then compared development of three muroid species: mouse (9 rugae), rat (8) and golden hamster (7). We showed that palatal growth rate, spacing and addition rate in mouse/rat were remark- ably similar (with respect to the embryo size difference), and that increase to 9 rugae in mouse is achieved by postponing the end of the addition process (hypermorphosis). Such a heterochronic shift may be typical of 1 variations observed among muroid rodents. In contrast, decrease to 7 rugae in golden hamster is attributed to early growth termination (progenesis) of the palate, which correlates with the severe shortening of gestation in this species. Our results provide an experimental support to the intu- itive view that terminal heterochronies are especially relevant to meristic evolution of traits that rely on a sequential addition process. doi:10.1016/j.mod.2009.06.653 15-P010 Evolutionary shifts in ALDH structure suggest transitions between pleiotropic and patterning functions Michael Schubert 1 , Marcos Simo ˜ es-Costa 2 , Ferdinand Marle ´ taz 1 , Tiago J. P. Sobreira 2 , Paulo S. L. de Oliveira 2 , Deborah Schechtman 2 , Fre ´de ´ ric Brunet 1 , Ricard Albalat 3 , Marianne Bronner-Fraser 4 , Vincent Laudet 1 , Jose ´ Xavier-Neto 2 1 Institut de Ge ´nomique Fonctionnelle de Lyon, Ecole Normale Supe ´rieure de Lyon, Lyon, France 2 Laborato ´rio de Gene ´tica e Cardiologia Molecular, InCor HC-FMUSP, Sa ˜o Paulo, Brazil 3 Departament de Gene `tica, Facultat de Biologia, Universitat de Barce- lona, Barcelona, Spain 4 Division of Biology 139-74, California Institute of Technology, Pasadena, CA, United States During development, major signalling pathways are con- trolled by morphogens, diffusible molecules whose evolutionary origins are very difficult to assess. Aldehyde dehydrogenase (ALDH) enzymes are attractive targets to study the evolution of morphogen signalling, because in addition to catabolizing highly reactive biogenic and xenobiotic aldehydes to their carboxylic acids, they also synthesize signalling molecules. A prime example is the conversion of the vitamin A derivative retinaldehyde to ret- inoic acid (RA), a morphogen that plays critical roles in the embryonic development of chordates. To understand the evolu- tion of substrate specificity of ALDH enzymes, we analyzed eukaryote ALDHs using phylogenetic and structural modelling approaches in combination with functional analyses in the ceph- alochordate amphioxus and the tunicate Ciona intestinalis, both of which possess a stereotypical chordate RA signalling cascade. We show that the ability of ALDHs to process small, toxic alde- hydes and the capacity to process larger aldehydes into signalling molecules is closely correlated to the presence of, respectively, small and large substrate access channels. Using ancestral sequence reconstruction, we provide evidence that the capacity to synthesize RA originated when a small, narrow-channelled eukaryotic ALDH1/2 ancestor accumulated specific mutations resulting in an increase in channel volume. Interestingly, we also found that some lineage-specific ALDH1 duplicates in amphioxus and Ciona intestinalis display channel signatures reminiscent of ALDH2s. Developmental gene expression surveys show that this constriction of the large ALDH1 substrate access channel is accompanied by changes in gene regulation resulting in func- tional shifts from spatially restricted roles in body patterning to more systemic, pleiotropic roles. doi:10.1016/j.mod.2009.06.654 15-P011 A typical relaxation of structural constraints in Hox gene clusters of squamates Nicolas Di-Poi 1 , Juan I. Montoya-Burgos 1 , Hilary Miller 2 , Nicolas Denans 3 , Olivier Pourquie ´ 3 , Michel C. Milinkovitch 1 , Denis Duboule 1 1 University of Geneva, Geneva, Switzerland 2 University of Wellington, Wellington, New Zealand 3 Stowers Institute for Medical Research, Kansas City, United States Hox genes control many aspects of embryonic development in metazoans. Previous analyses of this gene family has revealed a surprising diversity in terms of gene number and organization between various animal species. In vertebrates, Hox genes are grouped into tightly organized clusters, originally claimed to be devoid of repetitive sequences. Here, we report the genomic orga- nization of the posterior Hox loci (from Hox13 to Hox10) present in reptiles including the green anole lizard (Anolis carolinensis) and the corn snake (Pantherophis guttatus), and show that squa- mates have massively accumulated transposable elements, lead- ing to gene clusters larger in size when compared to other vertebrates. In addition, we show the disappearance of highly conserved regulatory sequences within both the HoxA and HoxD clusters, as well as the rapid evolution of Hox coding regions within the squamata lineages. Finally, in parallel with these mod- ifications in the genomic organization of squamata Hox clusters, we observe specific alterations in the expression patterns of pos- terior Hox genes during corn snake somitogenesis. Because trans- posable elements are major sources of genetic variations, we speculate that their insertion into Hox gene clusters, not reported so far for other vertebrates, may have associated with the evolu- tion of the spectacular realm of morphological variations in the body plans of squamates. doi:10.1016/j.mod.2009.06.655 15-P012 Expression of catshark Hox genes and evolution of vertebrate appendages Silvan Oulion , Ve ´ronique Borday-Birraux, Patrick Laurenti, Didier Casane LEGS – CNRS, Gif/Yvette, France Hox genes are thought to play a key role in the evolution of vertebrate body architecture and appendage development. In order to gain some insight in the evolution of body axis forma- S250 MECHANISMS OF DEVELOPMENT 126 (2009) S247 – S261