Effects of microsporjdia on the striated parietal muscle of Rhynchosciara angelae (Diptera: Sciaridae)

Nuclear hypertrophy and RNA synthesis were studied in the multinucleated striated muscle of Rhynchosciara angelae infected by the microsporidia Thelohania sp. Infection varied from fibers with few microsporidian cells to fibers completely occupied by the infective agent. Usually, the terminal portions of the fibers are free of microsporidia. The host-cell nuclei located in these regions (OM nuclei) are less affected than the nuclei of the same fiber that are completely surrounded by the cells of the parasite (IM). There is a correlation between the volume attained by the OM nuclei and their location in the fiber: the closer the nuclei to the microsporidia the higher the nuclear hypertrophy. Despite the errors peculiar to the autoradiography, grain counting showed that the nuclei of infected fibers are more active in the incorporation of RNA precursor than the normal noninfected fibers. Two additive mechanisms are suggested through which the microsporidia could influence the host-cell nucleus: (1) enhancement of the metabolism induced by juvenile hormone and (2) a further increase in the overall metabolism to repair the disruption imposed by the microsporidian development.