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

Abstract

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. © 1988 Springer-Verlag.