Characterization of two genes expressed in the salivary glands of the Hessian fly, Mayetiola destructor (Say)

Conserved and Unique Putative Effectors Expressed in the Salivary Glands of Three Related Gall Midge Species

Species in the stem gall midge genus Mayetiola (Diptera: Cecidomyiidae) cause serious damage to small grain crops. Among Mayetiola species are Hessian fly (Mayetiola destructor Say), barley midge (Mayetiola hordei Keiffer), and oat midge (Mayetiola avenae Marchal). Larvae of these species inject saliva into host tissues to manipulate plants. To identify putative effectors, transcriptomic analyses were conducted on transcripts encoding secreted salivary gland proteins (SSGPs) from first instar larvae of the barley and oat midges, since SSGPs are the most likely source for effector proteins delivered into host tissues. From barley midge, 178 SSGP-encoding unigenes were identified, which were sorted into 51 groups. From oat midge, 194 were obtained and sorted into 50 groups. Predicted proteins within a group had a highly conserved secretion signal peptide and shared at least 30% amino acid identity. Among the identified unigenes from both barley and oat midges, ~68% are conserved either among the three species or between two of them. Conserved SSGPs included members belonging to SSGP-1, SSGP-4, SSGP-11, and SSGP-71 families. Unconventional conservation patterns exist among family members within a species and among different gall midges, indicating that these genes are under high selection pressure, a characteristic of effector genes. SSGPs that are unique to each species were also identified. Those conserved SSGPs may be responsible for host manipulation since the three gall midges produce identical phenotypic symptoms to host plants, whereas the SSGPs unique to each species may be responsible for different host specificity.

Differential expression of candidate salivary effector proteins in field collections of Hessian fly, Mayetiola destructor

Evidence is emerging that some proteins secreted by gall-forming parasites of plants act as effectors responsible for systemic changes in the host plant, such as galling and nutrient tissue formation. A large number of secreted salivary gland proteins (SSGPs) that are the putative effectors responsible for the physiological changes elicited in susceptible seedling wheat by Hessian fly, Mayetiola destructor (Say), larvae have been documented. However, how the genes encoding these candidate effectors might respond under field conditions is unknown. The goal of this study was to use microarray analysis to investigate variation in SSGP transcript abundance amongst field collections from different geographical regions (southeastern USA, central USA, and the Middle East). Results revealed significant variation in SSGP transcript abundance amongst the field collections studied. The field collections separated into three distinct groups that corresponded to the wheat classes grown in the different geographical regions as well as to recently described Hessian fly populations. These data support previous reports correlating Hessian fly population structure with micropopulation differences owing to agro-ecosystem parameters such as cultivation of regionally adapted wheat varieties, deployment of resistance genes and variation in climatic conditions.

Gall midges (Hessian flies) as plant pathogens

Gall midges constitute an important group of plant-parasitic insects. The Hessian fly (HF; Mayetiola destructor), the most investigated gall midge, was the first insect hypothesized to have a gene-for-gene interaction with its host plant, wheat (Triticum spp.). Recent investigations support that hypothesis. The minute larval mandibles appear to act in a manner that is analogous to nematode stylets and the haustoria of filamentous plant pathogens. Putative effector proteins are encoded by hundreds of genes and expressed in the HF larval salivary gland. Cultivar-specific resistance (R) genes mediate a highly localized plant reaction that prevents the survival of avirulent HF larvae. Fine-scale mapping of HF avirulence (Avr) genes provides further evidence of effector-triggered immunity (ETI) against HF in wheat. Taken together, these discoveries suggest that the HF, and other gall midges, may be considered biotrophic, or hemibiotrophic, plant pathogens, and they demonstrate the potential that the wheat-HF interaction has in the study of insect-induced plant gall formation.

Serine proteases-like genes in the asian rice gall midge show differential expression in compatible and incompatible interactions with rice

The Asian rice gall midge, Orseolia oryzae (Wood-Mason), is a serious pest of rice. Investigations into the gall midge-rice interaction will unveil the underlying molecular mechanisms which, in turn, can be used as a tool to assist in developing suitable integrated pest management strategies. The insect gut is known to be involved in various physiological and biological processes including digestion, detoxification and interaction with the host. We have cloned and identified two genes, OoprotI and OoprotII, homologous to serine proteases with the conserved His⁸⁷, Asp¹³⁶ and Ser²⁴¹ residues. OoProtI shared 52.26% identity with mosquito-type trypsin from Hessian fly whereas OoProtII showed 52.49% identity to complement component activated C1s from the Hessian fly. Quantitative real time PCR analysis revealed that both the genes were significantly upregulated in larvae feeding on resistant cultivar than in those feeding on susceptible cultivar. These results provide an opportunity to understand the gut physiology of the insect under compatible or incompatible interactions with the host. Phylogenetic analysis grouped these genes in the clade containing proteases of phytophagous insects away from hematophagous insects.

A neo-sex chromosome that drives postzygotic sex determination in the hessian fly (Mayetiola destructor)

Two nonoverlapping autosomal inversions defined unusual neo-sex chromosomes in the Hessian fly (Mayetiola destructor). Like other neo-sex chromosomes, these were normally heterozygous, present only in one sex, and suppressed recombination around a sex-determining master switch. Their unusual properties originated from the anomalous Hessian fly sex determination system in which postzygotic chromosome elimination is used to establish the sex-determining karyotypes. This system permitted the evolution of a master switch (Chromosome maintenance, Cm) that acts maternally. All of the offspring of females that carry Cm-associated neo-sex chromosomes attain a female-determining somatic karyotype and develop as females. Thus, the chromosomes act as maternal effect neo-W's, or W-prime (W') chromosomes, where ZW' females mate with ZZ males to engender female-producing (ZW') and male-producing (ZZ) females in equal numbers. Genetic mapping and physical mapping identified the inversions. Their distribution was determined in nine populations. Experimental matings established the association of the inversions with Cm and measured their recombination suppression. The inversions are the functional equivalent of the sciarid X-prime chromosomes. We speculate that W' chromosomes exist in a variety of species that produce unisexual broods.

Localization and characterization of 170 BAC-derived clones and mapping of 94 microsatellites in the Hessian fly

Ninety-four microsatellites from enriched genomic libraries of Hessian fly (Hf, Mayetiola destructor [Say]) were localized to 170 cognate clones in an Hf bacterial artificial chromosome (BAC) library. These microsatellite-positive BAC clones were physically mapped to polytene chromosomes by fluorescent in situ hybridization. The mapped microsatellite loci can be used to study the genetic diversity and population structure of Hf.

A BAC-based physical map of the Hessian fly genome anchored to polytene chromosomes

Background

The Hessian fly (Mayetiola destructor) is an important insect pest of wheat. It has tractable genetics, polytene chromosomes, and a small genome (158 Mb). Investigation of the Hessian fly presents excellent opportunities to study plant-insect interactions and the molecular mechanisms underlying genome imprinting and chromosome elimination. A physical map is needed to improve the ability to perform both positional cloning and comparative genomic analyses with the fully sequenced genomes of other dipteran species.

Results

An FPC-based genome wide physical map of the Hessian fly was constructed and anchored to the insect's polytene chromosomes. Bacterial artificial chromosome (BAC) clones corresponding to 12-fold coverage of the Hessian fly genome were fingerprinted, using high information content fingerprinting (HIFC) methodology, and end-sequenced. Fluorescence in situ hybridization (FISH) co-localized two BAC clones from each of the 196 longest contigs on the polytene chromosomes. An additional 70 contigs were positioned using a single FISH probe. The 266 FISH mapped contigs were evenly distributed and covered 60% of the genome (95,668 kb). The ends of the fingerprinted BACs were then sequenced to develop the capacity to create sequenced tagged site (STS) markers on the BACs in the map. Only 3.64% of the BAC-end sequence was composed of transposable elements, helicases, ribosomal repeats, simple sequence repeats, and sequences of low complexity. A relatively large fraction (14.27%) of the BES was comprised of multi-copy gene sequences. Nearly 1% of the end sequence was composed of simple sequence repeats (SSRs).

Conclusion

This physical map provides the foundation for high-resolution genetic mapping, map-based cloning, and assembly of complete genome sequencing data. The results indicate that restriction fragment length heterogeneity in BAC libraries used to construct physical maps lower the length and the depth of the contigs, but is not an absolute barrier to the successful application of the technology. This map will serve as a genomic resource for accelerating gene discovery, genome sequencing, and the assembly of BAC sequences. The Hessian fly BAC-clone assembly, and the names and positions of the BAC clones used in the FISH experiments are publically available at .

Characterization and expression analysis of a gene encoding a secreted lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say)

In a salivary gland transcriptomics study we identified a cDNA with a full-length open reading frame for a gene (MdesL1) encoding a lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say). Fluorescent in situ hybridization on salivary polytenes positioned MdesL1 on the long arm of Autosome 1. BLASTp and conserved domain searches revealed the deduced amino acid sequence contained a lipase superfamily domain with similarity to lipases and phospholipases from other insects. A secretion signal peptide was identified at the amino terminus of the deduced amino acid sequence. Analysis of the transcript of MdesL1 in larval Hessian fly tissues by quantitative real-time PCR (qPCR) revealed the greatest abundance was in salivary glands. Analysis of transcript levels during development showed the greatest level was detected in feeding 1st-instar and early 2nd-instar larvae. Transcript levels increased dramatically over time in larvae feeding on susceptible wheat but were detected at low levels in larvae feeding on resistant wheat. These data suggest the protein encoded by MdesL1 is likely secreted into host-plant cells during larval feeding and could be involved in extra-oral digestion and changes in host-cell permeability or in generating a second messenger in a host-cell-signaling cascade.

Analysis of transcripts and proteins expressed in the salivary glands of Hessian fly (Mayetiola destructor) larvae

Hessian fly (Mayetiola destructor) larvae are thought to manipulate host growth and metabolism through salivary secretions. However, the transcriptome and proteome of Hessian fly salivary glands have not been systematically analyzed. In this research, we analyzed Expressed-Sequence-Tags (EST) representing 6106 cDNA clones randomly selected from four libraries made from dissected salivary glands. We also analyzed the protein composition of dissected salivary glands using one- and two-dimensional gel electrophoresis as well as LC-MS/MS analysis. Transcriptomic analysis revealed that approximately 60% of the total cDNA clones and 40% of assembled clusters encoded secretory proteins (SP). The SP-encoding cDNAs were grouped into superfamilies and families according to sequence similarities. In addition to the high percentage of SP-encoding transcripts, there was also a high percentage of transcripts encoding proteins that were either involved directly in protein synthesis or in house-keeping functions that provide conditions necessary for protein synthesis. Proteomic analysis also revealed a high percentage of proteins involved in protein synthesis either directly or indirectly. The high percentage of SP-encoding transcripts and high percentage of proteins related to protein synthesis suggested that the salivary glands of Hessian fly larvae are indeed specialized tissues for synthesis of proteins for host injection. However, LC-MS/MS analysis of 64 proteins did not identify any SPs corresponding to the cDNA sequences. The lack of accumulation of SPs in the salivary glands indicated the SPs were likely secreted as soon as they were synthesized.

Gene-for-gene defense of wheat against the Hessian fly lacks a classical oxidative burst

Genetic similarities between plant interactions with microbial pathogens and wheat interactions with Hessian fly larvae prompted us to investigate defense and counterdefense mechanisms. Plant oxidative burst, a rapid increase in the levels of active oxygen species (AOS) within the initial 24 h of an interaction with pathogens, commonly is associated with defenses that are triggered by gene-for-gene recognition events similar to those involving wheat and Hessian fly larvae. RNAs encoded by Hessian fly superoxide dismutase (SOD) and catalase (CAT) genes, involved in detoxification of AOS, increased in first-instar larvae during both compatible and incompatible interactions. However, mRNA levels of a wheat NADPH oxidase (NOX) gene that generates superoxide (O2-) did not increase. In addition, inhibiting wheat NOX enzyme with diphenyleneiodonium did not result in increased survival of avirulent larvae. However, nitro blue tetrazolium staining indicated that basal levels of O2- are present in both uninfested and infested wheat tissue. mRNA encoded by wheat genes involved in detoxification of the cellular environment, SOD, CAT, and glutathione-S-transferase did not increase in abundance. Histochemical staining with 3,3-diaminobenzidine revealed no increases in wheat hydrogen peroxide (H2O2) during infestation that were correlated with the changes in larval SOD and CAT mRNA. However, treatment with 2',7'-dichlorofluorescin demonstrated the presence of basal levels of H2O2 in the elongation zone of both infested and uninfested plants. The accumulation of a wheat flavanone 3-hydroxylase mRNA did show some parallels with larval gene mRNA profiles. These results suggested that larvae encounter stresses imposed by mechanisms other than an oxidative burst in wheat seedlings.

Genomic analysis of a 1 Mb region near the telomere of Hessian fly chromosome X2 and avirulence gene vH13

BACKGROUND

To have an insight into the Mayetiola destructor (Hessian fly) genome, we performed an in silico comparative genomic analysis utilizing genetic mapping, genomic sequence and EST sequence data along with data available from public databases.

RESULTS

Chromosome walking and FISH were utilized to identify a contig of 50 BAC clones near the telomere of the short arm of Hessian fly chromosome X2 and near the avirulence gene vH13. These clones enabled us to correlate physical and genetic distance in this region of the Hessian fly genome. Sequence data from these BAC ends encompassing a 760 kb region, and a fully sequenced and assembled 42.6 kb BAC clone, was utilized to perform a comparative genomic study. In silico gene prediction combined with BLAST analyses was used to determine putative orthology to the sequenced dipteran genomes of the fruit fly, Drosophila melanogaster, and the malaria mosquito, Anopheles gambiae, and to infer evolutionary relationships.

CONCLUSION

This initial effort enables us to advance our understanding of the structure, composition and evolution of the genome of this important agricultural pest and is an invaluable tool for a whole genome sequencing effort.

A super-family of genes coding for secreted salivary gland proteins from the Hessian fly, Mayetiola destructor

We have previously characterized a gene coding for the secreted-salivary-gland-protein 11A1 (SSGP-11A1) from the Hessian fly, Mayetiola destructor (Say) (Diptera Cecidomyiidae). Here we report the cloning and characterization of three new genes coding for proteins designated SSGP-11B1, SSGP-11C1, and SSGP-11C2, and their relationship with the SSGP-11A1-encoding gene. Based on their structural conservation, similar regulation, and clustered genomic organization, we conclude that the four genes represent a gene superfamily, designated SSGP-11, which originated from a common ancestor. Cloning, Southern blot and in situ hybridization data suggest that each of theSSGP-11 families has multiple members that cluster within short chromosome regions. The presence of a secretion signal peptide, the exclusive expression in the larval stage, and the clustered genomic organization indicate that this superfamily might be important for Hessian fly virulence/avirulence.

Molecular cloning and characterization of two digestive serine proteases from the Hessian fly, Mayetiola destructor

Full-length cDNA and genomic sequences for two genes (designated mdesprot-I and mdesprot-II) encoding digestive serine proteases in Hessian fly, Mayetiola destructor, have been cloned and characterized. The deduced amino acid sequences revealed similarity with trypsin-like digestive serine proteases from other Dipterans. Both mdesprot-I and mdesprot-II encoded proteins with secretion signal peptides at the N-terminals, indicating the proteins are secreted proteases that should function as midgut digestive proteases. A cytological analysis with fluorescent in situ hybridization revealed the cytological localization of mdesprot-I and mdesprot-II on the long arm of Autosome 2. Results are discussed in the context of the efficacy of potential protease inhibitors to develop Hessian fly resistant wheat through genetic engineering approaches.