On the genetic determination of the voltinism of silkworms

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That the voltinism of silkworms is under the genetic control is well demonstrated in the classical work of Watanabe (1918) who, having made analysis of the nature of the genes responsible for determination of the number of voltinism, observed that both the univoltine and the bivoltine are dominant to the tetravoltine, showing Mendelian ratio respectively. On the other hand, it is a matter of common observation that the voltinism is liable to be affected by changes of the environmental factors such as temperature and light. Also, in an earlier paper of the author (Umeya, 1925) it is pointed out, on the basis of the results of ovarian transplantation, that certain blood-born principe in the foster mother is significantly concerned with the determination of the voltinism.Incidentally it was disclosed that the eggs of the original bivoltine strain always hatch out as univoltine under laboratory conditions unless they are subjected to the incubation at 15°C. Toward 1915, some of the Japanese bivoltine strains were distributed among egg-manufactures as the parents for hybridization. At that time the eggs of the strains always gave rise to bivoltine animals when they were incubated at 15°C. However, since the introduction of the methods of artificial hatching, such bivoltine races have been distributed for nearly 30 years without being subjected to the test by incubation at the low temperature.Recently, an attempt was made in order to obtain non-wintering eggs from the bivoltine races widely distributed in Japan, to incubate the eggs at 15°C with an astonishing result that most of the eggs hatched out as univoltine. Taking into account various sorts of informations, the author has discussed the reason for such alternation. Two possible reasons have been conjectured, of which the one is interpreted on the assumption of unstable genes and the other is attributed to the artificial unconscious selection of bivoltine animals. Stress might be laid on the latter interpretation, but there is no concrete reason to reject the former possibility.A word is added as to the problem of the origin of bivoltine character in silkworms, of which no determinative concept has been advocated. In view of the fact that bivoltine is recessive to univoltine, it appears most plausible that the character in question might have originated through mutation from the dominant univoltine character. On the basis of such an assumption, it may be inferred accorning to a rather classical saying that the reduction to univoltine of the Japanese bivoltine race through the prolonged influence of high-temperature incubation would represent an example of "reversion".
日本遺伝学会の論文

On the genetic determination of the voltinism of silkworms

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