The function and development of the left-right asymmetric duck syrinx

Abstract

Dissertation Advisor: Dr. Cliff Tabin Darcy Mishkind The function and development of the left-right asymmetric duck syrinx Abstract Avian diversity, ranging from the stunning tailfeathers and advanced mimicry of the Lyrebird, to the bright red and varied calls of the Northern Cardinal, to the well camouflaged feathers and booming call of the American Bittern, has always captured the interest of biologists. This work explores another example of these varied and striking vocalizations, that of Anatidae (ducks and their relatives), where males are known to produce courtship whistle vocalizations. Unique to ducks, the avian vocal organ, the syrinx, demonstrates left-right asymmetry, and this left-right asymmetry is only in males. In males, there is a left-sided bulla that has long been hypothesized to be necessary for courtship vocalizations, though this has not been tested previously. Further, our understanding of left-right asymmetric development of organ primordia is limited, and the syrinx is a tissue yet to be examined with modern molecular approaches. Finally, the sexual dimorphism of this trait allows us to ask how pathways that are ordinarily separate, the left-right cascade and estrogen signaling, achieve crosstalk. First, we explore the possibility that the hollow bulla may be responsible for the whistling vocalizations common to male duck courtship displays. It has been hypothesized that the bulla functions as a Helmholtz resonator, and we began to test this by predicting bulla resonance frequencies in duck species based on measurements of the structure. When predictions are compared to frequencies emphasized in various call types in these species, we see evidence for bulla-influenced vocalizations, in particular in species with clear whistling vocalizations. We also see overlap with our predictions in non-courtship vocalizations as well as in female calls, an observation that should be explored in the context of other vocal tract features that alter vocalization frequencies. We find that bulla size is generally positively correlated with body mass. We conclude that there is support for the hypothesis that the bulla functions as a resonance chamber, in particular emphasizing courtship whistle vocalizations. Following this work, we investigate the development of the laterally asymmetric duck syrinx. We begin by examining literature that describes a possible mechanism of intra-syrinx inhibition by which the left side inhibits right side growth. Previous work observed that, when sliced in half, the growth of the right half is limited when grown in culture with the left half but grows as large as the left when grown in isolation1. Using ex vivo cultures, we did not observe any changes to right side growth as a result of either the culturing of right halves with left syrinx halves at various ratios or the presence of conditioned media taken from left-side cultures. This indicates an intrinsic developmental mechanism that is independent of any secreted factors produced by the opposing syrinx half. The next chapter explores how hormonal and left-right signaling interface to direct development of the duck syrinx. We characterize the cellular mechanisms at play and find cell division to be the main driver of early left-right asymmetry. Additionally, PITX2, a member of the canonical left-right asymmetry cascade, is present in the duck syrinx in males and females and we see evidence that left-sided expression in the syrinx is derived in ducks as it is not observed in other bird species examined. Results indicate asymmetric PITX2 expression occurs in two waves, the second of which is in the primordial syrinx. We see evidence that bilaterally-expressed BMP activates the second wave of left-sided PITX2 in the tissue primordium through laterally differentially accessible chromatin. Estrogen signaling is then responsible for establishing a sexually dimorphic developmental plan by activating left-sided ESR1 and reducing cell proliferation in the female syrinx and thus promoting bilaterally symmetric growth. Here, we characterize a novel system of left-right asymmetry at the organ level and how pathway integration can occur. Overall, this work describes the function and development of the duck syrinx. It adds to the discourse on sound production in birds and the question of how novel instances of left-right asymmetry are patterned.