Integration of sexual dimorphism and left-right asymmetry in the development of the duck syrinx

Embryonic morphogenesis is regulated across multiple dimensions. In ducks, the syrinx, the avian vocal organ, undergoes morphogenic processes that result in both left-right asymmetric and sexually dimorphic development. Although these properties are thought to be controlled by the NODAL-PITX2 left-right signaling cascade and the sex steroid pathways, how these mechanisms work together to produce an asymmetric structure in a sexually dimorphic manner remains unclear. Here, we first establish evidence for sexual selection driving the evolution of the duck syrinx. During its development, we observe that PITX2 is expressed on the left side in both male and female ducks, although not in other birds. Asymmetric activation of PITX2 in this domain is triggered by bilateral BMP signaling but is limited to the left side by left-specific stably accessible chromatin established during an earlier asymmetric wave of PITX2 expression. Ultimately, there is an induction of left-specific WNT and BMP signaling in the syrinx primordium, which synergistically elevates cell proliferation on the left, leading to asymmetric growth. Estrogen receptor expression is also shown to be induced on the left side of the forming syrinx. This has no effect in males, but in females where the hormone is present, estrogen signaling reduces left-sided cell proliferation, thus promoting bilaterally symmetric growth. These data demonstrate how sexually dimorphic left-right asymmetry can be integrated to produce an adaptive trait.