title: The mid-developmental transition and the evolution of animal body plans.
abstract: 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.
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Michal
authors.initials: M
authors.lastname: Levin
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Leon
authors.initials: L
authors.lastname: Anavy
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Alison G
authors.initials: AG
authors.lastname: Cole
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Eitan
authors.initials: E
authors.lastname: Winter
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Natalia
authors.initials: N
authors.lastname: Mostov
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Sally
authors.initials: S
authors.lastname: Khair
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Naftalie
authors.initials: N
authors.lastname: Senderovich
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Kovalev
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: David H
authors.initials: DH
authors.lastname: Silver
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Martin
authors.initials: M
authors.lastname: Feder
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
authors.forename: Selene L
authors.initials: SL
authors.lastname: Fernandez-Valverde
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
authors.forename: Nagayasu
authors.initials: N
authors.lastname: Nakanishi
authors.affiliation: Whitney Laboratory for Marine Bioscience, University of Florida, 9505 N Ocean Shore Blvd, St Augustine, Florida 32080-8610 USA.
authors.forename: David
authors.initials: D
authors.lastname: Simmons
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany.
authors.forename: Oleg
authors.initials: O
authors.lastname: Simakov
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany.
authors.forename: Tomas
authors.initials: T
authors.lastname: Larsson
authors.affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany.
authors.forename: Shang-Yun
authors.initials: SY
authors.lastname: Liu
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Ayelet
authors.initials: A
authors.lastname: Jerafi-Vider
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Karina
authors.initials: K
authors.lastname: Yaniv
authors.affiliation: Whitney Laboratory for Marine Bioscience, University of Florida, 9505 N Ocean Shore Blvd, St Augustine, Florida 32080-8610 USA.
authors.forename: Joseph F
authors.initials: JF
authors.lastname: Ryan
authors.affiliation: Whitney Laboratory for Marine Bioscience, University of Florida, 9505 N Ocean Shore Blvd, St Augustine, Florida 32080-8610 USA.
authors.forename: Mark Q
authors.initials: MQ
authors.lastname: Martindale
authors.affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany.
authors.forename: Jochen C
authors.initials: JC
authors.lastname: Rink
authors.affiliation: Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany.
authors.forename: Detlev
authors.initials: D
authors.lastname: Arendt
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
authors.forename: Sandie M
authors.initials: SM
authors.lastname: Degnan
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
authors.forename: Bernard M
authors.initials: BM
authors.lastname: Degnan
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Tamar
authors.initials: T
authors.lastname: Hashimshony
authors.affiliation: Department of Biology, Technion - Israel Institute of Technion, Haifa 32000, Israel.
authors.forename: Itai
authors.initials: I
authors.lastname: Yanai
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000235:genetics; D017124:Conserved Sequence / Q000235:genetics; D047108:Embryonic Development / Q000235:genetics; D019143:Evolution, Molecular; D018507:Gene Expression Regulation, Developmental; D053263:Gene Regulatory Networks; D050437:Genes, Developmental / Q000235:genetics; D008954:Models, Biological; D010641:Phenotype; D010802:Phylogeny; D013045:Species Specificity; D059467:Transcriptome / Q000235:genetics