Dinosaur-like lower leg created on bird through molecular experiment

Date: March 7, 2016

Source: Universidad de Chile

Any one that has eaten roasted chicken can account for the presence in the drumstick (lower leg) of a long, spine-like bone. This is actually the fibula, one of the two long bones of the lower leg (the outer one). In dinosaurs, which are the ancestors of birds, this bone is tube-shaped and reaches all the way down to the ankle. However, in the evolution from dinosaurs to birds, it lost its lower end, and no longer connects to the ankle, being shorter than the other bone in the lower leg, the tibia. In the 19th century, scientists had already noted that bird embryos first develop a tubular, dinosaur-like fibula. Only afterwards, it becomes shorter than the tibia and acquires its adult, splinter-like shape.

Brazilian researcher Joâo Botelho, working at the lab of Alexander Vargas (University of Chile) decided to study the mechanisms that underlie this transformation. In normal bone development, the shaft matures and ceases growth (cell division) long before the ends do. Botelho found that molecular mechanisms of maturation were active very early at the lower end, ceasing cell division and growth. When a maturation gene called Indian Hedgehog was inhibited, this resulted in chickens that kept a tubular fibula as long as the tibia and connected to the ankle, just like a dinosaur.

Botelho and collaborators believe that early maturation at the lower end of the fibula occurs because of the influence of a nearby bone in the ankle, the calcaneum. Unlike other animals, the calcaneum in bird embryos presses against the lower end of the fibula: They are so close they have even been confused with a single element by some researchers. Botelho proposes that at this stage, the lower end of the fibula receives signals more like those at the bone shaft. In normal development, the calcaneum then becomes detached from the fibula. However, its distal end has already become committed to shaft-like development, and matures early. In the chickens with experimentally dinosaur-like lower legs, the calcaneum was still attached to the fibula. Botelho also confirmed the calcaneum strongly expresses PthrP, a gene that allows growth at the ends of bones.

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