Morphological Characteristics of Conidiogenesis in Cordyceps militaris

Conidial development of Cordyceps militaris was observed from germinating ascospores and vegetative hyphae through light and scanning electron microscopy (SEM). Ascospores were discharged from fresh specimens of C. militaris in sterile water as well as Sabouraud Dextrose agar plus Yeast Extract (SDAY) plates. We observed ascospore germination and conidial formation periodically. Under submerged condition in sterile water, most part-spores germinated unidirectionally and conidia were developed directly from the tips of germinating hyphae of part-spores within 36 h after ascospore discharge, showing microcyclic conidiation. First-formed conidia were cylindrical or clavate followed by globose and ellipsoidal ones. Germination of ascospores and conidial development were observed on SDAY agar by SEM. Slimy heads of conidia on variously arranged phialides, from solitary to whorl, developed 5 days after ascospore discharge. Besides, two distinct types of conidia, elongated pyriform or cylindrical and globose, were observed in the same slimy heads by SEM. Conidia were shown to be uninucleate with 4,6-diamidino-2-phenylindole staining. Conidiogenous cells were more slender than vegetative hyphae, having attenuated tips. Microcyclic conidiation, undifferentiated conidiogenous hyphae (phialides), polymorphic conidia and solitary, opposite to whorled type of phialidic arrangement are reported here as the characteristic features of asexual stage of C. militaris, which can be distinguished from other Cordyceps species.

The analysis of the complete mitochondrial genome of Lecanicillium muscarium (synonym Verticillium lecanii) suggests a minimum common gene organization in mtDNAs of Sordariomycetes: phylogenetic implications

The mitochondrial genome (mtDNA) of the entomopathogenic fungus Lecanicillium muscarium (synonym Verticillium lecanii) with a total size of 24,499-bp has been analyzed. So far, it is the smallest known mitochondrial genome among Pezizomycotina, with an extremely compact gene organization and only one group-I intron in its large ribosomal RNA (rnl) gene. It contains the 14 typical genes coding for proteins related to oxidative phosphorylation, the two rRNA genes, one intronic ORF coding for a possible ribosomal protein (rps), and a set of 25 tRNA genes which recognize codons for all amino acids, except alanine and cysteine. All genes are transcribed from the same DNA strand. Gene order comparison with all available complete fungal mtDNAs-representatives of all four Phyla are included-revealed some characteristic common features like uninterrupted gene pairs, overlapping genes, and extremely variable intergenic regions, that can all be exploited for the study of fungal mitochondrial genomes. Moreover, a minimum common mtDNA gene order could be detected, in two units, for all known Sordariomycetes namely nad1-nad4-atp8-atp6 and rns-cox3-rnl, which can be extended in Hypocreales, to nad4L-nad5-cob-cox1-nad1-nad4-atp8-atp6 and rns-cox3-rnl nad2-nad3, respectively. Phylogenetic analysis of all fungal mtDNA essential protein-coding genes as one unit, clearly demonstrated the superiority of small genome (mtDNA) over single gene comparisons.

Genetic variability and phylogeny inferred from random amplified polymorphic DNA data reflect life strategy of entomopathogenic fungi

Our analysis of the genetic variability and phylogenetic relationships among the isolates of mitosporic entomopathogenic fungi Paecilomyces fumosoroseus (Wize) Brown & Smith (10 isolates), Paecilomyces farinosus (Holm ex S.F. Gray) Brown & Smith (3 isolates), Paecilomyces lilacinus (Thom) Samson (1 isolate), and Verticillium lecanii (Zimmerman) Viegas (6 isolates) was based on the use of random amplified polymorphic DNA (RAPD) markers. Neighbor-joining analysis characterized P. fumosoroseus and V. lecanii as monophyletic taxa. The analysis also supported monophyly of the genus Paecilomyces. We have used the distribution of genetic distances to demonstrate the genetic variability within studied species, in Paecilomyces spp. (Hyphomycetes) and Aschersonia spp. (Coelomycetes) in particular. We conclude that the RAPD-based analysis is useful for the reconstruction of intraspecific and interspecific relationships of the entomopathogenic fungi. Moreover, it can be employed to study the intergeneric relationships, especially in the case of closely related genera of the hyphomycetous entomopathogenic fungi. We propose that RAPD-based phylogeny and genetic variability reflect their life strategy. In our opinion, the mobility of spores represents a factor with major influence on the evolution of these entomopathogenic fungi.Key words: Paecilomyces, Verticillium, Aschersonia, RAPD, phylogeny, genetic variability.

Ultrastructural and cytochemical characterization of aphid invasion by the hyphomycete verticillium lecanii

Chronological events of the interaction between the hyphomycete Verticillium lecanii and the potato aphid Macrosiphum euphorbiae were investigated by light, scanning, and transmission electron microscopy. The parasitism of M. euphorbiae by V. lecanii appears to involve the following events: (i) adherence of conidia to the host cuticle through a thin mucilagenous matrix; (ii) germination of the conidia and production of mycelium that colonizes the surface of the cuticle; (iii) penetration of germ tubes into the aphid cuticle 24 h after application of the pathogen; (iv) extensive lateral development of hyphae accompanied by pronounced degradation of the cuticular layers by 72 h. Labeling with the WGA/ovomucoid-gold complex showed that penetration and colonization of the cuticle by the fungus resulted from localized enzymatic hydrolysis, likely through the synergistic action of chitinases and mechanical pressure; (v) production of blastospores and massive invasion of aphid internal tissues; (vi) assimilation of nutrients and accumulation of lipids by fungal cells; and (vii) production of conidiophores and release of the fungus from aphid cadavers. These observations bring further insights into the mechanisms by which V. lecanii parasitizes M. euphorbiae. They also provide a basis for comparing the modes of action of V. lecanii against hosts from fungal and animal origins. Copyright 1999 Academic Press.

Entomopathogenic potential of Verticillium and Acremonium species (Deuteromycotina: Hyphomycetes)

Hyphomycetes with conidia formed in slimy heads from hyaline mycelium were isolated from a range of insect hosts. Isolation on artificial medium and microscopic examination revealed that these fungi in many cases were not Verticillium lecanii despite a superficial resemblance to this common entomopathogen. The fungi were identified as Verticillium fusisporum, Verticillium psalliotae, Verticillium lamellicola, and species of Acremonium. Isolates of these fungi were bioassayed against the sweet-potato whitefly (Bemisia tabaci) and against the housefly (Musca domestica) to examine their entomopathogenicity. A test was also conducted with a coleopteran (lesser mealworm, Alphitobius diaperinus) to further evaluate the host range for some of the fungi. V. lamellicola was not pathogenic to the two species of insects treated, while varying levels of pathogenicity were shown for the other species. In general, V. lecanii was the most pathogenic species. Immature whiteflies appeared to be more susceptible to fungal infection than adult houseflies, and the host range for several of the fungi also included lesser mealworm.

Constraints associated with taxonomy of biocontrol fungi

An increasing number of fungi are being collected and screened for the biological control of pests, but nomenclature, taxonomy, and correct identification of many of them remain problematic. Trichoderma spp. and the rust fungi are promising candidates for biocontrol agents, yet they present serious taxonomic problems. Several groups of entomopathogenic fungi are also potential biocontrol sources of agents. Genera such as Cordyceps, Aschersonia, Verticillium, Beauveria, and Metarhizium are in need of urgent revision. Before their release can be considered, all details of their complex life cycles and taxonomy have to be elucidated to satisfy quarantine authorities. Formerly, the taxonomy and identification of biocontrol fungi has been based largely on morphological structures, but molecular techniques have been introduced to provide more objective criteria. Beauveria and Metarhizium spp. have been subjected to several molecular techniques that have resolved phylogeny and species concepts. In Metarhizium, a high degree of genetic diversity is present. Incorrect typification, lack of holotypes, and poorly resolved life cycles and unclear teleomorph – (syn)anamorph connections are serious taxonomic contraints. Only a small fraction of the fungi that can be used for biological control has been examined. The destruction of habitats and ecosystems for these fungi will result in the disappearance of fungal germ plasm of potential value, not only for biological control but also as a source of novel metabolites. This loss and the decline of taxonomic expertise in the fungi are major problems. Key words: biocontrol, taxonomy, Trichoderma, Beauveria, Metarhizium, Cordyceps.

Molecular systematics of the Hypocreales: a teleomorph gene phylogeny and the status of their anamorphs

Phylogenetic relationships among 40 species in the Hypocreales and Clavicipitales were inferred from sequence data obtained from the nuclear large-subunit ribosomal DNA. Cladistic analysis of these data support the monophyly of the Hypocreales, with the Clavicipitales derived from within the Hypocreales. Four groupings were resolved and are informally designated as the Hypocrea, Claviceps, Bionectria, and Nectria groups. Phylogenetic placement of teleomorphs including Melanospora and cleistothecial taxa, such as Heleococcum, Mycoarachis, and Roumegueriella, demonstrate the facility of molecular phylogenies to accommodate taxa with highly modified morphologies. Similarly, the hypocrealean origins of the anamorph species Verticillium lecanii and Acremonium chrysogenum illustrate the potential of the molecular phylogenetic approach to accommodate anamorph isolates within the context of a teleomorph phylogeny. Together these results suggest that a comprehensive classification of the Hypocreales, inclusive of teleomorph and anamorph states, is attainable through a molecular phylogenetic approach. Key words: Ascomycotina, Clavicipitaceae, Hypocreales, phylogeny, pyrenomycete, rDNA.