Development and evolution in the salamander family Plethodontidae

Development and evolution in plethodontid salamanders were studied. Most of the data are descriptive, including observations of the development of nine species whose development has not been described previously. These observations include developmental stages from first cleavage to hatching for all but one species. Three species (Batrachoseps attenuatus, Ensatina eschscholtzii, and Bolitoglossa subpalmata) are the subject of most of the descriptive embryology, with data on the other six species providing comparative material. These three species have direct development, large genome sizes, and relatively large eggs for amphibians. A robust phylogenetic hypothesis for plethodontids and comparable developmental data on other species of amphibians provide a phylogenetic context for the data.;There are four general conclusions: (1) Relatively simple changes in some developmental parameters can have dramatic impacts on later development. One example is the effect of interspecific differences on the morphogenetic events of gastrulation due to increases in egg size. (2) Different developmental mechanisms can produce similar morphologies. This conclusion can be reached whether one looks within the development of an individual organism (developmental redundancies) or compares the development of different species. The main example is an interspecific comparison of the evolution of direct development in a lineage of salamanders and in a lineage of frogs. Comparison of ontogenetic trajectories shows that a similar developmental pattern, the simultaneous appearance of front and hind limb buds characteristic of direct development in these two lineages, evolved by different heterochronic transformations. (3) Early development (those stages through neurulation) of the species studied is less similar than expected, based on previous studies. (4) Variation in early development does not correlate with the variation seen at the level of adult morphology. Later developmental events, particularly organogenesis, appear to offer better explanations of the variation in adult morphology.