Mutation of Silkworm

In the offspring of silkworm, the phenotype is different from the normal genetic phenomenon. There are two types of mutations in silkworms: natural mutations and induced mutations (man-made mutations). Characters change under natural conditions and passed on to children and grandchildren are called natural mutations. Artificial processing of silkworm eggs, larvae, and pupae, and subsequent mutations in their offspring are called induced mutations. Induced mutations are divided into physical and chemical mutations according to the nature of the inducing factors. The frequency of natural mutations is extremely low. Tajima Yataro (1958～1959) investigated the rate of occurrence of 100,000 times per genetic locus as shown in the attached table.

Natural mutation rate of silkworm genetic gene (Tajima 1966)

Despite this, the silkworm has formed a variety of morphological traits and local varieties with distinctive economic traits through long-term natural and artificial selection for thousands of years, and has effectively utilized natural variation. Of the more than 100 mutants that have been discovered so far, as many as 2/3 are derived from natural mutations. The frequency of induced mutations varies according to the intensity of radiation or the concentration of chemical substances, the maturity of germ cells and the variety, etc., and are generally higher than the frequency of natural mutations. Due to the wide range of mutation, induced mutation can produce more types of mutations. It has become an important means of modern breeding, creation of new types, and expansion of gene pools. In physical mutagenesis, X-rays, γ-rays and lasers were used to irradiate silkworm pupae, and the effect was more remarkable. Natural mutations and induced mutations are not essentially different in their occurrence mechanism and nature of mutations.

According to the location, degree and way of genetic material change, mutation can be divided into chromosomal mutation and gene mutation. Some of these two mutations originate in the process of gamete formation, and some originate in somatic cells. Chromosomal variation includes variation in chromosome structure and variation in chromosome number. Generally, chromosome structural variation is divided into 4 types: deletion, duplication, inversion, and translocation.

An example of the deletion of the missing silkworm is Shinta Mumu (1936), who discovered that the black silk (Sm) yellow-blooded silkworm with small white spots all over the body has disappeared on the second chromosome, and the Ps that was originally linked to Y disappeared, and the recessive gene p is present. . It was also found that Y was accompanied by lethal effects, and the exchange rate between p-Y was significantly reduced, showing the typical genetic effect of the second chromosome deletion. Jiang Tongqing, Xiang Zhonghuai et al. (1963) used X-rays and γ-rays to induce marking mutations in silkworms, and most of the obtained mutants were caused by deletions; Chikuzi Chunsheng and Doi Ryohiro (1971) induced slow growth by γ-rays (Rg) Silkworm (dominant mutation), larvae growth is significantly delayed and homozygous is lethal. Experiments have shown that Rg is a deletion in the middle of the third chromosome segment, the left break is at 24.9, and the right break is just to the left of Sm(S) locus 41.8, and about 17 units are missing.

Duplicate silkworm chromosomes have fewer repetitions on homologous chromosomes, while translocations to non-homologous chromosomes have more excesses. Tajima Yataro (1942～1943) reported that the second chromosome of the T102 system was translocated to homologous chromosomes; Aiga Hisao (1939) used X-ray to induce the No. 2 (DS-2) of the ps chromosome The segment is attached to the psY locus of the second chromosome; Takasaki Tsuneo (1947) irradiated the psY/p+Y female pupae with X-rays, mated with P+Y/p+Y, the next generation found the new Erhei (s2), which is considered to be ps The duplication of fragmented chromosomes is the result of ps transposition to the psY chromosome of another homologous chromosome.

Inversion (inversion) Yoshina Tanaka (1935) irradiated psY male moths with X-rays, mated +P+Y, F1 to get a chimera silkworm with common spots on the back and black silkworms on the ventral surface. The offspring produced plain spots. In silkworm yellow blood (pY), p and Y are completely linked, so male silkworms cannot exchange. He inferred that X-ray caused the ps of the ps-Y chromosome to fall off and inverted to form a complete pY linkage. Tajima Yataro (1938, 1943) studied the PsaY chromosome and compared the exchange rate of psa between heterozygous and homozygous. He found that the exchange rate of homozygous type increased significantly, and it is speculated that there is an inversion between psaY.

Translocation Tajima Yataro (1938) irradiated silkworm chrysalis with X-rays, and obtained the mutation of coal gray spot gene translocation to W chromosome. Later, from induced mutations and natural mutations, limited dark spots, limited black silkworms, limited new common silkworms, limited tabby (Hashimoto Haruo, 1943), and limited egg colors (Tajima Yataro, 1951) were successively obtained. , Limited cocoon color (Kimura Keisuke, 1971), balanced lethal system (Strunikov Струнников 1969, 1972) and limited red ant (Huang Junting, 1983), etc.; Kawaguchi Eisaku (1963) uses centrifugal treatment to give birth Egg, in the mutant, the ps fragment of the second chromosome is easily located on other ordinary chromosomes, which is extremely stable and not lost, so it is judged as a translocation; Aiga Hisao (1940) cut off the third chromosome by X-ray, which made Ze and lem lose linkage , Translocation to other chromosomes, and in the middle and late stages of the first spermatocyte division, three concatenated chromosomes, combined chromosomes and irregular chromosomal bridges were observed, which is considered to be a mutual translocation between the third chromosome and other chromosomes.

Mutations in the number of chromosomes: Tanaka Yoshina (1939) found a dysplastic male silkworm whose sex chromosome is ZO, which is a monomer; Takasaki Tsuneo (1940) found a monomer of the second chromosome O/p+Yod, which is dysplastic. Most of them are fatal; Tajima Yataro (1938) obtained 3-body (2n+1) and 4-body (2n+2) from the mutagenesis system, but they were meaningless mutations in terms of physiological and economic traits. In the study of ploidy, Yoshihiro Tanaka (1928) obtained an exceptional female silkworm with sex-linked inheritance in the female-male chimera system; Harutaro Sato (1929) mated an artificial parthenogenetic silkworm with a normal male, and found it in the offspring Triple and 4-ploid. Eisaku Kawaguchi (1936) centrifuged silkworm eggs to obtain tetraploid and hexaploid. In terms of economic value, polyploidy is not as good as 2ploidy.

Among the genetic mutations, the dominant mutations include black spots, dark spots, green eggs, lavender eggs, excess half-moon patterns, and excess feet. Recessive mutations include red ants, black pupae, small wings, and red eyes. , White eyes, red eggs, oily silkworms and most lethal mutations. In natural mutations, whether it is a dominant mutation or a recessive mutation, its lethality accounts for 10 to 16%. Approximately 55% of induced dominant mutations are sexually fatal. Gene mutations can be mutated into multiple alleles at the same position, forming a multiple allele system. For example, more than 20 alleles have been found in the E locus of chromosome 6. Gene mutations are reversible and repetitive.

Chemical mutagenesis was first performed by Hirobe Tatsudo (1939) coating silkworm eggs less than 2:30 in instar with 0.05-0.4% colchicine solution to obtain polyploid silkworms; Koichi Murachi, Yataro Tajima, Yoshio Nakao ( 1950) Coating silkworm eggs with nitrogen mustards induced deletion mutations; Machida (1975) injected female pupae with mitomycin C at the beginning of pupation, the frequency of mutagenesis was high, and it gradually decreased with the progress of pupal age: Murata Zhe Lang et al. (1977) used MMS to inject the female pupae of the moth 5 days before, and induced a dominant lethal mutation. Chemical mutagenesis has not yet reached the level of practicality.