So why is M. acridum so picky while its close relative is so indiscriminate? Amazingly, it appears to come down to a single gene call Mest1 - a gene present in M. robertsii, but is absent in M. acridum. To find out the function of this gene, a group of researchers in China created a mutant M. robertsii strain which has a non-functioning copy of Mest1. This mutant lost its ability to infect most insects - except grasshoppers and locusts - which happens to be the speciality of M. acridium. In parallel, the researchers also inserted functional copies of Mest1 into M. acridum. The insertion of this single gene allowed M. acridium to infect a wider range of insects.
What is so special about Mest1? In M. robertsii, Mest1 is expressed during spore germination, and plays an important role in initiating the infection process. Mest1 expression can be triggered by a range of stimuli including nutrient poor conditions or contact with insect cuticle. Metarhizium acridium has other genes playing the role of Mest1, but they are triggered by substances which are present only in the waxy coating of grasshoppers and locusts. So if its spores land on other insects such as caterpillars which have a different type of coating, M. acridum fails to germinate because the appropriate stimuli are absent. Thus, the insertion of Mest1 into M. acridium allows the fungus to bypass those usual stimuli and begin germinating under a wider range of conditions
Host specificity is one of the central question in the evolutionary biology of parasitic organisms. In this case, we can see how a single gene can changed this otherwise specialist pathogen into a broad-spectrum generalist.