Additional File 1.
Expression onset of MHB-specific key developmental genes in three different vertebrate species compared to orthologous factors around the otd/unpg interface in Drosophila. In vertebrates, the temporal order of genetic interactions among MHB-specific genes is closely reflected by the temporal order of the onset of their expression. Otx2 and Gbx2 are the first genes expressed (from gastrulation onwards), succeeded by the expression of further genes in the following order: Bts1 , Pax2, En1/2, Wnt1 and FGF8 . In Drosophila, comparable to vertebrates, otd is the first gene expressed at the presumptive region of the otd/unpg interface, already before gastrulation (stage 6). In contrast, unpg is expressed significantly later than otd, not before stage 11, when it is also found in the TC and in a segmental pattern in the ventral nerve cord. Similar to otd, the head gap gene btd is expressed before gastrulation; in the pNE and NBs at the later otd/unpg interface it is not detected before stages 7/8. Furthermore, wg and en are expressed in the region at about stage 8, thus later than otd and btd, but before unpg. This indicates that head segments can be distinguished already before unpg expression initiates. In contrast to vertebrates, D-pax2 and poxn are expressed significantly later than en, but neither in the pNE nor in brain NBs (indicated by the stippled line for poxn). Moreover, expression of the Fgf8-related genes bnl and pyr is lacking around the otd/unpg interface. Transient expression of ths (indicated by stippled line), is unlikely to reflect MHB-specific expression of Fgf8. The temporal onset of gene expression is in part reminiscent of the situation in vertebrates (for example, of otd,btd,en,wg), but otherwise discloses significant differences (for example, unpg,D-pax2,poxn,Fgf8-related genes), implying that the chronological order of possible genetic interactions at the otd/unpg interface seems to be different from those at the vertebrate MHB. Data for mouse, zebrafish and chick are according to Figure 1 in  and references therein. Mouse: HDF, head fold stage; s, somite stage. Zebrafish: tb, tail bud period; s, segmentation period; h, hatching period. Chick: HH, stages after Hamburger and Hamilton. n.e., not expressed in the investigated period until stage 13; n.d., not determined.
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Urbach Neural Development 2007 2:23 doi:10.1186/1749-8104-2-23