The morphological diversity of insects plays an important role in their survival,and in essence it results from the differential expression of genes during the insect body development.In the silkworm Bombyx mori,the ap (apodal) mutant exhibited degraded thoracic legs and that made their crawling and eating mulberry leaves difficult,and also showed female sterility,which is an ideal subject for the study of molecular mechanisms for morphogenesis.In this study,we confirmed that the infertility of ap female moths was due to the degradation of the bursa copulatrix,and report the identification and characterization of the gene responsible for the ap mutant.We mapped the ap locus using 384 F2 individuals with ap mutant phenotype and identified the candidate gene of the ap locus,Bombyx mori sister of odd and bowl (Bmsob).Analysis of the spatio-temporal expression pattern of Bmsob gene revealed strong expression in the silkworm larval genital gland and in the third day of wandering stage and the first day of pupa.This period is the key stage from larvae to pupae metamorphosis and also the important stage of genital gland development,implying that Bmsob gene plays an important role in the development of silkworm genital gland.Quantitative RT-PCR (qRT-PCR) revealed that the expression of Bmsob was both significantly reduced in the ap mutant compared with the wild-type in the embryo and 1st day pupa.We found that there were sequence differences in the Bmsob promoter region between Dazao and ap mutant.Then we detected the promoter activity in wild-type and ap mutant using the dual luciferase report system.The results showed that the activity of the Bmsob promoter in ap mutant was decreased significantly.Hence,we speculated that the sequence variation of Bmsob promoter region may be a significant reason for the decreased expression level of Bmsob gene.In addition,qRT-PCR showed that the expression levels of BmAntp,BmUbx and Bmabd-A genes were all up-regulated expression in the ap mutant compared with the wild-type.Therefore,we assume that Bmsob is the upstream regulatory gene of Hox gene.