open access

https://doi.org/10.11501/3122433

https://hdl.handle.net/2115/51436

The genetical and morphological features in the polyploid silkworms induced by low temperature treatment

日本語

1997-03-25

Hokkaido University

126p.

In the silkworm, the triploid progeny from the cross between tetraploid females induced by low temperature treatment and diploid males carrying the sex-linked recessive genes consisted of three types, ZZZ, ZZW and ZWW. The percentage of the ZWW females discriminated by sex-linked recessive phenorypes was only 1.5 in a batch. Three kinds of gametes, ZZ, ZW and WW, were considered to be produced in tetraploid females because a Z gamete was always transmitted from diploid males. The ratio of three types of triploids revealed that the sex chromosome pairing of Z with Z and W with W (ZZ+WW) was more preferential than that of Z with W (ZW+ZW). The preference that occurred in the tetraploids from the four strains harboring sex-limited marker genes was much higher than that in the tetraploids of three normal strains. Furthermore, within the tetraploid of sex-limited strains, the length of the translocated autosomes appeared to be one of factors to cause the preference in the pairing. By using the sex-limited "Homare" strain, the ratio of paring pattems ZZ+WW or ZW+ZW, was calculated both in Homotype (intra-strain mating) and Heterotype (inter-strain mating). In the tetraploid oocytes of sex-limited yellow cocoon strain (Sy), the pairing patterns was cytologically observed in Sy4n (Homotype) and Syr4n (Heterotype) and the ratio of two pairing patterns was calculated. These two experiments revealed that the sex chromosomes in Heterotype tetraploids paired in preference between the same kind at 5 fold of those in Homotypes. Deposition of large eggs by ZWW triploid females was confirmed in four strains. The results showed the presence of a quantitative gene (Egg size determining gene; Esd) on the W chromosome. Ovary transplantation between ZWW3n and ZZW3n females, as well as between the diploids with or without a giant egg gene (Ge) which is identical with Esd gene, was carried out in order to analyze the effects of environmental factors on egg production. Both donor and host ovaries produced eggs of original size. The fact revealed that the environmental factors had no effects on the gene expresslon. One ovary excision from an individual in the normal and Ge strains showed 10 to 20% increase in egg size in both cases. The increase in egg size within one ovary suggests some suppressing mechanism working in the normal condition where two ovaries are present. In the Ge strains, the increase of egg size was observed in the individuals which the operation was performed up to the pupa-5day. As to egg number, the increase continued in the individuals operated up to pupa-4day. The difference suggests that egg number was determined prior to egg size. The size of polygonal patterns was shown as the following equation, ZW(2n)=Zww(3n)<ZZW(3n)=ZZWW(4n)<ZZZW(4n). The size increase did not depend on ploidy but on the number of Z chromosomes. The fact leads to the conclusion that the polygonal pattern size detem tining gene (Pgd) is present on the Z chromosome. The Pgd gene on the Z is second quantitative gene discovered in the silkworm. The measurement of egg size and polygonal pattern size in Ge strain (TG) and normal strain (rw) were summarized as follows; rw2n<rw4n=TG2n<TG4n for the egg size and rw2n=TG2n<rw4n=TG4n for the polygonal pattern size. The expression of<the Esd and the Pgd appeared to be independent each other. This was confirmed in two kinds of F1 tetraploid females (ZZZW and ZZWW). In the silkworms, tetraploid males are sterile in nature. The experiments such as excision bf a corpus alattum, high temperature-moisture treatment and starvation were carried out in the tetraploid males during the 5th larval instar. These stimuli were effective to produce fertile tetraploid males. Especially, starvation of tetraploid males for 30h or 51h at 60h of 5th larval instar was found to be most effective. Start of meiosis in the 5th instar larvae is delayed 60h in the tetraploid males. It is noteworthy that the 60h of the delay well.coincided with,the most effective stage, 60h of the 5th instar, to induce fertile tetraploid males. The F1 tetraploids were produced by crossing tetraploid females and artificially induced fertile tetraploid males. In the F1 tetraploids, ZZZZ males and two types of females as to sex chromosome constitution, ZZZW and ZZWW, were expected. The ZZZW females showed quite high mortality at the stages of hatch, spinning, pupation and adult emergence. An even sex chromosome balance may be necessary in successful morphogenesis.

1997-03-25

乙第5158号

博士(農学)

博士論文

Version of Record