Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Cell Biology

Different sub-species of the beach mouse (Peromyscus polionotus) have different fur colors. These differences are best described by examining melanocyte cell function. Melanocytes are located at the base of the hair papilla and make a pigment molecule called eumelanin. Pigment is then transported from the base to the hair cortex where coloration becomes visible.

Illustration of an old field mouse and a melanocyte cell.

The Eumelanin Pigment

a diagram of part of the eumelanin pigment.

Eumelanin is a polymer of cross-linked dihydroxyindole carboxylic acids. It is composed of many units that can vary slightly in their composition.

Different sub-species of the beach mouse (Peromyscus polionotus) have different fur colors. These differences are best described by examining melanocyte cell function. Melanocytes are located at the base of the hair papilla and make a pigment molecule called eumelanin. Pigment is then transported from the base to the hair cortex where coloration becomes visible.

 

Eumelanin Production

Activation of the MC1R protein occurs when it successfully binds to a molecule called an α melanocyte stimulating hormone (α-MSH). Alternatively, if the MC1R protein binds to a different molecule, an agouti stimulating peptide, the biosynthetic pathway leading to eumelanin synthesis may not occur. However, assuming that α-MSH binding occurs, cAMP synthesis begins. cAMP is a common gene regulator and it aids in the expression of enzymes necessary in the biosynthetic pathway leading to eumelanin - it stimulates the expression of four genes: c(Tyr), Tryp1, Tyrp2 and p. The c(Tyr) gene is responsible for an enzyme called tyrosinase that catalyzes a reaction in a biosynthetic pathway that converts the amino acid tyrosine into a molecule called dopaquinone. The Tyrp1 and Tryp2 genes hold the blueprint for tyrosine-related enzymes that catalyze reactions in a subsequent pathway that converts dopachrome into eumelanin. The p gene is responsible for a protein that also aids in this process. Eumelanosomes are then made in the golgi apparatus of the melanocyte that are then transported out of the melanocyte to the hair cortex.

Without the successful binding of an α-MSH molecule by the MC1R protein, cAMP is not adequately regulated and the multistep biosynthetic pathway for eumelanin synthesis cannot take place. Structural changes to the MC1R protein as a result of a genetic mutation make it essentially non-functional and result in a different coat color due to changes in pigment production. This is often the case in coastal populations of beach mice where coat colors are lighter than their inland neighbors.