About This Project

Most mammals have penis bones, also called bacula. Our recent review indicated that penis bones evolved 9 independent times. Based on this study, the origin of a rodent penis bone is separate from the origin in carnivores. This got us thinking, do these independent events utilize the same set of genes to develop penis bones? In this project we will investigate the expression of a critical bone development gene across species that evolved bacula independently.

Ask the Scientists

What is the context of this research?

After our review which supported that penis bones evolved independently, our focus turned to understanding the molecular mechanisms involved in these independent derivations. Previous research indicates that both the penis itself and bones evolved long before the most recent common ancestor of mammals. The genes involved in the initiation of these structures have been maintained over time. This leads us to believe that the expression of a conserved set of bone-related genes may be modified in different mammal groups leading to the independent gains and losses of bacula; almost like turning on or off a light switch. Here, we plan to address this by investigating the regulation of a key skeletal gene(Sox9) which is involved in the early development of bones.

What is the significance of this project?

Much of what we know about skeletal evolution pertains to changes in morphology, leaving a giant void in comparing the mechanisms for how new bones evolve. The numerous independent gains of penis bones provides us with a unique model for understanding how new bones can form. Bones are known to develop either from a cartilaginous precursor (endochondral ossification) or directly into bone cells (membranous ossification). Studies on the development of penis bones show that they generally form via a cartilaginous precursor, similar to how long bones like femurs develop. Given these developmental similarities studying penis bones may allow us to learn different regulatory mechanisms not yet identified in other parts of the skeleton.

What are the goals of the project?

We will see if Sox9, a key gene for early cartilage development, is expressed in the developing penises of three mammal species; mice, ferrets, and rabbits. While mice and ferrets have penis bones, rabbits do not. According to our study, mice and ferrets evolved penis bones independently. To start, we will look for Sox9 expression in the genitalia of developing mice using In Situ Hybridization. These comparisons will help us address two critical questions: 1) Do independently evolved penis bones utilize Sox9 in early development? 2) Is Sox9 still expressed in species that never develop penis bones? Confirming the presence of Sox9 in the developing genitalia of these three species is the first step in understanding how a key skeletal gene like Sox9 may be regulated in different species.