Can mechanical forces be responsible for novel bone development and evolution in fishes?

Abstract

Summary Mechanical forces influence the induction, growth and maintenance of the vertebrate skeleton. Using the zebrafish, Danio rerio, we explore the hypothesis that mechanical forces can ultimately lead to the generation of skeletal evolutionary novelties by modifications of the mechano-responsive molecular pathways. Locomotion and feeding in zebrafish larvae begin early in ontogeny and it is likely that forces incurred during these behaviours affect subsequent skeletal development. We provide two case studies in which our hypothesis is being tested: the kinethmoid and intermuscular bones. The kinethmoid is a synapomorphy for the order Cypriniformes and is intricately linked to the bones of the protrusible upper jaw. It undergoes chondrogenesis within a ligament well after muscular forces are present within the head. Subsequent ossification of the kinethmoid occurs at sites of ligamentous attachment, leading us to believe that mechanical forces are involved. Unlike the kinethmoid, which has evolved only once, intermuscular bones have evolved several times during teleostean evolution. Intermuscular bones are embedded within the myosepta, the collagenous sheets between axial muscles. The effect of mechanical forces on the development of these intermuscular bones is experimentally tested by increasing the viscosity of the water in which larval zebrafish are raised. Since locomotion in high viscosity requires greater muscular forces, we can directly test the influence of mechanical forces on the development of intermuscular bones. Using developmental techniques paired with outgroup comparison for the kinethmoid, and direct experimentation for intermuscular bones, our case studies provide complementary insights into the effects of mechanical forces on the evolution of skeletal novelties in fishes.