Abstract: A new morphological variant strain was produced by infecting the mulberry looper, Hemerophila atrilineata [Phthonandria atrilineata], with Nosema bombycis successively for 24 times, and named 24Nbh. Comparative studies on morphology, infectivity and spore surface protein were conducted. 24Nbh was short and wide, and had a significant difference (P<0.01) over the Nb spores. The infectivity tests conducted on second instar silkworm larvae showed that IC50 of 24Nbh was 1.98×104 spores/ml and of Nb was 1.72×103 spores/ml, thus indicating that the infectivity of Nb decreased 11.5 times after multiplying in mulberry looper for 24 times. The IC50 of spores from silkworm infected with 24Nbh was 6.9 time less than Nb, showing that the infectivity of 24Nbh spores rejuvenated very fast when reinfected to silkworms, furthermore, the length and width of such spore was larger (P<0.01) than 24Nbh and smaller (P<0.05) than Nb. The SDS-PAGE profiles of Nb and 24Nbh were generally the same, distinct 4 proteins of 12 kD, 17 kD, 30 kD, 33 kD were obtained with differences in quantity. When 120 μg of protein was used for 2D-PAGE, five suspected different proteins with difference in quantity was observed. These results demonstrate that these differential proteins maybe associated with variation in infectivity.

Key words: Nosema bombycis, Morphological variation, Infectivity, Surface protein, Two dimensional electrophoresis