Silkworm developmental stages

FIGURE 1. Schematic illustration of gametogenesis of the silkworm in relation to the developmental stages in both sexes. Duration represented by a dashed line indicates a period during which differentiation of the same cell type continues but utilizable gametes are not produced, because of the shortness of time. (I, II,. . . ) First, second,. . . larval instars. Meiosis denotes meiotic metaphases I and II. 

Experimental data with chemical mutagens have been scanty in the silkworm. Recently, Datta et studied the induction rate of dominant lethals with the well-known mutagen EMS. They used as material Nistari, a strain indigenous to India. The chemical was administered by injection at a dose of 0.04 ml per insect. Three different concentrations, 0.05, 0.10, and 0.15%, were prepared in 0.75% aqueous solution of NaCl, and all these concentrations were injected at three different developmental stages of the silkworm the 5th instar larvae Just after the 4th molt and 2-day-old and 7-day-old pupae. 

TABLE 5. Induction of Specific Locus Mutations Induced by EMS at Several Developmental Stages of Silkworm Male Germ Cells... 

Strain difference in radiosensitivity has been studied extensively in the silkworm, and seven representative strains, sensitive, intermediate, and resistant, have been established.These were first extracted from the worldwide collection of silkworm strains maintained at the National Seri-cultural Experiment Station, Tokyo, using embryonic killing as a criterion. The difference in mutation response between the most and the least sensitive strains is about 7- to 9-fold when spermatogonia are irradiated. However, it becomes smaller with the progress of the developmental stage of those germ cells, namely, 2- to 4-fold in spermatids and almost unity when mature sperm were irradiated (for details, see Tazima ). 

FIGURE 4. Variation in the mutagenic efficiency of EMS and MMC with the progress of developmental stage of the male silkworm. For comparison, a similar curve obtained for X-ray treatment is shown. Courtesy of the Indian Society of Genetics and Plant Breeding. 

To elucidate the mechanism(s) underlying the discordance, Murakami and Tazima compared the metabolic activation potentialities of these two insects using the mutagenicity to Salmonella as a criterion. In this experiment, microsome fractions of the silkworm were prepared from the whole-body homogenates of silkworms at various developmental stages. The technique was almost the same as that reported by Ames et except that no enzymatic... 

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