油蚕

The normal skin of the silkworm larvae is opaque and white in color and contains a large amount of uric acid, the endproduct of the nitrogen metabolism, in the hypodermis. In the " oily skin " mutants, however, the skin of the larvae is more or less translucent due to the reduced amount of uric acid contained in the hypodermis. It has been generally considered that the more transparent the skin becomes, the less uric acid it contains in the hypodermis. More than twenty such mutants are known so far and are located on the widely dispersed linkage groups, i. e., about half of the already identified twenty groups. This is in conspicuous contrast to the increasing evidences of existence of genic factors, with similar or related functions, occurring closely linked together on the chromosomes of the silkworm. Concerning the manifestation of genes, distribution of genes in the chromosomal set, and differentiation of chromosomes and genes, this group may provide a very fascinating problem to the genetics of the silkworm. From these points of view, we have been carrying out a genetical and biochemical analysis of the manifestation of this character on the basis of uric acid content in various genic combinations. Results of such an analysis are reported in this paper. The effects o f single and double doses of an autosomal oily gene, oa, on the uric acid contents were investigated by utilizing N1-deficiency which involves the oa locus. Results showed that the haplo-oa (oaf-) individuals contained more uric acid than the diplo-oa (oa/oa) ones (Tables 1 and 9). This suggests that oa gene is an antimorph in the sense that it inhibits the uric acid accumulation in the hypodermis. In the cas e of sex-linked genes, od and os, higher uric acid content was also obtained in the hemizygous individuals, suggesting the antimorphic nature of the genes (Tables 3 and 4). The normal alleles of oil y skin mutants are considered to be haplo-sufficient for the accumulation of uric acid since no or very few differences were found between single and double doses of the normal alleles (Tables 1, 2, 3, 4 and 5). The interactions of non-allelic oily genes on the uric acid accumulation were studied with the double oily mutants. Each of the oa (autosomal), os or od (sex-linked) genes was combined with one of the w3 allelic genes, w3, w01 and oew. It has been considered that the transparency in double mutants is determined predominantly by a gene having stronger effect on the transparency. This, however, does not hold true for all cases examined. In the combination of od with one of the w_3 a lleles, slight cumulative effect was observed toward reduction of uric acid content (Table 6) ; while in the case of oa combined with w_3 alleles much severe cumulative effect was observed, no or very few uric acid was detected in the double mutants (Table 9). In the case of os combined with w_3 alleles, however, higher uric a cid content as well as lower transparency was noted in the double mutants than in the single mutants, suggesting a quite distinct type of interactions (Tables 6, 7 and 8). In this case, a further interesting phenomenon was noted. Normally, uric acid contents in the w3 series mutants are in the order of w_3>w^M>oew. However, when each mutant is combined with os, it becomes equal in the double mutants, that is, os ; w_3=os; w^ol=os; oew. This result is interpreted to mean that all the w3 alleles must be dimorphic in nature of the locus ; one is a " common oily factor , " nonspecific to each alleles, and the other is a " specific oily factor, " peculiar to each mutant. This interpretation is consistent with the hypothesis of the complexity of this locus proposed by Chikushi previously . In the heterozygotes between w_3 alleles, a mutant having more translucent skin seems to be expressed as dominant (Tables 10 and 11). But careful observation suggests that this is not due to dominancy but to differences in transtype position effect caused by their inherent " specific oily factors." It is clear that the interaction of these genes is quite different depending on the combinations. From these results it is suspected that in spite of the phenotypic similarities, i. e., transparent skin and reduced uric acid accumulation in the hypodermis, the mode of action of each genes responsible for the oily skin characters is not al ways the same. Recently we have obtained some evide nces suggesting that individual genes controlling oily skin characters are primarily concerned with the alterations of the structure of the tissue proteins present in the hypodermis. It seems that the genic factors responsible for the production and differentiation of tissue proteins may be distributed on the various regions of different chromosomes. This may be the main reason why the genes in question are distributed so widely dispersed on the chromosomes of the silkworm.