UC Berkeley

Despite the variety of organisms across all families of life, the process of organ formation is remarkably similar across vertebrate species. The homology in mechanisms of epithelial appendage formation and regeneration allows the use of model organisms to better study longstanding questions about how organs form and regenerate. The developmental genetic circuitry underlying formation of epithelial appendages such as hair, skin, nails, and teeth are largely conserved across mammalian and non-mammalian vertebrates. Thus, continuous tooth replacement in the genetically tractable threespine stickleback fish (Gasterosteus aculeatus) has made teeth an attractive model for studying the developmental genetic basis of epithelial appendage formation and regeneration. Further, the evolutionary history of threespine sticklebacks, having undergone adaptive radiation, enables them to serve as a system to study how population-driven phenotypic divergence shifts throughout development. While evolved changes in the pharyngeal dentition have been shown to evolve repeatedly in multiple derived freshwater populations, how the oral dentition has evolved over time between populations is less known. Further, although the primary pharyngeal tooth sequence has been established, the spatiotemporal patterning of oral tooth formation in sticklebacks remains unknown. Here, we characterize the spatiotemporal patterning during formation of the oral dentition in freshwater and marine sticklebacks. Further, we show that differences in oral tooth number are present between the derived marine and freshwater populations. We address the onset of population-derived divergence in tooth number and the ontogenetic trajectory of tooth development. We find that sexual dimorphism in tooth number is present in both the ancestral marine and the derived freshwater populations, arises in the late juvenile stage, and is stronger in the ancestral marine population than in the derived freshwater population. Further, we determine the spatiotemporal sequence of the primary oral dentition and clarify the effect of mutations in a key signaling gene, Wnt10a, over developmental time. Altogether, these findings deepen our understanding of sexual dimorphism and population-driven divergence throughout development. In addition, these findings advance the stickleback oral jaw as a new model system to study the developmental genetic basis of epithelial appendage formation and regeneration.