Akio Murakami

Cytological evidence for holocentric chromosomes of the silkworms, Bombyx mori and B. mandarina, (Bombycidae, Lepidoptera)

The nature of the centromere and the orientation in meiosis of silkworm chromosomes were investigated using the trivalent of the F1 hybrid between the wild and domestic silkworm and X-ray-induced aberrant chromosomes as well as normal silkworm chromosomes. The results of the experiments were as follows: (1) Pro-metaphase chromosomes showed no distinct primary constriction even after treatment with hypotonic solution, (2) sister chromatids separated in parallel along the entire length of the chromosome at mitotic anaphase, (3) chiasmata underwent complete terminalization during diakinesis and thus chromosome dyads were always connected end-to-end by a terminal chiasma at metaphase I, (4) radiation-induced aberrant chromosomes were stably transmitted throughout a number of cell generations, and (5) although the homomorphic bivalents generally orientated axially at metaphase I and equatorially at metaphase II, this normal sequence tended to be inverted or modified in the X-ray-induced aberrant chromosomes and in the trivalent of the F1 hybrid silkworms. These observations may be best interpreted by assuming the holocentric nature of silkworm chromosomes.

Comparison of the Stage Sensitivity to X-rays during Meiosis in the Egg of the Silkworm, Bombyx Mori

SummaryOocytes of the silkworm, Bombyx mori were irradiated with x-rays at various phases of the meiotic stage. The radiosensitivity was determined by scoring the hatchability of irradiated eggs in comparison with unirradiated control eggs.Cells in the dividing phase were more sensitive to x-rays than those in any other phases in both the first and the second meioses, and cells in the first meiosis were more resistant than those in the second meiosis. Furthermore, the sensitivity of the second meiotic cells almost corresponded to that of early mitotic cells. These differences in radiosensitivity may be interpreted as being due to (1) whether or not cells are associated with DNA synthesis or chromosome duplication, and/or (2) the difference in the chromosomal structure at the time of irradiation.It can be concluded that the radiosensitivity in cell-killing during meiosis of silkworm oocytes shows the same pattern as in other insect orders, although there are some differences in the degree of sensitivity.

Mutagenesis of acridine orange in mitotic cleavage nuclei of the silkworm, bombyx mori ☆

Abstract The present study was done to determine whether acridine orange (AO) is mutagenic for the mitotic cleavage nuclei in the silkworm. The mutation frequency was estimated by the specific locus method using egg-color genes. AO was injected into the body cavity of marked female pupae (homozygous for pe and re genes) in active vitellogenesis (prophase I oocytes). The moths emerging from the treated pupae were mated to wild type male moths. AO increased the frequency of mosaic type mutations, indicating that AO has a positive mutagenic action on the paternal chromosomes in the mitotic cleavage nuclei in the silkworm.

X-ray-induced recombination during oogenesis in the silkworm (Bombyx mori l.).

SummaryThe physical induction of recombinational events has been studied in the female silkworm (Bombyx mori), in which crossing-over does not normally occur. Female silkworms heterozygous in the trans type of two egg-color genes,pe (V-0.0) andre (V-31.7), received a single dose of X-rays (1000 R) at various developmental stages. Then they were crossed to marked males homozygous for both genes. The results indicated that X-rays increase the occurrence of recombinational events in silkworm females from first instar larvae to late stage pupae. The spontaneous frequency of exchange type recombinants in the control series was 2.5 x 10−5, while after irradiation the frequency of these recombinants was up to 38.8 x10−5. The sensitive stage to X-ray-induced recombinational events was late stage larvae from fourth to fifth instar. Exchange (cross-over) type recombinants were about three times more frequent than interchange types among the 122 recombinants recovered. The biological significance of the present finding is discussed.

A comparison of mutation induction by 14 MeV fast neutrons and 137Cs γ-rays in meiotic spermatocytes in the silkworm

Abstract This paper presents the results of experiments on the relative biological effectiveness (RBE) of 14 MeV fast neutrons as compared with 137Cs γ-rays for recessive visible mutation induction on the primary meiotic spermatocytes in the silkworm. 1-day-old fifth instar silkworm larvae were treated with five different doses (167, 250, 500, 684 and 1000 R) of γ-rays, or three different doses (160, 250 and 540 rad) of 14 MeV neutrons. Mutations at the pe and re loci were detected by the egg-colour specific locus method. Primary meiotic spermatocytes showed an extremely high radiosensitivity of cell killing which has not been previously found in postmeiotic cells. With 1000 R of γ-rays and 540 rad of neutrons, more than 90% of the cells in meiotic divisions were killed. However, these effects were negligible with lower doses either for neutrons (160 and 250 rad) or γ-rays (167, 250 and 500 R). Hence the results presented are from the remaining low doses for both radiations. The average mutation rate obtained on the two loci was 0.9·10−6 per rad for γ-rays and 1.9·10−6 per rad for neutrons. The mutation rate for γ-rays was somewhat lower than that of spermatids which have the highest rate throughout spermatogenesis. The RBE of 14 MeV neutrons was determined only for whole-body mutants because the frequency of fractional-body mutants was nearly the same as the control. The initial dose mutation frequency for both loci and both radiations were almost linear at lower doses, though at higher doses they tended to reach a maximum followed by a decline in mutation frequency. The RBE was calculated on the assumption that the dose-response curve was linear in lower dose regions. Thus, RBE values, as the ratio of the slope of the weighted linear regression line for neutrons to γ-rays, were estimated to be 1.7 at the pe locus and 2.2 at the re locus. These values are about one-third of those for mature sperm and are much the same as those in primordial germ cells and in spermatogonia.