Pigment pattern is a defining characteristic of many animals that is more than just beautiful to look at; stripes, spots, and bright colors function in many behaviors such as warning coloration, mate recognition, and camouflage. Even among closely related species, pigment patterns can be stunningly diverse. In the Parichy lab, we use the pigment pattern of the adult zebrafish, Danio rerio, to study molecular and cellular mechanisms of pattern formation and how these processes evolve between species.
The stripes on a zebrafish are composed of three different cell types: black melanophores, yellow orange xanthophores, and iridescent iridophores. We know from previous work that interactions between pigment cells are crucial for stripe formation, but cues from the environment tell the pigment cells when and where to show up.
In this paper, we discuss the role of thyroid hormone in the development of different pigment cell lineages and metamorphosis using genetic mutants as well as a new technique to ablate the thyroid follicles in living fish. We find that no thyroid hormone results in no xanthophores, more melanophores, and a host of other developmental delays. On the other hand, too much thyroid hormone in the opallus mutant yields many more xanthophores, fewer melanophores, and other heterochronies in developmental milestones. Interestingly, the pigment pattern and anatomical features of the opallus mutant resemble D. albolineatus, a species that is closely related to D. rerio suggesting that the thyroid hormone pathway is involved in the divergence of these danios. By ablating the thyroid follicles in the other species, we learned that D. albolineatus have evolved a distinct pigment cell population that is independent of thyroid hormone while other functions of the hormone remain conserved.
These are just a few of the exciting new findings in this jam-packed report. Be sure to check it out (even if it’s just for the stellar pictures)!
~Emily
