Comparative development of two microsporidian species: Enterocytozoon bieneusi and Enterocytozoon salmonis, reported in AIDS patients and salmonid fish, respectively.

Enterocytozoon bieneusi and Enterocytozoon salmonis are reported in HIV-infected patients and in salmonid fish, respectively. Both species share the early development of the extrusion apparatus of the spores, which is completed prior to fission of the sporogonic syncytium into sporoblasts, and the early synthesis of polar tube constituents, but they differ in other developmental and sporogenetic processes. Enterocytozoon bieneusi develops in direct contact with the cytoplasm of epithelial cells whereas E. salmonis occurs only in the nucleus of leucocytes and epithelioid cells. Sporogonic nuclei, which are scattered throughout the sporont in E. bieneusi, are located in the periphery in E. salmonis. The multilamellar structures associated with the nuclear envelopes and the endoplasmic reticulum cisternae are specific for E. bieneusi. Additionally, the evolution of the polar tube precursors proceeds differently in the two parasites. In E. bieneusi, they transform into electron-dense bodies associated with a reticulum and polar tubes derive from these structures according to a process similar to that reported in other microsporidia. In E. salmonis, polar tube precursors fuse directly at their ends and the polar tubes appear to be formed by the assemblage of these fused precursors with a material previously synthesized in the vicinity of nuclei. In conclusion, both species appear to be less closely related than was supposed in earlier descriptions.