To investigate the mechanism of cocoon formation in the silkworm, Bombyx mori, we placed medical tape around larval segments to restrict movement. Spinning behavior was then captured on videotape from two angles and analyzed using three-dimensional (3D) computer simulation software. No significant difference in ability to spin silk was observed in larvae with restricted mobility. However, larvae produced more spherical cocoons that were twice as large as control cocoons when tape was applied to the 4th or 5th segment. Conversely, larvae spun cocoons that were slightly longer when tape was applied around the posterior segment. Generally, the wider the tape applied to the larvae was, the more pronounced the effect. Analysis by 3D computer simulation showed that the larvae taped at the 5th segment moved their heads backward at a smaller angle, and spun more slowly than control larvae. In addition, the frequency of turn behavior and cocoon expansion behavior were slightly lower. Conversely, the larvae taped at the 9th segment showed reduced restriction of backward movement. Turn behavior was observed more frequently, but was not rhythmical. In addition, the time required for one turn varied significantly. Restriction of segment mobility by tape thus changes cocoon shape by modifying spinning behavior. In addition to our previous findings, these results suggest that the main behavioral factor affecting cocoon size and shape is spinning posture, especially the backward bending behavior of the head and thoracic region.
The genetic characterization of the cocoon color of the silkworm Bombyx mori is little known, because the cocoon color formation is entangled with various complicated factors, such as pigments synthesis, pigments transport, 1ight reflexivity etc. However, the silkworm is appropriate to study on the genetics of the cocoon color, since there are many known mutants. We found that the green cocoon color strain, Ryokuken ichi-go was composed by two factors: 1) greenish yellow color (Gy) , 2) fluorescent color (Fy) enhanced by ultraviolet light. This finding was observed for the first time under the black light. To investigate the genetics and gene mapping about Fy and Gy, the original strain (G01) of Ryokuken ichi-go was crossed with the strain RF02 whose cocoon color is white and no enhanced fluorescent color under ultraviolet light. The cocoon color of F1 were nearly same color to the strain G01, so these greenish yellow color and fluorescent color of the cocoon seemed to be dominant characters. Then F1 was backcrossed to the strain RF02 and the individuals of this BF1 were segregated to four kinds of cocoon color phenotypes. These results confirmed that these two kinds of cocoon color, greenish yellow and fluorescent color, were independent dominant characters. Gene mapping for Gy and Fy were carried out by RFLP. We report here that Gy is located at 5.4cM from a marker m72 on the chromosome RFLG5 and Fy is located at 7.5cM from a marker e96 on the chromosome RFLG20.