The effect of temperature and relative humidity on the formation of Metarhizium anisopliae chlamydospores in tick eggs

The influence of ambient conditions on the development of Metarhizium anisopliae chlamydospores in tick eggs is reported for the first time. The infection of tick eggs by M. anisopliae involves common events, such as adhesion, conidial germination, appressoria formation, invasion, and development within the eggs. However, the final stage of fungal development differs according to the environmental conditions. At high humidity (close to 100 %) and moderate temperature (25 °C) the fungus emerged from the eggs and formed conidiophores and conidia externally on the dead eggs. Elevating the temperature to 30 °C or reducing humidity to 55–75 % induced the production of chlamydospores inside the eggs, without conidiogenesis. When eggs with mature chlamydospores were returned to the appropriate conditions (25 °C and 100 % RH), conidiogenesis was recovered. Formation of chlamydospores, observed by means of histology and TEM, began with the thickening and septation of hyphae. As the chlamydospore wall thickened a new external undulated wall layer appeared. The mature chlamydospore in eggs has an oval shape (5.3 ± 0.9 μm long, 2.5 ± 0.2 μm wide); its wall comprises three distinct layers. The ability of M. anisopliae to produce chlamydospores under harsh conditions is advantageous and should be considered in application.