Heterochronic shifts during early cranial neural crest cell migration in two ranid frogs

We describe the development of the cranial neural crest cell streams relative to embryonic events such as neural tube formation and somite appearance in two Eurasian frog species belonging to the Ranidae, Rana temporaria and Sylvirana nigrovittata, and demonstrate developmental heterochronies. The mandibular stream appeared well developed in R. temporaria at a time when the embryo was still spherical, the neural folds were elevated, and the neural plate was wide open, thus earlier than known from any frog species so far. The appearance of the second stream and its division into hyoid and branchial portions was clearly accelerated in R. temporaria relative to other embryonic events when compared to S. nigrovittata. For example, in R. temporaria, the hyoid and branchial portions of the cranial neural crest cell streams were separated before the neural folds had started to fuse, whereas in S. nigrovittata this event took place only after the neural folds had fused completely. Such ostentatious heterochronies related to the characters used herein have formerly only been reported from comparisons between species belonging to different higher taxa. Our results re‐confirm that to understand the full dynamics of the evolution of development, studies need to implement comparative embryological approaches, and include phylogenetically relatively closely related taxa.