Variation in anuran embryogenesis: differences in sequence and timing of early developmental events.

Comparative embryology of closely related species can shed light on the evolution of developmental processes. An important mechanism in the evolution of developmental processes, which can lead to significant changes in larval or adult form, is variation in the sequence and timing of developmental events. We compared the development of 12 species of anurans, including a wide taxonomic range as well as a number of congeneric species. The comparison consisted of monitoring a series of external morphological markers and histological markers. For each species we noted the timing of each of the markers, using a uniform parameter of normalized time. We compared the normalized time of each of these events among the species, as well as the sequence of the events. Our analysis revealed many differences in sequence and in timing of developmental events. We mapped these differences on a cladogram of the studied species, using sequence units as discrete characters. The differences do not seem to be connected to the phylogenetic relations between the species or to any obvious ecological factors. We suggest a hypothetical ancestral sequence of developmental events, and discuss the possible factors that could have caused the observed variations from the ancestral sequence.