The phylotypic stage of Zebrafish

The phylotypic stage, as part of the embryonic period, is the stage where embryos of different species of a phylum show a high degree of similarity. Johann Friedrich Meckel, Karl Ernst von Baer and Ernst Haeckel already described it for vertebrates in the 19th century. They observed that vertebrate embryos pass through a period of morphological similarity. Since then, scientists have researched the field of the phylotypic stage and it was subject of many controversial discussions. The name “phylotypic stage” was coined by Klaus Sander in 1983 and describes not only the stage of the highest similarity but also the stage, typical (characteristic) for a phylum. The following study examines the phylotypic stage of zebrafish (Danio rerio). Looking at different conserving mechanisms like internal constrains and stabilizing selection, different hypothesis and concepts by several researchers were tested. To test if the phylotypic stage is accessible to selection (although it generally is considered a conserved evolutionary stage) I have studied patterns of variation during embryogenesis. I have looked at the phenotypic variance and the number of significant correlations among embryonic traits and described the phylotypic stage as a period characterized by a high number of internal correlations and declining phenotypic variance. Then, I tested if changes in the raising conditions could elicit phenotypic changes. Therefore, zebrafish embryos have been raised under different experimental conditions to see if developmental plasticity can be induced during the early developmental period and if clearly defined modules can be identified. Eggs of zebrafish were raised in: (1) different temperatures; (2) different salinities; and (3) different levels of oxygen concentration. Up to 14 characters of individual embryos were measured during early development, encompassing the phylotypic stage. In particular I found a considerable degree of heterochrony and modularity. Embryos grew slower at lower temperatures and lower oxygen levels. Plasticity was detected in the overall size of the embryo and the size of somites in the oxygen and temperature experiment. The development of the eye and otic vesicle was shifted to a later x stage under severe hypoxia. Thus, eye and otic vesicle could be identified as modules, which can be dissociated from other characters of the developing embryo (heterochrony). Changes in raising condition affect early development of the zebrafish on three levels: (1) developmental rate (2) size and shape, and (3) dissociation of modules. Thus, plasticity and modularity are effective during early embryonic development. Finally I studied the heritability of embryonic traits to examine how inheritance contributes to the stabilization of the phylotypic stage in variable environments. Following the heritabilities of certain traits reveals that the phylotypic stage is not characterized by a certain pattern of decreased heritability and thus decreased additive genetic variance. The results suggest that the phylotypic stage of zebrafish is constrained by multiple internal correlations when embryos are developing in standard conditions. However, under marginal developmental conditions so far ineffective modules become effective and buffer the embryo against disruptive effects of the environment. Patterns of family resemblance are present, indicating an inherited genetic portion of the phylotypic stage. However, under strong environmental influence it is dominated by variation associated with phenotypic plasticity. My general conclusion is that the phylotypic stage is not established because additive genetic variance is exhausted during the early period of vertebrate development but that it is under environmental and genetic influence, thus is accessible to selection. Internal constraints could be identified to stabilize morphology during the phylotypic stage, but a certain degree of phenotypic variation can be observed.