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Nature. 2016 Mar 31;531(7596):637-641. doi: 10.1038/nature16994. Epub 2016 Feb 17.

The mid-developmental transition and the evolution of animal body plans.

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Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
Whitney Laboratory for Marine Bioscience, University of Florida, 9505 N Ocean Shore Blvd, St Augustine, Florida 32080-8610 USA.
Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany.
Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany.
Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
Contributed equally


Animals are grouped into ~35 'phyla' based upon the notion of distinct body plans. Morphological and molecular analyses have revealed that a stage in the middle of development--known as the phylotypic period--is conserved among species within some phyla. Although these analyses provide evidence for their existence, phyla have also been criticized as lacking an objective definition, and consequently based on arbitrary groupings of animals. Here we compare the developmental transcriptomes of ten species, each annotated to a different phylum, with a wide range of life histories and embryonic forms. We find that in all ten species, development comprises the coupling of early and late phases of conserved gene expression. These phases are linked by a divergent 'mid-developmental transition' that uses species-specific suites of signalling pathways and transcription factors. This mid-developmental transition overlaps with the phylotypic period that has been defined previously for three of the ten phyla, suggesting that transcriptional circuits and signalling mechanisms active during this transition are crucial for defining the phyletic body plan and that the mid-developmental transition may be used to define phylotypic periods in other phyla. Placing these observations alongside the reported conservation of mid-development within phyla, we propose that a phylum may be defined as a collection of species whose gene expression at the mid-developmental transition is both highly conserved among them, yet divergent relative to other species.

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