Meeting Abstract
Adult phenotypes result from dynamic interactions among developmental stability mechanisms (e.g., canalization, morphological integration, modularity) and developmental instability. Among the stability mechanisms, modularity has been targeted as a primary evolutionary developmental component facilitating the evolvability of complex systems. Among the various definitions of modularity (genetic, developmental, morphological, individual, ecological), a module is a network of interacting elements behaving as quasi-independent units, or as semi-autonomous sets of highly correlated traits within larger units. Here, we explore modularity as it pertains to the evolution of the Weberian apparatus, and test if it is a functional, developmental, and/or evolutionary module. The Weberian apparatus, diagnostic of the ostariophysan subgroup Otophysi, is a mechanical linkage (modified anterior centra, neural arches, supraneurals, and pleural ribs) that transmits motion of the swimbladder wall directly to the inner ear. This linkage enables otophysans to hear frequencies up to several thousand hertz and increases sensitivity throughout the frequency range. Based on cleared and double-stained ontogenetic series of larval–juvenile cypriniform fishes, chondrification and ossification sequences of the Weberian apparatus elements were compared in a phylogenetic framework. We show that both chondrification and ossification sequences provide phylogenetic signal, and heterochronic patterns can be identified at various levels. This suggests that the Weberian apparatus evolved from a non-modified vertebral column in a modular framework, and that heterochronic events occurred during the development of various otophysans. These have resulted in the disparity among cypriniforms and extreme specialization in hearing among otophysans.
