Ideias antigas sobre desenvolvimento embrionário em nossos livros didáticos de Biologia do ensino médio vão ter que passar por profundas revisões: é muito mais 'complexo' do que antes imaginado.
O bom da ciência é isso: o que foi 'verdade científica' um tempo, deixa de ser pouco tempo depois.
Nada como um dia atrás do outro em ciência...
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Researchers Revise Long-Held Theory of Fruit-Fly Development
ScienceDaily (Dec. 19, 2009) — For decades, science texts have told a simple and straightforward story about a particular protein -- a transcription factor -- that helps the embryo of the fruit fly, Drosophila melanogaster, pattern tissues in a manner that depends on the levels of this factor within individual cells.
"For 20 years, this system of patterning has been used in textbooks as a paradigm for patterning in embryos, controlled by transcription factors," says Angelike Stathopoulos, assistant professor of biology at the California Institute of Technology (Caltech).
Now Stathopoulos and her Caltech colleagues, reporting in the online edition of the Proceedings of the National Academy of Sciences (PNAS), have called that paradigm into question, revealing a tale that is both more complicated and potentially more interesting than the one previously described.
A) This cross-section of an early Drosophila embryo shows the levels of the transcription factor Dorsal, which is present in a nuclear gradient. B) The predominant view in the field had been that different levels of Dorsal support the expression of distinct target genes along the dorsal-ventral axis of embryos. C) The Caltech team found that levels of nuclear Dorsal cannot account for the fact that the gene ind ceases to be expressed in dorsal regions of the embryo. (Credit: PNAS/Stathopoulos et al, Caltech)
The football-shaped embryo of the fruit fly has a dorsal (back/top) side and a ventral (front/bottom) side. During development, the cells in each of these regions begin to differentiate and take on specific, specialized roles.
Those decisions are influenced, at least in part, by chemical signals in the cells' environment, including signals called transcription factors -- proteins that, by promoting the transcription of particular DNA sequences, regulate whether specific genes are turned on or off.
In Drosophila, the textbooks said, decisions in the early embryo are made by a transcription factor called Dorsal (which, confusingly, is found primarily in the cells in the ventral part of the embryo, and is absent in those in the dorsal part). Dorsal was said to be the key determinant of the ultimate fate of the cells in which it is present -- as long as it is present in high enough concentrations to be noticed by the nuclei.
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Read more here/Leia mais aqui.
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Journal Reference:
Quantitative imaging of the Dorsal nuclear gradient reveals limitations to threshold-dependent patterning in Drosophila
Louisa M. Liberman1, Gregory T. Reeves1 and Angelike Stathopoulos2
- Author Affiliations
Division of Biology, California Institute of Technology, 1200 East California Boulevard, MC114-96, Pasadena, CA 91125
↵1L.M.L and G.T.R contributed equally to this work.
Edited by Eric H. Davidson, California Institute of Technology, Pasadena, CA, and approved November 3, 2009 (received for review June 4, 2009)
Abstract
The NF-κB-related transcription factor, Dorsal, forms a nuclear concentration gradient in the early Drosophila embryo, patterning the dorsal-ventral (DV) axis to specify mesoderm, neurogenic ectoderm, and dorsal ectoderm cell fates. The concentration of nuclear Dorsal is thought to determine these patterning events; however, the levels of nuclear Dorsal have not been quantified previously. Furthermore, existing models of Dorsal-dependent germ layer specification and patterning consider steady-state levels of Dorsal relative to target gene expression patterns, yet both Dorsal gradient formation and gene expression are dynamic. We devised a quantitative imaging method to measure the Dorsal nuclear gradient while simultaneously examining Dorsal target gene expression along the DV axis. Unlike observations from other insects such as Tribolium, we find the Dorsal gradient maintains a constant bell-shaped distribution during embryogenesis. We also find that some classical Dorsal target genes are located outside the region of graded Dorsal nuclear localization, raising the question of whether these genes are direct Dorsal targets. Additionally, we show that Dorsal levels change in time during embryogenesis such that a steady state is not reached. These results suggest that the multiple gene expression outputs observed along the DV axis do not simply reflect a steady-state Dorsal nuclear gradient. Instead, we propose that the Dorsal gradient supplies positional information throughout nuclear cycles 10-14, providing additional evidence for the idea that compensatory combinatorial interactions between Dorsal and other factors effect differential gene expression along the DV axis.
development gene expression
Footnotes
2To whom correspondence should be addressed. E-mail: angelike@caltech.edu
Author contributions: L.M.L. and A.S. designed research; L.M.L. and G.T.R. performed research; L.M.L. and G.T.R. analyzed data; and L.M.L., G.T.R., and A.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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