Among strains of the silkworm, Bombyx mori, large differences exist in susceptibility to the entomopathogenic fungus Beauveria brongniartii when it is injected into the larval hemocoel. This infection route bypasses the integument, suggesting that internal factors, such as those for cellular and humoral defenses, may correlate with resistance to B. brongniartii. Here, we investigate causes of this resistance. We injected the conidial suspensions into larvae of susceptible and resistant strains; the difference in susceptibility resulted from the difference in the speed and quantity of multiplication of hyphal bodies in the hemocoel. Phenoloxidase activity of the hemolymph declined after hyphal bodies increased in the larval hemocoel of both strains. There were no differences in the mortality rates when the larvae were injected with conidial compared with hyphal body suspensions in both strains; therefore, the mechanism of silkworm resistance to B. brongniartii was likely independent of differences between conidia and hyphal bodies. Furthermore, the germination rate and germ tube length of the conidia and multiplication of the hyphal bodies within the larval hemolymph in vitro did not differ significantly between strains. In addition, granulocytes of the susceptible and resistant strains phagocytosed fluorescent latex beads. We transformed B. brongniartii to express green fluorescent protein; the transformed hyphal bodies were phagocytosed by the larval granulocytes of both strains. Then the hyphal bodies grew from the granulocytes in the susceptible strain but not in the resistant strain; therefore, the abilities of the strains to suppress the fungus within the granulocytes differed.
The domesticated silkmoth, Bombyx mori, and the wild silkmoth, B. mandarina, are close relatives. Fertile hybrids can form by the mating of female moths of B. mori with male moths of B. mandarina. Recent studies, however, have found no gene flow from domesticated silkmoths into natural populations. Here, we directly analyzed the rate of interspecific hybridization in conventional sericulture by examining the mitochondrial genotype of silkmoths caught around sericulture farms. Results showed that all of the 3,750 moths had B. mandarina-type mitochondrial genomes; there were no F1 hybrid moths. This result was supported by experiments showing that no moths emerged after 2,964 F1 larvae had been released on the ground close to a mulberry tree. These results indicate practically no genetic introgression from B. mori to B. mandarina, therefore suggesting that transgenic silkworm rearing in sericulture farms does not have adverse effects on biological diversity through hybridization.
We previously found that rRNA of BM-N cells derived from the silkworm Bombyx mori undergoes rapid and extensive degradation through site-specific cleavage during abortive infection with nucleopolyhedroviruses (NPVs) of Autographa californica (AcMNPV), Hyphantria cunea (HycuMNPV), Spodoptera exigua and S. litura. Here, we demonstrated that rRNA degradation also occurs in Bme21 and Bm-aff3 cells, which are derived from B. mori embryo and fat body, respectively, during infection with AcMNPV. rRNA degradation in Bme21 cells was also observed following HycuMNPV infection, but was not detected in Bm-aff3 cells. We further showed that rRNA in a cell line derived from B. mandarina, an ancestor of B. mori, underwent degradation in response to cellular infection with AcMNPV and HycuMNPV. In contrast, no rRNA degradation was observed in a cell line derived from Antheraea pernyi. Taken together, these results indicate that NPV-triggered rRNA degradation represents a mechanism of innate antiviral immunity that is unique to B. mori and B. mandarina cells.