K. Uma Devi

A study of host specificity in the entomopathogenic fungus Beauveria bassiana (Hypocreales, Clavicipitaceae)

Abstract Beauveria bassiana (Balsamo – Crivelli) Vuillemin based mycoinsecticides are used against agricultural, veterinary and medical insect pests. The fungus has a very diverse and extensive host range. Variation in virulence among isolates of B. bassiana to different insect species has been abundantly documented. Given the effect of multiple factors on virulence, it is not certain whether the observed difference in virulence can be labelled as host specificity. Environmental conditions and susceptibility of the insect population are two main factors that affect successful fungal infection. Keeping the environmental factors constant, if virulence of an isolate to different insect species and different populations within an insect species is compared, the scale of difference between the two responses can be estimated. If differences in virulence of an isolate to different insect species are greater than the difference in virulence to different insect populations within an insect species, then, the isolate can be considered as exhibiting specific preference to those insect species towards which it exhibits high virulence. To examine this feature, a worldwide sample of B. bassiana was bioassayed on nine insect species and two different populations within two insect species. Laboratory bioassays were done on: Bombyx mori (Lepidoptera), Spodoptera litura (Lepidoptera), Chilo partellus (Lepidoptera), Helicoverpa armigera (Lepidoptera), Epilachna vigintioctopunctata (Coleoptera), Mylabris pustulata (Coleoptera), Aphis craccivora (Homoptera), Maconellicoccus hirsutus (Hemiptera) and Oecophylla smaragdina (Hymenoptera). The range of variation in virulence of a B. bassiana isolate to different insect species was not more than that observed with different populations within a single insect species. B. bassiana is thus a generalist with no strict host preference. B. bassiana based biopesticide can be used as a broad spectrum insecticide against a myriad of insect pests.

Laboratory evaluation of the virulence of Beauveria bassiana isolates to the sorghum shoot borer Chilo partellus Swinhoe (Lepidoptera: Pyralidae) and their characterization by RAPD-PCR

Beauveria bassiana has long been used as a mycopesticide. It has a wide host range; isolates have been reported to differ in host range and virulence to a given insect species. Identification of a molecular marker linked to a virulent phenotype to a target pest would be useful in screening for isolates effective against it. Twenty B. bassiana isolates were tested for their virulence to the second instar larvae of Chilo partellus Swinhoe in laboratory bioassays and their DNA fingerprints were generated by RAPD-PCR. Three arbitrary categories of aggressiveness were chosen; isolates that caused >70%, between 70 and 40% and <40% larval mortality were grouped as highly, medium and less aggressive types, respectively. In the random amplified polymorphic DNA (RAPD) analysis a 30% variability was observed among the isolates; which clustered into three major groups. The groups based on virulence rating did not match with the RAPD clusters. One of the highly aggressive isolates clustered with less aggressive isolates in one cluster and the other grouped along with the medium aggressive isolates in a different cluster. The B. bassiana isolates were classified phenotypically based on the taxonomic order of the original insect host and the climatic zone (tropical/temperate) from which they were isolated. No correlation between the aggressiveness of the isolate and the relatedness of the original insect host to the tested insect was observed; both the highly aggressive isolates were from coleopteran insects. A correlation was found between the RAPD grouping and the phenotypic classification of the isolates. All the lepidopteran isolates grouped into one major cluster, most sub clusters were constituted by isolates from the same climatic zone.

Laboratory Evaluation of the Pathogenicity of Three Isolates of the Entomopathogenic Fungus Beauveria bassiana (Bals.) Vuillemin on the American Cockroach (Periplaneta americana)

Mycopesticides can be ideal for the biocontrol of cockroaches because the habitat of these insects promotes initial fungal infection and its subsequent spread. The pathogenicity of three isolates of Beauveria bassiana to the American cockroach was tested. The insects were treated in three different ways, by direct contact with spore mass, a spore-wheat flour mixture and a spray of an aqueous spore suspension. A mortality of 100% in the first treatment, 67-100% in the second treatment and 17-75% in the third treatment was observed. These results suggest that B. bassiana spore formulations in food baits can be developed for cockroaches.

The optimum and tolerance pH range is correlated to colonial morphology in isolates of the entomopathogenic fungus Beauveria bassiana - a potential biopesticide

Beauveria bassiana is an insect pathogenic fungus used as a biopesticide in crop pest management. It exists saprophytically in the soil. The pH of the soil and the insect cuticle and its haemocoel may influence fungal survival and infection potential. Therefore the tolerance and optimum pH ranges were studied in 29 isolates of B. bassiana. Germination and growth bioassays in liquid culture medium adjusted to pH values in the range 3–14 at unit intervals were studied. A pH of 3 was found to be toxic to all isolates – conidia germinated at this pH but growth was totally inhibited. All isolates tolerated a pH of 5–13. Some isolates showed tolerance to a pH of 4 and/or 14 as well. Sixteen isolates showed a wide range of pH optimum of 5–13 while others had a narrower optimal range. The relation of the pH tolerance and optimum range of an isolate to its phenotypic characters (colonial morphology and growth-rate) was studied. All isolates with a ‘chalky’ type colonial morphology had an intermediate growth rate and showed a wide optimum range of pH 5–13 or 5–14. All isolates with ‘dusty’ type colonial morphology were found to have a high growth rate and a wide pH tolerance range of 4–14. Both chalky and dusty colonial morphology are due to a growth pattern characterized by iterated conidiation with a very brief hyphal stage. The correlation observed between these traits based on external phenotype would facilitate prediction of the pH tolerance characters of an isolate of B. bassiana.

Screening for tolerance to bavistin, a benzimidazole fungicide containing methyl benzimidazol-2-yl carbamate (MBC), in Beauveria bassiana: Sequence analysis of the β-tubulin gene to identify mutations conferring tolerance

The entomopathogenic Beauveria bassiana is a potential biopesticide. Fungicide-tolerant isolates of this fungus would have an added advantage of being compatible with the conventional chemical methods of pest control. Therefore, 30 isolates of the fungus were screened for tolerance to bavistin a commonly used benzimidazole fungicide containing methyl benzimidazol-2-yl carbamate (MBC). Germination and growth bioassays in the presence of 0.05% bavistin were conducted for screening. Three tolerant isolates were identified, showing tolerance up to 2% bavistin. Mutation in the beta-tubulin gene is known to confer tolerance to MBC; the nine known mutation sites in the gene involved were sequenced in the tolerant strains. The beta-tubulin gene from codons 1-405 was amplified using two pairs of degenerate primers, designed for the conserved region of the beta-tubulin gene and sequenced. From the sequence suitable primers were designed for the regions flanking the nine known sites involved in mutations conferring MBC tolerance. The amplified products with these primers from the MBC-tolerant isolates were sequenced and in two (ARSEF 1315 and ARSEF 1316) a mutation was detected in the 198 codon resulting in replacement of glutamic acid with lysine. In the third tolerant isolate (ITCC 913) no mutation could be detected in any of the nine known sites conferring tolerance to MBC. To complete the sequencing of the beta-tubulin gene, the remaining part (from codon 405 onwards) of the gene was isolated by a three-prime gene walk. The gene sequence showed a close homology to fungal beta-tubulin genes with four introns.

Effect of combination treatment with entomopathogenic fungi Beauveria bassiana and Nomuraea rileyi (Hypocreales) on Spodoptera litura (Lepidoptera: Noctuidaeae)

Abstract In biocontrol of insect pests, efficacy of treatment with multiple pathogens has not been frequently investigated but may have potential for effective management. The possible advantage of a combination treatment with two entomopathogenic fungi – Beauveria bassiana and Nomuraea rileyi – was assessed in laboratory bioassays on second instar Spodoptera litura. From among the fungal isolates of an epizootic population, two isolates of each fungus differing in virulence to S. litura were chosen, one highly virulent and the other with low virulence. The bioassays were carried out at either a continuous temperature of 25±1°C or at a temperature cycle of 32±2°C 8 h/21±2°C 16 h to mimic the field temperatures during the epizootic. Treatments with the two fungi were done both simultaneously and sequentially. In combination treatments at 25±1°C, in all isolate combinations, a majority of the larvae showed N. rileyi induced mycosis; the percentage mortality and speed of kill of insects in these treatments was similar to the N. rileyi isolate used in the combination treatments. At the temperature cycle of 32±2°C 8 h/21±2°C 16 h, in all combination treatments, all the dead insects exhibited B. bassiana mycosis; the mortality pattern was similar to the B. bassiana isolate used in the combination treatments. The adverse effect of high temperature on virulence of N. rileyi was however, not evident in in vitro growth assays. Combination treatment with both fungi did not have a synergistic effect on insect mortality.Abstract In biocontrol of insect pests, efficacy of treatment with multiple pathogens has not been frequently investigated but may have potential for effective management. The possible advantage of a combination treatment with two entomopathogenic fungi – Beauveria bassiana and Nomuraea rileyi – was assessed in laboratory bioassays on second instar Spodoptera litura. From among the fungal isolates of an epizootic population, two isolates of each fungus differing in virulence to S. litura were chosen, one highly virulent and the other with low virulence. The bioassays were carried out at either a continuous temperature of 25±1°C or at a temperature cycle of 32±2°C 8 h/21±2°C 16 h to mimic the field temperatures during the epizootic. Treatments with the two fungi were done both simultaneously and sequentially. In combination treatments at 25±1°C, in all isolate combinations, a majority of the larvae showed N. rileyi induced mycosis; the percentage mortality and speed of kill of insects in these treatments w...

Susceptibility to Fungi of Cotton Boll Worms Before and After a Natural Epizootic of the Entomopathogenic Fungus Nomuraea rileyi (Hyphomycetes)

Epizootics caused by Beauveria bassiana and Nomuraea rileyi have been observed on boll worms and Spodoptera litura in south Indian fields during winter since the last 15 years. During the N. rileyi-induced natural epizootics, some boll worms were found surviving without infection. Whether they represent pathogen-resistant genotypes was investigated. Two insect populations, collected 3 months prior to and during the epizootic were established. Their sensitivity to both the fungi was compared in laboratory bioassays. No significant difference in sensitivity was observed between the two populations. It was concluded that the boll worm population surviving the epizootic was not genotypically resistant.

Coleoptile length, gibberellin sensitivity and concentrations in five non-allelic dwarf mutants of pearl millet — Pennisetum glaucum (L.) R. Br.

The concentrations of endogenous gibberellins (GAs) were determined by combined gas chromatography-mass spectrometry in shoots of five non-allelic dwarfs of pearl millet Pennisetum glaucum (L.) R. Br. One mutant (d3), with an extreme dwarf phenotype, was found to be deficient in all GAs measured; the others (d1, d2, d4 and the quantitatively inherited dwarf) had similar levels of GAs to the tall genotype. Only the GA-deficient dwarf recovered the tall phenotype in response to applying GA3 up to the adult stage, while the others showed slight to moderate responses at the seedling stage, depending on the season, and no response at later stages. The d1, d3 and d4 dwarfs had short coleoptiles. A wide range of coleoptile lengths with a normal distribution pattern was observed in the tall, d2 and the quantitatively inherited dwarf, suggesting that there is polygenic control of this trait.

Allelic relationship of four male sterility genes and nucleo-cytoplasmic interactions in the expression of male sterility in pearl millet, Pennisetum americanum (L.) leeke

Male sterility genes isolated in four inbred lines of pearl millet were found allelic. The differences between male fertile and male sterile phenotypes is mainly due to a single gene. Presence of a dominant gene (Ms) resulted in male fertility and double recessiveness (ms ms) in male sterility. However, genic male sterility (GMS) in Pennisetum is not a simply inherited case of monogenic recessive condition but is influenced by cytoplasmic and several nuclear factors. In a male sterile, the stage at which the male sterility gene is expressed during the development of the male gametophyte resulting in breakdown of the cells is influenced by cytoplasmic and other nuclear factors. Two types of cytoplasm, C-1 and C-2, are recognized. Presence of any two recessive male sterility alleles in C-1 led to breakdown of male development before differentiation of an archesporium in the anther (Arc-type); in C-2 cytoplasm, degeneration started during meiosis with fusion of meiocytes and syncyte formation (Syn-type), or at post-meiotic stages terminating in abortion of microspores before first pollen mitosis (PGM type). The triggering of activity of recessive male sterility genes in C-2 cytoplasm appeared to be regulated by two nuclear factors, R 1 and R 2 with duplicate gene action. Recessiveness for both the R factors in C-2 cytoplasm resulted in PGM-type expression. The action of R 1 and R 2 is specific to C-2 cytoplasm. Mutation of cytoplasm from C-1 to C-2 and C-2 to C-1 was observed.

Microalgae in Bioremediation: Sequestration of Greenhouse Gases, Clearout of Fugitive Nutrient Minerals, and Subtraction of Toxic Elements from Waters

Microalgae, the microscopic, predominantly unicellular eukaryotes of wide diversity, are the most common inhabitants of Earth’s water bodies. They have a very short generation time and multiply exponentially under favorable environmental conditions. Being photosynthetic, they act as massive carbon sinks. They also have the capacity to take up metal ions from the surrounding medium. Different microalgal species are distinct with respect to the biomolecules they accumulate. These biomolecules—pigments, lipids, and proteins—constitute high-value nutrients in human and animal diets. Considering these attributes of microalgae, their potential in bioremediation of toxic metal pollutants and minerals like nitrates and phosphates in water and the greenhouse gases (CO2 and N2O) in the flue gas emissions from fossil fuel consumption in transport vehicles and industries is being extensively explored. The algal biomass thus created during biosequestration of greenhouse gases and nutrient minerals can be a source of nutraceuticals for humans or animals and also probably a biofuel. The literature on this topic is herewith reviewed.