Cytogenetic Analysis Shows that the Unusually Large Chromosome in the Sex-Limited pB Silkworm (Bombyx Mori) Strain Consists of three Chromosomes

Abstract

We have discovered an inordinately large chromosome pair at the pachytene stage in the oocyte of the sex-limited p B (black larval marking) silkworm (Bombyx mori) strain (TWPB). We have analyzed the composition and arrangement of this large chromosome. A genetic linkage analysis shows that the large chromosome is made up of the W chromosome, the second chromosome fragment (p fragment), and the fifth chromosome (linkage group) containing at least the region from map position 0.0 to 40.8. We also observed a sex heterochromatin body (SB) that we deduced to be made up of condensed W chromosomes. The number of SBs in each female nucleus among the sucking stomach cells of the TWPB strain was variable. Evidently, the W chromosome of the TWPB strain is attached to another chromosome. The composition of the W chromosome, the second chromosome fragment, and the fifth chromosome was studied through linkage analysis for these three chromosomes. We used two strains derived from the TWPB strain, the sex-limited p M (moricaud barVal marking)-hke (TWPML) and the autosomal p M-like (TSPML). The results show that the TWPML strain originates through a detachment of the fifth chromosome from the large chromosome of the TWPB strain, and the TSPML strain originates through a detachment of the W chromosome from that. Accordingly, the large chromosome of the TWPB strain is arranged in the order W chromosome-second chromosome fragment-fifth chromosome.

Cytogenetic Characterization and AFLP-Based Genetic Linkage Mapping for the Butterfly Bicyclus anynana, Covering All 28 Karyotyped Chromosomes

“…Much longer meiotic chromosomes in the pachytene stage provide better resolution, but their chromomere patterns are usually not fully distinctive [11], [21]. In addition, lepidopteran chromosomes are holokinetic, i.e.…”

Identification of 2nd chromosome region translocated onto the W chromosome by RFLP with EST-cDNA clones in the Gensei-kouken strains of the mulberry silkworm, Bombyx mori L

“…The sex-limited p Sa sable marking strain had a translocation between the W and the second chromosome, whereas the normal marking strain (+ P ) had evolved by eliminating the excess part of the translocated second chromosome (Tazima, 1944). Tanaka et al (2000) found that the sex limited Pb silkworm strain had 27 chromosome pairs whereas a normal strain had 28 bivalents. By backcross-breeding, it was found that the large chromosome was composed of a W chromosome to which portions of the second and fifth chromosome had been translocated.…”

“…Several cases of spontaneous chromosomal fusion or attachment are reported in Bombyx mori , most of which having been identified cytogenetically (Itikawa, 1952; Chikushi, 1959; Sakaida et al , 1996; Doira et al , 1987; Banno et al , 1993; Onimura and Tazima, 1983) and/or by genetic linkage analysis (Tanaka et al , 2000). Neither genetic linkage analysis nor cytological preparation was of help in identifying the attachment region of the autosome to the W chromosome, thus necessitating an alternate strategy.…”

Identification of the key stages for sex determination in the silkworm, Bombyx mori

“…For this strain, the pink-eyed white egg ( pe / pe ) is male (Z/Z) while the black-eyed black egg (+ pe / pe ) is female (Z/W) and homozygous for pnd . This strain has a large chromosome composed of the W chromosome, chromosome 2 bearing the p B gene, and chromosome 5 bearing the + pe gene (Tanaka et al 2000). Therefore, eggs of this sex-limited strain, in which females have a genotype of T (W; 2, 5) pB + pe , pe (egg, black; larva, black) and males have a genotype of pe (egg, white; larva, white), were examined.…”