marker [45].

Furthermore, as the main regulator of RNAs in nucleoli, small nucleolar RNAs can also respond to cell stress. Michel et al. found that in mice, box C/D snoRNAs origin from the introns of the ribosomal protein L13a (Rpl13a) are key mediators of cell death under oxidative stress [46]. Cytoplasmic accumulation of box C/D snoRNAs can be induced by oxidative stress doxorubicin [47]. Li et al. found that a C/D box snoRNA Bm-15 can target the 2′-O methylation modification at C434 of 18S rRNA in silkworm B. mori [48]. Bm-15 is located in both the nucleus and cytoplasm of silkworm cells [49] and can translocate dynamically from the nucleolus to the cytoplasm under the abiotic stress of nutritional deficiency or UV radiation, which might slow down the maturation of rRNAs and synthesis of ribosomes to enhance stress resistance of cells [50].

snRNAs and snoRNAs are known to be functionally and evolutionarily conserved elements of transcript processing machinery. But recently, with abundance measuring of snRNAs and snoRNAs in the frontal cortex of humans, chimpanzees, rhesus monkeys, and mice, 44% of the 185 measured snoRNA and 40% of the 134 snoRNA families showed significant expression divergence among different species. A 10-fold elevated expression of U1 snRNA and a 1000-fold drop in expression of SNORA29 were shown in humans compared to the other species, indicating that snoRNA abundance changes specific to the human lineage and suggests a possible mechanism underlying these changes [51].

Identification of snoRNAs was accelerated by high-throughput sequencing in Lepidoptera. With library construction and sequencing, 141 snoRNAs were identified in silkworm B. mori, and many snoRNAs were orphan snoRNAs [48]. Knocking down the homolog of C/D box snoRNA Bm-15 in Spodoptera frugiperda can induce the apoptosis of Sf9 cells [52]. Bm-152 might participate in the development of silkworms through the ecdysone and juvenile hormone signaling pathways [53]. Two silk gland-enriched ncRNAs Bm-102 and Bm-159 can be found in the histone modification complex and might play roles through epigenetic modifications in silkworms [54].

3.3 MicroRNA

In 1993, Victor Ambros and his colleagues, Rosalind Lee and Rhonda Feinbaum, found that lin-4 does not code for a protein to control the timing of C. elegans larval development, but produces small RNA lin-4 [55], which had antisense complementarity to multiple sites in the 3′ UTR of the lin-14 gene [55, 56], which mediate the repression of lin-14 by the lin-4 gene product [57]. With more and more discoveries of miRNAs, the functional roles of them were revealed. miRNA plays regulatory roles by targeting mRNAs for cleavage or translational repression. More than 50% of protein-coding genes in animals are regulated by miRNAs [58].

The earliest report of insect miRNAs was in 2001, Lagos Quintana et al. first found 22 miRNAs in the Drosophila melanogaster [59]. From then on, a large number of miRNAs were identified from bacteria, viruses, and plants to Metazoa. Now, 3857 mature miRNAs were identified through 31 species of Hexapoda according to the miRBase database (Table 1), includes 123 mature miRNAs in Acyrthosiphon pisum, 155 in Aedes aegypti, 130 in Anopheles gambiae, 254 in Apis mellifera, 80 in Bactrocera dorsalis, 2 in Biston betularia, 487 in Bombyx mori, 74 in Culex quinquefasciatus, 102 in Dinoponera quadriceps, 1470 in Drosophila, 92 in Heliconius melpomene, 7 in Locusta migratoria, 98 in Manduca sexta, 32 in Nasonia giraulti, 28 in Nasonia longicornis, 53 in Nasonia vitripennis, 133 in Plutella xylostella, 73 in Polistes Canadensis, 122 in Spodoptera frugiperda, and 342 in Tribolium castaneum [60, 61, 62, 63, 64, 65].

Species	Number of rniRNA	Species	Number of miRNA
Acyrthosiphon pisutu	123	Drosophila sechellia	103
Aedes aegypti	155	Drosophila simulans	148
Anopheles gambiae	130	Drosophila virilis	180
Apis mellifera	254	Drosophila willistoni	77
Bactrocera dorsalis	80	Drosophila yakuba	89
Biston betularia	2	Heliconius melpomene	92
Bombyx mori	487	Locusta migratoria	7
Culex quinquefasciatus	74	Manduca sexta	98
Dinoponera quadriceps	102	Nasonia giraulti	28
Drosophila ananassae	76	Nasonia longicornis	53
Drosophila erecta	101	Nasonia vitripennis	32
Drosophila grimshawi	82	Plutella xylostella	133
Drosophila melanogaster	258	Polistes Canadensis	73
Drosophila mojavensis	71	Spodoptera frugiperda	122
Drosophila persimilis	75	Tribolium castaneum	342
Drosophila pseudoobscura	210	Total	3857

Table 1.

Mature miRNAs in insect according to miRbase 22.1.

In Lepidoptera, research on miRNAs focused more on B. mori, M. sexta, P. xylostella and S. frugiperda. miRNAs contribute to the modulation of a wide range of biological processes of insects, such as development [66, 67], metamorphosis [68], metabolism and longevity [69], reproduction [70], sexual dimorphism [71], cast determination [72], memory formation [73], behavior [74], insecticide resistance [75], endosymbiosis [76], and host-pathogen interactions and immunity [77, 78].

In silkworm B. mori, microRNA-14 regulates larval development time; the hormone receptor E75 (E75) and the ecdysone receptor isoform B (ECR-B) were the putative target genes of miR-14 [79]. let-7 is required for the developmental transition of B. mori through coordinating the biosynthesis of ecdysone and JH. The let-7 knockout larvae were developmentally arrested in the prepupal stage and became pupal-adult intermediates after apolysis [80]. Genetic disruption of miR-2738 increased the levels of BmPSI and BmMasc transcripts and might be a minor regulator of sex-determination genes in the silkworm [81]. bmo-miR-2739 and the miR-167 coordinately regulate the expression of the vitellogenin receptor in B. mori oogenesis, disruption of bmo-miR-2739 or miR-167 resulted in increased amounts of BmVgR protein in the ovaries and high level of mRNA expression of BmVgR in the fat body [82]. In H. armigera, microRNA-277 regulates dopa decarboxylase to control larval-pupal and pupal-adult metamorphosis [83].

Recently, microRNAs were found to play key roles in the host-pathogen interaction of insects. Cellular miRNAs could affect virus infection by regulating the expression of virus genes. A cellular lncRNA Lnc_209997 inhibits the proliferation of Bombyx mori nuclear polyhydrosis virus (BmNPV), but miR-275-5p can facilitate the replication of BmNPV, the expression of miR-275-5p can be induced by BmNPV through inhibiting the expression of Lnc_209997 [84]. Overexpression of bmo-miR-2819 could suppress BmNPV replication by down-regulating the expression of BmNPV ie-1 gene [85]. BmCPV-miR-1 could enhance viral replication by suppressing the expression of its target gene, the inhibitor of nuclear factor kappa-B kinase subunit beta of B. mori (BmIKKβ), the key gene of the NF-κB signaling pathway [86].

3.4 Long non-coding RNAs (lncRNAs) in Lepidoptera

Long non-coding RNAs (lncRNAs) were discovered in the early 1990s. Brannan and Brockdorff found that two lncRNAs, H19 and Xist, can be involved in the epigenetic regulation of mammals [87, 88]. Then, with genomic screening of mouse [89, 90] and other organisms, a large number of lncRNAs were identified [91].

lncRNAs are RNAs longer than 200 nucleotides that do not code for proteins [92, 93]. Based on Kung and Lee described [94], lncRNAs can be divided into three categories: (1) Long intervening non-coding RNA (lincRNA), which can be transcribed independently, such as H19 [87], Xist [88], HORTAIR [95], MALAT1 [96], etc. (2) Intronic lncRNAs are transcribed from the introns of protein-coding genes. Such as COLDAIR, which is transcribed from the intron of FLC [97]. (3) Sense-antisense pair, lncRNAs are transcribed from the antisense strand of protein-coding genes, including completely non-overlapping dispersed antisense transcripts; completely overlapping inverse semantic transcripts, and partial overlapping antisense transcripts.

lncRNAs can regulate gene expression in multiple levels [98, 99], such as epigenetic regulation of chromatin [100, 101], transcriptional [102, 103, 104, 105], post-transcriptional [106, 107], and translational regulation [108], as well as having effects on protein transportation or location [109, 110]. Moreover, some lncRNAs function as a decoy/sponge or precursor of miRNAs [107, 111, 112, 113]. Such as H19, the oncofetal lncRNA is the precursor RNA of miR-675 [114, 115]. The association of H19 in tumorigenesis and invasion is attributable to the regulation of miR-675 [116, 117].

In Lepidoptera, lncRNAs were identified more in B. mori, H. armigera and P. xylostella. In P. xylostella, 3844 long intergenic ncRNAs (lincRNA) were identified, 280 and 350 lincRNAs are differentially expressed in Chlorpyrifos and Fipronil-resistant larvae [118]. 1309 lncRNAs were identified in one susceptible and two chlorantraniliprole-resistant P. xylostella strains, of which 877 were intergenic lncRNAs, 190 were intronic lncRNAs, 76 were anti-sense lncRNAs and 166 were sense-overlapping lncRNAs [119]. A total of 8096 lncRNAs were identified and classified into three groups. Expression profiling identified 114 differentially expressed lncRNAs during the development and found that the majority were temporally specific. Many lncRNAs are microRNA precursors or competing endogenous RNAs involved in micro-RNA regulatory pathways [120]. A total of 3,463 H. armigera, 6,245 P. xylostella differentially expressed lncRNAs were identified; the differentially expressed lncRNAs were mainly enriched in the metabolic, digestive, and synthetic signaling pathways [121].

In B. mori, 11,810 lncRNAs were identified from different tissues and developmental stages, including 474 intronic lncRNAs (ilncRNAs), 6250 intergenic lncRNAs (lincRNAs), and 5086 natural antisense lncRNAs (lncNATs) [122]. Using RNA sequencing technology data, 599 lncRNAs were identified in the silk gland of domestic and wild silkworms [123]. The functional roles of lncRNAs in silkworm were revealed gradually. lncRNA Bmdsx-AS1, origin from the antisense strand of sex-determining gene Bmdsx of silkworm, binding to the splicing factor hnRNPH, then interacted with BmPSI, one of the upstream regulating factors of Bmdsx. The splicing pattern of Bmdsx pre-mRNA was altered in male silkworms after Bmdsx-AS1 knockdown, but overexpression of Bmdsx-AS1 induced the male-specific splicing form of Bmdsx in the females [124]. A nucleus-enriched lncRNA lncR26319 regulates Endophilin A through competitively binding to miR-2834 and regulates oogenesis of silkworms [125].

A recent study showed that lncRNAs also play roles in pathogen-insect interaction. A total of 1845 candidate lncRNAs were identified in the virus-infected and non-infected midgut of silkworm larvae, 41 lncRNAs were differentially expressed, the apoptosis, autophagy, and antiviral response genes, such as ATG3, PDCD6, IBP2, and MFB1, could be targeted lncRNAs with differential expression [126]. LincRNA_XR209691.3 could promote BmNPV replication by interacting with BmHSP70 [127]. Overexpression of LINC5438 promoted the proliferation of BmNPV, while interference with LINC5438 inhibited its proliferation, LINC5438 can regulate the expression of Bmiap, BmDronc, BmICE, and its predicted target gene BmAIF [128].

3.5 PIWI-interacting RNA (piRNA)

piRNAs are generally 24–32 nucleotides in length and bind specifically to the PIWI subfamily of Argonaute proteins. So far, the large-scale sequencing of the sRNAs from different organisms has discovered a variety of piRNA sequences [129, 130, 131, 132, 133]. Mature piRNAs have a strong preference (≥60%) for the 5′ uridine residue, with adenosine signature at position 10, and 2′-O-methylation at 3′ end, assembling as clusters in the genome [134]. Over 80% of piRNAs have unique genomic locations, from which nearly 75% are mapped to transposon loci [135], the others are mapped to multiple genomic locations. piRNAs exist in both germline and somatic cells, including neurons. The most widely-recognized and well-characterized function of piRNAs is to suppress the activities of transposable elements in the germline development, where piRNAs are highly abundant, and in somatic cells like neurons, piRNAs are modestly abundant [136, 137, 138, 139, 140, 141, 142, 143, 144, 145]. For the low conservation of individual piRNAs, the deductions of their functions are challenging. Piwi-piRNAs have important functional roles in suppressing transposon [142], preserving genomic integrity [144, 146], regulating translation [147], regulating target mRNAs [140], modulating mRNA stability [147] through epigenetic modifications including DNA methylation, and histone modifications [148, 149].

In silkworm, a systematic discovery of transposable-element (TE)-associated small RNAs in the genome were carried out, 182, 788, and 4990 TE-associated small RNAs in the miRNA, siRNA, and piRNA species were identified, respectively [150]. In female embryos, the mRNA of the male-determining gene Masculinizer (Masc) can be cleaved by the fem piRNA-Siwi complex, disruption of Masc directed to the female-determining pathway. But in male embryos, Masc activates the male-determining pathway for the absence of Fem piRNA [151]. In P. xylostella, the W chromosome-derived piRNAs complementary to Masc mRNA have also been identified, indicating the convergent evolution of piRNA-dependent sex determination in Lepidoptera [152]. But this is not the case in the Asian corn borer, Ostrinia furnacalis (Pyraloidea) [153].

Recently, piRNA pathway has also been shown to be involved in the host-pathogen interaction of insects. In mosquitoes, besides germline transposon control, the piRNA pathway also plays a very important role in antiviral immunity [154]. Piwi-like-1 and Piwi-like-2-3 could inhibit AcMNPV replication in B. mori, while Piwi-like-4-5 could promote virus replication in Sf9 cells [155].

3.6 Circular RNA (circRNA)

circRNAs are covalently closed single-stranded RNAs (ssRNAs) that have recently re-emerged as a widespread class of RNA species. Circular RNA was first identified in plant viroids [156, 157], followed by the discovery of eukaryotic circular transcripts via electron microscopic evidence showing a circular morphology with unknown function [158]. Circular RNAs extensively exist in eukaryotes from yeast [159, 160, 161], worms [162], insects [163, 164], plants [165, 166], and mice to humans [161, 167]. Eukaryotic circular RNAs are usually classified into three groups according to their biogenesis pathways [168]: the exonic circRNAs are produced from pre-mRNA back-splicing, a downstream 5′ splice site joined to an upstream 3′ splice site, resulting in an RNA molecule in a circular format, which is the major biogenesis pathway of circRNAs, a, named. Another type of circRNA is the circular intronic RNA (ciRNA) generated from intronic lariats that failed to be debranched after splicing, this type of circular RNAs ligated with a 20–50 phosphodiester bond [169, 170]. Furthermore, circRNAs with inside unspliced introns were also found and named the exon-intron circRNAs (EIciRNAs) [171].

Circular RNAs play roles in gene expression by modulating transcription in the nucleus and regulating translation in the cytoplasm. In the nucleus, circRNAs play a diverse biological role including chromatin looping, transcription regulation, and alternative splicing [170, 171, 172, 173, 174]. Circular RNAs can form three-stranded structures harboring a DNA:RNA hybrid named R-loops with their producing locus to impact transcription [171]. ci-ankrd52 originated from the second intron of the ANKRD52 locus. Degradation of ci-ankrd52 by RNase H1 resolves the transcribing R-loop to enhance transcriptional elongation [175]. Conn et al. discovered circSEP3 in the cell nucleus of Arabidopsis thaliana regulated the splicing of SEPALLATA3, a homeotic MADS-box transcription factor important for floral homeotic phenotypes [173]. circRNAs play various biological roles in cytoplasm, they can act as decoys for miRNA, serve as protein scaffolds, or sequestering proteins. Several abundant circRNAs act as miRNA sponges, or competing endogenous RNAs (ceRNAs), that subsequently suppress their bio-accessibility and thereby targeted mRNAs [176, 177]. circRNA supercont3.352:252102|253283 acted as sponge of cpp-miR-1671 and decreased the expression of cpp-miR-1671 target gene CYP4G15, then participated in deltamethrin resistance of Culex pipiens pallens (L.) [178]. In Aedes albopictus, aal-circRNA-407 acts as a sponge of aal-miR-9a-5p to promote the expression of its target gene Foxl, then regulate the ovarian development of mosquito, knockdown of circRNA-407 resulted in a decreasing number of developing follicles and a reduction in follicle size after a blood meal [179].

CircRNAs play roles in development [180], reproduction [181, 182], metamorphosis [183, 184], life-span [163], insecticide resistance, aging [185], and host-pathogen interactions and immunity [186, 187]. The biological role of circRNAs in host-pathogen interactions has been broadly investigated. In Lepidoptera, the expression levels of circRNAs in the silkworm midgut are altered after BmNPV infection, these altered circRNAs can modulate various immune pathways, such as the Notch, ABC transporters, and the endocytosis pathway, indicating circRNA might be an anti-viral factor [188]. Another vcircRNA_000048 encoded by BmCPV can translate a small peptide vsp21 with 21 amino acid residues–, which attenuates the viral replication [189].

CircRNAs have recently been reported to be implicated in the regulation of anti-bacteria [190], anti-fungal [191], and anti-parasite immunity [192]. circRNAs modulate various immune pathways, including lysosomes, phagosomes, endocytosis, ubiquitin-mediated proteolysis, the metabolism of xenobiotics by cytochrome P450, and insect hormone biosynthesis [193], as well as cellular renewal and structure and carbohydrate and energy metabolism [194]. Pathogen-encoded circRNAs hijack the host system for proliferation [195], and the host immune system also has the ability to hijack circRNAs encoded by pathogens to inhibit their infection [196, 197].

4. Conclusion

Over the last decade, we have gained a deeper understanding of the biological role of ncRNAs in the development, sex determination, oogenesis, spermatogenesis, and pathogen-interaction in insects. Now a growing body of evidence demonstrating that ncRNAs are involved in immune regulation in insects, especially miRNAs, piRNAs, circRNAs, and lncRNAs, which provide new insight into the immuno-interaction of host and pathogen. Recently, new techniques are likely to improve our understanding of the biogenesis and biological roles of ncRNAs. As more and more functional roles of ncRNAs are revealed, ncRNAs will be used as the potential target of pest control for Lepidoptera in the near future.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 31970480), the Natural Science Foundation of Henan province (No. 212300410063), and the Young Elite Scientist Sponsorship Program by China Association for Science and Technology (YESS 20150026).

Conflict of interest

The authors declare no conflict of interest.