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Unformatted text preview: Chapter 20 Development and Evolutionary Change (Evo-Devo) The great diversity of organisms is, to a large degree, the result of: Different genes producing different features in different organisms The differential expression of a relatively small number of regulatory genes that many organisms share along with the genes that they regulate These regulatory genes comprise part of the molecular tool kit other components are the products of those genes: transcription factors and extracellular signals These regulatory genes have been conserved over evolutionary time but may produce different results in different organisms Figure (20.1) The first of the tool kit genes to be discovered (in the 80s) were the homeotic genes Which were associated with mutants with aberrant segmental features Figure (20.2) How can mutations with large effects change only one part of a body? This can occur because of the modularity of organisms (including their embryos) A functional module consists of the genes that affect that module (via stimulation or inhibition of signaling pathways) and the physical structures that result from these signaling cascades The attributes of a module are determined by genetic switches which turn on or off particular genes E.G., the development of wing or haltere (on thoracic segments 2 and 3) in drosophila is a function of whether or not the genes for wing development are expressed They are in segment 2; they are not in segment 3 (blocked by the product of the gene ultrabithorax a genetic switch) Figure (20.3) Complexity in physical structure is enhanced by the fact that: Multiple switches control each gene of the tool kit Many elements of the tool kit influence more than one developmental process Modularity allows differences in the timing and spatial pattern of gene expression (within or between species) There can be a shift in the relative timing of gene expression in the same module in different species (heterochrony) E.G., the delay (stopping) of apoptosis resulting in the retention of webbing between the toes of salamanders Figure (20.4) There can be changes resulting from differential spatial expression of regulatory genes in the same module of different species E.G., the role of the apoptosis inhibiting gene gremlin in duck feet (active in the webbing and blocks the apoptosis promoting gene that codes for BMP4) Figure (20.5 & 20.6) How can differences among species evolve? New Genes? Probably a small role among closely related species; a larger role among distantly related species Modified functions of genes? Probably a significant role among closely related species and an even greater role among distantly related species Different patterns of expression of shared regulatory genes?...
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- Fall '10