title: Somitogenesis in the anole lizard and alligator reveals evolutionary convergence and divergence in the amniote segmentation clock.
abstract: The axial skeleton is a defining feature of vertebrates and is patterned during somitogenesis. Cyclically expressed members of the notch and other signaling pathways, described as the 'segmentation clock', regulate the formation of somite boundaries. Comparisons among vertebrate model systems have revealed fundamental shifts in the regulation of expression among critical genes in the notch pathway. However, insights into the evolution of these expression differences have been limited by the lack of information from non-avian reptiles. We analyzed the segmentation clock of the first Lepidosaurian reptile sequenced, the green anole lizard, Anolis carolinensis, for comparison with avian and mammalian models. Using genomic sequence, RNA-Seq transcriptomic data, and in situ hybridization analysis of somite-stage embryos, we carried out comparative analyses of key genes and found that the anole segmentation clock displays features common to both amniote and anamniote vertebrates. Shared features with anamniotes, represented by Xenopus laevis and Danio rerio, include an absence of lunatic fringe (lfng) expression within the presomitic mesoderm (PSM), a hes6a gradient in the PSM not observed in the chicken or mouse, and EGF repeat structure of the divergent notch ligand, dll3. The anole and mouse share cycling expression of dll1 ligand in the PSM. To gain insight from an Archosaurian reptile, we analysed LFNG and DLL1 expressions in the American alligator. LFNG expression was absent in the alligator PSM, like the anole but unlike the chicken. In contrast, DLL1 expression does not cycle in the PSM of the alligator, similar to the chicken but unlike the anole. Thus, our analysis yields novel insights into features of the segmentation clock that are evolutionarily basal to amniotes versus those that are specific to mammals, Lepidosaurian reptiles, or Archosaurian reptiles.
authors.affiliation: School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ 85287, USA.
authors.forename: Walter L
authors.initials: WL
authors.lastname: Eckalbar
authors.forename: Eris
authors.initials: E
authors.lastname: Lasku
authors.forename: Carlos R
authors.initials: CR
authors.lastname: Infante
authors.forename: Ruth M
authors.initials: RM
authors.lastname: Elsey
authors.forename: Glenn J
authors.initials: GJ
authors.lastname: Markov
authors.forename: April N
authors.initials: AN
authors.lastname: Allen
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Corneveaux
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Losos
authors.forename: Dale F
authors.initials: DF
authors.lastname: DeNardo
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Huentelman
authors.forename: Jeanne
authors.initials: J
authors.lastname: Wilson-Rawls
authors.forename: Alan
authors.initials: A
authors.lastname: Rawls
authors.forename: Kenro
authors.initials: K
authors.lastname: Kusumi
mesh_terms: D000489:Alligators and Crocodiles / Q000196:embryology / Q000235:genetics*; D000595:Amino Acid Sequence; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000145:classification / Q000235:genetics; D056926:CLOCK Proteins / Q000145:classification / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000196:embryology / Q000378:metabolism; D019143:Evolution, Molecular; D005260:Female; D018507:Gene Expression Regulation, Developmental; D014644:Genetic Variation; D017403:In Situ Hybridization; D047908:Intracellular Signaling Peptides and Proteins / Q000145:classification / Q000235:genetics; D008116:Lizards / Q000196:embryology / Q000235:genetics*; D008297:Male; D008648:Mesoderm / Q000196:embryology / Q000378:metabolism; D008969:Molecular Sequence Data; D010802:Phylogeny; D019170:Somites / Q000196:embryology / Q000378:metabolism*; D059467:Transcriptome / Q000235:genetics
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D056926:CLOCK Proteins; D047908:Intracellular Signaling Peptides and Proteins