Insect-parasitic fungi as a model system to investigate coinfections

Living organisms are often infected simultaneously by different parasites. The outcomes of such coinfections can be based largely on the identity of the parasites involved, which renders investigation of other variables difficult. Additionally, most coinfection studies use pairs of very different parasites making it even more challenging to investigate any fundamental aspect of coinfection itself. To address this problem, we report here the establishment of a model system composed of two entomopathogenic fungi, Beauveria bassiana and Metarhizium robertsii, and larvae of the beetle Tenebrio molitor to study coinfections. These two fungi interact in a similar fashion with the host and this allows us to examine coinfections experimentally while largely excluding parasite identity and host-exploitation strategy as variables. Coinfection between both fungi in standard conditions - inoculated at the same time and in the same place - did not enhance virulence towards the host, larvae of the beetle Tenebrio molitor. In this scenario, B. bassiana was the dominant fungus, emerging in 60 % of infected larvae, versus M. robertsii in 12 %, and both parasites emerging in 28 %. We then tested if intrinsic aspects of infection - location of infection, order of inoculation and time lag between infections - could modify this outcome. We found that while host virulence was pratically unchanged, parasite fitness was strongly affected by infection-specific variables. When the fungi were inoculated on different parts of the host body and both fungi sporulated, parasite fitness was greatly reduced. Order of inoculation also had an effect on coinfection, with the first parasite to arrive dominating the host, even for M. robertsii which proved to be a weak competitor in simultaneous infection. However, the time lag between infections did not alter this within-host priority effect. Our findings support the use of the proposed system to study coinfections since we can investigate a wide range of environmental and infection factors, including neglected ones, that can alter host and parasite fitness. We intend to generate a body of theory that also can be used in other biological systems that investigate coinfections. These results may also inform the use of these fungi as biocontrol agents in agriculture, given they may be used in conjunction.