Habitat association in two genetic groups of the insect-pathogenic fungus Metarhizium anisopliae: Uncovering cryptic species?

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 biol ogical 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 pop ulation genetics analysis of the insect-pathogenic fungus Metarhizium aniso pliae from forested and agricultural habitats in Ontario, Canada, showed a nonrandom association of alleles between two distinct, reproductively isola ted groups (index of multilocus association = 1.2). Analyses of the mitocho ndrial DNA showed no differences between the groups. The two groups were as sociated with different habitat types, and associations with insect hosts w ere not found. The group from forested areas showed an ability for cold-act ive growth (i.e., 8 degreesC), while the group from the agricultural area s howed an ability for growth at high temperatures (i.e., 37 degreesC) and re silience to UV exposure. These results represent a significant paradigm shi ft; habitat selection, not host insect selection, drives the population str ucture 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. W orldwide, M. anisopliae may be an assembly of cryptic species, each adapted to certain environmental conditions. The association of fungal genotypes w ith habitat but not with host insects has implications on the criteria for utility of this, and perhaps other, fungal biocontrol agents.