The problem of spontaneous formation of reliable patterns in embryology is largely unexplained. The fruitfly Drosophila has emerged as one of the currently most important species for which detailed experimental data have accumulated to such an extent that a beginning is made in understanding the processes that govern early embryogenesis. Genetics studies on Drosophila have been carried out for decades and about 1980 it was established that a hierarchy of genes seemed to control the initial transition from the egg to a segmented embryo. From mutants defficient in one or more genes, fate maps were constructed that gave an initial guide to possible interactions of genes and products. Regions of activity of specific genes and their proteins are now available and this has caused some revision of previous models.
How a number of independent particular stripe generators could cooperate
to form the observed equally spaced stripes is a much more serious problem.
Theoreticans have pointed out that the zebra stripes may alternatively
be generated by Turing's mechanism, that is, by an autocatalitic reaction-diffusion
system that is known to be capable of producing such stripes autonomously.
The particular pair-rule stripes could then be activated by a combination
of maternal, gap, and Turing pattern interactions.
An overview of the genes of discussion here will be given shortly.
The metameric prepattern in the Drosophila blastoderm is generated through a series of DNA-protein interactions that is initiated by at least three transcription factor gradients in the egg. In the terminal regions, one (or several) unknown transcription factor(s) appear to be activated through a signal transduction cascade. In the anterior region of the preblastoderm embryo, two morphogen gradients are formed.
After transcription in the nurse cells, bcd mRNA is transferred into the growing oocyte, and it eventually becomes localized in the anterior pole region of the egg. After translation, the bcd protein (BCD) forms a concentration gradient along the longitudinal axis of the embryo and acts as the anterior morphogen. Maternal hunchback is present as well... Similarly, the posterior part contains nanos mRNA and presumably a similar gradient form this end in nos gene products.
While BCD is likely to provide only an activating function that is limited to the anterior portion of the egg (with the central gap gene Kr likely to represent the posterior-most gene controlled by BCD activity), hb activity is likely to both activate and repress gene expression and reaches into the prospective abdominal region, as it marks the anterior border of gt expression within the posterior region of embryo.
The gene hb is activated also in the egg's own DNA and thus this zygotic hb places this gene the first level in the hierarchy activated by the maternal genes: the gap genes. Activation of zygotic hb is due to bcd. Subsequently, hb activates the gap gene Kruppel. The result is an expression of Kr in the middle of the embryo. Finally, gap gene knirps may be activated by Kr at moderate concentrations of Kr but repressed by large concentrations. Gene hb represses kni so the result is expression of kni on the posterior side of the Kr region with some overlap. The giant gene is expressed in two broad bands on both sides of the central peak of Kr. The expression of gt (and kni) is believed to be due to global activation (by a so far unknown factor) followed by repression of gt (and kni) by hb. The repression of gt is overruled by bcd anteriorly.
Expression of the terminal gap genes tll and hkb in the posterior pole region in the embryo is almost entirely controlled by the maternal terminal organizer system, and their expression is functional zygotic response to the activity of a signal transduction pathway (Weigel et al., 1990; Bronner and Jackle, 1991). In this pathway, the product of the gene torso probably acts as a receptor for an extracellular signal that is produced at the two poles of the egg. torso encoodes a putative transmembrane receptor tyrosine kinase which, in response to a localized extracellular signal (Springer et al., 1989; Stevens et al., 1990), activates downstream gene products including the as yet unknown transcription factor which eventually activates tll and hkb.
The mechanisms by which the spatially localized gap gene expression patterns are regulated are fundamentally different. hb expression occurs in response to the BCD gradient likely to be regulated through the affinity of the BCD binding sites within the hd promotor: in regions where the BCD concentration is high enough too bind (through the site with the highest affinity within the promotor), hb becomes activated. The activation then leads to a pattern which extends from the anterior pole region, where bcd concentration is highest, to the position in the gradient where BCD fails to bind. This mechanism may also account for other genes which are expressed in regions of the embryo where the slope of the BCD gradient is steep.
In more posterior regions of the embryo, i.e. in the more shallow portions of the BCD gradient, this mechanism for gene activation might not be sufficent to provide sharp boundaries of the expression domain. For this reason, a different mechanism for BCD-dependent gene action might be used, as in the case of Kr. In view of the fact that none of the BCD-binding sites of the Kr730-element shows a higher affinity binding site for BCD than in the hb promotor region, we favor a model in which cooperative interactions may lead to a crowding effect to activate Kr by BCD. The use of such a mechanism would be consistent with the finding that gap gene proteins such KNI, GT and TLL (ehich act as repressors to delimit the Kr expression domain) have target sequences that overlap the BCD binding sites within Kr730 DNA: binding of either TLL, GT or KNI would interfere with BCD binding and thus would lead to repression. This effect would also interfere with cooperative interactions facilitating the filling of the additional BCD binding sites in the Kr regulatory region, and the binding of the repressor instead of the activator would shift Kr gene expression towards regions of higher BCD concentrations. In the extreme situation, the binding of several repressors should knock out BCD-dependent activation even if high BCD concentrations were present, as for in the anterior pole position. The way in which HB activates and represses Kr gene expression is concentration-dependent manner.
An interplay between activators and repressors as observed for Kr expression s also observed for kni. However, the mechanism y which kni is spatially regulated in the posterior region again differs significantly from the one that regulates hb and Kr. kni activation occurs throughout the entire embryo through the activity of a general, as yet unidentified activator. The spatial borders are then actively set through repression by factors that are regionally localized: HB, GT and TLL repress through separate modular cis-acting elements that are different from the activation element within the kni regulatory region.
Another feature, common to both Kr and kni regulation, is interesting
to note(!) Regulatory inputs of the same kind are provided by more then
one gene product. For example, Kr can be activated by HB when BCD is absent,
and vice versa. Furthermore, TLL, HB and GT in the anterior and KNI, GT,
TLL and possibly also HKB (Weigel et al., 1990) in the posterior can provide
local repression of Kr. Currently we do not know wether these multiple
activation and repressor functions represent a fine tuning system to determine
and specify the precise position n the embryo where the target gene is
active (or repressed), whether the regulatory inputs are redundant, or
whether some of the regulatory pathways represent just evolutionary relicts
which are not decisive any in the Drosophila wild-type embryo.
The next level in the gene hierarchy is the primary pair-rule genes hairy, runt and even-skipped. After the gap genes stert to be expressed but before they reach quasistationary spatial positions, the primary pair-rule genes generate seven zebra stripes in the midle of the embryo. Gene runt is expressed between the stripes of hairy, whereas eve is slightly phase shifted from hairy to the posterior site.
Partial deletions in hairy DNA caused particular hairy stripes to vanish.
The discovery of these region-specific hairy alleles and the subsequent
discovery that similar fragments of eve gene expressed particular stripes
have given rise to the model most widely used at present.
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A.Hunding, Modeling Spatial Pattern Formation in Drosophila, Comments Theoretical Biology, 1993, V.3, No3, P.141-167.
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