Altered Expression of Chemosensory and Odorant Binding Proteins in Response to Fungal Infection in the Red Imported Fire Ant, Solenopsis invicta

Social insects have evolved acute mechanisms for sensing and mitigating the spread of microbial pathogens within their communities that include complex behaviors such as grooming and sanitation. Chemical sensing involves detection and transport of olfactory and other chemicals that are mediated by at least two distinct classes of small molecular weight soluble proteins known as chemosensory- and odorant binding proteins (CSPs and OBPs, respectively) that exist as protein families in all insects. However, to date, a systematic examination of the expression of these genes involved in olfactory and other pathways to microbial infection has yet to be reported. The red imported fire ant, Solenopsis invicta, is one of the most successful invasive organisms on our planet. Here, we examined the temporal gene expression profiles of a suite of S. invicta CSPs (SiCSPs1-22) and OBPs (SiOBPs1-16) in response to infection by the broad host range fungal insect pathogen, Beauveria bassiana. Our data show that within 24 h post-infection, i.e., before the fungus has penetrated the host cuticle, the expression of SiCSPs and SiOBPs is altered (mainly increased compared to uninfected controls), followed by suppression of SiCSP and select SiOBP expression 48 h post-infection and mixed responses at 72 h post-infection. A smaller group of SiBOPs, however, appeared to respond to fungal infection, with expression of SiOBP15 consistently higher during fungal infection over the time course examined. These data indicate dynamic gene expression responses of CSPs and OBPs to fungal infection that provide clues to mechanisms that might mediate detection of microbial pathogens, triggering grooming, and nest sanitation.

Red Imported Fire Ant (Solenopsis invicta) Chemosensory Proteins Are Expressed in Tissue, Developmental, and Caste-Specific Patterns

The red imported fire ant, Solenopsis invicta, is a eusocial invasive insect that has spread worldwide. Chemosensory proteins (CSPs) are ligand-binding proteins that participate in a diverse range of physiological processes that include olfaction and chemical transport. Here, we performed a systematic survey of the expression of the 21 gene S. invicta CSP family that includes at least two groups of apparent S. invicta-specific gene expansions. These data revealed caste, tissue, and developmental stage-specific differential expression of the SiCSPs. In general, moderate to high SiCSP expression was seen in worker antennae and abdomen tissues with lower expression in head/thorax regions. Male and female alates showed high antennal expression of fewer SiCSPs, with the female alate thorax showing comparatively high SiCSP expression. SiCSP expression was lower in male alates tissues compared to workers and female alates, albeit with some highly expressed SiCSPs. SiCSP expression was low during development including in eggs, larvae (early and late instars), and pupae. Global analyses revealed examples of conserved, divergent, and convergent SiCSP expression patterns linked to phylogenetic relationships. The developmental and caste-specific variation seen in SiCSP expression patterns suggests specific functional diversification of CSPs that may translate into differential chemical recognition and communication among individuals and/or reflect other cellular roles of CSPs. Our results support a model for CSPs acting as general ligand carriers involved in a wide range of physiological processes beyond olfaction. As compared to the expression patterns of the S. invicta odorant binding proteins (OBPs), an inverse correlation between SiOBP and SiCSP expression was seen, suggesting potential complementary and/or compensatory functions between these two classes of ligand carriers.

Whole genome sequence of the emerging oomycete pathogen Pythium insidiosum strain CDC-B5653 isolated from an infected human in the USA

Pythium insidiosum ATCC 200269 strain CDC-B5653, an isolate from necrotizing lesions on the mouth and eye of a 2-year-old boy in Memphis, Tennessee, USA, was sequenced using a combination of Illumina MiSeq (300 bp paired-end, 14 millions reads) and PacBio (10  Kb fragment library, 356,001 reads). The sequencing data were assembled using SPAdes version 3.1.0, yielding a total genome size of 45.6 Mb contained in 8992 contigs, N₅₀ of 13 Kb, 57% G + C content, and 17,867 putative protein-coding genes. This Whole Genome Shotgun project has been deposited at DDBJ/EMBL/GenBank under the accession JRHR00000000.

Interaction between TATA-Binding Protein (TBP) and Multiprotein Bridging Factor-1 (MBF1) from the Filamentous Insect Pathogenic Fungus Beauveria bassiana

TATA-binding protein (TBP) is a ubiquitous component of eukaryotic transcription factors that acts to nucleate assembly and position pre-initiation complexes. Multiprotein bridging factor 1 (MBF1) is thought to interconnect TBP with gene specific transcriptional activators, modulating transcriptional networks in response to specific signal and developmental programs. The insect pathogen, Beauveria bassiana, is a cosmopolitan fungus found in most ecosystems where it acts as an important regulator of insect populations and can form intimate associations with certain plants. In order to gain a better understanding of the function of MBF1 in filamentous fungi, its interaction with TBP was demonstrated. The MBF1 and TBP homologs in B. bassiana were cloned and purified from a heterologous E. coli expression system. Whereas purified BbTBP was shown to be able to bind oligonucleotide sequences containing the TATA-motif (Kd ≈ 1.3 nM) including sequences derived from the promoters of the B. bassiana chitinase and protease genes. In contrast, BbMBF1 was unable to bind to these same target sequences. However, the formation of a ternary complex between BbMBF1, BbTBP, and a TATA-containing target DNA sequence was seen in agarose gel electrophoretic mobility shift assays (EMSA). These data indicate that BbMBF1 forms direct interactions with BbTBP, and that the complex is capable of binding to DNA sequences containing TATA-motifs, confirming that BbTBP can link BbMBF1 to target sequences as part of the RNA transcriptional machinery in fungi.

A putative methyltransferase, mtrA, contributes to development, spore viability, protein secretion and virulence in the entomopathogenic fungus Beauveria bassiana

The filamentous fungus, Beauveria bassiana, is a ubiquitously distributed insect pathogen, currently used as an alternative to chemical pesticides for pest control. Conidiospores are the means by which the fungus disseminates in the environment, and these cells also represent the infectious agent most commonly used in field applications. Little, however, is known concerning the molecular basis for maintenance of spore viability, a critical feature for survival and persistence. Here, we report on the role of a putative methyltransferase, BbmtrA, in conidial viability, normal fungal growth and development, and virulence, via characterization of a targeted gene knockout strain. Loss of BbmtrA resulted in pleiotropic effects including reduced germination, growth and conidiation, with growing mycelia displaying greater branching than the WT parent. Conidial viability dramatically decreased over time, with <5 % of the cells remaining viable after 30 days as compared with >80 % of the WT. Reduced production of extracellular proteins was also observed for the ΔBbmtrA mutant, including protease/peptidases, glycoside hydrolases and the hyd1 hydrophobin. The latter was further confirmed by hyd1 gene expression analysis. Insect bioassays using the greater wax moth, Galleria mellonella, further revealed that the ΔBbmtrA strain was attenuated in virulence and failed to sporulate on host cadavers. These data support a global role for mtrA in fungal physiological processes.

Action on the Surface: Entomopathogenic Fungi versus the Insect Cuticle

Infections mediated by broad host range entomopathogenic fungi represent seminal observations that led to one of the first germ theories of disease and are a classic example of a co-evolutionary arms race between a pathogen and target hosts. These fungi are able to parasitize susceptible hosts via direct penetration of the cuticle with the initial and potentially determining interaction occurring between the fungal spore and the insect epicuticle. Entomogenous fungi have evolved mechanisms for adhesion and recognition of host surface cues that help direct an adaptive response that includes the production of: (a) hydrolytic, assimilatory, and/or detoxifying enzymes including lipase/esterases, catalases, cytochrome P450s, proteases, and chitinases; (b) specialized infectious structures, e.g., appressoria or penetrant tubes; and (c) secondary and other metabolites that facilitate infection. Aside from immune responses, insects have evolved a number of mechanisms to keep pathogens at bay that include: (a) the production of (epi) cuticular antimicrobial lipids, proteins, and metabolites; (b) shedding of the cuticle during development; and (c) behavioral-environmental adaptations such as induced fever, burrowing, and grooming, as well as potentially enlisting the help of other microbes, all intended to stop the pathogen before it can breach the cuticle. Virulence and host-defense can be considered to be under constant reciprocal selective pressure, and the action on the surface likely contributes to phenomena such as strain variation, host range, and the increased virulence often noted once a (low) virulent strain is "passaged" through an insect host. Since the cuticle represents the first point of contact and barrier between the fungus and the insect, the "action on the surface" may represent the defining interactions that ultimately can lead either to successful mycosis by the pathogen or successful defense by the host. Knowledge concerning the molecular mechanisms underlying this interaction can shed light on the ecology and evolution of virulence and can be used for rational design strategies at increasing the effectiveness of entomopathogenic fungi for pest control in field applications.

Targeting of insect epicuticular lipids by the entomopathogenic fungus Beauveria bassiana: hydrocarbon oxidation within the context of a host-pathogen interaction

Broad host range entomopathogenic fungi such as Beauveria bassiana attack insect hosts via attachment to cuticular substrata and the production of enzymes for the degradation and penetration of insect cuticle. The outermost epicuticular layer consists of a complex mixture of non-polar lipids including hydrocarbons, fatty acids, and wax esters. Long chain hydrocarbons are major components of the outer waxy layer of diverse insect species, where they serve to protect against desiccation and microbial parasites, and as recognition molecules or as a platform for semiochemicals. Insect pathogenic fungi have evolved mechanisms for overcoming this barrier, likely with sets of lipid degrading enzymes with overlapping substrate specificities. Alkanes and fatty acids are substrates for a specific subset of fungal cytochrome P450 monooxygenases involved in insect hydrocarbon degradation. These enzymes activate alkanes by terminal oxidation to alcohols, which are further oxidized by alcohol and aldehyde dehydrogenases, whose products can enter β-oxidation pathways. B. bassiana contains at least 83 genes coding for cytochrome P450s (CYP), a subset of which are involved in hydrocarbon oxidation, and several of which represent new CYP subfamilies/families. Expression data indicated differential induction by alkanes and insect lipids and four CYP proteins have been partially characterized after heterologous expression in yeast. Gene knockouts revealed a phenotype for only one (cyp52X1) out of six genes examined to date. CYP52X1 oxidizes long chain fatty acids and participates in the degradation of specific epicuticular lipid components needed for breaching the insect waxy layer. Examining the hydrocarbon oxidizing CYP repertoire of pathogens involved in insect epicuticle degradation can lead to the characterization of enzymes with novel substrate specificities. Pathogen targeting may also represent an important co-evolutionary process regarding insect cuticular hydrocarbon synthesis.

A fungal homologue of neuronal calcium sensor-1, Bbcsa1, regulates extracellular acidification and contributes to virulence in the entomopathogenic fungus Beauveria bassiana

Neuronal calcium sensor proteins and their homologues participate in transducing extracellular signals that affect intracellular Ca(2+) levels, which in turn regulate enzyme activities, secretion, gene expression and other biological processes. The filamentous fungus Beauveria bassiana is a broad-host-range pathogen of insects that acidifies the extracellular milieu during growth and pathogenesis towards target hosts. A collection of B. bassiana random insertion mutants were screened on pH indicator plates and one mutant was isolated that displayed reduced acidification. The random insertion site was mapped to a gene that displayed homology to the neuronal calcium sensor/frequenin protein family and was designated Bbcsa1. To validate the role of Bbcsa1 in B. bassiana, a targeted gene-knockout was constructed. Data confirmed that Bbcsa1 was not an essential gene and the ΔBbcsa1 strain displayed delayed acidification of the medium when grown in Czapek-Dox medium, as compared with the wild-type parent. HPLC profiling of secreted metabolites did not detect any major changes in the production of organic acids, although downregulation of the membrane H(+) pump/ATPase was noted in the mutant. A slight growth-deficient phenotype was observed for the ΔBbcsa1 strain on Czapek-Dox and potato dextrose media, which was accentuated at high calcium concentrations (500 mM) and 1.5 M sorbitol, but was unaffected by EDTA or SDS. Perturbations in vacuole morphology were also noted for the mutant. Insect bioassays using Galleria mellonella as the target host revealed decreased virulence in the ΔBbcsa1 mutant when applied topically, representing the natural route of infection, but no significant effect was observed when fungal cells were directly injected into target hosts. These results suggest that Bbcsa1 participates in pre-penetration or early penetration events, but is dispensable once the insect cuticle has been breached.

Genetic analyses place most Spanish isolates of Beauveria bassiana in a molecular group with word-wide distribution

Background

The entomopathogenic anamorphic fungus Beauveria bassiana is currently used as a biocontrol agent (BCA) of insects. Fifty-seven Beauveria bassiana isolates -53 from Spain- were characterized, integrating group I intron insertion patterns at the 3'-end of the nuclear large subunit ribosomal gene (LSU rDNA) and elongation factor 1-alpha (EF1-α) phylogenetic information, in order to assess the genetic structure and diversity of this Spanish collection of B. bassiana.

Results

Group I intron genotype analysis was based on the four highly conserved insertion sites of the LSU (Ec2653, Ec2449, Ec2066, Ec1921). Of the 16 possible combinations/genotypes, only four were detected, two of which were predominant, containing 44 and 9 members out of 57 isolates, respectively. Interestingly, the members of the latter two genotypes showed unique differences in their growth temperatures. In follow, EF1-α phylogeny served to classify most of the strains in the B. bassiana s.s. (sensu stricto) group and separate them into 5 molecular subgroups, all of which contained a group I intron belonging to the IC1 subtype at the Ec1921 position. A number of parameters such as thermal growth or origin (host, geographic location and climatic conditions) were also examined but in general no association could be found.

Conclusion

Most Spanish B. bassiana isolates (77.2%) are grouped into a major phylogenetic subgroup with word-wide distribution. However, high phylogenetic diversity was also detected among Spanish isolates from close geographic zones with low climatic variation. In general, no correlation was observed between the molecular distribution and geographic origin or climatic characteristics where the Spanish B. bassiana isolates were sampled.

Insect-toxic secreted proteins and virulence of the entomopathogenic fungus Beauveria bassiana

Fungal virulence has been mostly associated with cuticle-degrading enzymes that can be regulated depending on nutrient conditions. However, few studies have related fungal virulence to insect-toxic secreted proteins. Here, we describe how the presence of secreted toxic proteins may be linked to conidial virulence, which can be affected by nutrient factors. In this study we evaluated: (1) the virulence of the conidia of four Beauveria bassiana strains (EABb 01/103-Su, EABb 01/12-Su, EABb 01/88-Su and EABb 01/110-Su) grown on three different media (malt extract agar (MA), Rice (Rice), Sabouraud dextrose agar (SDA) and harvested from the cadavers of fungal-infected Galleria mellonella larvae (CAD) and (2) the toxicity of the crude soluble protein extracts (CSPEs) obtained from Adamek's liquid medium inoculated with these conidia. Conidial suspensions were obtained from the four media, assessed on G. mellonella larvae and used to produce CSPEs that were injected into healthy G. mellonella larvae. The larvae were also injected with conidia obtained from MA and CAD cultures to expose them to in vivo-secreted proteins. For all isolates, the CAD conidia were by far the most virulent, followed by conidia grown on SDA, Rice and MA. The injected CSPEs showed the same toxicity trends as the conidial suspensions. In addition, the outcomes of injection of the in vivo-secreted proteins showed that the toxic proteins secreted in vitro by the EABb 01/110-Su strain are not produced in vivo. However, the other strains produced toxic proteins both in vivo and in vitro, suggesting that these toxic proteins may be virulence factors involved in invertebrate pathogenesis.

Effects of cultural conditions on fungal biomass, blastospore yields and toxicity of fungal secreted proteins in batch cultures of Metarhizium anisopliae (Ascomycota: Hypocreales)

BACKGROUND

Recently, two fungal proteins with apparent molecular masses of 11 and 15 kDa and insecticidal activity against Ceratitis capitata (Wied.) have been purified from the crude soluble protein extract (CSPE) secreted by the entomopathogenic fungus Metarhizium anisopliae (Metsch.) Sorokin (strain EAMa 01/58-Su) in Adamek's liquid medium. The feasibility of culturing this strain in fermentation facilities in order to harvest and formulate the insecticidal proteins for C. capitata control is mainly dependent on the ability to produce high concentrations of the active proteins at a reasonable cost.

RESULTS

These studies report that, in batch cultures of EAMa 01/58-Su strain, the carbon (C) and nitrogen (N) ratios and sources are important considerations with respect to fungal biomass production, blastospore yield and secretion of insecticidal proteins against C. capitata adults. The data indicate that the type and concentration of N source in the medium influence the production of insecticidal protein and thus the toxicity of the CSPEs. The electrophoretic analysis suggests that the monomer of 11 kDa plays an important role in the insecticidal effect described. Concerning biomass production, no clear differences were found between media with different C and N sources and C:N ratios in total biomass production at day 7. Conversely, important differences were found among the media in terms of blastospore yields.

CONCLUSIONS

By optimising the culture media, the insecticidal effect of the CSPE against C. capitata can be improved. In the CSPE from G(40):P(20) (40 g L(-1) glucose and 20 g L(-1) peptone in dH(2)O), the LC(50) and the LT(50) were 7 and 4.5 times lower than in the CSPE obtained from Adamek.

Purification and characterisation of proteins secreted by the entomopathogenic fungus Metarhizium anisopliae with insecticidal activity against adults of the Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae)

BACKGROUND

The control of the Mediterranean fruit fly (medfly) Ceratitis capitata (Wied) is usually performed with protein bait sprays incorporating chemical insecticides that may have adverse effects on humans, non-target organisms and the environment. In recent years, scientists have sought more environmentally friendly insecticides for medfly control, such as plant- and microorganism-derived compounds. Among these compounds, entomopathogenic fungi are an unexplored source of natural insecticides.

RESULTS

The crude soluble protein extract (CSPE) of the entomopathogenic fungus Metarhizium anisopliae (Mestch.) (strain EAMa 01/58-Su) shows chronic insecticidal activity when administered per os. Mortality in flies exhibits a dose response. The CSPE produces an antifeedant effect in adult flies, a result probably due to a progressive deterioration of the fly midgut after ingestion of the extract. Protease and temperature treatments show that insecticidal activity against C. capitata is due to proteinaceous compounds that are highly thermostable. Four monomeric proteins from this crude extract have been purified by liquid chromatography and gel electroelution. Although all four monomers seem to be involved in the insecticidal activity of the CSPE, the 15 kDa and the 11 kDa proteins appear to be mainly responsible for the observed insecticidal effect.

CONCLUSIONS

Four new fungal proteins with insecticidal activity have been purified and identified. These proteins might be combined with insect baits for C. capitata biocontrol.

Factors affecting the occurrence and distribution of entomopathogenic fungi in natural and cultivated soils

Factors affecting the occurrence and distribution of entomopathogenic fungi in 244 soil samples collected from natural and cultivated areas in Spain were studied using an integrated approach based on univariate and multivariate analyses. Entomopathogenic fungi were isolated from 175 of the 244 (71.7%) soil samples, with only two species found, Beauveria bassiana and Metarhizium anisopliae. Of the 244 soil samples, 104 yielded B. bassiana (42.6%), 18 yielded M. anisopliae (7.3%), and 53 soil samples (21.7%) harboured both fungi. Log-linear models indicated no significant effect of habitat on the occurrence of B. bassiana, but a strong association between M. anisopliae and soils from cultivated habitats, particularly field crops. Also, irrespective of habitat type, B. bassiana predominated over M. anisopliae in soils with a higher clay content, higher pH, and lower organic matter content. Logistic regression analyses showed that pH and clay content were predictive variables for the occurrence of B. bassiana, whereas organic matter content was the predictive variable for M. anisopliae. Also, latitude and longitude predicted the occurrence of these same species, but in opposite directions. Altitude was found to be predictive for the occurrence of B. bassiana. Using principal component analysis, four factors (1 to 4) accounted for 86% of the total variance; 32.8, 22.9, 19.6 and 10.4% of the cumulative variance explained, respectively. Factor 1 was associated with high positive weights for soil clay and silt content and high negative weights for soil sand content. Factor 2 was associated with high positive weights for soil organic matter content and high negative weights for soil pH. Factor 3 was associated with high positive weights for latitude and longitude of the sampled localities and factor 4, had high positive weights only for the altitude. Bi-plot displays representing soil samples were developed for different factor combinations and indicated that, irrespective of geographical location, absence of both fungal species was determined by alkaline sandy soils with low organic matter content, whereas heaviness of soil texture, acidity and increasing organic matter content led to progressively higher percentages of samples harbouring entomopathogenic fungi. These results could aid decision-making as to whether or not a particular cultivated or natural soil is suitable for using entomopathogenic fungi as a pest control measure and for selecting the fungal species best suited to a particular soil.