Abstract
BackgroundThe formation of the bicoid (bcd) mRNA gradient is a crucial step for Bcd protein gradient formation in Drosophila. In the past, a microtubule (MT)-based cortical network had been shown to be indispensable for bcd mRNA transport to the posterior.ResultsWe report the identification of a MT-binding protein CLASP/Chb as the first component associated with this cortical MT network. Since CLASPs in vertebrates were shown to serve as an acentriolar microtubule organization center (aMTOC) in concert with trans-Golgi proteins, we examined the effect of the Drosophila trans-Golgins on bcd localization and gradient formation. Using a genetic approach, we demonstrate that the Drosophila trans-Golgins dGCC88, dGolgin97 and dGCC185 indeed affect bcd mRNA localization during oocyte development. Consequently, the bcd mRNA is already mislocalized before the egg is fertilized. The expression domains of genes downstream of the hierarchy of bcd, e.g. of the gap gene empty spiracles or of the pair-rule gene even-skipped are changed, indicating an altered segmental anlagen, due to a faulty bcd gradient. Thus, at the end of embryogenesis, trans-Golgin mutants show bcd-like cuticle phenotypes.ConclusionsOur data provides evidence that the Golgi as a cellular member of the secretory pathway exerts control on bcd localization which indicates that bcd gradient formation is probably more intricate than previously presumed.
Highlights
In Drosophila, the Bicoid (Bcd) concentration gradient along the cortex provides essential positional information on the development of the anterior-posterior axis where it functions as a morphogen
Chromosome bows is part of the MT network that forms the bcd mRNA gradient To explain the observation of the bcd mRNA gradient [9] during early nuclear cycles of Drosophila development, a search for a MT-based transportation system was initiated, leading to the discovery of a specific anterior MT network shown to be indispensable for bcd mRNA gradient formation [10]
Attempts to define the directionality of the MTs by co-staining the cortical MT threads with minus-end and plus-end markers failed for most markers, possibly because there is no ‘conventional’ microtubule organizing center (MTOC) at the cortex or because the harsh fixation conditions that allowed for the staining of the anterior cortical network were not suitable for antibodies directed against MT-polarity-defining proteins
Summary
In Drosophila, the Bicoid (Bcd) concentration gradient along the cortex provides essential positional information on the development of the anterior-posterior axis where it functions as a morphogen. Bcd as a maternal factor is at the top of the hierarchy of segmentation genes that controls via its gradient, the expression of the gap genes which are expressed in broader domains. The gap genes in turn control genes of the level of the hierarchy, the pair-rule genes. Members of this class are usually expressed in 7 stripes and provide a double-segment identity. The pair-rule genes control the segmentation genes which provide cues in every single segment. A microtubule (MT)-based cortical network had been shown to be indispensable for bcd mRNA transport to the posterior
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