Developmental and functional interactions structure patterns of variational modularity in the lunar wrasse skull.

Trait modularity is a defining feature of complex life. However, the drivers of modularity across different scales of biological organization remain opaque. Studies have shown that a combination of developmental and functional interactions can structure patterns of trait covariation at the developmental, population, and even macroevolutionary level. However, it remains unclear how developmental and functional interactions may translate or influence macroevolutionary patterns of trait covariance and diversification. Pharyngognathy is a striking evolutionary innovation that has evolved multiple times in acanthomorph fishes and has resulted in the evolution of robust pharyngeal jaws that are used to process hard prey. Recent studies have found strong patterns of evolutionary integration among the jaw systems in pharyngognathous fishes suggesting that this innovation was brought about by the evolutionary coupling of two otherwise distinct trait complexes. Furthermore, the pharyngeal jaws have been hypothesized to act as a constraining force on the evolution of the oral jaws potentially due to their developmental origins in the more conserved hox-positive region of the skull. While multiple studies have recovered strong evolutionary integration between the jaw systems, patterns of modularity at the population (variational) level appear to differ where a high degree of modularity has been found between the oral and pharyngeal jaws suggesting a disconnect between patterns of evolutionary modularity and patterns of variational modularity. Here we are using three-dimensional geometric morphometrics to test for modularity between the oral and pharyngeal jaws at the variational level in a population of Lunar wrasse collected from Kagoshima, Japan and additionally test for differences in morphological disparity between the oral and pharyngeal jaws. We find strong support for a developmental hypothesis of modularity that separates the jaw systems into distinct modules. We additionally find mixed support for the constraint hypothesis of the pharyngeal jaws where some elements of the pharyngeal jaws were found to exhibit less morphological disparity than the oral jaws while others exhibited more morphological disparity. Our findings suggest that developmental and functional interactions at the variational level may impart patterns of covariation that are distinct from evolutionary patterns of modularity that are found between species.