3/6/11 The Goal of Evo-Devo Types of Evidence To understand the mechanisms by which Developmental genetic data: development has evolved, in terms of: Gene expression patterns developmental processes (i.e., what new cell or tissue Qualitative and Quantitative expression of gene transcripts interactions are responsible for novel morphologies in or proteins certain taxa) ex. Neural crest Cis-Regulatory Reporter Constructs evolutionary processes (e.g., what selection pressures Comparative Embryological data promoted the evolution of these novel morphologies) Paleontological data Together these data help infer the developmental genetic origins and histories of morphological characters. Phylogenetic tree Metazoan Phylogeny Homology – A Phylogenetic Concept A hypothesis of the evolutionary Homologous features are those that have been inherited, with more or less modification, from relationships among taxa. a common ancestor in which the feature first evolved. If they are shared by all members of a group/clade, these homologous structures are called synapomorphies Details the history of descent of groups of (shared derived characters) taxa such as species from their common Homology may be suggested by a combination of similarity in : ancestors position structure Consensus derived from the amalgamation of different biological fields (e.g., Paleontology, Molecular Biology, and Developmental genetics) 1 3/6/11 Biological Homology concept Macroevolution The Toolkit & its Evolution A feature that is homologous among species at one Refers to the evolution of significant phenotypic The “genetic toolkit” level of organization (e.g., phenotypic), may or may changes Transcription factors & Signaling factors not be homologous at another level (e.g., genetic or Great enough to place the changed lineage and its developmental). descendants in a distinct genus or higher taxon. Toolkit Evolution Multicellular organisms are constructed from a “genetic Duplication and divergence Expands Toolkit! toolkit”. Homologous genes within a species are called paralogues. These changes are made possible by evolving utility The toolkit consists of: Their sequence similarities are due to descent from a common of the genetic toolkit ancestor and are not the result of convergence for a particular Regulatory genes Gene regulatory networks function. Toolkit “tinkering” provides the basis of Homologous genes between species are called orthologues. Morphological evolution (Macroevolution) Example: Hox gene cluster Pitx1 expression in Three-spine Sticklebacks and Hox genes Modularity evolutionary pelvic reduction Modular units allow certain parts of the body to change PARALOGS PARALOGS without interfering with the functions of other parts. They are discrete and interacting modules. Types of modularity: Anatomical (e.g., imaginal discs, morphogenetic fields, parasegments, vertebrate organ rudiments) Cellular (e.g., signal transduction pathways) DNA (e.g., enhancers [cis-reg. modules]) Example: Pitx1 expression in the Three-spine Stickleback (Gasterosteus aculeatus) Carroll et al. 2001 ORTHOLOGS Shapiro et al. 2004 2 3/6/11 Heterotopy – Evolutionary changes in the Cis-reg construct and function Mechanisms of Macroevolution location of development • A reporter construct consists of: Duck feet and Bat wings: Extra skin membrane evolution Toolkit “tinkering” Merino et al. 1999 • regulatory DNA of interest including promoter • reporter gene - encodes protein (e.g., GFP) whose expression can be Mutations that affect regulatory regions easily visualized under a microscope Chicken Heterotopy - changes in location Heterochrony - changes in time Enhancer Promoter Heterometry - change in amount Duck Mutations that affect coding regions Bmp4 Gremlin Apoptosis Heterotypy - change in kind Weatherbee et al. 2006 Alberts et al. 2002. Heterochrony – Evolutionary changes in the Heterochrony: the paedomorphic Axolotl Heterometry - Evolutionary changes in the timing of development (Ambystoma mexicanum) amount during development Tiger Salamander (Ambystoma tigrinum) The retention of juvenile characteristics into adulthood (e.g., tail fin and gills) Wildtype Terrestrial Adult Bmp4 Aquatic Adult overexpressed Example: Salamanders exhibit heterochronic changes by which the larval Bmp4 stage is either retained or truncated are caused by genetic changes in the inhibited ability to induce or respond to hormones initiating metamorphosis. Lets consider the Axolotle! The TRH Cascade BMP4 expression is correlated with the Thyroxin-releasing hormone (TRH) Thyroid-stimulating hormone (TSH) Thyroxin breadth and depth of the beak! Thyroxin METAMORPHOSIS! 3 3/6/11 Co-option and the evolution of novel Co-Option: Hox-a9-13 genes Summary characters Recruitment (co-option) – refer to the evolution of Evo-Devo novel functions for pre-existing genes and Allows us to understand Homology by uniting developmental pathways. Morphological and Genetic Data Provides insight into the developmental underpinings of ba fl hl au Hox-a9 the mechanisms of macroevolution Involves the addition or evolution of novel enhancer elements that allow genes and signaling pathways Highlights the importance of gene regulation (i.e., utilizing enhancer modularity) in morphological to have multiple developmental roles. ba fl hl Hox-a9 evolution. ba Hox-a9 4 .