Habitat Association in Two Genetic Groups of the Insect-Pathogenic FungusMetarhizium anisopliae: Uncovering Cryptic Species?

Abstract

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...

Species associations and distributions of soil entomopathogenic fungi Metarhizium spp. in Japan

“…The observation of Mpi predominance across different habitats in the current study is similar to a previous finding in western Canada (British Columbia) where two closely related genotypes are predominant across different habitats (Inglis et al 2008). The restriction of Mb and Mpe to forest habitats in the current study is partly comparable to the findings of a previous study in eastern Canada in which two genotypic groups separately occupied agricultural and forest environments (Bidochka et al 2001). We also observed high genetic diversity of Metarhizium in a small area.…”

“…In eastern Canada, Mb was associated with the rhizosphere soil of trees rather than with the rhizospheres of plants in open fields (Wyrebek et al 2011), indicating the association of Mb with forested habitat. Furthermore, the cold-active and forest-dominant genotypic group of M. anisopliae sensu lato had been reported in the same area (Bidochka et al 2001), which partially corresponds to Mb (Bischoff et al 2009). Accordingly, Mb in Canada and Japan seem to have similar habitat and temperature preferences.…”

“…Although the habitat types of soil samplings should be considered when discussing the dominant species, because some groups of Metarhizium spp. show clear habitat preferences (Bidochka et al 2001), it appears that Mpi is not as abundant in those research areas as in Japan, because Mpi was most frequently detected in both forest and agricultural environments in Japan. The reason for these geographical differences is as yet unknown.…”

“…Bidochka et al (2001) demonstrated that cryptic species of M. anisopliae s. l. were associated with specific habitat type preferences rather than host insects. They showed that the genotypic groups of M. anisopliae s. l. in eastern Canada were linked to habitat type rather than to insect hosts; isolates from open field habitats (OG1) belonged to a different genetic group compared with those from forest soil samples (OG2).…”

“…This is because, particular entomopathogenic fungal species will adapt to the abiotic conditions of their specific habitat so as to persist in that habitat, as seen in the case of M. anisopliae s. l. in eastern Canada (Bidochka et al 2001). Fungal strains that exhibit good persistence in the introduced environments will have a higher probability of contact with a sufficient quantity of propagules to cause disease and will therefore be effective in pest management.…”