Osamu Ninaki

Nucleolus organizers in the wild silkworm Bombyx mandarina and the domesticated silkworm B. mori

Two types (Ra1 and Ra2) of nucleolus organizers were identified in the genome of Bombyx mandarina (Japan) which occurs in Japan. Genetical analysis of a hybrid with B. mori suggested that the loci of both nucleolus organizers are allelic and correspond to the R0 locus of B. mori. These nucleolus organizers segregated and were inherited by the progeny in a Mendelian fashion. The majority of the Ra1 rDNA units were 10.6 kb in length and had an additional EcoRI site in the transcribed spacer region when compared with the same size unit of R0. On the other hand, the KpnI site present in the non-transcribed spacer region of the R0 rDNA was not detected in the Ra1 unit. The 15.1 kb unit observed in the Ra2 locus was the same as the unit with the type II intron of R0. The four major components of Ra2 rDNA, with lengths of 10.6, 15.1, 15.7 and 20.8 kb, were also found in the R0 locus, and thus the Ra2 and the R0 loci were considered to be closely related. Usually the functional unit of rDNA in the nucleolus organizers of homologous or non-homologous chromosomes cannot be easily distinguished. However, in the case of B. mandarina (Japan), distinct functional 10.6 kb units were observed in the allelic Ra1 and Ra2 loci. Therefore the existence of the two distinct units suggest the possibility of introducing the chromosomes of the interspecies in the genus Bombyx.

Stability and Telomere Structure of Chromosomal Fragments in Two Different Mosaic Strains of the Silkworm, Bombyx mori

Abstract Mottled mosaic strains in the silkworm, induced by X-ray irradiation, contain chromosomal fragments carrying the larval body marking genes that are lost occasionally during larval development. In one of the mosaic strains, mottled zebra (Zem), the somatic loss of the chromosomal fragment is presumed to cause the mosaic pattern, but the fragment has not yet been identified. Here, we showed that Zem individuals have an extra small chromosomal fragment (Ze fragment) using genetical and cytological methods. The rate of loss of the Ze fragment, calculated based on the data of segregation distortion, is higher than one from a different mottled strain, mottled striped (pSm). Fluorescent in situ hybridization with theBombyxtelomeric sequence (TTAGG)n as a probe demonstrated that the Ze fragment has a telomeric repeats at only one chromosomal end, although the fragment of pSm (pS fragment) has the repeats at both ends. These data show that the broken ends of chromosomal fragments generated due to X-ray irradiation could be basically healed by de novo addition of the telomeric repeats and the structural difference of telomere may be related to the stability of chromosomal fragments.

Chromosomal fragment responsible for genetic mosaicism in larval body marking of the silkworm, Bombyx mori

Several genetic mosaics for larval body marking of the silkworm, Bombyx mori , have been induced by X-ray irradiation. It is hypothesized that the occasional loss of chromosomal fragments carrying the genes for body marking during development may give rise to this type of mosaicism. Using pulsed field gel electrophoresis (PFGE), we find that a DNA molecule of about 2·5 megabases (Mb) is present in one type of mosaic (mottled striped strains p Sm 788 and p Sm 872), and not in any other strain. This DNA fragment hybridizes strongly with some chorion genes which are less than 6·9 cM away from the p s locus, and hence it corresponds to a chromosomal fragment containing genes for both striped marking ( p s ) and the chorion. In the non-mottled p s strain, the phenotype before X-ray irradiation, no band was detected either on a PFGE gel or after hybridization with the chorion probe. These results suggest that the mottled p s strains carry short chromosome fragments which are lost differentially during cell divisions.

Comparative studies on the rDNA of the silkworm, Bombyx mori and its presumed ancestor

Abstract 1. 1. The transcribed region of Bombyx mandarina rDNA was identical to that of B. mori rDNA when the restriction maps and partial nucleotide sequences were compared. The result supports the assumption that Bombyx mandarina is an ancestor of Bombyx mori and that the two were subdivided very recently. 2. 2. Non-transcribed spacer (NTS) of four clones derived from the two insects was slightly different from one another, which seemed to be due to the difference in the number of repeated sequences distributed in the spacer. 3. 3. The five clones from B. mandarina had the type I insertion sequence (IS) homologous to that in Bombyx mori 28S rDNA. There was micro-heterogeneity in the structure of IS.