title: Axial patterning in snakes and caecilians: evidence for an alternative interpretation of the Hox code.
abstract: It is generally assumed that the characteristic deregionalized body plan of species with a snake-like morphology evolved through a corresponding homogenization of Hox gene expression domains along the primary axis. Here, we examine the expression of Hox genes in snake embryos and show that a collinear pattern of Hox expression is retained within the paraxial mesoderm of the trunk. Genes expressed at the anterior and most posterior, regionalized, parts of the skeleton correspond to the expected anatomical boundaries. Unexpectedly however, also the dorsal (thoracic), homogenous rib-bearing region of trunk, is regionalized by unconventional gradual anterior limits of Hox expression that are not obviously reflected in the skeletal anatomy. In the lateral plate mesoderm we also detect regionalized Hox expression yet the forelimb marker Tbx5 is not restricted to a rudimentary forelimb domain but is expressed throughout the entire flank region. Analysis of several Hox genes in a caecilian amphibian, which convergently evolved a deregionalized body plan, reveals a similar global collinear pattern of Hox expression. The differential expression of posterior, vertebra-modifying or even rib-suppressing Hox genes within the dorsal region is inconsistent with the homogeneity in vertebral identity. Our results suggest that the evolution of a deregionalized, snake-like body involved not only alterations in Hox gene cis-regulation but also a different downstream interpretation of the Hox code.
authors.affiliation: Department of Molecular Cell Biology, Institute of Biology, Leiden University, 2333 AL, Leiden, the Netherlands. jmwoltering@hotmail.com
authors.forename: Joost M
authors.initials: JM
authors.lastname: Woltering
authors.forename: Freek J
authors.initials: FJ
authors.lastname: Vonk
authors.forename: Hendrik
authors.initials: H
authors.lastname: Müller
authors.forename: Nabila
authors.initials: N
authors.lastname: Bardine
authors.forename: Ioana L
authors.initials: IL
authors.lastname: Tuduce
authors.forename: Merijn A G
authors.initials: MA
authors.lastname: de Bakker
authors.forename: Walter
authors.initials: W
authors.lastname: Knöchel
authors.forename: I Ovidiu
authors.initials: IO
authors.lastname: Sirbu
authors.forename: Antony J
authors.initials: AJ
authors.lastname: Durston
authors.forename: Michael K
authors.initials: MK
authors.lastname: Richardson
mesh_terms: D000423:Alcian Blue / Q000378:metabolism; D000663:Amphibians / Q000196:embryology* / Q000235:genetics; D000818:Animals; D000880:Anthraquinones / Q000378:metabolism; D019521:Body Patterning; D001842:Bone and Bones / Q000033:anatomy & histology / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D008116:Lizards / Q000196:embryology / Q000235:genetics; D008648:Mesoderm / Q000378:metabolism; D051379:Mice; D012911:Snakes / Q000196:embryology* / Q000235:genetics; D019170:Somites / Q000378:metabolism; D020825:T-Box Domain Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D000880:Anthraquinones; D018398:Homeodomain Proteins; D020825:T-Box Domain Proteins; C103172:T-box transcription factor 5; C010078:alizarin; D000423:Alcian Blue