Evo-Devo PDF

What mechanisms might generate complex characteristics? Evolutionary Developmental Biology “Evo-Devo” 1 Development allows a multicellular individual to form from one cell: Introduction to Genes,...

What mechanisms might generate complex characteristics? Evolutionary Developmental Biology “Evo-Devo” 1 Development allows a multicellular individual to form from one cell: Introduction to Genes, Development, – A zygote is a fertilizedand egg: Evolution Divides and forms a ball of cells, called an embryo Eventually forms an organism with many different types of cells Developmental biology integrates genetics, biochemistry, cell biology, and evolution M odified from Pearson Education 2020 2 2 1  21.1 Genetic Equivalence and Differential Gene Expression in Development Cells acquire specialized properties during differentiation: – Distinct cell types have different structures and functions – This is because they contain different molecules Different cells in an individual contain the same genes: – Differential gene expression accounts for cell differentiation; only a subset of genes are expressed Modified from Pearson Education 2020 3 3 Evidence That Differentiated Plant Cells Are Genetically Equivalent All cells in a plant are genetically equivalent—they contain the same genes: – Some branch cells can de-differentiate and form root cells – Entire plants can be grown from a single adult cell – A genetically identical copy of an organism is a clone Modified from Pearson Education 2020 4 4 2  FIGURE 21.1 GENETIC EQUIVALENCE IS DEMONSTRATED BY PLANT CELLS MODIFIED FROM PEARSON EDUCATION 2020 5 5 FIGURE 21.2 GENETIC EQUIVALENCE IS DEMONSTRATED BY THE ABILITY TO CLONE MAMMALS 6 MODIFIED FROM PEARSON EDUCATION 2020 6 3  21.4 Establishing the Body Plan Fate of a cell depends on its position along the three body axes: – One axis runs anterior to posterior – One axis runs dorsal to ventral – One axis runs left to right MODIFIED FROM PEARSON EDUCATION 2020 7 7 Genetic Regulatory Cascades Provide Increasingly Specific Positional Information Genetic regulatory cascades supply progressively detailed information about: – Where cells are located – What they are to become Modified from Pearson Education 2020 8 8 4  Genetic Regulatory Cascades A genetic regulatory cascade is a set of linked regulatory genes: – One activated gene turns on the expression of other regulatory genes – These trigger expression of yet more regulatory genes Modified from Pearson Education 2020 9 9 Genetic Regulatory Cascade in Fruit Fly Embryos Maternal effect genes encode morphogens that define anterior– posterior axis of the early embryo Gap genes products control formation of large body regions along the anterior–posterior axis Pair-rule genes are expressed in alternating bands and control formation of individual segments Segment polarity genes are expressed in parts of each segment and create regions within segments Hox genes specify each segment’s identity Effector genes lead to development Modified from Pearson Education 2020 10 10 5  FIGURE 21.11 A GENETIC REGULATORY CASCADE IN DROSOPHILA Modified from Pearson Education 2020 11 11 Master genes Genes that control where, when, and how other genes are expressed – Make proteins that signal, activate, mark, or otherwise communicate with other genes and their products – Often sequentially expressed in a set pattern – Also called “homeotic” (HOX) or developmental regulatory genes 12 6  Certain genes control where and when other genes are expressed Not all genes code for “building materials” such as keratin Tell the cells of the fly when and where to start building wings *modified from Berkeley “Understanding Evolution” 13 Homeotic genes Expression of body parts—”Antennapedia” the most famous Antennapedia is a HOM-C gene first discovered in Drosophila which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of the second leg pair into ectopic antennae. By contrast gain-of-function alleles convert antennae into ectopic legs. Genes regulate where, when, and how genes downstream will be expressed Highly conserved and often occurring in a multi-gene families (result of ancestral duplications?) 14 7  Normal fruit fly Homeotic mutant Homeotic mutant Haltere Antenna Wings in place of halteres Legs in place of antennae © 2017 Pearson Education, Inc. 15 Figure 21.13 Hox Genes in Vastly Different Species Are Similar in Organization and Expression Set up Anterior-Posterior and Dorsal-Ventral axes Colinear on chromosomes Not fully understood why this is the case!! Modified from Pearson Education 2020 16 16 8  Conservation of Hox Gene Function (1 of 2) Hox genes play a key role in specifying which body parts to build in most animals, including humans McGinnis et al. introduced the Hoxb6 gene from mice into fruit fly eggs: – Gene is like the Antp gene, which specifies leg development in Drosophila – The Hoxb6 gene did not have its normal regulatory sequences – Caused the same phenotypes as Antp in the flies—legs in place of antennae Modified from Pearson Education 2020 17 17 Conservation of Hox Gene Function (2 of 2) Biologists hypothesize that Hox genes are homologous: – Similar because they are descended from genes in a common ancestor – This implies that Hox genes arose very early in animal evolution Pattern-formation genes have been highly conserved during animal evolution Modified from Pearson Education 2020 18 18 9  Master genes also specify similar kinds of structures Eye development spurred by PAX genes Even though eyes form differently, all initiated by similar genes Zucker, 1994 19 (a) Pattern of gene expression in tetrapods. (b) Pattern of gene expression in snakes. In the area where Hoxc6 In the areas where Hoxc6 and Hoxc8 are is expressed by itself, Hoxc6 and Hoxc8 always expressed together, the forelimb forms are expressed so no forelimbs form together, ribs form Chick Hoxc8 Snake Hoxc8 embryo Forelimb Hoxc6 embryo Hoxc6 © 2017 Pearson Education, Inc. 20 10  But remember! Variants in the regulatory genes still arise through random mutation Selection (natural or otherwise) still acts to shape the effects of the mutation in terms of fitness and frequency within the population 21 “It’s been said that classical evolutionary theory looks at survival of the fittest, evo-devo looks at the ARRIVAL of the fittest.” 22 11  Gene duplication Most important source of new genes Polyploidy – More copies of ALL genes Misalignment in meiosis cause unequal crossing over – One chromosome gets extra copies of a gene 23 Consequences of gene duplication “New” copy may not have same functional constraints as ancestral copy Mutations may accumulate in new (daughter) copy with fewer consequences Mutations may allow daughter to perform new functions 24 12  Gene families Clusters of genes similar in structure and sequence – HOX genes, Actins, Tubulins, Histones, Keratins, Heat-Shock Proteins, Globins, Immunoglobulins, MHC complex 25 Globin genes—an example of a gene family 7 α-globin genes on chromosome 16 6 β-globin genes on chromosome 11 Similar in structure – All have 3 exons separated by 2 introns, repeated sequence motifs that allow us to recognize them as similar Duplications WITHIN α and β globin lineages lead to more copies of each Family includes “pseudogenes” that are non-functional as globin genes – Ususally not transcribed – Pseudogenes may eventually evolve new function or role through changes in sequence or in transcription pattern 26 13  Globin genes Different genes expressed at different times in development – Genes can be “turned on” or “off” Each gene product slightly different – Fetal product has higher affinity for oxygen, etc Selection acts independently on each gene 27 Complex change may have an extremely simple basis!! 28 14 29 15