Strains of insect-pathogenic fungi with high virulence toward certain pest insects have great potential for commercial biological control applications. Identifying such strains has been a central theme in using fungi for biological control. This theme is supported by a persistent paradigm in insect pathology which suggests that the host insect is the predominant influence on the population genetics of insect-pathogenic fungi. In this study, a population genetics analysis of the insect-pathogenic fungus Metarhizium anisopliae from forested and agricultural habitats in Ontario, Canada, showed a nonrandom association of alleles between two distinct, reproductively isolated groups (index of multilocus association ‫؍‬ 1.2). Analyses of the mitochondrial DNA showed no differences between the groups. The two groups were associated with different habitat types, and associations with insect hosts were not found. The group from forested areas showed an ability for cold-active growth (i.e., 8°C), while the group from the agricultural area showed an ability for growth at high temperatures (i.e., 37°C) and resilience to UV exposure. These results represent a significant paradigm shift; habitat selection, not host insect selection, drives the population structure of these insect-pathogenic deuteromycetous fungi. With each group we observed recombining population structures as well as clonally reproducing lineages. We discuss whether these groups may represent cryptic species. Worldwide, M. anisopliae may be an assembly of cryptic species, each adapted to certain environmental conditions. The association of fungal genotypes with habitat but not with host insects has implications on the criteria for utility of this, and perhaps other, fungal biocontrol agents.Insect-pathogenic fungi have genetic features related to insect infection (17), and the population genetics of these fungi are also assumed to be influenced primarily by host insect taxa (5,6,14,19,20,24,28,33,35). Metarhizium anisopliae is an insect-pathogenic, haploid, deuteromycetous fungus that is assumed to reproduce clonally, and it is also assumed that certain genotypes are related to an insect host (5,6,14,19,20,24,28,33,35). It also has the potential for parasexual reproduction (36), although an analysis of clonality versus recombination has not been undertaken. One of the distinctive features of a clonal population is the widespread occurrence of identical genotypes (23). Here we have undertaken to determine the population structure of M. anisopliae and to test the paradigm that certain genotypes are related to insect hosts.M. anisopliae is a recognized pathogen of more than 200 insect species, including several major pests (29). It is a recurrent paradigm in the literature that the insect host drives the population structure, i.e., that there are fungal isolates or genotypes adapted for pathogenesis toward certain species or taxa of insects (5,6,14,19,20,24,28,32,35). Because this fungus offers an environmentally safe alternative to chemical pesticides, it is of...

A persistent paradigm in insect pathology is one that relates the insect host to certain genetic groups of insect-pathogenic fungi. This paradigm assumes that the genotype of an insect-pathogenic fungus coevolves with a certain taxon of insect host that it infects. The insect-pathogenic fungus Beauveria bassiana shows a wide host range and is considered to be a facultative insect pathogen. In this study, a population genetics analysis of B. bassiana from forested and agricultural habitats as well as from the Canadian Arctic showed distinct genetic groups associated with the three different habitats. Within each group, recombining population structures and clonally reproducing lineages were observed. The B. bassiana isolates were also assessed for their abilities to grow at 8, 15, 25 and 37 degrees C and for their tolerances to UV exposure. The genetic groups from the Arctic and from the forested habitats grew at lower temperatures, while the genetic group from the agricultural habitat grew at 37 degrees C and was tolerant to UV exposure. There were no clear associations between the genetic group and the ability to infect coleopteran or lepidopteran insect larvae. There is increasing evidence that such studies represent a significant paradigm shift; habitat selection, not insect host selection, drives the population structure of deuteromycetous insect-pathogenic fungi. We suggest that adaptation to a certain habitat type is an important criterion for identifying insect-pathogenic fungal strains for use in insect biocontrol efforts.

Aims:  Conidium production by three species of insect pathogenic fungi, Metarhizium anisopliae , Beauveria bassiana and Verticillium lecanii , was assessed on various depths and types of commercially available agars. Methods:  Conidium production was assessed after 14 d of growth on commercially available media as well as at three different agar depths. Results:   Metarhizium anisopliae and B. bassiana isolates showed greatest conidium production on potato dextrose agar (PDA) at a depth of 2 mm, whereas V. lecanii showed greatest conidium production on yeast extract‐peptone‐dextrose agar (YPDA) regardless of agar depth. Optimum conidium production for M. anisopliae and B. bassiana was not only dependent upon the isolate used but also on the medium type and agar depth. Significance and Impact of the Study:  Conidia are the infective structures for insect pathogenic fungi and this study suggests a rationale basis for consistent conidium production for laboratory and commercial practices.

From a large expressed sequence tag (EST) database representing several developmental stages of Puccinia triticina, we discovered a mitogen-activated protein kinase (MAPK) with homology to kinases with known pathogenic functions in other fungi. This PtMAPK1 is similar to the Ustilago maydis MAPK, Ubc3/Kpp2, but has a longer N-terminal extension of 43 amino acids (aa) with identities to U. maydis Kpp6, a homolog of Ubc3/Kpp2 with a 170-aa N-terminal extension. Ubc3/Kpp2 is involved in mating and subsequent pathogenic development, whereas Kpp6 functions during invasive growth in corn tissue. PtMAPK1, expressed from a Ustilago sp.-specific promoter, was able to complement a ubc3/kpp2 deletion mutant and restore mating. It also substantially increased virulence on corn, measured as tumor formation, of a kpp6 deletion mutant. Moreover, this construct restored to near-full pathogenicity a ubc3/kpp2 kpp6 nonpathogenic double deletion mutant. Complementation of the ubc3/kpp2 mutant with the complete PtMAPK gene and verification of expression by reverse-transcription polymerase chain reaction indicated that the rust promoter is recognized in U. maydis. Phylogenetically, these basidiomycete plant pathogens are related, which was reflected in comparison of P. triticina ESTs to U. maydis gene sequences. The U. maydis heterologous expression system allows functional analysis of rust genes, currently frustrated by the lack of efficient transformation and selection procedures.

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