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Molecular and genetic analysis of the
extracellular matrix during Drosophila development.
Mechanisms of bicoid mRNA gradient formation.
Cancer in Drosophila: a novel genetic
screen.
Molecular
and genetic analysis of Drosophila dystroglycan, a regulator of
epithelial polarity.
Embryology
and molecular biology of the blow fly Lucilia sericata.
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a bicoid mRNA gradient dictates the Bicoid protein gradient: the SDD model is incorrect!
The Bicoid (Bcd) protein gradient is generally believed to be established in the pre-blastoderm Drosophila embryo by the diffusion of Bcd after translation of its maternal mRNA, which serves as a strictly localized source of Bcd at the anterior pole (also referred to as the SDD model, localized synthesis, diffusion, and spatially uniform degradation of the Bcd protein). The crux, however, is: there is a serious error in the SDD model. Moreover, already when proposed in 1988, this model ignored previous conflicting results (Frigerio et al., Cell 47, 735-746 (1986), which demonstrated that the Bcd protein gradient is preceded by a bcd mRNA gradient.
Recently, the SDD model, whose name refers to the localized synthesis , diffusion , and spatially uniform degradation of the Bcd protein, has been subjected to a critical test (Gregor et al., Cell 130, 141-152 (2007) . A series of ingenious experiments, measuring the diffusion constant of Bcd in the cortex of embryos, uncovered a serious difficulty of the model: the diffusion constant was too low by two orders of magnitude to explain the observation that the steady state of the Bcd gradient profile is reached within 1.5 hours (Gregor et al., 2007).
INCORRECT!!!
We extended our old results from 1986 by showing that the bcd mRNA and protein gradient profiles are virtually identical at all times, which confirms our previous conclusion that the Bcd gradient is produced by its mRNA rather than by diffusion (Spirov et al., (2009). Based on our observation that bcd mRNA colocalizes with Staufen (Stau), we conclude that the bcd mRNA gradient is formed by a novel mechanism of a quasi-random active transport of a Stau- bcd mRNA complex through a non-polar microtubular network, which confines the bcd mRNA to the cortex of the embryo.
Films:
1) Watch bicoid mRNA gradient formation on You Tube (click on film):
2) Watch the transition from basal to apical localization of bicoid mRNA during early cycle 14 on You Tube (click on film):
