Early events in the infection of a Helicoverpa zea cell line by Nosema furnacalis and Nosema pyrausta (Microspora: Nosematidae)

We compared the early events in the in vitro development of two microsporidia, Nosema pyrausta and Nosema furnacalis, that infect the European and Asian corn borers, Ostrinia nubilalis and Ostrinia furnacalis, respectively. Spores of both species, produced in O. nubilalis larvae, were activated in alkaline saline solutions and used to infect the Helicoverpa zea cell line BCIRLHZAM1. In both N. pyrausta and N. furnacalis incubated at 31°C, the first sporogonic cycle resulted in the differentiation of early spores that germinated intracellularly. N. pyrausta showed a 24-hr lag phase followed by a growth phase during which meronts had an approximate doubling time of 8 hr. Early spore differentiation began at 48 hr pi, and by 72 hr pi infected cells were filled with empty spores. There was a single increase in the percentage of N. pyrausta infected cells at 72 hr pi as early spore production ceased and the late spores that differentiated subsequently did not germinate intracellularly. In contrast, N. furnacalis had a 12-hr lag phase followed by a growth period when meronts had an approximate doubling time of 4 hr. Early spore differentiation was observed 24 hr pi, and at 34.5 hr pi most infected cells were filled with empty spore cases. Subsequently, N. furnacalis also differentiated late spores but continued to produce early spores leading to continued cross-infection of host cells by N. furnacalis during a 7-day culture period. These results indicate that N. pyrausta and N. furnacalis differ from one another in their patterns of growth and sporogony in H. zea cell cultures.