title: Analysis of embryonic development in the unsequenced axolotl: Waves of transcriptomic upheaval and stability.
abstract: The axolotl (Ambystoma mexicanum) has long been the subject of biological research, primarily owing to its outstanding regenerative capabilities. However, the gene expression programs governing its embryonic development are particularly underexplored, especially when compared to other amphibian model species. Therefore, we performed whole transcriptome polyA+ RNA sequencing experiments on 17 stages of embryonic development. As the axolotl genome is unsequenced and its gene annotation is incomplete, we built de novo transcriptome assemblies for each stage and garnered functional annotation by comparing expressed contigs with known genes in other organisms. In evaluating the number of differentially expressed genes over time, we identify three waves of substantial transcriptome upheaval each followed by a period of relative transcriptome stability. The first wave of upheaval is between the one and two cell stage. We show that the number of differentially expressed genes per unit time is higher between the one and two cell stage than it is across the mid-blastula transition (MBT), the period of zygotic genome activation. We use total RNA sequencing to demonstrate that the vast majority of genes with increasing polyA+ signal between the one and two cell stage result from polyadenylation rather than de novo transcription. The first stable phase begins after the two cell stage and continues until the mid-blastula transition, corresponding with the pre-MBT phase of transcriptional quiescence in amphibian development. Following this is a peak of differential gene expression corresponding with the activation of the zygotic genome and a phase of transcriptomic stability from stages 9-11. We observe a third wave of transcriptomic change between stages 11 and 14, followed by a final stable period. The last two stable phases have not been documented in amphibians previously and correspond to times of major morphogenic change in the axolotl embryo: gastrulation and neurulation. These results yield new insights into global gene expression during early stages of amphibian embryogenesis and will help to further develop the axolotl as a model species for developmental and regenerative biology.
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States.
authors.forename: Peng
authors.initials: P
authors.lastname: Jiang
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States.
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Nelson
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States.
authors.forename: Ning
authors.initials: N
authors.lastname: Leng
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States.
authors.forename: Michael
authors.initials: M
authors.lastname: Collins
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States.
authors.forename: Scott
authors.initials: S
authors.lastname: Swanson
authors.affiliation: Department of Biostatistics and Medical Informatics, University of Wisconsin, Madison, WI, United States.
authors.forename: Colin N
authors.initials: CN
authors.lastname: Dewey
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States; Department of Cell and Regenerative Biology, University of Wisconsin, Madison, WI, United States; Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA, United States.
authors.forename: James A
authors.initials: JA
authors.lastname: Thomson
authors.affiliation: Regenerative Biology, Morgridge Institute for Research, Madison, WI, United States. Electronic address: rstewart@morgridge.org.
authors.forename: Ron
authors.initials: R
authors.lastname: Stewart
mesh_terms: D000558:Ambystoma mexicanum / Q000196:embryology*; D000818:Animals; D020451:Contig Mapping; D047108:Embryonic Development / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D063990:Gene Ontology; D009024:Morphogenesis / Q000235:genetics; D005810:Multigene Family; D012333:RNA, Messenger / Q000235:genetics; D017423:Sequence Analysis, RNA; D014158:Transcription, Genetic; D059467:Transcriptome
chemical_list: D012333:RNA, Messenger
keywords: Axolotl; Development; Transcriptome