Figure 3. Hormonal inputs into insect sexual dimorphism.

Two principal mechanisms exist through which hormones can deliver sex-specific effects in insects. (A) Sex differences in hormone titer. Developing eyespot cells in the butterfly Bicyclus anynana express ecdysone receptor. The titer of circulating 20-hydroxyecdysone in females leads to a binding threshold being exceeded, which causes the cells to proliferate and the eyespot to grow. The lower titer in males fails to exceed the binding threshold and the cells fail to proliferate. What generates the divergence in hormone titer is unclear, but one potential mechanism is the direct or indirect regulation of enzymes in the ecdysone biosynthesis pathway by Dsx^M and/or Dsx^F. (B) Sex differences in sensitivity to hormones. Expression of dsx in the developing prepupal mandibles of the stag beetle Cyclommatus metallifer changes the sensitivity of mandibular cell proliferation to juvenile hormone. Dsx^M increases sensitivity, leading to enlarged mandibles in males. Dsx^F reduces sensitivity, leading to small mandibles in females. Figure created using BioRender.