Temperature-Dependent Spore Dimorphism in Burenella dimorpha (Microspora: Microsporida)

Burenella dimorpha, a microsporidian parasite of the tropical fire ant, Solenopsis geminata, produces two morphologically distinct types of spores. The binucleate free spores (spores not bound by a pansporoblast membrane) develop normally at temperatures at least as low as 20°C and as high as 32°C. The uninucleate octospores (spores bound in octets by a pansporoblast membrane), however, develop in a restricted range of temperature. Octospores constituted 35.9%± 2.6 of the spores in 25 pupae held at 28°C. Raising the temperature to 30°C reduced octospores to < 1% of the total spore population. Lowering the temperature to 25° or 22°C reduced the octospore population to 8.5%± 6.5 or 0.4 ± 0.5, respectively. Inhibition of octospore development was complete at 20°C. In contrast, the octospores of Vairimorpha necatrix and Vairimorpha plodiae are reported to be abundant at 16°C and 21°C, respectively. The critical event blocked in octospore development may be meiosis, as evidenced by an abundance of binucleate sporonts in the octospore sequence of development, and absence of more advanced sporogonic stages in hosts held at inhibitory temperatures. Free spore size is not affected by temperature although yield may be slightly reduced at elevated temperature.