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Origin of Animals Carol E. Lee University of Wisconsin 1 Copyright©2020; do not upload without permission Evolution of Development: Evolution of Animal Body Plans as an Example Or, another way to conceptualize today’s lecture: Evolution of Gene Regulatory Networks: Evolution of Development as an Example C.H. Waddington --his resurgence • Largely dismissed during the Evolutionary Synthesisà attacked for being “Lamarckian” for his ideas on “Genetic Assimilation” Conrad Hal Waddington • Father of Developmental Biology – Introduced the concept of “Canalization” – Coined the term “Epigenetics” Interested in the interplay between phenotypic plasticity (response to stress) and selectionà “Genetic Assimilation” Why did Waddington become popular starting in the 1990’s? • Evo-Devo: Evolution of Development as playing an important role in the evolution of phenotypes – Evolution of developmental program could cause radical phenotypic change – The idea of evolution of canalization and decanalization of a developmental program – Genetic Assimilation: Stress could cause decanalization, and the phenotypes that are exposed could then be under selection à creation of “Hopeful Monsters” • What is an Animal? • What makes them different from other organisms? • When did they Evolve? • How did they Evolve? What is an Animal? Multicellular (metazoan) Heterotrophic (eat, not photo or chemosynthetic) Eukaryote No Cell Walls, have collagen Nervous tissue, muscle tissue Particular Life History-developmental patterns (this lecture) • Some animals (Plankton): • https://www.youtube.com/watch?v=xFQ_fO2 D7f0 Are there differences between plant and animal evolution? • Greater diversity in sexual systems in plants – Abundant asexuality • More chemistry less behavior in plants • Development is less rigid and regulated in plants: perhaps allowing for more evolution by “hopeful monsters,” as developmental abnormalities are more tolerable in plants • Polyploidy is tolerated more readily and common in plants Outline • Today: Bigger picture on how radical changes in body plan come about • Evolution of Development • Evolution of Developmental Gene Regulatory Networks (GRNs) • Hierarchy in Evolution of GRNs • Evolution of GRNs leading to evolution of major phylogenetic breaks in Earth History Review concepts from previous lectures: • cis- and trans-regulation • Transcription factors • Pleiotropy • Cambrian “Explosion” • Phylogeny Evolution of Development: • What is it? • How can it lead to evolution of radical changes in body plan? • How can different types of developmental changes (mutations at different developmental stages) lead to different hierarchical evolutionary changes (that distinguish phylum, class, order, family, genus, species)? Ontogeny Recapitulates Phylogeny Ernst Haeckel (1834-1919) • Ontogeny is the course of development of an organism from fertilized egg to adult; phylogeny is the evolutionary history of a group of organisms. • Haeckel observed that as embryos of vertebrates developed, they passed through stages that resembled the adult phase of more ancestral (“primitive”) organisms. For example, at one point each human embryo has gills and resembles a tadpole. • Haeckel’s idea was that a species’ biological development, or ontogeny, parallels and summarizes the species’ evolutionary history, or phylogeny Ontogeny Recapitulates Phylogeny Ernst Haeckel (1834-1919) • Some of his analogies have been discredited (in favor of Von Baer’s ideas) • However, Haeckel's general concept, that the developmental process reveals some clues about evolutionary history, appears to often hold for the evolution of developmental genes Romanes's 1892 copy of Ernst Haeckel’s embryonic drawings The Cambrian “Explosion” 65 mya: Cretaceous Extinction (dinosaurs go extinct) 230 mya: Permian Extinction 570 mya: Cambrian “Explosion” Evolution of Animal Body Plans • True Tissues • Tissue Layers (Diplo vs Triploblasts) • Body Symmetry • Evolution of body cavity (Coelom) • Evolution of Development Cambrian “Explosion” Evolutionary and comparative physiology Zoology 611/612 Instructor: Scott Hartman 12:05-12:55 pm MWF (3 credits) / Optional capstone lab 1:20-5:20 M (2 credits) This course is offered in the Spring and is designed for upper-level undergraduates interested in an integrated approach to animal physiology, ecology and evolution. We examine general physiological principles by comparing taxa from diverse evolutionary histories and ecological adaptations to see how evolutionary processes shaped life today and explore how the same processes are shaping the future in our rapidly changing world. We read key papers in evolution, physiology, and ecology that are further explored in student- driven discussion sections. The course also offers an optional 2 credit capstone research lab (Zoo 612), where students design, conduct and statistically analyze physiological experiments on invertebrates. How could this happen? (genetic mechanism?) The Evolution of Development (Freeman& Herron, Chapter 19) • The tremendous increase in diversity during the Cambrian radiation appears to have been caused by evolution of developmental genes • Changes in developmental genes can result in radically new morphological forms • Developmental genes control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult • The discovery of Hox genes – Not the “most important” development genes – Not the only developmental genes – But, among the first studied Hox genes are types of Homeotic genes, which are genes that control the patterns and order of development in plants and animals. For example, homeotic genes are involved in determining where, when, and how body segments develop in organisms. Hox genes encode transcription factors. Examples of Homeotic genes: Hox genes, paraHox genes, MADS-box containing genes, etc. Changes in a few regulatory genes could have big impacts • Most new features of multicellular organisms arise when preexisting cell types appear at new locations or new times in the embryo. • Changes in the specification of cell fates are a major mechanism for the evolution of different organismal forms. • For example, small changes in gene regulation could cause changes in timing of developmental events (heterochrony), which could then lead to dramatic changes in morphology • Stephan Jay Gould in 1977 proposed this as a mechanism for evolutionary change So, what happened during the Cambrian “Explosion”? (1) Precambrian-Paleozoic Boundary (~570 MYA) All major Animal Phyla (different body plans) evolved within a relatively narrow window of time Cambrian “Explosion” Million Years AGo Annelida Agnatha 0 Mollusca Arthropoda Gnathostomata 200 Echinodermata Cambrian “Cambrian Explosion” 600 Based on phylogeny of 800 animals based on DNA sequence data, the radiation of animals 1000 predates the geological record of the Cambrian 1200 Explosion Precambrian 1400 Wray et al. 1996 The Grand Mystery • How did all the animal phyla appear within a relatively short period of time? • How can different types of developmental changes lead to different hierarchical evolutionary changes (over longer periods of time, that distinguish phylum, class, order, family, genus, species) The Grand Mystery Why has there been so little change in major animal body plans since the Cambrian “Explosion”??? Davidson & Erwin. 2006. Gene Regulatory Networks and the Evolution of Animal Body Plans. Science. 311: 796-800. Big phylogeny “Kernels” “Gene Batteries” Different Hierarchical Components of Gene Regulatory Networks 1. ‘‘Kernels’’ of the GRN: Evolutionarily inflexible subcircuits (of regulatory genes) that perform essential upstream functions in building given body parts à main differences among phyla 2. ‘‘Plug-ins’’ of the GRN: Certain small subcircuits (of regulatory genes), that have been repeatedly co-opted for diverse developmental purposes 3. Input/Output (I/O) devices within the GRN: Switches that allow or disallow developmental subcircuits to function in a given context (e.g. Hox genes) 4. Differentiation Gene Batteries: Consist of groups of protein- coding genes under common regulatory control, the products of which execute cell type–specific functions à Species differences First, Basics on Developmental Gene Regulatory Networks Developmental Gene Regulatory Network • The binding of transcription factors to regulatory DNA sequences controls the spatial and temporal expression of genes in the developing organism • Because each transcription factor regulates the expression of multiple genes, regulatory gene interactions form a network S. Sinha Developmental Gene Regulatory Network • The binding of transcription factors to regulatory DNA sequences controls the spatial and temporal expression of genes in the developing organism • Because each transcription factor regulates the expression of multiple genes, regulatory gene interactions form a network. Developmental Gene Regulatory Network Example shown for neural development Developmental Gene Regulatory Networks (GRNs) • Development is controlled directly by progressive changes in the regulatory state in the spatial domains of the developing organism. • As regulatory genes regulate one another as well as other genes, and because every regulatory gene responds to multiple inputs while regulating multiple other genes, the total map of their interactions has the form of a network. • Gene Regulatory Networks consist of: • Regulatory genes, which encode transcription factors • Signaling genes, which encode ligands and receptors for
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