BILD 3 - Lecture 16

BILD 3 - Lecture 16 - Homeobox genes are expressed in...

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Homeobox genes are expressed in different parts of the body, and when that expression is altered inappropriate structures can result. For example, some of the genes of the Bithorax complex are normally turned on more strongly at the posterior end of the body. When these genes are damaged or silenced so that they do not turn on, they unable to repress other developmental pathways, which permits inappropriate organs to develop in the posterior segments. As animals have diversified, the numbers and groupings of homeobox genes have also changed. These genes are centrally important to differences in the body plans of animals, but they are not the whole story — many other developmental pathways are involved. Large Phenotypic Changes During the Evolution of Eyes Darwin acknowledged in the Origin of Species that it is difficult to see how an organ as complex as the vertebrate eye could have evolved through small steps. But he also pointed out that many different types of eye, exhibiting different degrees of complexity, have been found in different animal lineages. Perhaps, he thought, these different types of eye could have descended from a common ancestor and diverged through many small evolutionary steps.
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If Darwin was right, then eyes must have evolved only once or a few times. But until recently the evidence seemed to suggest otherwise. Eyes in different animals are so extremely different from each other that biologists had concluded that they have evolved independently as many as 40 different times in different evolutionary lineages. New genetic evidence suggests that different eye structures such as lenses, irises and ommatidia (the subunits that make up the compound eyes of organisms such as the arthropods) have indeed evolved independently, sometimes in similar ways in different evolutionary lineages, but that the first simple light-sensing organ that was the ancestor of these different eyes evolved only once (Figure below). , found in flatworms Simple lens eye, found in cockles Cockle, Din o c a r d iu m r o bu s t u m Compound eye, found in some bivalves Scallop, P ec t en ma x imu s Turkey-wing mussel, A r c a z ebr a Camara-like eye, found in the cephalopods (squids and octopus) Compound eye, found in many arthropods ranging from the extinct trilobites to present-day insects Camera-like eye of vertebrates , the structures of which evolved independently of those in cephalopod eye 1 2 3 4 5 6 7 Eye with a resemblance to a reflecting telescope The structures of most of these eyes resemble those of a variety of light-gathering
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BILD 3 - Lecture 16 - Homeobox genes are expressed in...

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