Development, dimorphism, and divergence of the oral dentition in threespine sticklebacks ( Gasterosteus aculeatus )

How morphology forms during development and changes during evolution remain major questions in biology. In vertebrates, teeth have long served as model systems to address these questions. In threespine stickleback fish ( Gasterosteus aculeatus ), repeated and convergent increases in pharyngeal tooth number in derived freshwater sticklebacks occur, suggesting increased tooth number is adaptive in freshwater environments, likely due to a diet of larger prey in freshwater. Whether changes in oral tooth patterning also occur in freshwater sticklebacks was unknown. Here we describe oral tooth number and patterning in a dense developmental time course of lab-reared ancestral marine and derived freshwater fish. We address three major questions. First, is the spatial sequence of early oral tooth formation invariant as we previously described for the pharyngeal dentition? Second, is oral tooth patterning in the upper and lower jaw sexually dimorphic, and if so, when during development does this dimorphism arise? Third, have freshwater fish evolved increases in oral tooth number? We find that (1) unlike the pharyngeal dentition, the oral jaw early spatial sequence is variable, especially in the lower jaw (2) sexual dimorphism in both oral jaws arises at the late juvenile stage with males having more teeth and (3) freshwater fish have evolved more oral teeth similar to the evolved tooth gain in the pharyngeal jaw. Together our morphological descriptions advance the stickleback oral jaw as a model system to study how morphology forms during development and evolves in nature.