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.