家蚕遺伝学への寄与 : 特に形質発現の機構について(4)
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1. On the conception of " Compound Locus " The 14th linkage group in the silkworm was established by Hasimoto in 1941 from the fact that the recombination value between U (Ursa marking) and odk (E-15-translucent) is 8.0. The present author discovered that U is allelic to Di (Dirty marking), Nl (No-lunule), les (light eye-spot) and to oa (aojuku-nl translucent). In the larva homozygous for Di, no crescents and star-spots are found, while they emerge in the heterozygote, that is, the dorsal pattern of Di is dominant to the Normal marking (*), and it is recessive as to the development of crescents and star-spots. On the other hand, oa and les are simple recessives to their Normals. All of these three recessive characters, oa, les and the disappearance of crescents and star-spots of Di, usually behave as the pseudo-dominants in F1 when crossed with No-lunule. Ni is lethal when homozygous and in spite of its allelic relation to U, the recombination value between odk and Ni is about 2.7. These facts evidently show that No-lunule individual is heterozygous for the chromosome involving a deficiency. The loci of Di and oa on t he 14th chromosome can not be the same, because map distances from odk to Di and to oa are 10.7 and 9.7 respectively (Fig. 1). Nevertheless, no crossing over is usually found between U, Di and oa. It is postulated, therefore, that this may be a case of pseudoallelism. This assump tion is somewhat different from the general concept of pseudoalleles, because the mechanism of manifestation of Di or U, a type of the marking, and of oa, a physiological trait, do not belong to the same category.【Figure1】 The Ursa marking is a character which reveals a black side pattern, eyespot, crescents and star-spots besides a black dorsal pattern, while Di reveals a dorsal pattern and eye-spot, but no side pattern, and both crescents and starspots disappear in homozygous constitution. Di and U hold, therefore, the dorsal pattern in common. On the other hand, the Normal marking consists of eye-spot, crescents and star-spots, while les lacks only eye-spot from the Normal (Fig. 2). Thus each of U, Di, les and - - consists of several elements which are manifested pleiotropically, not being separated usually. It can, however, be supposed that various combinations of those elements in different marking types may show that each of the elements is nothing else than a unit of mutation.【Figure2】 From the differences in recombination values between Di, U and oa, pseudoallelic behavior of U, Di, NI, les and oa each other, pseudodominant phenomena of these recessive phenotypes as well as the mutual differences in these characters, it is assumed that these units of mutation are arranged near to each other on the 14th chromosome, constructing the U-locus as shown in the following schema (Fig. 3). Experimentally oa unit must be considered to be involved in the LT-locus. But the author prefers, for the moment, to reserve this conclusion owing to a peculiar behavior of oa unit (Cf. later paragraphs). In conclusion, the assumption above-mentioned has a strong resemblance either to the pseudoallelism hypotheses in which a locus consists o f a series of genes showing similar action with each other (Dunn and Caspari 1945 ; Lewis 1951 ; McClintock 1951 ; Green and Green 2956), or to the hypotheses in microorganisms in which a locus (segment) is further capable of subdividing into several sites (regions) (Pontecorvo 1952 ; Pritchard 1955 ; Demerec 1957 ; Benzer 1957). The case of the U-locus is, however, different from these in the fact that the units of mutation belonging to the same locus control functionally different elements which are seen combined in one individual larva. The author desires to propose to call such a locus a " Compound Locus. " 2. On selection of the No-lunule No-lunule (NI) is a recessive lethal owing t o a chromosomal de ciency. In this mutation the crescents and star-spots are usually incomplete and fairly variable in their manifestation or entirely disappeared in extreme cases. Selecting the individuals of incomplete crescents, we were able to obtain them in 100 % of the segregating NI-genotype in several generations, while we failed to obtain a strain, all larvae of which entirely lacked the crescent pattern, segregating ingividuals with incomplete crescents at about 43 % on average, even after a continued selection carried out for 11 years or more. This means more efficiency of selecting the type with incomplete crescents than selecting the type with no trace of crescents. When the larvae with incomplete crescents are selected in an original culture which produced them in very lower ratio, it was possible to increase their ratio up to 90 % in only two generations. Though it is difficult to explain this fact in the state of our present knowledge, the author would like to infer as follows : only in diploid constitution of the normal 14th chromosome, the individuals may be capable of complete manifestation of crescents and star-spots, while these patterns generally form incomplete crescents and star-spots in haploid constitution due to pairing with a deficient chromosome, as in the case with Ni strain. Heterochromatin in the U-locus may not always divide cleanly at mitosis, and one of the separated chromosomes may happen to receive more heterochromatin than the other. The different quantities of heterochromatin means the variation in the quantity of RNA or nucleoprotein which is assumed to transmit the genetic material from the U-locus. The more quantity is contained in a separating chromosome, the greater the degree of the anlage for crescents and star-spots is formed. Hence the variation in the intensity of crescent patterns among the Ni individuals takes place. When the quantity of heterochromatin decreases below a certain threshold, the anlage of crescents and star-spots will become so small that they are insufficient to be observed phenotypically. This may be why the individuals with incomplete crescents and star-spots are always found in typical No-lunule stocks. 3. On the pleiotropy in the silkworm Many examples of pleiotropic phenomena hav e been observed in the silkworm. For instance, the incomplete manifestation of patterns and lethality in Ni. the dorsal and side patterns and the Normal marking in U, and the dorsal pattern and no-crescents and no-star-spots in Di are nothing but the compound manifestation, in other words, the recombination of characteristics affected by several hereditary units contained in the same locus. The author has demonstrated the linkage relation between the white egg characteristic and the translucent skin in oew (white-egg-translucent) and w°' (aojuku white-egg-translucent) of the 10th linkage group in which both characteristics emerge pleiotropically. He also obtained the evidence that in some of the E-alleles belonging to the 6th linkage group, in which supernumerary crescents and extra-legs used to show pleiotropy, the linkage is observed between two characteristics. From these facts, it is believed that most cases of the pleiotropic manifestation in the silkworm may be explained with the concept of the compound characters. The author is in opinion that the standard type of larval pattern, for which the Normal marking has been generally adopted up to present, consists of three elements, i. e., eye-spot, crescents and star-spots which are determined independently, the Normal marking being a compound character. 4. On the oa-translucent skin character As is well known, the genes for the translucent skin character of the silkworm are located in the 1st, 2nd, 5th, 7th, 9th, 10th, 14th and 17th chromosomes. These genes belong to a category of the analogous genes which give rise to abnormal accumulation of crystals of urate in hypodermal cells, accordingly most of the translucent genes are semi-lethal. These facts contrasted with the case of the egg color genes related to tryptophane metabolism, the number of which amounted to eight, and all located on the 10th chromosome, but not on any others. It is expected that there exist many factors relating to the nitrogen metabolism of silkworm larvae in extremely wide range on chromosomes, and any change of very small portion of chromosome may upset the nitrogen metabolic routes resulting in a reduced accumulation of crystals of urate in hypodermal cells. As to the case of oa translucent, perhaps a small aberration in the chromosome structure took place in that locus, which was able to change the nitrogen metabolism, and to give rise to translucent skin, but not sufficient in amount to cause further phenotypical variations. Thus, a tentative interpretation for t he existence of so many analogous translucent genes in a vast distribution on different chromosomes, and for the lower rates of their viability, seems possible. our present knowledge is, however, not sufficient to draw any definite conclusion for analysis of translucent genes, and further investigations are needed. 5. Induction o f m utations by high temperature shock and ultrashort wave treatment By the treatment of high temperature shock against moths of the silkworm, three kinds of mutants, White-thorax striped (S^(1)), Light crescent (p') and Double crescent (E^ca), have been obtained. These mutants occurred at the loci each of which has been known to contain multiple alleles. Besides the author has obtained a mutant, Grey egg, by the ultrashort wave treatment against the egg soon after oviposition. There exist several kinds of gray egg, but their loci seem to center about one locus on the 2nd chromosome, therefore it is expected that newly obtained Grey egg mutant might be put together with their group of multiple alleles. It is assumed that the rrile of high temperature shock is to affect the mutable portions of the chromosome, its effect being negligible at other portions. Su' has transmuted from S (New striped) as a recessive mutation, and p' from p (plain marking) as a dominant mutation, in both cases dominance being incomplete. These facts suggest that each of these loci involving multiple alleles also consists of units of mutation, and one or more of these units may be transformed by temperature shock, so that the mutants reveal the almost similar traits to the original one. Acknowledgement The author wishes to express his thanks to Dr. Y. Tanaka, the National Institute of Genetics, Mishima, for his valuable advice and constructive criticism of the manuscript, and also for many mutant stocks handed over from him to the author. He is also grateful to Professor T. Hayashi, Kyushu University, Fukuoka, for his help and advice throughout these experiments.1927年Mullerによつて突然変異の誘発が人為的に可能であることが証明され,この方面の研究が進展するにつれてその重要性は日を逐うて加わり,今日に至るもその研究は広汎に行われているが,その研究方向は誘発源からいつて放射線によるものがその主流をなしているというも過言ではない.近時Auerbach等(1947)のmustardgasによる人為突然変異誘発成功以来,薬品による誘発実験研究がその後を追つている.一時華々しかつた温度処理による研究は現今殆ど見かけなくなつたが,放射線による誘発効果が極めて鋭敏である反面,蚕ではmutantの生理的機能が破壊されて,幼虫飼育上困難が伴い易いのに対し,温度処理に因る誘発効果は逆に顕著とはいえないが,爾後の飼育,研究,保存等には却つて好都合な程度に生理的影響は少ない。このことは突然変異の実際的利用という面からは重要視されねばならぬ点である.家蚕に於ける温度衝撃に因る突然変異の誘発は田中及橋本(1928)が卵と蛹とを材料として行なつたのが最初で,その後さらに田中(1928),田島(1939)によつて行われている.それらの結果では産卵直後の卵に高温度衝撃を与えると,1)2精子メロゴニー或は2精子メロゴニー核と正常受精核との混在, 2)倍数性, 3)単為生殖或は卵核,極体核の受精による異常核と正常核との混在,といつた異常を生ずることは歴然としているが,その間に於て遺伝子突然変異が問題にされたことはない.著者は産卵前の雌蛾又は交尾前の雄蛾に高温度処理を施して実験した結果,遺伝子突然変異を劣性へも優性へも起し得ることを実証した.
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