Characterization of Metarhizium species and varieties based on molecular analysis, heat tolerance and cold activity

Aims:  The genetic relationships and conidial tolerances to high and low temperatures were determined for isolates of several Metarhizium species and varieties.
Methods and Results:  Molecular-based techniques [AFLP and rDNA (ITS1, ITS2 and 5·8S) gene sequencing] were used to characterize morphologically identified Metarhizium spp. isolates from a wide range of sources. Conidial suspensions of isolates were exposed to wet heat (45 ± 0·2°C) and plated on potato dextrose agar plus yeast extract (PDAY) medium. After 8-h exposure, the isolates divided clearly into two groups: (i) all isolates of Metarhizium anisopliae var. anisopliae ( Ma-an ) and Metarhizium from the flavoviride complex ( Mf ) had virtually zero conidial relative germination (RG), (ii) Metarhizium anisopliae var. acridum ( Ma-ac ) isolates demonstrated high heat tolerance ( c . 70–100% RG). Conidial suspensions also were plated on PDAY and incubated at 5°C for 15 days, during which time RGs for Ma-an and Ma-ac isolates were virtually zero, whereas the two Mf were highly cold active (100% RG).
Conclusions:  Heat and cold exposures can be used as rapid tools to tentatively identify some important Metarhizium species and varieties.
Significance and Impact of the Study:  Identification of Metarhizium spp. currently relies primarily on DNA-based methods; we suggest a simple temperature-based screen to quickly obtain tentative identification of isolates as to species or species complexes.