The Drosophila melanogaster T-box genes midline and H15 are conserved regulators of heart development.

The Drosophila melanogaster genes midline and H15 encode predicted T-box transcription factors homologous to vertebrate Tbx20 genes. All identified vertebrate Tbx20 genes are expressed in the embryonic heart and we find that both midline and H15 are expressed in the cardioblasts of the dorsal vessel, the insect organ equivalent to the vertebrate heart. The midline mRNA is first detected in dorsal mesoderm at embryonic stage 12 in the two progenitors per hemisegment that will divide to give rise to all six cardioblasts. Expression of H15 mRNA in the dorsal mesoderm is detected first in four to six cells per hemisegment at stage 13. The expression of midline and H15 in the dorsal vessel is dependent on Wingless signaling and the transcription factors tinman and pannier. We find that the selection of two midline-expressing cells from a pool of competent progenitors is dependent on Notch signaling. Embryos deleted for both midline and H15 have defects in the alignment of the cardioblasts and associated pericardial cells. Embryos null for midline have weaker and less penetrant phenotypes while embryos deficient for H15 have morphologically normal hearts, suggesting that the two genes are partially redundant in heart development. Despite the dorsal vessel defects, embryos mutant for both midline and H15 have normal numbers of cardioblasts, suggesting that cardiac cell fate specification is not disrupted. However, ectopic expression of midline in the dorsal mesoderm can lead to dramatic increases in the expression of cardiac markers, suggesting that midline and H15 participate in cardiac fate specification and may normally act redundantly with other cardiogenic factors. Conservation of Tbx20 expression and function in cardiac development lends further support for a common ancestral origin of the insect dorsal vessel and the vertebrate heart.

Drosophila T box proteins break the symmetry of hedgehog-dependent activation of wingless.

BACKGROUND: Segmentation of the Drosophila embryo is a classic paradigm for pattern formation during development. The Wnt-1 homolog Wingless (Wg) is a key player in the establishment of a segmentally reiterated pattern of cell type specification. The intrasegmental polarity of this pattern depends on the precise positioning of the Wg signaling source anterior to the Engrailed (En)/Hedgehog (Hh) domain. Proper polarity of epidermal segments requires an asymmetric response to the bidirectional Hh signal: wg is activated in cells anterior to the Hh signaling source and is restricted from cells posterior to this signaling source. RESULTS: Here we report that Midline (Mid) and H15, two highly related T box proteins representing the orthologs of zebrafish hrT and mouse Tbx20, are novel negative regulators of wg transcription and act to break the symmetry of Hh signaling. Loss of mid and H15 results in the symmetric outcome of Hh signaling: the establishment of wg domains anterior and posterior to the signaling source predominantly, but not exclusively, in odd-numbered segments. Accordingly, loss of mid and H15 produces defects that mimic a wg gain-of-function phenotype. Misexpression of mid represses wg and produces a weak/moderate wg loss-of-function phenocopy. Furthermore, we show that loss of mid and H15 results in an anterior expansion of the expression of serrate (ser) in every segment, representing a second instance of target gene repression downstream of Hh signaling in the establishment of segment polarity. CONCLUSIONS: The data we present here indicate that mid and H15 are important components in pattern formation in the ventral epidermis. In odd-numbered abdominal segments, Mid/H15 activity plays an important role in restricting the expression of Wg to a single domain.