蠶の黒縞, 黄血兩因子交叉價の變異に關する研究
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Y. TANAKA is the first author who discovered the linkage groups in the silkworm, Boanbyx mori, and a remarkable fact that no crossing over takes place in the female of this insect. According to his experiments, the S (striped. larval marking) and Y (yellow blood color) are linked with each other, the coupling ratio being very near to a 3 : 1 ratio, which corresponds to 25 crossing over. He noted however, that the ratio is susceptible to considerable variation, it came so low as 2 : 1 at least in a single family. In the review of TANAKA'S paper, STURTEVANT suggested the presence of a modifying factor as a cause of variation in the crossing over value. In order to study the influence of X rays upon the crossing over value, I started my experiments with a. single F1 egg-batch produced by mating between a striped yellow female (race Cltinayang, a Chinese bivoltine) to a normal white male (race Yamatoniskiki, a Japanese bivoltine). Both races had been obtained from different sources from those of the strains used by TANAKA, and no blood relation could be suspected between them. When the F1 striped yellow males were back cro ssed to the double recessive females, I found that c. o. v. (crossing over value) is markedly different in different families, and that it might be desirable to study, at first, the nature of the variation in question. I selected "high" class, i. e. the families that showed a high value of crossing over, and "low" class, i. e. the families that showed a lower crossing over percentage, in F1 generation, and the mating was accomplished among the same class respectively. Thus I continued the selective breeding so far as to sixth generation. It goes without saying that the c. o. v. of the SY/sy individuals in a certain generation is calculated from the result of the next generat on obtained by mating them to the double recessive sy/sy females. The "high" series maintained a high c. o. v. in succes s ive generations, segregating no "low" class from it. The average c. o. v. was alw a ys low in the "low" series, but it often segregated families of intermediate or still higher class. The intermediate class produced the offspring of high, intermediate and low values, roughly in a 1 : 2 : 1 ratio. From the results mentioned above, we may assume a dominant modifying factor CII which reduces the S-Y crossing over value, in a homozygous condition, by 12 %. Consequently c. o. v. in males with the genetic constitution SYCII/syCII is about 12 %, while it is about 24 % in normal case or in SYcII/sycII. Owing to the imperfect dominancy of CII the individuals heterozygous for this factor give rise to an intermediate c. o. v., i. e. 17-18 %. Besides CII gene, which I may call the major modifier, I have found one or more minor modifiers that influence, but in a slight degree or by 1-4 %, the c. o. v. of the "high" class as well as that of the "low" class. Seasonal fluctuation of c. o. v. is also noted. There is some indication that the essential factor of the seasonal fluctuation is the influence of temperature, which is especially striking in the "high" class. The average c. o. v. of the "high" class was 21.48 % in 30℃, while it was 25.86 % in 19°C, and it was intermediate when reared in an intermediate temperature. Thus it is evident that high temperature reduces c. o. v. between the striped and yellow. The influence of temperature is hardly perceivable in the "low" class.1. 蠶の第二染色體の交叉價の變異の一部分は遺傳的原因に因りて惹起せらるるものなり。2. 而して高區の固定は容易にして F3 までに於て24%を示す。3. 低區は12%を示し其固定は困難なり。其は常に中間交叉價のものを伴ひ往々高き交叉價のものを分離すればなり。4. 高區も低區も尚各遺傳的に二三の系統に分離することを得。5. 中區な平均17.67%を示し該蛾區が相互交雜をなしたる時は高きもの, 中間のもの, 低きものを大凡 1:2:1 の此に分離す。6. 以上に據り第二染色體には多分S-Yの交叉價を24.01%より11.90%まで減する一個の優性の major modifier あることを知る。之をCIIと記す。尚其他に少くとも一個の minor modifier が高及び低區の中に存在し小さき遺傅的變異を司るものなりとす。7. 第二染色體の交叉價の變異の他の一部は蠶の飼育時期寧ろ飼育温度に因る。即ち温度高き時即ち30℃の時は交叉價低く温度低き時即ち19℃の場合は交叉價高し。8. 温度に因る變異は高區及中區に於て顯著にして其差4-5%に及ぶ。9. 低區に於ては温度により僅少なる變異あれども是低區が中間交叉價のものを伴ひ其中間交叉價のものが温度に因り變異を生するものにして純粹の低區に於ては變異は顯著に非ざるべし。
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