Recently, Sericin has attracted a lot of attention as a functional material. Practical sericin silkworms which produce sericin effectively have been developed by cross breeding Nd mutant and normal cocoon strains. It has become clear that the states of cocoons and pupae should be surveyed during cocooning and pupation to discover whether silkworms have an Nd gene. Furthermore, it is difficult to distinguish between homozygous and heterozygous strains in the case of dominant genes, such as the Nd gene. However, it was found that the homozygous transformation of Nd genes can be easily distinguished by differences in the behavior of the first instar larvae when they are reared on artificial diets.

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We aimed to identify virulence-associated genes of Serratia liquefaciens FK01 against Bombyx mori. Among 1,200 transconjugants from a transposon library, 4 (ET0234, ET0373, ET0418, and ET0964) showed decreased virulence towards B. mori. Southern hybridization revealed that the transposon was inserted at a single site of the S. liquefaciens genome. The flanking sequences of the transposon indicated that it disrupted the lipopolysaccharide (LPS) synthesis gene in all mutants. The complemented strain restored virulence completely or partially. Thus, LPS contributes to the virulence of S. liquefaciens against B. mori. Serum killing assays indicated that the bacterium was probably killed by the complement system. Since the innate immunity of a host is triggered by bacterial recognition, LPS might inhibit recognition by modifying the bacterial surface in B. mori.

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Gonads develop as testes in males or ovaries in females. In the silkworm, B. mori, little is known about testes and ovaries in the embryonic stages and early larval stages. In this study, morphological and histomorphological observations were performed of ovaries and testes from the late embryonic stage to the first instar larval stage. Results obtained with lack of accurate information on the sex of the examined individuals may be misleading, and thus phenotypic observations of gonads were also performed by utilizing sex-limited strains that enables easy discrimination of female embryos from male ones based on the egg colors. In the testes, four testicular follicles were clearly observed at the first instar larval stage, and boundary layers were formed between the testicular follicles. At the late embryonic stage, the testes consisted of four testicular follicles, while the boundary layers were still obscure. In the ovaries, four ovarioles were easily recognizable at the first instar larval stage, and boundary layers were formed between the ovarioles. However, in the late embryonic stage, it was quite difficult to identify four ovarioles. Morphological characteristics were almost similar between the testes and ovaries in the early developmental stages. The present study demonstrates that the most reliable difference between the testes and the ovaries in the early developmental stages is the attaching point of the duct. Formation and development of the duct may be sensitive to the sex-determining signal and may display sexual dimorphism in early embryonic stages.

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