Figure 1.

Nosema/Vairimorpha life cycles. 1. Diplokaryotic meront 2. Primary (autoinfective) diplokaryotic spore 3. Within-host transmission or vertical transmission 4. Secondary (environmental) diplokaryotic spore (not N. granulosis, N. empoascae) 5. Horizontal transmission 6. Formation of sporophorous vesicle (Vairimorpha only) 7. Meiosis and karyokinesis 8. Nuclear division to form plasmodia 9. Monokaryotic sporogony 10. Monokaryotic environmental spore. Sexual species undergo stages 1–10, producing diplokaryotic mitospores and monokaryotic meiospores. The fate of meiospores has never been directly observed in Nosema/Vairimorpha but is assumed to be similar to that within the Amblyosporidae (Figure 2), in which monokaryotic spores differentiate into gametes and undergo cytoplasmic fusion, restoring the diplokaryotic state and completing the life cycle. In V. imperfecta monokaryotic sporogony is aborted at stage 9. Asexual lepidopteran Nosema species such as N. bombycis retain only the diplokaryotic cycles (stages 1–5). The life cycle of N. granulosis is further reduced, retaining only the primary diplokaryotic cycle (stages 1–3).