Logistic growth of the host-specific obligate insect pathogenic fungusEntomophthora muscaein house flies (Musca domestica)

Insect pathogenic fungi (IPF) need to overcome the host immune system in order to sporulate and ensure transmission to new hosts. Some IPF produce immunosuppressive toxins, whereas others rely on rapid fungal proliferation to kill the host by sheer fungal mass, resulting in a trade-off between allocating resources to toxin production and fungal proliferation. The obligate entomopathogenic fungus, Entomophthora muscae sensu stricto, is host specific to the common house fly, Musca domestica. E. muscae grows as protoplast cells without cell walls and is not known to produce toxins. Here, we assessed the growth of E. muscae, in vivo, using real-time PCR to measure the amount of a single-copy actin gene. We find that E. muscae exhibits S-shaped logistic growth between time post-exposure and the number of fungal nuclei. The results show that E. muscae initially grows exponentially inside the host until depletion of available nutrient sources signifies the ‘limiting capacity’ where after the host is killed. This growth pattern differs markedly from toxin-producing IPF species of Metarhizium and Beauveria in which maximal (plateau) growth and sporulation do not occur until well after the death of the host.