bild 3 lecture 16

bild 3 lecture 16 - Ectopic eyes Eyes develop ectopically...

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Ectopic eyes Eyes develop ectopically (in abnormal places) when a functional allele of the eyeless gene is expressed in a variety of tissues during the development of Drosophila melanogaster . Homologues of eyeless genes from mice and from squid could also be used to turn on the Drosophila eye formation pathway. As with the signal that causes teeth to develop in the vertebrate jaw, the Eyeless gene normally induces eye development only in the flies’ heads. Its expression is repressed elsewhere in the fly. But the pathway that leads to eye development is intact in other parts of the flies’ bodies, and can be induced when the transposable element begins to make Eyeless protein in these tissues. Of course, the eyes that appeared on odd places on these flies were not like those of the organisms from which the genes came! They were always Drosophila compound eyes. The foreign genes induced the developmental pathway that was already present in the fly tissues. Eyeless and its homologs are master control genes that are capable of inducing eye development pathways in a wide variety of organisms, even though the developmental pathways themselves differ. In general master regulatory genes like eyeless (Pax-6) have many cis- regulatory elements (“cis” meaning that they are linked on the same
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chromosome) that regulate their expression in different tissues and at different times of development (see below). But genes that are only turned on in one tissue or in all tissues have much simpler cis-regulatory elements, like the Rhodopsin gene shown here. Conservation of regulatory genes These patterns of regulation appear to be evolving much more quickly than the regulatory genes themselves. If we look at the patterns of homeo box genes in the Protostomes and the Deuterostomes, we find that early in Deuterostome evolution the set of homeo box genes was duplicated and duplicated again. But within the Protostomes and within the Deuterostomes, after this early event, the pattern of homeo box genes had changed very little.
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Thus, organisms as different as humans and coelacanths (see movie of a coelacanth “walking” on the web site) have the same organization of homeo box genes. What has changed is the cis-regulatory regions of these genes, so that they now often do different jobs in different tissues. Some Large Evolutionary Changes Can Draw on Preexisting Genetic Variation and Evolve Repeatedly
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Ssometimes, similar large phenotypic changes can take place in parallel in different evolutionary lineages, provided that similar pools of genetic variation are present in the different lineages. Some changes in small fish called sticklebacks have now been show to arise from selection of preexisting genetic variation.
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This note was uploaded on 11/05/2008 for the course BILD BILD 3 taught by Professor Woodruff during the Spring '08 term at UCSD.

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bild 3 lecture 16 - Ectopic eyes Eyes develop ectopically...

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