Unique synteny and alternate splicing of the chitin synthases in closely related heliothine moths

Reference Genes for Expression Analyses by qRT-PCR in Phthorimaea operculella (Lepidoptera: Gelechiidae)

Due to a lack of effective internal references, studies on functional genes in Phthorimaea operculella, a serious Lepidopteran pest attacking potatoes worldwide, have been greatly limited. To select suitable endogenous controls, ten housekeeping genes of actin (ACT), α-tubulin (α-TUB), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), elongation factor 1α (EF1α), 18S and 28S ribosomal RNA (18S, 28S), ribosomal protein genes RPL4, RPL13 and RPL27 and superoxide dismutase (SOD) were tested. Their expression levels were determined under three different experimental conditions (developmental stages, tissues/organs and temperatures) using qRT-PCR technology. The stability was evaluated with five methods (Ct value, geNorm, NormFinder, BestKeeper and RefFinder). The results clarified that RPL13, EF1α and RPL27 are ranked as the best reference gene combination for measuring gene expression levels among different developing stages and under various temperatures; EF1α and RPL13 are recommended to normalize the gene expression levels among diverse tissues. EF1α and RPL13 are the best reference genes in all the experimental conditions. To validate the utility of the selected reference pair, EF1α and RPL13, we estimated the tissue-biased expression level of chitin synthase A gene (PoChSA). As expected, PoChSA was abundantly expressed in ectodermally derived epidermal cells, and lowly transcribed in the midgut. These findings will lay the foundation for future research on the molecular physiology and biochemistry of P. operculella.

New genus of gelechiid moths (Lepidoptera: Gelechiidae) from Borneo, and its four new species

Tawaya gen. n. and its four new species (T. flaventia sp. n., T. rutila sp. n., T. luteola sp. n. and T. armeniaca sp. n.) are described from Borneo. A key to the described species is provided. The taxonomic position of a new genus within the subfamily Dichomeridinae (Gelechiidae) is proposed based on a combination of molecular and morphological analyses.

Insecticidal and synergistic activity of dsRNAs targeting buprofezin-specific genes against the small brown planthopper, Laodelphax striatellus

The small brown planthopper, Laodelphax striatellus, is a dangerous pest in rice fields. Although buprofezin has been used to control L. striatellus for more than a decade, the occurrence of buprofezin-resistant L. striatellus has been recently reported. To develop an alternative pest control strategy, comparative transcriptome analysis of buprofezin-treated and nontreated L. striatellus was performed to screen the buprofezin-specific target genes for RNA interference (RNAi) application. Among six genes downregulated in the buprofezin-treated L. striatellus, RNAi-based silencing of the lipophorin precursor, endocuticle structure glycoprotein, and chitin synthase significantly induced the lethality of L. striatellus in a concentration-dependent manner. In addition, a cocktail of double-stranded RNAs against these three genes showed synergistic effects with buprofezin. These results provide RNAi-based effective approaches to control L. striatellus as well as an efficient method to identify novel target genes for RNAi application.

Cloning, functional characterization and screening of potential inhibitors for Chilo partellus chitin synthase A using in silico, in vitro and in vivo approaches

Chitin synthase (CHS) is one of the crucial enzymes that play an essential role in chitin synthesis during the molting process, and it is considered to be the specific target to control insect pests. Currently, there are no potent inhibitors available in the market, which specifically target this enzyme. Pyrimidine nucleoside peptide, nikkomycin Z, binds to nucleotide-binding sites of fungal and insect CHS. But, their mode of action is still fragmentary due to the lack of a 3Dstructure of CHS. Chilo partellus is a severe pest insect of major food crops such as maize and sorghum, in an attempt to target integument expressed cuticular CpCHS. The CpChsA cDNA was cloned, and subsequently, their developmental and tissue-specific expression was studied. The 3D structure of the CHS catalytic domain was modeled, after which natural compounds were screened using a virtual screening workflow and resulted in the identification of five hit molecules. Molecular dynamics simulations were performed to investigate the dynamics and interactions of hits with CpCHS. The obtained results revealed that the compounds kasugamycin, rutin and robinin could act as potent inhibitors of CpCHS. All three molecules were observed to significantly reduce the chitin production as validated using in vitro and in vivo studies. Thus, this study aims to provide a set of novel inhibitor molecules against CpCHS for controlling the pest population. Communicated by Ramaswamy H. Sarma.

Molecular cloning, expression, and functional analysis of the chitin synthase 1 gene and its two alternative splicing variants in the white-backed planthopper, Sogatella furcifera (Hemiptera: Delphacidae)

Chitin synthase is responsible for chitin synthesis in the cuticles and cuticular linings of other tissues in insects. We cloned two alternative splicing variants of the chitin synthase 1 gene (SfCHS1) from the white-backed planthopper, Sogatella furcifera. The full-length cDNA of the two variants (SfCHS1a and SfCHS1b) consists of 6408 bp, contains a 4719-bp open reading frame encoding 1572 amino acids, and has 5′ and 3′ non-coding regions of 283 and 1406 bp, respectively. The two splicing variants occur at the same position in the cDNA sequence between base pairs 4115 and 4291, and consist of 177 nucleotides that encode 59 amino acids but show 74.6% identity at the amino acid level. Analysis in different developmental stages showed that expression of SfCHS1 and SfCHS1a were highest just after molting, whereas SfCHS1b reached its highest expression level 2 days after molting. Further, SfCHS1 and SfCHS1a were mainly expressed in the integument, whereas SfCHS1b was predominately expressed in the gut and fat body. RNAi-based gene silencing inhibited transcript levels of the corresponding mRNAs in S. furcifera nymphs injected with double-stranded RNA of SfCHS1, SfCHS1a, and SfCHS1b, resulted in malformed phenotypes, and killed most of the treated nymphs. Our results indicate that SfCHS1 may be a potential target gene for RNAi-based S. furcifera control.

Chitin in Arthropods: Biosynthesis, Modification, and Metabolism

Chitin is a structural constituent of extracellular matrices including the cuticle of the exoskeleton and the peritrophic matrix (PM) of the midgut in arthropods. Chitin chains are synthesized through multiple biochemical reactions, organized in several hierarchical levels and associated with various proteins that give their unique physicochemical characteristics of the cuticle and PM. Because, arthropod growth and morphogenesis are dependent on the capability of remodeling chitin-containing structures, chitin biosynthesis and degradation are highly regulated, allowing ecdysis and regeneration of the cuticle and PM. Over the past 20 years, much progress has been made in understanding the physiological functions of chitinous matrices. In this chapter, we mainly discussed the biochemical processes of chitin biosynthesis, modification and degradation, and various enzymes involved in these processes. We also discussed cuticular proteins and PM proteins, which largely determine the physicochemical properties of the cuticle and PM. Although rapid advances in genomics, proteomics, RNA interference, and other technologies have considerably facilitated our research in chitin biosynthesis, modification, and metabolism in recent years, many aspects of these processes are still partially understood. Further research is needed in understanding how the structural organization of chitin synthase in plasma membrane accommodate chitin biosynthesis, transport of chitin chain across the plasma membrane, and release of the chitin chain from the enzyme. Other research is also needed in elucidating the roles of chitin deacetylases in chitin organization and the mechanism controlling the formation of different types of chitin in arthropods.

RNA interference of chitin synthase genes inhibits chitin biosynthesis and affects larval performance in Leptinotarsa decemlineata (Say)

Dietary introduction of bacterially expressed double-stranded RNA (dsRNA) has great potential for management of Leptinotarsa decemlineata. Identification of the most attractive candidate genes for RNA interference (RNAi) is the first step. In the present paper, three complete chitin synthase cDNA sequences (LdChSAa, LdChSAb and LdChSB) were cloned. LdChSAa and LdChSAb, two splicing variants of LdChSA gene, were highly expressed in ectodermally-derived epidermal cells forming epidermis, trachea, foregut and hindgut, whereas LdChSB was mainly transcribed in midgut cells. Feeding bacterially expressed dsChSA (derived from a common fragment of LdChSAa and LdChSAb), dsChSAa, dsChSAb and dsChSB in the second- and fourth-instar larvae specifically knocked down their target mRNAs. RNAi of LdChSAa+LdChSAb and LdChSAa lowered chitin contents in whole body and integument samples, and thinned tracheal taenidia. The resulting larvae failed to ecdyse, pupate, or emerge as adults. Comparably, knockdown of LdChSAb mainly affected pupal-adult molting. The LdChSAb RNAi pupae did not completely shed the old larval exuviae, which caused failure of adult emergence. In contrast, silencing of LdChSB significantly reduced foliage consumption, decreased chitin content in midgut sample, damaged midgut peritrophic matrix, and retarded larval growth. As a result, the development of the LdChSB RNAi hypomorphs was arrested. Our data reveal that these LdChSs are among the effective candidate genes for an RNAi-based control strategy against L. decemlineata.