Genomic innovations, transcriptional plasticity and gene loss underlying the evolution and divergence of two highly polyphagous and invasive Helicoverpa pest species

BACKGROUND

Helicoverpa armigera and Helicoverpa zea are major caterpillar pests of Old and New World agriculture, respectively. Both, particularly H. armigera, are extremely polyphagous, and H. armigera has developed resistance to many insecticides. Here we use comparative genomics, transcriptomics and resequencing to elucidate the genetic basis for their properties as pests.

RESULTS

We find that, prior to their divergence about 1.5 Mya, the H. armigera/H. zea lineage had accumulated up to more than 100 more members of specific detoxification and digestion gene families and more than 100 extra gustatory receptor genes, compared to other lepidopterans with narrower host ranges. The two genomes remain very similar in gene content and order, but H. armigera is more polymorphic overall, and H. zea has lost several detoxification genes, as well as about 50 gustatory receptor genes. It also lacks certain genes and alleles conferring insecticide resistance found in H. armigera. Non-synonymous sites in the expanded gene families above are rapidly diverging, both between paralogues and between orthologues in the two species. Whole genome transcriptomic analyses of H. armigera larvae show widely divergent responses to different host plants, including responses among many of the duplicated detoxification and digestion genes.

CONCLUSIONS

The extreme polyphagy of the two heliothines is associated with extensive amplification and neofunctionalisation of genes involved in host finding and use, coupled with versatile transcriptional responses on different hosts. H. armigera's invasion of the Americas in recent years means that hybridisation could generate populations that are both locally adapted and insecticide resistant.

Isolation and functional characterization of two independently-evolved fatty acid Delta12-desaturase genes from insects

We report the first isolation and characterization of insect fatty acid Delta12-desaturase genes, AdD12Des from house cricket (Acheta domesticus) and TcD12Des from the red flour beetle (Tribolium castaneum), responsible for the production of linoleic acid from oleic acid. Sequence analysis shows the cricket and flour beetle Delta12-desaturase genes have evolved independently from all previously known Delta12-desaturases and are much more closely related to the archetypal stearoyl-Coenzyme A-acting desaturase from rat than to the phospholipid-acting Delta12-desaturases widely reported in plants. Phylogenetic and functional analysis indicates the cricket AdD12Des gene may have evolved from an ancestral Delta9-desaturase. By contrast, the beetle Delta12-desaturase is distantly related to the cricket genes and beetle Delta9-desaturases suggesting evolution by an independent route. Linoleic acid has key physiological roles in insects and this is the first report of genes capable of producing this essential fatty acid in higher animals.

Direct comparison of fluorescence- and bioluminescence-based resonance energy transfer methods for real-time monitoring of thrombin-catalysed proteolytic cleavage

In this study, a representative FRET system (CFP donor and YFP acceptor) is compared with the BRET(2) system (Renilla luciferase donor, green fluorescent protein(2) (GFP(2)) acceptor and coelenterazine 400a substrate). Cleavage of a thrombin-protease-sensitive peptide sequence inserted between the donor and acceptor proteins was detected by the RET signal. Complete cleavage by thrombin changed the BRET(2) signal by a factor of 28.9+/-0.2 (R.S.D. (relative standard deviation), n=3) and the FRET signal by a factor of 3.2+/-0.1 (R.S.D., n=3). The BRET(2) technique was 50 times more sensitive than the FRET technique for monitoring thrombin concentrations. Detection limits (blank signal+3sigma(b), where sigma(b)=the standard deviation (S.D.) of the blank signal) were calculated to be 3.05 and 0.22nM thrombin for FRET and BRET(2), respectively. This direct comparison suggests that the BRET(2) technique is more suitable than FRET for use in proximity assays such as protease cleavage assays or protein-protein interaction assays.

Enzymatic bioremediation: from enzyme discovery to applications

1. Enzymatic bioremediation is potentially a rapid method of removing environmental pesticide residues. Applications include the treatment of residues resulting from agricultural production and processing industries, such as the treatment of irrigation waters, surface-contaminated fruit and vegetables and spent dip liquors. 2. A specific application for some organophosphate-degrading enzymes involves detoxification of nerve agent stockpiles. Effective and affordable remediation requires highly specialized enzymes, so protein engineering techniques are being used to improve properties of various source enzymes to enhance catalytic rates, stability and substrate range. 3. Trials with an optimized organophosphate-degrading enzyme have shown the feasibility of such technology in various applications. 4. The enzymes developed for environmental remediation for specific pesticide classes also have applications as antidotes for high-dose pesticide poisonings and as prophylaxis for people at risk of high pesticide doses.

Evolution of an organophosphate-degrading enzyme: a comparison of natural and directed evolution

Organophosphate-degrading enzyme from Agrobacterium radiobacter P230 (OPDA) is a recently discovered enzyme that degrades a broad range of organophosphates. It is very similar to OPH first isolated from Pseudomonas diminuta MG. Despite a high level of sequence identity, OPH and OPDA exhibit different substrate specificities. We report here the structure of OPDA and identify regions of the protein that are likely to give it a preference for substrates that have shorter alkyl substituents. Directed evolution was used to evolve a series of OPH mutants that had activities similar to those of OPDA. Mutants were selected for on the basis of their ability to degrade a number of substrates. The mutations tended to cluster in particular regions of the protein and in most cases, these regions were where OPH and OPDA had significant differences in their sequences.

Isolation and characterization of a Mycobacterium strain that metabolizes the insecticide endosulfan

AIM

The aim of this study was to isolate and characterize a bacterium capable of metabolizing endosulfan.

METHODS AND RESULTS

A endosulfan-degrading bacterium (strain ESD) was isolated from soil inoculum after repeated culture with the insecticide as the sole source of sulfur. Analysis of its 16S rRNA gene sequence, and morphological and physiological characteristics revealed it to be a new fast-growing Mycobacterium, closely related to other Mycobacterium species with xenobiotic-degrading capabilities. Degradation of endosulfan by strain ESD involved both oxidative and sulfur-separation reactions. Strain ESD did not degrade endosulfan when sulfite, sulphate or methionine were present in the medium along with the insecticide. Partial degradation occurred when the culture was grown, with endosulfan, in the presence of MOPS (3-(N-morpholino)propane sulphonic acid), DMSO (dimethyl sulfoxide), cysteine or sulphonane and complete degradation occurred in the presence of gutathione. When both beta-endosulfan and low levels of sulphate were provided as the only sources of sulfur, biphasic exponential growth was observed with endosulfan metabolism being restricted to the latter phase of exponential growth.

CONCLUSIONS

This study isolated a Mycobacterium strain (strain ESD) capable of metabolizing endosulfan by both oxidative and sulfur-separation reactions. The endosulfan-degrading reactions are a result of the sulfur-starvation response of this bacterium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This describes the isolation of a Mycobacterium strain capable of degrading the insecticide endosulfan. This bacterium is a valuable source of enzymes for use in enzymatic bioremediation of endosulfan residues.

Development of a simple and sensitive fluorimetric method for isolation of coumaphos-hydrolysing bacteria

AIMS

To develop a simple, rapid and sensitive fluorimetric assay to detect, isolate and characterize a soil bacterium capable of degrading the organophosphorus pesticide, coumaphos.

METHODS AND RESULTS

A high throughput microtitre plate-based method was used to quantify coumaphos hydrolysis by the bacterium. The fluorescent hydrolysis product of coumaphos, chlorferon, was detected at levels as low as 10 nmol l(-1). Incorporation of coumaphos into agar plates allowed the rapid detection of coumaphos-hydrolysing bacteria when exposed to an excitation wavelength of approximately 340 nm. The coumaphos-hydrolysing enzyme could be visualized when bacterial cell extracts were separated on SDS-PAGE, incubated with coumaphos and exposed to an excitation source as above.

CONCLUSIONS

This method is 100-fold more sensitive than the currently used spectrophotometric method for coumaphos.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is a unique and versatile tool to screen for bacteria possessing phosphotriesterase activity.

Enrichment of a microbial culture capable of degrading endosulphate, the toxic metabolite of endosulfan

AIMS

The aim of this study was to isolate a source of enzymes capable of degrading endosulphate (endosulfan sulphate), the toxic metabolite of the pesticide endosulfan.

METHODS AND RESULTS

A microbial broth culture capable of degrading endosulphate was enriched from endosulfan-contaminated soil by providing the metabolite as the sole source of sulphur in broth culture. No microbial growth was observed in the absence of endosulphate. In the presence of endosulphate, growth of the culture occurred with the concomitant formation of three chlorine-containing compounds. Thin layer chromatography and gas chromatography--mass spectral analysis identified these metabolites as endosulfan monoaldehyde, 1,2,3,4,7,7-hexachloro-5,6-bis(methylene)bicyclo[2.2.1]-2-heptene and 1,2,3,4,7,7-hexachloro-5-hydroxymethylene-6-methylenebicyclo[2.2.1]-2-heptene. The second and third compounds have not been reported in previous metabolic studies. The enriched culture was also able to utilize alpha- and beta-endosulfan as sulphur sources, each producing the hydrolysis product endosulfan monoaldehyde as the sole chlorine-containing metabolite. Alpha-endosulfan was more readily hydrolysed than the beta-isomer.

CONCLUSIONS

This study isolated a mixed microbial culture capable of degrading endosulphate. The products of degradation were characterized as novel endosulfan metabolites.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study describes the isolation of a mixed microbial culture that is potentially a valuable source of hydrolysing enzymes for use in enzymatic bioremediation, particularly of endosulphate and alpha-endosulfan residues.

Enrichment of an endosulfan-degrading mixed bacterial culture

An endosulfan-degrading mixed bacterial culture was enriched from soil with a history of endosulfan exposure. Enrichment was obtained by using the insecticide as the sole source of sulfur. Chemical hydrolysis was minimized by using strongly buffered culture medium (pH 6.6), and the detergent Tween 80 was included to emulsify the insecticide, thereby increasing the amount of endosulfan in contact with the bacteria. No growth occurred in control cultures in the absence of endosulfan. Degradation of the insecticide occurred concomitant with bacterial growth. The compound was both oxidized and hydrolyzed. The oxidation reaction favored the alpha isomer and produced endosulfate, a terminal pathway product. Hydrolysis involved a novel intermediate, tentatively identified as endosulfan monoaldehyde on the basis of gas chromatography-mass spectrometry and chemical derivatization results. The accumulation and decline of metabolites suggest that the parent compound was hydrolyzed to the putative monoaldehyde, thereby releasing the sulfite moiety required for growth. The monoaldehyde was then oxidized to endosulfan hydroxyether and further metabolized to (a) polar product(s). The cytochrome P450 inhibitor, piperonyl butoxide, did not prevent endosulfan oxidation or the formation of other metabolites. These results suggest that this mixed culture is worth investigating as a source of endosulfan-hydrolyzing enzymes for use in enzymatic bioremediation of endosulfan residues.