Spatio-temporal patterns of Hox paralog group 3–6 gene expression during Japanese medaka (Oryzias latipes) embryonic development Adam Davis * , Edmund J. Stellwag Department of Biology, Howell Science Complex, East Carolina University, Greenville, NC 27858, USA article info Article history: Received 14 April 2010 Received in revised form 6 May 2010 Accepted 8 May 2010 Available online 13 May 2010 Keywords: Japanese medaka Hox PG3-6 gene expression Embryonic development Pharyngeal arches Hindbrain Cranial neural crest cells Cis-regulatory elements Evolution Teleosts abstract Clustered Hox genes encode transcription factors that pattern the regional identities of tissues along the anterior–posterior (A-P) axis of animals during embryonic development. They are expressed along the A-P axis collinear with their physical location within a cluster. Several studies have examined the expres- sion of teleost Hox genes from paralog groups (PG) 1-4 in the rhombomeres of the hindbrain and the ante- rior pharyngeal arches. However, little is known about Hox gene expression within the posterior pharyngeal arches (PA3-7) of teleosts. Here we present the spatio-temporal expression patterns of Hox paralog group (PG) 3-6 in the neural tube and posterior arches of the Japanese medaka (Oryzias latipes). We show that medaka Hox gene expression patterns in the hindbrain are divergent spatially from orthol- ogous genes in mouse and other evolutionarily divergent teleosts, which suggests divergence of cis-reg- ulatory sequences directing hindbrain expression. Further, our study is the first to show the complete teleost Hox PG3-6 expression code in the posterior arches up to the chondrogenic stage of the cranio- facial skeletal elements. This study will provide the basis for comparative teleost Hox gene expression profiles in PA3-7 and may help in understanding the mechanisms underlying development of divergent morphological pharyngeal arch skeletal structures among evolutionarily divergent teleosts. Ó 2010 Elsevier B.V. All rights reserved. 1. Results and discussion Hox genes are a family of evolutionarily-related developmental regulatory genes that serve as critical genetic determinants of re- gional tissue identities along the anterior–posterior (A-P) axis of animal species (McGinnis and Krumlauf, 1992). They are organized in clusters in the chordate genome and are expressed along the A-P axis during animal embryonic development collinear with their physical location within a cluster (Holland and Garcia-Fernandez, 1996; Ferrier et al., 2000; Powers and Amemiya, 2004). Multiple whole-genome duplications have expanded the total number of Hox clusters to 4 in tetrapods, 7–8 in most teleosts and even 13 in salmoniformes (Stellwag, 1999; Amores et al., 2004; Moghadam et al., 2005; Hoegg et al., 2007; Mungpakdee et al., 2008a). Post- genome duplication independent gene loss has resulted in Hox gene clusters and paralog groups (PGs) that differ in gene numbers across evolutionarily divergent species depending on the historical timing of gene losses relative to genome duplications (Amores et al., 2004; Le Pabic et al., 2007; Davis et al., 2008). The bony derivatives arising from the pharyngeal arches (PA) in teleosts aid in feeding and breathing and include the oral jaws aris- ing from PA1, the oral jaw support structures arising from PA2 and the pharyngeal jaw apparatus, which originates from the posterior pharyngeal arches (PA3-7) (Schaeffer and Rosen, 1961; Kimmel et al., 2001). Many of the bony elements, especially those derived from the posterior arches, have been shown to differ in structure across evolutionarily divergent teleosts (e.g., Le Pabic et al., 2009). The difference in structure of pharyngeal jaw components among these teleosts may be related to alteration of nested expres- sion patterns of Hox genes in the posterior pharyngeal arches, which have been shown experimentally to function both cell autonomously and non cell autonomously in patterning post- migratory cranial neural crest cells (CNCCs) into specific cartilagi- nous structures within the pharyngeal arches (Santagati et al., 2005; Crump et al., 2006; Minoux et al., 2009). Unfortunately, very few studies have examined Hox gene expression patterns in the posterior pharyngeal arches of teleosts (Miller et al., 2004; Le Pabic et al., 2007, 2009; Davis et al., 2008), and of these, none were con- ducted using a comprehensive collection of the Hox genes from paralog groups 3 through 6 over a developmental period that ex- tended into the chondrogenic stages of posterior pharyngeal arch development. Until the patterns of anterior expressing Hox genes are observed and documented for divergent teleost species, little will be understood regarding how Hox genes have contributed to the morphological diversity in the posterior pharyngeal arch deriv- atives in teleosts. 1567-133X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gep.2010.05.003 * Corresponding author. Tel.: +1 484 678 9582; fax: +1 252 328 4178. E-mail address: ad0314@ecu.edu (A. Davis). Gene Expression Patterns 10 (2010) 244–250 Contents lists available at ScienceDirect Gene Expression Patterns journal homepage: www.elsevier.com/locate/gep