What is metamorphosis?

Metamorphosis (Gr. meta- "change" + morphe "form") as a biological process is generally attributed to a subset of animals: most famously insects and amphibians, but some fish and many marine invertebrates as well. We held a symposium at the 2006 Society for Integrative and Comparative Biology (SICB) annual meeting in Orlando, FL (USA) to discuss metamorphosis in a comparative context. Specifically, we considered the possibility that the term "metamorphosis" could be rightly applied to non-animals as well, including fungi, flowering plants, and some marine algae. Clearly, the answer depends upon how metamorphosis is defined. As we participants differed (sometimes quite substantially) in how we defined the term, we decided to present each of our conceptions of metamorphosis in 1 place, rather than attempting to agree on a single consensus definition. Herein we have gathered together our various definitions of metamorphosis, and offer an analysis that highlights some of the main similarities and differences among them. We present this article not only as an introduction to this symposium volume, but also as a reference tool that can be used by others interested in metamorphosis. Ultimately, we hope that this article-and the volume as a whole-will represent a springboard for further investigations into the surprisingly deep mechanistic similarities among independently evolved life cycle transitions across kingdoms.

Induction of a secondary embryonic axis in zebrafish occurs following the overexpression of beta-catenin.

Formation of the vertebrate axis may involve a Wnt signaling cascade similar to the Drosophila wingless pathway. Zebrafish wnt8 is a candidate for involvement in axis specification insofar as it is expressed maternally and when overexpressed it can induce goosecoid, a transcription factor normally expressed in the embryonic shield. In this study we demonstrate that beta-catenin, a cadherin associated protein in the Wnt pathway, is expressed maternally in zebrafish and is widely distributed in the early embryo. Overexpressing beta-catenin in early zebrafish embryos induces goosecoid and ntl, ultimately leading to a duplication of a complete secondary axis. These data are consistent with the involvement of beta-catenin in a Wnt signaling pathway which is involved in mesoderm induction in zebrafish.

Zebrafish wnt8 and wnt8b share a common activity but are involved in distinct developmental pathways.

The specification of the vertebrate body plan is dependent on numerous signaling molecules, including members of the Wnt family. We have identified two zebrafish wnt8 paralogs related to Xwnt-8B and Xwnt-8, respectively. A RT-PCR assay demonstrated that wnt8 is expressed maternally, with transcripts detected throughout embryogenesis, whereas wnt8b transcripts were first detected during late gastrulation. The wnt8 transcripts at 50% epiboly are spatially restricted to those cells at the blastoderm margin, overlying gsc-expressing cells in the axial hypoblast. During late gastrulation, wnt8 was no longer detected in the marginal cells at the dorsal midline and by mid-segmentation, transcripts were found in the presumptive tail bud. In contrast, wnt8b expression is spatially restricted to prospective neuroepithelium, and later to neural-specific structures. Overexpression of both wnts results in two major phenotypes: radialized embryos and embryos with anterior defects. These phenotypes were preceded by significant changes in the spatial expression patterns of gsc and ntl transcripts, reminiscent of activities of Xwnt-8 in Xenopus, and consistent with a role for wnt8 in the specification or patterning of mesoderm.