Micropylar structure of chorion of the female sterile mutation, bd, in Bombyx mori

Micropylar structures of chorion in the bd mutant group of Bombyx mori were observed under a scanning electron microscope, with special reference to the external features of the micropylar rosette, orifice and internal channel tubes. These structures were normal in the control eggs, i.e., heterozygous for bd, bd^sw and bd^f alleles, but were deformed in the bd and bd^sw homozygous eggs, both of which were sterile. In particular, the channel tubes were not present in these eggs. On the other hand, the micropylar architecture of the bd^f homozygous eggs, which were fertile, was normal like control eggs. Based on these results, we suggest that the sterility of the bd and bd^sw females is caused by the abnormality of the micropylar apparatus, which may be crucial for fertilization processes.