PHYLOGENETICS OF MODEL ORGANISMS - THE LABORATORY AXOLOTL, AMBYSTOMA-MEXICANUM

The axolotl, Ambystoma mexicanum, has been an important model system i n both developmental and evolutionary biology for over 100 years. The axolotl is a recently derived member of the Ambystoma tigrinum complex (Ambystomatidae), a rapidly evolving clade of 17 closely related spec ies ranging from southern Canada to central Mexico. The relationships of the A. tigrinum complex to the remaining species of ambystomatids a re controversial; morphological data are ambiguous, although allozyme data suggest a sister-group relationship with A. macrodactylum-A. jeff ersonianum with moderate statistical support. At the familial level, r ibosomal RNA sequence data strongly suggest that Dicamptodon is the si ster group to the Ambystomatidae, followed by the newts (Salamandridae ). Two primary foci of laboratory research on the axolotl are the deve lopmental mechanisms of pigment pattern and the physiological control of metamorphosis. Both pigment pattern and metamorphosis are extremely variable within the A. tigrinum complex and are important systematic characters at the inter- and intraspecific levels. Current comparative studies on postmetamorphic color pattern and metamorphosis across the A. tigrinum complex will be particularly important in elucidating the developmental bases of rapid divergence and speciation, and our exist ing phylogenetic information provides a historical framework to help g uide the choice of taxa for additional developmental analyses.