Thursday, March 10, 2011
Growing Limbs
Shubin explains how a molecule organizes limbs in chickens. He explains how the Sonic hedgehog gene affects limb development, which is by diffusing from pinky to thumb. Are there any other factors that play a role in the development of limbs not only at the ZPA, but also at the apical ectodermal ridge (AER)? What is a possible experiment to demonstrate the universality of the Sonic hedgehog gene? From an evolutionary standpoint, what conclusion can be drawn from the fact that sharks have an AER in their median fins? Explain why or why not the genes for growing a limb from a chicken can be used on a shark using the answers from the previous questions.
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ReplyDeleteThis question reminds me of an earlier point that Shubin made early in the book. Shubin was addressing the problem of how to find relatives of the first fish to walk on land. He proposes an answer by saying that fish are "somewhere between the 'Everythings' and the 'Everythings with limbs." He implies that ancient relationships can be uncovered by looking at the occurance of features over time from simpler to complex. It is through the continuity and change of features over time, and the mechanisms that faciliate this change, that the basis of why organisms diverge and become as diverse as they are now is formed.
ReplyDeleteThe question Ben is asking is about what these mechanisms that facilitate the divergence of species are and how we can prove their existance. I think the short term answer to his question is Hox genes. Hox genes are the control switches of all organismal genetic codes: "The genes in mice and humans are very similar in number and chromosomal arrangement...only about 40 genes out of a total of about 100,000 control most of the development,architecture, and appearance of the body plan of complex mammalian species."(Zihlman, 2001). With this specialized power, Hox genes have important roles such as "specifying the place where the limbs will form" and "specifying whether a particular mesenchymal cell will become stylopod, zeugopod, or autopod." (Sinauer, 2000).
A major application of Hox genes is how they play an important impact on the homologous structure in limbed species called the apical ectodermal ridge. It is from the AER that limbs grow during embryonic development, and it is here a high concentration of Hox genes are present. No experiment is needed to test for the existance of Hox genes. Rather, observational studies can easily prove whether an organism has the structure or not.
Hox genes are impressionable by continuity and change. Analysis shows that "there were at least about 88 homeobox genes in the common ancestor of bilateral animals." (Nam, 2005). This ascertains that vertabrates and invertabrates started out with similar numbers of Hox genes in their nervous system. A major twist to this fact is that "About 50–60 genes of them [Hox genes] have left at least one descendant gene in each of the 11 species studied, suggesting that about 30–40 genes were lost in a lineage-specific manner. Although similar numbers of ancestral genes have survived in each species, vertebrate lineages gained many more genes by duplication than invertebrate lineages, resulting in more than 200 homeobox genes in vertebrates and about 100 in invertebrates." (Nam, 2005). Just as they change so much in the development of vertabrate nervous systems, they also change significantly to facilitate the divergent development of limbs and other features such as shark fins. Since AER are homologous among organisms, it would be no doubt that genes from the AER of one organism can be transplanted into the AER of another organism, causing the development of foreign features on an organism.
https://homes.bio.psu.edu/people/faculty/nei/lab/2005-nam-nei.pdf
http://biology.ucsd.edu/classes/bild10.WI08/documents/AnimalBodyPlanHoxcoloring.pdf
http://www.ncbi.nlm.nih.gov/books/NBK10102/
Troy Glickstern
cleverstar8@comcast.net