COMPARISON OF GASTRULATION IN FROGS AND FISH

Comparative embryological studies of frogs and fish provide valuable i nformation about the mechanisms and evolution of vertebrate developmen t. First, by mapping developmental data from a range of species onto a cladogram, one can distinguish general features of a ground plan from variation within it. Two studies illustrate this: comparison of gastr ulation mechanisms in sturgeon and Xenopus, and morphogenesis of the d orsal mesoderm in five species of anurans. Second, phylogenetic analys is of developmental data makes it possible to identify radical departu res from the ground plan among related groups. Teleost gastrulation is a highly derived process that appears to have little in common with t he ancestral version. However, teleost gastrulation may have evolved a s a result of two specific developmental changes: loss of bottle cells in the surface layer, and changes in the yolk. The phylogenetic distr ibution of developmental characters forms the basis for mechanistic hy potheses about the origins of major evolutionary changes in developmen t.