Local adaptation of a fungal pathogen introduced into a soil community

Identifying the genetic basis for natural selection is a fundamental research goal, and particularly significant for soil fungi because of their central role in ecosystem functioning. Here, we identify rapid evolutionary processes in the plant root colonizing insect pathogen Metarhizium robertsii. While adapting to a new soil community, expression of TATA box containing cell wall and stress response genes evolved at an accelerated rate, whereas virulence determinants, transposons and chromosome structure were unaltered. The survival of diversified field isolates was increased, confirming that the mutations were adaptive, and we further show that large populations of Metarhizium are principally maintained by associations with plant roots rather than insect populations. These results provide a mechanistic basis for understanding mutational and selective effects on soil microbes. A loop design was used to compare differential gene expression between the original input strain Metarhizium robertsii ARSEF 2575 (designated as Control) and 5 field strains (designated as Re01, Re02, Re03, Re04 and Re05). The control samples contained samples of input strain Mr2575 were compared to themselves to establish a base-line for natural variation in gene expression and standardize the data and confirm there were no technical issues with the design. These design generated seven-condition experiment: Control vs. Control (five biological replicates, no dye swapping), Control vs. Re01 (three biological replicates, dye swapping), Re01 vs.Re02 (three biological replicates, dye swapping), Re02 vs. Re03 (three biological replicates, dye swapping), Re03 vs. Re04 (three biological replicates, dye swapping), Re04 vs. Re05 (three biological replicates, dye swapping) and Re05 vs. Control (three biological replicates, no dye swapping). Biological replicates were independently grown and harvested. Three replicates per array (in-slide replicates). Dye-swap was performed as specified in parenthesis.