Somitogenesis is the key developmental step, which divides the vertebrate body axis into segmentally repeated structures. It requires an intricate process of pre-patterning, which is driven by an oscillator mechanism consisting of the Delta–Notch pathway and various
hairy- and
Enhancer of split-related (
her) genes. The subset of
her genes, which are necessary to set up the segmentation clock, reveal a complex scenario of interactions. To understand which
her genes are essential core players in this process, we compared the expression patterns of somitogenesis-relevant
her genes in zebrafish and medaka (
Oryzias latipes). Most of the respective medaka genes (
Ol-her) are duplicated like what has been shown for zebrafish (
Dr-her) and pufferfish genes (
Fr-her). However, zebrafish genes show some additional copies and significant differences in expression patterns. For the paralogues
Dr-her1 and
Dr-her11, only one copy exists in the medaka (
Ol-her1/11), which combines the expression patterns found for both zebrafish genes. In contrast to
Dr-her5, the medaka orthologue appears to play a role in somitogenesis because it is expressed in the presomitic mesoderm (PSM). PSM expression also suggests a role for both
Ol-her13 genes, homologues of mouse
Hes6 (
mHes6), in this process, which would be consistent with a conserved
mHes6 homologue gear in the segmentation clock exclusively in lower vertebrates. Members of the
mHes5 homologue group seem to be involved in somite formation in all vertebrates (e.g.
Dr- and
Ol-her12), although different paralogues are additionally recruited in zebrafish (e.g.
Dr-her15) and medaka (e.g.
Ol-her4). We found that the linkage between duplicates is strongly conserved between pufferfish and medaka and less well conserved in zebrafish. Nevertheless, linkage and orientation of several
her duplicates are identical in all three species. Therefore, small-scale duplications must have happened before whole genome duplication occurred in a fish ancestor. Expression of multiple stripes in the intermediate PSM, characteristic for the zebrafish orthologues, is absent in all somitogenesis-related
her genes of the medaka. In fact, the expression mode of
Ol-her1/11 and
Ol-her5 indicates dynamism similar to the
hairy clock genes in chicken and mouse. This suggests that
Danio rerio shows a rather derived clock mode when compared to other fish species and amniotes or that, alternatively, the clock mode evolved independently in zebrafish, medaka and mouse or chicken.An erratum to this article can be found at
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