The embryonic development of Pleurodeles waltl (Michah). Reappraisal of the Gallien-Durocher stage series.

Abstract

Among the Urodelan amphibians that were chosen for study by developmental biologists during the second half of the present century, the Spanish newt (Pleurodeles waltl Michahelles) was and still is widely used in European countries and especially in France. Concomitantly with other European newts, it had already been selected by Vogt (1929) to establish the map of presumptive areas in the Urodelan gastrula: compared to the Triturus species, the Pleurodeles waltl offers a great advantage by providing hundreds of eggs in one and the same spawning, an abundance shared by the Mexican Axolotl, whose eggs and embryos are slightly bigger. A decisive step forward was taken when laboratory facilities were established to breed Pleurodeles. The moving spirit behind this task was Professor Louis Gallien (University of Paris) who published in 1952 a short article describing the mating behavior of adult Pleurodeleskept in aquaria, the collection of eggs, the rearing of larvae and young post-metamorphic animals. The latter were prevented from leaving the water. which enable Gallien to acquire a population of salamanders that were constantly maintained in running water and could spawn, not only in winter and spring. like the wild populations at Portugal and Spain, but also in autumn. A selection at such animals probably occurred at the beginning, because young adult wild animals obviously suffer and eventually die when they are constantly maintained in water. Even young metamorphosed animals. when obtained from wild parents, are not quite able to adapt to a permanently aquatic existence. Gallien's original population of Spanish newts in 1948 was obtained from a few adults kept in the Museum National d'Histoire Naturelle in Paris. Later on, crosses with wild animals from Portugal were performed to avoid the deleterious effects of perpetuated inbreeding. In the eariy fifties. standard developmental stages had already been described for several species of frogs, toads and salamanders (Ambystoma, Triturus). It was necessary to obtain a similar stage series for Pleurodeles, which Gallien had detinitely chosen as a developmental model at that time, when his investigations on trag sexual differentiation shifted to the Urodeles, in parallel to Humprey's work on Ambystoma. I was still a student in October 1950 and I had to work on a biological research topic for one year. Gallien allowed me a choice of two topics. The first was an investigation of the regenerative power of limbs in post-metamorphic Xenopus, which had not yet been described. The second was the time-table of Pleurodeles embryonic development. I preferred the first alternative because it was an experimental topic and, moreover, at that time, there were no temperature regulated incubators (18' or 20'C, 25'C) in the newly established laboratory: Pleurodeles eggs still had to be kept in more or less well insulated glass containers and heated with electric lamps, a situation which was not satisfactory for precise timestaging. Only a few years later came another student. Micheline Durocher, from the Ecole Normale Superieure at Fontenay-aux-Roses, where Gallien used to give regular lectures. For several months, she devoted herself conscientiously to the study of Pleurodeles development. She found evidence for a slight variability in the duration of stages from one spawning to another and she calculated the limits. She accurately drew the various embryonic