Zebrafish in context: uses of a laboratory model in comparative studies

With the recent interest in the reintegration of evolutionary and developmental biology has come a growing need for understanding the phylogenetic relations and degree of generality of the model organisms upon which we rely so heavily. In vertebrate biology the zebrafish Danio rerio has become a paradigmatic system for studies at levels of organization from molecular to interspecific. Studies of model systems in development are often techniques-driven rather than questions-based; however, informative hypotheses for developmental research can be derived from phylogenetic distributions of characters. With some understanding of how general the characters of interest are, a thoughtful comparison of the requirements of the questions with the lists of available embryos, reagents, and protocols can guide choices of new vertebrate models. We describe here the phylogenetic placement of zebrafish within the vertebrate world and discuss how generally observations on zebrafish can be taken to apply. We outline a practical protocol for investigating development in a comparative context, illustrated with an example from an ongoing study of teleost tail fin evolution. The principles and procedures presented here apply equally well to any comparative study with an interest in evolution, at any level of phylogeny from intraspecific studies to comparisons across phyla.