Yataro Tazima

The sequence around the 5′ end of the fibroin gene from the wild silkworm, Bombyx mandarina, and comparison with that of the domesticated species, B. mori

SummaryThe fibroin gene of the wild silkworm Bombyx mandarina was identified through hybridization with the fibroin DNA of the domesticated species, B. mori. Using their cross homology, we isolated the clone carrying the fibroin gene from a B. mandarina genomic library in λ Charon 30. A resultant recombinant phage contained the DNA fragment corresponding to the 5′-end region of the B. mori fibroin gene. After subcloning the DNA fragment into pUC12, its DNA sequence was partially determined. Comparison of the nucleotide sequence of the 5′-end region from the two species showed that the B. mandarina fibroin gene is highly homologous to that of B. mori but the extent of homology varies in different functional domains within this region. In particular, the promoters and enhancer-like element sequences are identical in the two species, indicating that these regions are essential for fibroin gene expression irrespective of species. In contrast, there is an appreciable amount of base change in the region far upstream from the enhancer-like element sequence and in the intron-region. These results support the view that the B. mandarina which exists in nature at present is a possible ancestor of the domesticated silkworm, B. mori.

Naturally occurring mutagens of biological origin: A review

Summary Many naturally occurring mutagens have been discovered either from a sudden outbreak of toxicosis in livestock and poultry or from the frequent incidence of hepatomas or cancers among inhabitants of certain districts. These are pyrrolizidine and some other alkaloids contained in higher plants (Senecio, Crotararia, Heliotropium, etc.), cycasin in cycads, bracken toxins in ferns and several kinds of mycotoxins produced by fungi. Chemical structures and major biological effects of those substances are discussed in relation to their carcinogenicity and mutagenicity. Some, for instance cycasin and sinigrin, are not mutagenic by themselves but their enzymatic metabolites are known to be mutagenic. From the view-point of genetic hygiene the most important group may be that of mycotoxins, because those are contained as contaminants of several sources in human food-stuffs. Since species and strains of fungi that produce toxic substances are so large in number and their chemical structures vary widely, it is almost impossible to describe the general features of those substances. Mutagenic investigations of those substances are as yet very limited, but increasing evidence for a close relationship between carcinogenicity and mutagenicity suggests that most of them are mutagenic. In view of the probable significance of mycotoxins on the incidence of spontaneous mutations as well as etiology of cancel, the necessity for further investigation is stressed.

Frequency pattern of mosaic and whole-body mutants induced by ionizing radiations in post-meiotic cells of the male silkworm

Abstract This paper is concerned with mutation spectra induced by γ- or X-rays in postmeiotic germ cells of male silkworm with special regard to the change in frequency pattern of mosaic (fractional) and whole-body (complete) mutants. When silkworm spermatozoa are irradiated with γ-rays, the frequency of whole-body mutants increases linearly with increasing dose, whereas that of mosaics increases exponentially. The latter relation is in striking contrast to that observed in Drosophila, where the incidence of mosaics has been known to reach a plateau at a relatively low dose level. Several possibilities are considered for the interpretation of the difference. Mosaics produced after irradiation of silkworm spermatozoa appear to form a mixed class of at least two types, single-helix type and chromosome-aberration type. The proportion of both may vary depending on the irradiated dose. The incidence of mosaics in comparison to that of whole-body mutants varies considerably with the progress of spermatogenesis. On a per R-basis, spermatogonia yield the lowest frequency of mosaics; the frequencies of both complete and mosaic mutations increase sharply through meiotic prophase up to V-4.5 (fifth instar larvae, day 4.5). Until this stage relatively more completes than fractionals are produced. Around V-4.5 the ratio of mosaics to completes reaches unity. From this stage onward, during spermiogenesis, relatively more mosaics than completes are produced, although their absolute frequencies of induction are far lower than in V-4.5. This observation may be interpreted as resulting from a greater capacity for repair of premutational damage leading to mosaic mutations in the early than in the late stages of spermiogenesis.

Chemical Mutagenesis in the Silkworm

For screening environmental mutagens, several convenient systems have been developed, using bacteria or cultured mammalian cells. Most of these systems deal with mutagenic events that occur in somatic cells. From the genetic viewpoint, however, we need systems that can be applied for the detection of mutagenicity that affects germ cells in the gonad and produces mutations to be transmitted to the offspring. Systems adequate for these requirements are rather limited, and thus, Drosophila has been frequently used in a number of laboratories. We have, however, developed for this purpose a convenient detection system using the domesticated silkworm, Bombyx mori L.

Specificity of interactions between the individual gene locus and the structure of chemical mutagens

SummaryRelative frequencies of visible mutations (egg colour mutants) induced by two similar mutagenic agents of the mustard group, nitromine (methyl-bis-β-chloroethyl-amine N-oxide) and N. M.-alanine (N-bis-β-chloroethyl-alanine), have been studied in the silkworm. The solution of either agents was injected into the wild male moths, which were then mated to double recessivepere/pere female moths. (Normal eggs are black,pere/pere andpe/pe eggs are white andre/re eggs, red.) Eggs produced from these matings were examined for their colour and the mutation rates were estimated by the ratio of uncoverings at the marked loci to the total pigmented eggs.Analysis of the results has revealed that the ratio of the mutation rates at thepe-locus to those at there-locus varies markedly according to whether nitromine or N. M.-alanine is injected. When nitromine was used the ratios ofpe-aberrants tore-aberrants were 1.06 and 1.58, while in the case of N. M.-alanine the ratios were 0.59 and 0.60. In many other experiments in which ♀♀ moths were treated with nitromine, thepe/pe ratio was consistently greater than 1 and often as high as 2. The same was true for experiments with irradiated male moths. These results have been discussed in relation to possible specific interactions between chemical mutagens and chromosome loci.

Sable, A new Mutant Type of Bombyx mori, induced by X-rays

(1) ホモの黒縞黄血の雌蛹に X 線を照射して之に形蠶白血の雄を配した F1 にセーブル斑紋蠶 (Saにて表示) と名付ける1頭の突然變異體が見出された。(2) セーブル斑紋は姫蠶に對し上位, 形蠶•暗色蠶に等位, 黒縞蠶に對し下位である。(3) Sa は P と對立因子をなすにも拘らず Y と Sa との交叉價は1.8%に過ぎない。(4) Sa は形態的特徴の外に劣性致死作用を伴ふ。致死作用は卵の點青期以前に發現する。致死の原因は缺失を伴ふ逆位の爲めと考へられる。(5) Sa, Y 兩因子を同一染色體上に有する個體に Py を交雜すれば F1 に PSaY を得る。此の PSaY 雄を py 雌に配すれば再び少數ながら PSaY が得られる。後の場合に得られる PSaY 相互交配に於ては致死作用は消失する。(6) 此の致死作用を失つた PSaY の出現は多分 Py 染色體と SaY 染色體の轉座, 或は兩染色體の永久的不分離現象に基くものであらう。(7) Sa の成因に關して二つの想像説が存在し得る。一つは Ps→Sa なる因子突然變異であり, 他は Ps を主因子とする變更因子が存在し, 其の缺如に基因すると假定するのであるが, 恐らく後者の方がより妥當であらう。