Evolutionary growth of the fruit fly' pair-rule and the segment-polarity
networks has features which reminiscent my model. However, my
scheme apparently applicable only for the thoracic and abdominal
region segmentation mechanisms (in vicinity of the Kruppel
band of expression). Particularly, the engrailed activation
cascade apparently recruited new pair-rule type genes, even-skipped,
first of all, on the evolutionary way from the long to the shot
germ-band embryogenesis.
The HUNCHBACK morphogenetic gradient activates, in concentration-dependent manner, the Kruppel gene. In turn, bell-shaped morphogenetic gradient of the KRUPPEL activates two stripes of the primary pair-rule gene hairy and acts on activity of the even-skipped gene. Finally, bell-shaped morphogenetic fields of the EVE activate pairs of the engrailed gene expression stripes. This finding of Fujioka et all (1995) adds to the previous discoveries of embryo-length morphogenetic gradients, composed of maternal gene products and the more localised gradients of gap gene products, and shows that Drosophila uses the same principle again to further refine pattern information.
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