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Thus perhaps the key to understanding the evolution of development is the study the evolution of the

Thus perhaps the key to understanding the evolution of development is the study the evolution of the

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Thus perhaps the key to understanding the evolution of development is the study the evolution of the genetic regulatory mechanisms that control development. Now the question becomes: what do we know about genetic regulation of development? A fair amount is known in Drosophila. The exciting point here is that in recent years there have been increasing numbers of papers describing the existence of gradients across the egg or early embryo in the concentration of specific proteins encoded by a handful of loci. These proteins can be thought of as morphogens ("form creators"), molecules that, for years, were postulated to exist by embryologists. With a gradient across the embryo of such a morphogen, there is the possibility the other proteins that might interact with such a morphogen can obtain position information from the gradient such that high concentration means "anterior" (or "limb end" in vertebrate limb bud)
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Unformatted text preview: and low concentration means "posterior" (or "limb base"). The significant point in all this is that Drosophila geneticists have been able to identify specific developmental mutations (mutations in the genes that code for morphogens, or genes that code for molecules that interact with morphogens) that disrupt specific events in development. One such example is the bicoid gene: when this gene is mutated, its normal gradient is disrupted and the embryo has two tails (bi-caudal). The point is that there are specific genes that determine the major body axes and one can envision that evolution of major new developmental programs might proceed by naturally occurring mutations in these genes that would move/alter the gradient , or, equally as significant move/alter the cellular localization of the receptor of a morphogen ....
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