A possible candidate for this additional regulatory element is the homeotic gene Abd-B , since this gene is reponsible for specification of posterior segments. Under normal conditions, the female genital primordium can develop either into genitalia or remain in the repressed state, producing no adult structures. In mutant conditions for Abdominal-B m transcript, it can develop into an abdominal tergite plus sternite. Similarly, under normal conditions, the male genital primordium can develop into either normal genitalia or remain in the repressed state, forming no adult strucures. Cell proliferation refers to the capacity of each genital primordium to grow or to be kept in the repressed state.
Sexual differentiation refers to the type of adult structure formed by each genital primordium.
It is proposed that the control of cell proliferation in the male and female genitalia requires the concerted action of Abd-B and doublesex , either directly or indirectly, through the expression of the genes dpp and wingless . Thus, in female genital discs, the repressed male primordium does not express dpp whereas the repressed female primordium of the male genital discs expresses a reduced level of dpp . This reduced level seems to be insufficient to stimulate cell growth. The genes dpp and wg , however, do not participate in the sexual differentiation process, which depends on sexual cytodifferentiation genes.