Cytogenetical studies on the chromosomes of silk gland cells of the silkworm with special reference to the structure and behavior of the sex chromosomes.

Abstract

1) The structure of the nuclei in the silk gland cells of the silkworm, Bombyx mori, was investigated with special reference to the fine structure of the sex-heterochromatin body (SB), using both diploids and experimentally induced polyploids.2) Microscopic observations revealed that the SB was not a homogeneous mass, but it was surrounded by a layer of chromatin substances formed by the coiling and folding of fibrils.3) The SB was clearly observed in the silk gland nuclei in acetic orcein squashed preparations, being 100% in the female (SB1) and 0% in the male (SB0) in diploids. In the tetraploids, the female nucleus had two SBs (SB2), and the male nucleus contained no SB (SB0).4) On the observation of the segregation of the number of the SB in triploid offsprings from the backcross of the tetraploid female, it was revealed that the ratio of SB2:SB1:SB0 was 1:4:1. The behavior of sex chromosomes in meiosis of the tetraploid female was considered according to the above results.5) Using triploid offsprings from the strain having a translocated Y chromosome marked with visible genes, it is apparent that the SB is corresponding to a Y chromosome.