The size and shape of the vertebrate skull can reveal a great deal about an animal's lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don't. Where did ear bones come from? Gene expression patterns can be used to test hypotheses based on morphology. For example, the regulatory gene Bapx1 is expressed in the hinge of the developing lower jaw in fishes and reptiles.
Where would you predict Bapx1 expression to occur in mammals?

Verified step by step guidance

1. Understand the context: The problem discusses the evolutionary origin of ear bones in mammals and the expression of the Bapx1 gene, which is involved in jaw development in fishes and reptiles.
2. Identify the key concept: Bapx1 is a regulatory gene expressed in the hinge of the developing lower jaw in fishes and reptiles. This suggests a role in jaw formation and possibly in the evolution of structures related to the jaw.
3. Consider evolutionary relationships: In mammals, the ear bones (malleus, incus, and stapes) are derived from jaw bones found in ancestral species. This indicates a transformation of jaw elements into ear bones during mammalian evolution.
4. Predict gene expression in mammals: Given the evolutionary transformation, Bapx1 expression in mammals might be expected in areas related to both jaw development and the formation of ear bones, such as the region where the jaw and ear structures are derived.
5. Formulate a hypothesis: Based on the evolutionary link between jaw bones and ear bones, predict that Bapx1 expression in mammals could occur in the region of the developing jaw that contributes to the formation of the ear bones, reflecting its ancestral role in jaw development.

Key Concepts

Evolution of Ear Bones

The evolution of ear bones in mammals is a significant adaptation that distinguishes them from reptiles. The malleus, incus, and stapes are derived from jaw bones found in ancestral species, reflecting a shift in function from feeding to hearing. This evolutionary change highlights the complex relationship between morphology and function in vertebrates.

Gene Expression Patterns

Gene expression patterns are crucial for understanding developmental processes and evolutionary changes. In this context, the expression of specific genes like Bapx1 can indicate the developmental pathways that lead to the formation of structures such as ear bones. By comparing gene expression across species, scientists can infer evolutionary relationships and functional adaptations.

Regulatory Gene Bapx1

Bapx1 is a regulatory gene involved in the development of the jaw and associated structures. In fishes and reptiles, Bapx1 is expressed in the hinge of the developing lower jaw, suggesting its role in jaw formation. In mammals, predicting Bapx1 expression involves understanding its evolutionary shift, potentially indicating its involvement in the development of ear bones due to their origin from jaw structures.