The evolution of allometric growth patterns in the genus Danio (Cypriniformes: Cyprinidae)

The zebrafish Danio rerio is fast becoming the most important vertebrate model organism in genetics and developmental biology. Establishing the phylogenetic position of a model organism is crucial for setting the framework to generalize from it to related organisms. In this study, I: (1) used a data set assembled from a traditional revision of the genus Danio to formulate a hypothesis of relationship among 17 species of Danio; (2) used allometric parameters to describe the growth trajectories of body shape characters for three species of Danio (D. rerio, D. albolineatus and D. aequipinnatus) and an outgroup {Rasbora sumatrana); (3) used the heterochronic changes in the allometric growth parameters to infer the phylogenetic relationships among the three species; and (4) mapped the heterochronic parameters on an independent estimate of the phylogeny to infer ancestral growth trajectories and the rates of character evolution occurring along the different lineages. The phylogenetic tree for the 17 species of Danio contradicts the conventional division of the genus into the subgenera Danio and Brachydanio. Significant shifts in the allometric parameters indicate a rearrangement in the developmental plan of D. rerio as it approaches sexual maturity; such developmental shifts were not observed in the other two species, presumably because only adults were sampled. The heterochronic growth parameters unequivocally support the hypothesis that D. rerio and D. albolineatus are more closely related to one another than either is to D. aequipinnatus. The rates of morphological evolution in D. rerio and D. albolineatus seem to be comparable, while D. aequipinnatus seem to be evolving at a much slower rate, being only as morphologically derived as the ancestor of D. rerio and D. albolineatus.