Promoter Structure

The twist gene is a good candidate for regulating expression of buttonless. There are a total of 6 consensus binding sites for TWI within a 300 base pair region of genomic sequence immediately upstream of the btn structural gene (Chiang, 1994).

Transcriptional Regulation

Notch signaling may regulate the restricted expression of buttonless. Dorsal midline cells in Notch mutant cells differentiate with approximately twice the wild-type number. The phenocritical period for hypertrophy of the DM cells in the temperature sensitive Notch mutant occurs between 4 and 6 hours after fertilization (Hartenstein, 1992).



Expression of btn is first detected after 4 hours of development, and by stage 11 (the extended germ band at 5.5 to 6 hours of development) BTN mRNA is found in a group of two to four midline mesodermal cells in each of the three thoracic segments and in the first seven abdominal segments. These cells are rounded in shape and are located along the dorsal midline of the mesodermal bridge that links the right and left halves of the mesoderm (Chiang, 1994).

The Drosophila ventral midline cells generate a discrete set of CNS lineages, required for proper patterning of the ventral ectoderm. The CNS midline cells also exert inductive effects on the mesoderm. Mesodermal progenitors immediately adjacent to the midline progenitor cells give rise to three types of structures: ventral somatic muscles, cells associated with the salivary glands (presumably fat cells), and a pair of unique cells that come to lie dorsomedially on top of the ventral nerve cord, the so-called dorsal median (DM) cells. DM cells are molecularly distinct mesodermal cells, as they express and require the homeobox gene buttonless in addition to a number of other markers (neuroglian, laminin, Glutactin and collagen IV). Cell ablation as well as cell transplantation experiments indicate that formation of the DM cells is induced by midline progenitors in the early embryo. Each pair of DM cells derives from one mesodermal progenitor. In cyclin A mutants there is only one DM cell per segment, suggesting that in the wild type, the DM cells appear in the lineage after the second postblastodermal mitosis. Expression of a buttonless reporter construct reveals expression at early stage 11, in a cluster of 2-4 cells at the midline on top of the CNS. One of these cells expresses the marker significantly more strongly than the others and a short time later divides (Luer, 1997).

These results are corroborated by genetic analyses. Mutant single minded embryos lack the CNS midline as well as the DM cells. Transplanted ventral midline induce DM cells, which are recognized by their expression of a buttonless reporter. Embryos mutant for any of the spitz group genes, which primarily express defects in the midline glial cell lineages, show reduced formation of the DM cells. Since spitz group genes, including pointed, star and rhomboid most prominently affect glial lineages, a test was made of the effect of mutation of orthodenticle on DM formation. otd determines midline neural cell fate. In all otd embryos inspected, DM cell numbers correspond to wild type, suggesting that the glial lineage is responsible for DM induction. Directed overexpression of secreted Spitz by some or all CNS midline cells leads to the formation of additional DM cells. Supernumerary midline cells are triggered by additional mitoses and not by recruiting additional DM progenitors. DM cell development does not depend on the absolute concentration of the local inducer (likely to be Spitz), but appears to require a graded source of an inducing signal. Thus, the Drosophila CNS midline cells play a central inductive role in patterning the mesoderm as well as the underlying ectoderm (Luer, 1997).

Effects of Mutation or Deletion

buttonless mutation specifically eliminates the dorsal medial (DM) cells; this genetic ablation reveals a requirement for DM cells as cellular cues for axonal guidance during transverse nerve outgrowth and bifurcation of the median nerve. Transverse nerves are absent in btn mutants. By stage 16 [Images] of development, abnormally long and thick axon bundles expressing Fasciclin II can be seen in mutant flies, extending along the midline at the dorsal surface of the ventral cord. It is presumed that these bundles result from outgrowth and fasciculation of axons that contribute to the median nerve in multiple segments (Chiang, 1994).


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buttonless: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 5 MAR 97 

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