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Lecture 5

BIO153 Lecture 5.pdf

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Christoph Richter

2009 BIO153: Lecture 5 Development in Evolution January 19th, 2009 Topics: ▯ role of development in macroevolution and the generation of diversity ▯ differential gene expression and transcription factors ▯ the animal “toolkit” Development has an important role in macroevolution ▯ evolutionary change in morphology, function, organization & performance of the organism involves development ▯ the study of development helps answer the question: how do diverse descendents arise from a common ancestor? Development and diversity: 1. Changes in developmental processes result in the changes in phenotype seen in diversification in a lineage ▯ especially evident in adaptive radiations (rapid speciation within a lineage; usually as the result of colonization of a new environment with several vacant niches) ▯ e.g. Hawaiian honeycreepers; cichlid fish in Africa;eliconius butterflies 2. Shared developmental pathways show relatedness ▯ lineages descended from an ancestor evolve in similar ways because they share an inherited developmental pathway (parallelism) 1 ▯ e.g. within the frog and salamander lineages, both show digit reduction in forelimb (5 ▯ 4) ▯ in frogs: first thgit is lost; in salamanders, 5 digit is lost ▯ frogs share a particular developmental mechanism through inheritance; salamanders share another mechanism ▯ thus, same result (digit loss) is achieved through 2 different pathways conserved by the 2 lineages (frogs and salamanders diverged ~135 million years ago (Mya)) 3. Development puts constraints on the types of variants possible: ▯ natural selection can only act on the array of variants that are produced ▯ developmental pathways are complex and integrated; thus, certain variations in phenotype are simply not possible (viable) ▯ e.g., the wings of winged tetrapods are modified forelimbs; a winged horse would have to be built through the duplication of the forelimb; this is not viable in embryonic development; thus Pegasus is not possible! Process of development: how do we go from a single cell to an adult? ▯ fertilized cells all look alike! ▯ Ontogeny (development) is a combination and integration of 3 processes: growth (increase in cell number through cell division), cell differentiation (diversification of cell types), and cell organization (morphogenesis, or generation of form). 2 Morphogenesis also depends on apoptosis (programmed cell death). Differentiation: ▯ “Dolly” – cloned from differentiated mammary cell ▯ thus, differentiation does not alter nuclear genome, but expression of genes ▯ differentiation depends on differential gene expression How do cells express different genes? ▯ different genes are turned “on” and “off” in different cells ▯ requires a class of genes: regulatory genes (developmental control genes) ▯ most encode transcription factors (proteins that control the transcription of a gene) How do transcription factors work? ▯ transcription factors bind to DNA in the promoter region of a gene (upstream – closer to the 5’ end – of the coding part of a gene) ▯ transcription factors allow for the binding of RNA polymerase ▯ by binding RNA polymerase, they initiate the transcription of the gene How does development build an organism? To build an animal, the following things need to be defined: ▯ symmetry: none, radial or bilateral ▯ cell layers: one, two or three ▯ body axes: front– back, left – right, top – bottom ▯ skeleton: internal or external 3 ▯ organ systems: digestive, circulatory, respiratory, endocrine…. Plant development differs from animals development because plant cells do not move during development In plants: need to define: ▯ growth axis: up—down ▯ body parts: roots, shoots, leaves etc. ▯ body systems: reproductive structures, transport system, etc. For normal development: ▯ genes must be activated in appropriate sequence (broad patterns ▯ parts of pattern ▯ details) ▯ cells need positional information so that differentiation occurs at the appropriate place in the body ▯ activity of a regulatory gene triggers activity of other regulatory genes ▯ combinatorial control: a structural gene may have many binding sites; transcription factors may act together at different times during development e.g. in Drosophila: ▯ on average, genes have 10 – 20 binding sites in promoter region ▯ a transcription factor may bind to hundreds of genes ▯ Drosophila has ~13, 000 genes ▯ allows for a lot of possibilities! There are several families of control genes that encode transcription factors in multicellular organisms ▯ in animals & fungi: homeotic genes (HOX genes) ▯
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