oca2 Mutation and its Effects on Pigmentation and Behavior in Astyanax Mexicanus and Danio Rerio
Understanding the genetic basis of trait evolution is critical for determining how and why traits evolve. While many genes, hypothesized to underlie the evolution of traits, have been identified, few have been functionally evaluated in an evolutionary relevant organism. Albinism is the loss of melanin pigmentation, a genetic condition that has evolved in many animals, including populations of Astyanax mexicanus, a small teleost fish, which has both pigmented surface-dwelling and albino cave-dwelling forms. Loss of function mutations in the oculocutaneous albinism II (oca2) gene in cavefish are hypothesized to result in albinism. Intriguingly, loss of OCA2 may also be responsible for changes in behavior, as transient knock-downs of this gene lead to increased amounts of dopamine, a catecholamine neurotransmitter. Catecholamines underlie several behaviors. Our study examined (1) the pigmentation and (2) behavior in oca2 mutant and wildtype fish. These studies were performed in A. mexicanus and another teleost fish, Danio rerio, the zebrafish. We found that the oca2 mutation also affects melanin pigment in zebrafish and surface A. mexicanus. We demonstrated that loss of melanin pigmentation occurs by disrupting the first step of the melanin synthesis pathway. In addition, we found an increase in the number of another pigment cell type, iridophores, in the albino oca2- mutant zebrafish. By crossing oca2 mutant surface fish to cavefish, we definitively demonstrated that oca2 mutations cause albinism in these populations. To study the relationship between pigmentation and behavior, we examined a catecholamine-regulated behavior, schooling, in zebrafish. Our preliminary results did not demonstrate differences in schooling behavior in oca2-mutant zebrafish. Future work will examine alternative behaviors. This is one of the first studies in vertebrates functionally testing a candidate gene underlying the evolution of a trait in the organism itself. Furthermore, extensions of this work may demonstrate a link between albinism and behavior.