Pathobiology of Burenella dimorpha Jouvenaz and Hazard (Microspora: Microsporida)

Abstract

Burenella dimorpha Jouvenaz and Hazard is a host-specific, dimorphic microsporidium that parasitizes the tropical fire ant, Solenopsis geminata (F.). The red and black imported fire ants, Solenopsis invicta Buren and Solenopsis richteri Forel, can be infected perorally by B. dimorpha, but the parasite survives only a few generations in colonies of these factitious hosts. Nine species of ants other than fire ants (representing eight genera) were refractory to infection. Infected pupae develop pathognomonic signs (eye teratology and blister-like clearings in the occiput and petiole) that are due to destruction of the cuticle. In an advanced stage of disease, the pupa ruptures and is cannibalized by worker ants. The spores and other particulate matter are not ingested, but are diverted to the infrabuccal cavity, formed into a pellet, expelled, and fed to fourth-instar larvae only. The intracolonial cycle of infection is thus from ruptured, diseased pupae to fourth-instar larvae via the adults, who are mechanical vectors. The binucleate, nonpansporoblast membrane-bounded (NMB) spores are infective perorally for larvae; the uninucleate, pansporoblast membrane-bounded (MB) spores are not infective, and their function is unknown. These spore types are tissue specific: NMB spores develop from disporous sporonts in the hypodermis; MB spores develop in octets from multinucleate sporonts in the fat body. The development of MB spores is temperature-dependent. The lower thermal threshold for MB spore development is between 20 and 22.5 C; the upper thermal threshold is below 32 C. A hypothesis is advanced that these limits are set by the stability of an enzyme(s), and that the physiological function blocked in MB sporulation is meiosis. Burenella dimorpha has two cycles of merogony (vegetative multiplication). The meronts of the first cycle were originally described as uninucleate cells that become binucleate and divide. However, these nuclei are actually pairs of nuclei in a diplocaryotic arrangement. Surface structure has been seen for the first time in MB spores. The surface of NMB spores is smooth.