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Induction of the otic placode involves a number of regulatory interactions. Early studies revealed that the induction of this program is initiated by instructive signals from the mesendoderm as well as from the adjacent hindbrain. Further investigations on the molecular level identified in zebrafish Fgf3, Fgf8, Foxi1, Pax8, Dlx3b and Dlx4b genes as key players during the induction phase. Thereafter an increasing number of genes participates in the regulatory interactions finally resulting in a highly structured sensory organ. Based on data from zebrafish we selected medaka genes with presumptive functions during early ear development for an expression analysis. In addition we isolated Foxi1 and Dlx3b gene fragments from embryonic cDNA. Altogether we screened the spatio-temporal distribution of more than 20 representative marker genes for otic development in medaka embryos, with special emphasis on the early phases. Whereas the spatial distribution of these genes is largely conserved between medaka and zebrafish, our comparative analysis revealed several differences, in particular for the timing of expression.

Nodal-related genes have been implicated in mesendoderm induction, establishment of embryonic axes, neural patterning and left-right development among vertebrates. Here we report the isolation of three Nodal-related genes in medaka (Oryzias latipes). Based on sequence analysis and in accordance to zebrafish orthologues we named the isolated genes Ndr1, Ndr2 and Spaw. Gene expression analysis throughout medaka development confirmed this assignment. Ndr1 and Ndr2 are detectable during gastrulation whereas Ndr2 and Spaw are expressed asymmetrically during somitogenesis. In accordance with its zebrafish orthologue, Spaw is expressed as the first asymmetric marker in the left lateral plate mesoderm (LPM) and Ndr2 displays asymmetric expression domains in the brain and the LPM. In general, the spatial distribution of Nodal transcripts resembles those reported for zebrafish, in case of Ndr2, however, we report a novel left-right asymmetry in the posterior paraxial mesoderm flanking the Kupffer's vesicle.