Transcriptomic Evidence for Cell-Autonomous Sex Differentiation of the Gynandromorphic Fat Body in the Silkworm, Bombyx mori

Abstract

The classic model of sex determination in insects suggests that they do not have sex hormones and that sex is determined in a cell-autonomous manner. On the other hand, there is accumulating evidence that the development of secondary sexual traits is controlled in a non-cell-autonomous manner through external factors. To evaluate the degrees of the cell-autonomous and non-cell-autonomous regulation of secondary sexual trait development, we analyzed the dynamics of the sexually dimorphic transcriptome in gynandromorphic individuals of the mo mutant strain in the silkworm Bombyx mori. The silkworm possesses a female heterogametic sex-determination system (ZZ = male/ZW = female), where the master regulatory gene for femaleness, Feminizer (Fem), is located in the W chromosome. As a secondary sexual trait, we focused on the fat body, which shows remarkable differences between the sexes during the last instar larval stage. A comparison of the transcriptomes between the fat bodies of male and female larvae identified 232 sex-differentially expressed genes (S-DEGs). The proportions of ZZ and ZW cells constituting the fat body of the gynandromorphic larvae were calculated according to the expression level of the Fem. Based on the obtained values, the expression level of each S-DEG was estimated, assuming that the levels of S-DEG expression were determined according to the proportion of ZZ and ZW cells. The estimated expression levels of 207 out of 232 S-DEGs were strongly correlated with the corresponding S-DEG expression level of the gynandromorphic fat body, determined by RNA-seq. These results strongly suggest that most of the sexually dimorphic transcriptome in the fat body is regulated in a cell-autonomous manner.