Further evidence that mirr is responsible for the repression of ventral genes that direct the dorsal-ventral polarity of the embryo comes from experiments in which mirr is ectopically expressed in the posterior of the egg chamber. Ectopic expression of DER AF in this region causes the loss of both posterior embryonic segments and the posterior twi expression domain. When mirr is ectopically expressed in the posterior follicle cells, a similar phenotype is observed, with both posterior embryonic denticle belts and the twi expression domain missing. The ecdysone response hierarchy mediates egg chamber maturation during mid-oogenesis. Decreasing or increasing the ovarian ecdysone titer using a temperature-sensitive mutation or exogenous ecdysone results in corresponding changes in early gene expression.
Genetic analysis indicates a germline requirement for ecdysone-responsive gene expression.
Germline clones of E75 mutations arrest and degenerate during mid-oogenesis and EcR germline clones exhibit a similar phenotype, demonstrating a functional requirement for ecdysone responsiveness during the vitellogenic phase of oogenesis. Finally, the expression of Drosophila Adrenodoxin Reductase increases during mid-oogenesis and clonal analysis confirms that this steroidogenic enzyme is required in the germline for egg chamber development. During stage 10, the follicle cell expression of E75 becomes enriched in the dorsal anterior cells. This suggested that inputs in addition to ecdysone are needed to refine E75 expression. To determine whether E75 expression is under control of the dorsoventral signaling pathway, ovarian E75 mRNA distribution was examined in dorsalized and ventralized mutant backgrounds.