Biology Reference
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Drosophila embryo, expression of the gene engrailed depends on the
protein Wingless (the equivalent of Wnt in this organism). In the
absence of Wingless, engrailed can still be expressed initially but
this is soon interrupted instead of being maintained as is the case
when Wingless is present. The result has been confirmed by a series
of further observations of similar significance (Arias and Hayward,
2006). In line with the Darwinian model, these data show that Wnt
proteins do not play the role of inducer but rather of stabiliser in
the stochastic expression of genes (Kupiec, 1996).
Major mortality occurs among blood cells, which are continu-
ously replaced by cells that differentiate from stem cells. This
haematopoiesis is regulated by a large number of proteins called
cytokines which transduce their signal within cells via enzymes
known as Janus kinases (JAKs) and gene transcription regulators.
There are more than 50 cytokines for just four JAKs and seven
transcriptional regulators. This system thus provides a good exam-
ple of the non-specificity of molecules which underlies cell differen-
tiation. We have already mentioned the experiment in which the
erythropoietin (EPO) receptor was replaced by that of prolactin,
resulting in blood cells differentiating normally under the action
of prolactin, which usually acts in a different cell line (chapter 4,
ยง4.3.2). A series of similar experiments has been performed with
other cytokines and other haematopoietic cells and confirms that
it is the state of the cell which determines its differentiation and
not the chemical nature of the signal it receives. Furthermore, dif-
ferentiation of haematopoietic cells can also take place normally
if any cytokine deficiency is compensated for by the expression of
a survival factor (bcl-2). These experiments indicate that the
function of cytokines is not to specifically induce differentiation
but simply to allow the survival and multiplication of cells
(Robb, 2007).
6.3.6
Testable predictions of the Darwinian model
A theoretical model should propose predictions that can be tested
and initiate a research programme, which is the case with the
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