Development and evolution of pigmentation patterns in fishes of the genus Danio (Teleostei: Cyprinidae)

The developmental basis of a morphological character can affect the range of variation that can be produced in that character and therefore, its evolution. In this dissertation, I conduct a comparative study of pigment pattern development in six species of Danio and one outgroup (Tanichthys albonubes), and examine the habitats in which these species were found, to assess the selective forces that may have played a role in shaping those pigment patterns. I raised and examined growth series of all species and found that they formed identical larval patterns and followed a common sequence of events from which elements were eliminated or altered to produce differing pigment patterns. Species differed in the rate of pigment cell proliferation, in pigment cell differentiation, and in coalescence of pigment cells into patterns. Species also differed in the ultimate number and shape of stripes that formed. Differences in patterns between species are similar in several cases to described mutants of the zebrafish. This suggests that some aspects of interspecific pigmentation pattern variation may be under relatively simple genetic control. Morphometric analysis of all species showed that patterning differences between species were correlated with growth, size and shape change in a manner consistent with the predictions of theoretical models. This indicates that variation in pigment pattern may arise in part through differential growth between species. Those characters similar to single-gene mutants of the zebrafish tended to be homoplastic, characters potentially linked to other aspects of morphology are more phylogenetically conservative. I suggest that the integration of pigment and other morphological characters imposes a constraint, whereas the repeated occurrence of characters similar to those caused by a single gene may be the result of the repeated loss of function of that gene. Finally, analysis of habitat, environmental light, and fish reflectance data suggest that pigment patterns in this group may largely function in crypsis, although there may be competing pressures to be visible to conspecifics.