Biology 443- Exam 1
Progression of A-P axis differentiation: maternal effect to Hox genes
1. Maternal Effect Genes - egg & earlyembryo 2. Gap Genes - first zygotically induced genes; regional specification 3. Pair-rule genes - Respond to gap genes to make 7 stripes/gene 4. Segment Polarity Genes - specify all14 segments and their polarity 5. Hox Genes - specify regional identity
Homeobox
180 bp. conservedDNA-binding sequence.
Hox genes (what they are, you do not need to know the different gene names)
8 genes that control homeotic mutants and segmental fate - all contain a homeobox and are located in the same chromosomal region
Understand how a wing eyespot can change size and shape due to the amount and threshold responses to various morphogens.
A central morphogen diffuses to turn on white vs. black vs. orange pigment at different thresholds of morphogen concentration. To get a bigger eyespot the body would have to create more morphogens
developmental constraint
A lineage's development may limit the sorts of phenotypes that it can evolve.
vestigial structures
A structure in an organism that has lost all or most of its original function in the course of evolution, such as human appendixes.
Advantages and disadvantages of multicellularity
Advantages: Two advantages of being multicellular are a longer lifespan and the ability to adapt to different environments. Disadvantages: Needs more food for energy
What features make an organism "multicellular"?
Aggregation of cells Cellular communication effecting cell fate Division of labor Partitioning of reproduction: germ line vs. somatic cells The boundary of what is multicellular is fairly nebulous and influences the number of developmental origins inferred
analogous structures, convergence
Analogous: Structures are similar structures that evolved independently-they are convergently evolved. Convergence: Trait has an independent origin.
General trends in species diversity in Metazoa
Arthropods have the most diversity with 1,170,000 different species which is the highest at 84%. The lowest is Lancets at 33 different species.
Which animal lineages are most diverse and what are some of their key innovations?
Arthropods, Exoskelaton, segmented body, jointed appendages.
Diversity of cleavage patterns across Metazoans and yolks impact on cleavage patterns. I do not need to know which taxa have which cleavage but understand that cleavage patterns are very diverse, are impacted by the amount and type of yolk (which is adaptive!), and that this is not the most conserved stage of development.
Cell Cleavage: Zygote to Blastula 1) Cell division without growth-maternal effects, fast 2) Zygotic genome takes over, slower growth No strong phylogenetic patterns, prone to variation
Choanocytes
Cells that line the interior of asconoid, syconoid and leuconoid body type of sponges that contain a central flagellum, or cillium, surrounded by a collar of microvilli which are connected by a thin membrane.
Understand the convergence in multicellular morphology for multicellular bacteria, charophytes, and fungi (vegetative cells and spores) and how animal multicellularity had a different multicellular form (ball of cells + gastrulation), and slime molds a different one as well (colonial)
Charophyte: Multicellularity begins with- Cell wall shared between daughter cells-incomplete cell division Cell division on one axis to form filaments Differentiation of terminal reproductive structures. Slime molds: Multicellular through coloniality and sophisticated cell-cell communication Fungi: At least 3 independent origins within fungi of multicellularity - Form filamentous hyphae with tip extension- no true tissues and organs, mycelium. Functional/Morphological convergence in the origins of multicellular. Animals: Exaptation for multicellularity Choanaflagellates-cell adhesion proteins, some cell-cell signaling genes-for adhesion of prey, mating, or interactions with other cells.
Choanoflagellates and the origins of animal multicellularity
Choanoflagellates-cell adhesion proteins, some cell-cell signaling genes-for adhesion of prey, mating, or interactions with other cells
Chordates, Vertebrates
Chordates: Lancelets, Urochordates, Vertabrates Chordate evolution: Synapomorphies: notochord, dorsal hollow neural tube, somites, post-anal tail Notochord: A ridgid structure for muscle attachment; supporting larval tail; formed from mesoderm Vertebrates: Notochord appears in embryogenesis and is replaced in vertebrates by vertebral column.
How does chromatin packaging impact gene expression? Understand role of histone modifications and nucleosomes.
Chromatin changes structure during development (more exposure, more expression) Histone: Any group of basic proteins found in chromatin Histone modifications affect gene transcription by chromatin remodeling complexes changes chromatin architecture and gene activtion. Without it transcriptional machinery can't access DNA, which can result in gene silencing. Nucleosomes: A structural unit of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones
Cis vs. Trans modifications
Cis-regulatory change- change in the flanking sequence that regulates a genes expression. Impacts gene regulation at the locus. Changes the amount of transcription. Trans-regulatory change- change in the protein-coding sequence. Impacts gene regulation at another genetic location. Change protein structure and binding affinity.
Difference between cleavage, gastrulation, organogenesis (neurula, pharyngula). Understand that there are diverse means of gastrulation and types of cell movements that generate germ layers, but once established those germ layers go on to make very similar structures during a more conserved stage of embryogenesis. They don't need to remember each type and which organisms they have but rather have a sense of diversity.
Cleavage: The first stage of embryogenesis. Cleavage continues until blastula is formed. Gastrulation: A cellular dance; germ layers (endo-, ecto-, mesoderm) form through cell movement Organogenesis: The production and development of the organs of an animal or plant.
How pair-rule gene stripes in general are generated from other genes at cis-regulatory level (do not need to know gene names)
Complex cis-regulation: a regulatory module for every segment. Even-skipped - stripe 2: activated by Bicoid and Hb, repressed by Giant and Krüppel. Other stripes are regulated by different sets of maternal & gap genes. Binding self - common for maintaining signal long-term Primary pair rule genes expressed first and trigger secondary pair-rule
Cnidaria and Ctenophores - examples of taxa. Phylogenetic position; Diploblasts with radial symmetry; Vary in whether the medusa or polyp are the predominant life history stage
Ctenophores- The Comb Jellies; Locomotion via comb-like cillia, gelatinous, Predators of plankton, Capture prey with colloblasts (often on tentacles) that secrete glue, Nerve net. Cnidaria- Coral, Sea anemone, Sea pens, Jellyfish. Predators that capture prey using cnidocytes on tentacles-toxic harpoon, lasso-like barb cells. Symbiotic algae. Diploblasts- 2 germ layers:ectoderm and endoderm; Ctenophores, Cnidarians Shifts in predominate life stages: medusa and sessile polyps.
Why should we not expect a common developmental toolkit for multicellular life?
Different origins of development
How cell-adhesion facilitates development & the role of cadherins
Differential Adhesion Hypothesis Stronger adhesion->Center Cadherins major call adhesion molecules Cadherins guide cell adhesion and movement. Transmembrane protein, connects cells with intracellular scaffold (actin + myosin) More cadherins, More adhesion Different types (100s) of cadherins -> like cells aggregate Shift in expression create folding & shifts tissue fate
Dorsal as a dorsal-ventral regulator and the concentration-dependent way that it triggers other genes to specify germ layers
Driven by Dorsal: Initiated by gürken; Dorsal expressed ventral; activates ventral genes and repressed dorsal genes 40 Genes respond differently to dorsal, primarily by [ ]gradient Snail represses non-mesoderm and Twist activates mesoderm;non-overlapping region expresses Fgf8 receptor which drives gastrulation
Gastrulation forms the three germ layers: ectoderm, mesoderm, endoderm
During gastrulation the three germ layers form; the cell mass is now known as a gastrula Week 3 of development is the week of gastrulation into germ layers. A germ layer is a layer of cells that will go on to form one of our organizational tubes.
The three germ layers and the types of tissue they become
Ectoderm-skin, central nervous system Endoderm-digestive tissue, lungs Mesoderm-muscles, circulatory system, bone
DNA methylation and histone modifications as two types of epigenetic modifications
Epigenetics: Heritable change in gene expression that occurs independently of alteration of DNA sequence. DNA methylation: 1. Environment triggers enzymes to add methyl groups to CpG nucleotides in promoter region. 2. The gene cannot be expressed. 3. Methylation can turn off genes long term, passed to the next generation. Histione modification: 1. Environment triggers histone modification 2. Accessibility of DNA shifts 3. Turns genes on off, more temporary, can trigger DNA methylation; heritable.
What segment polarity genes do; how wingless (wg) and engrailed (en) interact in general to create polarity
Established by pair-rule genes; eve and ftz activate en; wg is present where these are low Maintain their own and each others gene products to keep genes on and border distinct Establish polarity within segments
When and how Evo-Devo began as a field
Evo-Devo began with revelation of conservation of homeotic gene sequences: Homeotic Mutants - Mutants where one structure is replaced by another Major homeotic genes (Hox genes) share a common sequence element (homeobox) and the same genes are used for segment identity across animals Hox genes conserved in both sequence and function across animals
Exaptation; Exaptations for multi-cellularity: cell-cell communication and adhesion
Exaptation: Trait that evolved to serve one function but was subsequently advantageous for another.
The basic components of a gene (e.g., exon, intron, promoter, enhancers, insulators, etc.), their function, position in a gene, and how changes in them impact gene expression
Exon: Any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. Intron: An nucleotide sequence within a gene that is not expressed or operative in the final RNA product. Promoter: Where the RNA polymerase II that orchestrates transcription binds. Enhancers: Where transcription factors bind to help turn on transcription; can occur 5', 3', introns Insulators: DNA sequence elements that help to prevent inappropriate interactions between adjacent regions of the genome. There are 2 types-one that is involved in enhancer-blocking activity and one that provides a barrier to the spread of heterochromatin.
The way differential repression and activation and different threshold responses to morphogens allow discrete gap genes to be generated from maternal effect genes
First zygotically expressed genes Turned on/off by Bicoid,Caudal, or Hunchback protein expression Hunchback effects Krüppel in concentration dependent manner Gap genes reinforce boundaries by repressing each other
genetic drift and natural selection as mechanisms of evolution
Genetic drift: Variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce. Natural selection: The process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution.
Difference between genome, transcriptome, and proteome
Genome: A molecular code for building and maintaining an organsim-most of the changes in development derives from this one(mostly) static code Transcriptome: The sum total of all the messenger RNA molecules expressed from the genes of an organism Proteome: The entire complement of proteins that is or can be expressed by a cell, tissue, or organism
genotype, phenotype, allele, recombination
Genotype: the genetic constitution of an individual organism. Phenotype: The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. Allele: One or two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Recombination: Shuffles DNA to create novel combinations of alleles that may result in different phenotypes.
evolution operates on heritable genetic variation generated from mutation and recombination
Heritable variation must exist, generated by mutation and recombination. Recombination: Shuffles DNA to create novel combinations of alleles that may result in different phenotypes.
Primitive streak. Anterior-Posterior gene gradients set up axis and Hox genes.
Hox genes laid down with migration of primitive streak, anterior genes first
How 4 Hox clusters work together to give identity to segments; Redundancy Homeotic mutants replace vertebrae types
Hox genes partially determined prior tosomite formation Like Drosophila, controlled by complex cis-regulation involving auto regulation and regulation from upstream genes Strict posterior prevalence creates discrete Hox protein domains
Degree of conservation between Drosophila and vertebrate Hox genes
Hox genes repress each other Complex interaction with AP-axis genes - e.g. Ubx Hox genes control each other Hox genes are transcription factors Not an organizer - no diffusible morphogen/paracrine factor (a.k.a. signaling ligand)Somit
Why might some genes in development be similar between independent origins of multicellularity?
If it works and is successful will see different convergens
Lophotrochozoan, Ecdysozoan
Lophotrochozoan: Spiral cleavage; Flatworms, molluscs, Annelids Ecdysozoan: Shed exterior skeleton; Nematodes, Arthropods
Epithelial vs. mesenchymal cells.
Mesenchymal cells: small spindle-shaped cells with large nuclei, prominent nucleoli and fine chromatin. These are multipotent stem cells that differentiate as progenitor cells for all types of connective tissue, such as fibroblasts, osteoblasts, chondroblasts and preadipocytes. Epithelial: Epithelial cells make up the lining of many tissues in the human body. Packed tightly in sheets, they create a barrier to the outside world and protect you. Tightly packed in various organ systems, such as your skin.
microevolution vs. macroevolution
Microevolution: A change in allele (gene variant) frequencies over time. Studied within species or close relatives. Macroevolution: Changes across species over time. Studied over deep time.
understand microevolutionary to macroevolutionary processes: how allele frequencies and their phenotypes can shift in populations across space and time as a result of selection and drift and how speciation can impact the sorting of these alleles
Microevolution: Evolution starts with a mutation in an individual, hemoglobin-related mutation. Given its rarity and chance (drift), the mutation most likely not survive. Through natural selection a mutation can spread.
Why Drosophila is a model system for evo-devo
Model organism; easy to rear, short generation time Easy to genetically manipulate- roles of many development genes from mutants Baseline for genetic study of other arthropods Insights (princples, genetic basis) generalizable
Modularity and importance for evo-devo and diversification
Modularity: Developmental evolution involves modifying number and kind of repeating structures The body is composed of developmentally discrete units, like building blocks These units are often repeating and can evolve independent of one another Modules can be modified without effecting the whole, providing more evolutionary potential Modularity is an important factor driving specialization and diversification across life and occurs at multiple levels
Understand what a morphogen is and how it results in concentration gradients and how other genes can have threshold responses to morphogens.
Morphogens- Diffusible substance that varies in concentration across a field. Cells respond to different thresholds of morphogen concentration The center of the eyespot is an orgnizer and contains a morphogen
Understand the role of Cambrian radiation in animal evolution. Timing of Cambrian.
Most of these lineages are about the same age Some of these lineages are much more speciose ("successful") than others
Understand the multiple origins of multicellularity: know the major lineages and general aspects of how multicellularity evolved and the sister lineages to each major origin.
Multicellular evolved numerous times in some lineages; 25 convergent origins
Homeotic mutants
Mutants where one structure is replaced by another (1894, Bateson).
Segmentation Clock & Somite formation: Involves clock-like waves of Notch signaling followed by induction of repressor, embryonic gene gradients promote location of segments forming
Notch (& other genes) moves in periodic waves ending with somite formation negative feedback loop - Notch activates selfin neighboring cells (juxtacrine) but also its own repressor, Lunatic Fringe, creating waves Somite Formation - Segmentation Clock Form Rostral to Caudal at regular intervals FGF & Wnt gradients establish boundaries
Fate of the notochord in vertebrates
Notochord appears in embryogenesis and is replaced in vertebrates by vertebral column.
Ontogeny recapitulates phylogeny, biogenetic law, Haeckel; why it is not a valid hypothesis
Ontogeny recapitulates phylogeny: Shared phylotypic stage (~pharyngula stage-tail, nerve cord, pharyngeal arches) Haeckel law: Proposed development is a window into evolution. He placed organisms on a developmental hierarchy. Law of Correspondence In development, an organismpasses through the forms of its ancestors Law of Terminal Addition More advanced stages are added at the end of development. Law of Truncation Some stages are removed. Law was rejected because organisms share embryonic stages not adult ones.
Concept of an organizer; Spemann-Mangold, AER, ZPA organizers
Organizers: Signaling centers that influence the development of neighboring tissues. Apical Epidermal Ridge (AER)- Specifies distal limb elements Zone of Polarizing Activity (ZPA)- Specifies Polarity of limbs Spemann-Mangold organizer: Its fate is determined early (by grey crescent) and determines the fate of cells around it
Origin of nervous system (nerve net, central nervous system)
Origin of nervous tissue in cnidarians and ctenophores; signal transduction and Na++ channels- nerve net.
Paracrine vs. juxtracrine signaling.
Paracrine signaling: a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Juxtracrine signaling: involves communication between cells that are in direct contact with each other. This communication is often mediated by gap junctions in animal cells and plasmodesmata in plant cells.
Be able to interpret relatedness, trait homology, and convergence given a phylogeny monophyly, paraphyly, polyphyly
Phylogeny: The evolutionary history of a group of organisms; A diagram of speciation events over time. Monophyly: The condition of a taxonomic grouping being a clade Paraphyly: it consists of the grouping's last common ancestor and most of its descendants, but excludes a few monophyletic subgroups. Polyphyly: An assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor.
Understand the various factors that activate and repress Hox genes, understand how complex cis-regulation is involved in this polycomb and Tri thorax genes and their function in Hox gene regulation
Polycomb - maintains repression Trithorax - maintains activation
Stromatolites
Prokaryote biofilms 3.4 bya. They peak at 1.2 bya, when the first burrows (mat grazers) were present (Metazoans) then decline; precipitous declines in Cambrian
General mechanisms of post-translational modification
Protein folding: Addition of chemical groups; cleavage. Typically need to be activated via phosphorylation. Degraded by ubiquitin labeling. Histones modified to loosen chromatin. Work in complexes-need cofactors: protein-protein interactions.
The various functions of proteins
Proteins have multiple functions including building cellular structures, which makes them the basic building blocks of our body, facilitate chemical reactions as enzymes, guide gene expression as transcription factors, and interact with each other as complexes. Protein folding and modification important for function: heat denatures proteins and they undergo diverse post-translational modifications that alter function.
Protostomes vs. Deuterostomes and which key taxa belong to them
Protosomes: Flatworms, Molluscs, Annelids, Nematodes, Arthropods Deuterosomes: Echinoderms, Hermichordates, Lancelets, Urochordates, Vertabrates.
Radial vs. Bilateral symmetry (Bilaterians)
Radial symmetry: Being symmetrical around a central point. Bilateral symmetry: Organisms that are mirror images.
Role of development in evolutionary thinking from 1800s to today
Rise of the Modern Synthesis to explain evolution: • genetics + systematics + paleontology • Macroevolution a result of microevolution • Mutation, selection and gradualism Embryology 1920s-1980s: Largely descriptive; focus on homology
Understand the three components of a signaling pathway: signaling ligand, signaling receptor, transcription factor. Which of these usually are the molecular organizers?
Signaling ligands: Ligands interact with proteins in target cells, which are cells that are affected by chemical signals; these proteins are also called receptors. Ligands and receptors exist in several varieties; however, a specific ligand will have a specific receptor that typically binds only that ligand. Signaling receptors: Cellular receptors are proteins either inside a cell or on its surface which receive a signal. Transcription factor: a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. Signaling ligands are usually molecular organizers. Signaling ligands=morphogens.
Somites & vertebrate segmentation
Somites flank neural tube + notochord Somites produce future skeletal muscles, ribs, vertebrae
Temporal and spatial collinearity of Hox genes in vertebrates (spatial in Drosophila)
Spatially colinear but not expressed the same way in all tissues Hindbrain to tail Temporal colinearity - Hox genes turned on anterior to posterior. (Drosophila is only spatial!)
Mechanisms of post transcriptional modification: Splice variation, miRNA
Splice variants can perform different functions. Example is in bats where an alternative heat-sensative splice variant promotes heat detection. miRNA: Small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Involved in RNA silencing and post-transcriptional regulation of gene expression. contain matching complementary sequence to specific mRNAs; bind them, and silence them by chopping them (Dicer complex) or preventing translation.
Porifera as a transitional animal, knows general characteristics.
Sponges; Single cell layer: Bacterial filter feeders; rely on flow of water for nutrient, oxygen, and waste transfer. They lack tissues or organs: no nervous system (cells can be sensory and have calcium signaling), no circulatory system, no muscles; Egg and sperm cells.
Homology
Structure that are modified from a common ancestral structure.
systematics, phylogenetics
Systematics: The branch of biology that deals with classification and nomenclarture; taxonomy Phylogenetics: The evolutionary development and history of a species or trait of a species or of a higher taxonomic grouping of organisms: the phylogeny of Calvin cycle enzymes.
Understand the hourglass model and how it applies at both a molecular and morphological level and whether it applies across animals.
Testing the Vulnerability of the Phylotypic stage Goal: Testing out the vulnerability of the phylotypic stage Results showed lots of abnormalities in both the mice and rats If the animal developed a cleft lip, they showed more modularity. A "Development Hourglass" in Fungi Focuses on molecular level of the certain stages in the hourglass method Goal: Whether this fungus follows an hourglass pattern during development Results showed more changes from early to mid and mid to late Spiralian Development Goal: To find out if Spiralia really follow the hourglass method Results showed that it is a reverse hourglass when all the genes are considered; showed in all of the spiralians
Hourglass model and hypotheses behind why it happens
The beginning stage the organisms are simple with just being an egg and in the end stage the organisms are similar. The middle stage is when the hox genes are expressed and has a lot of change, which makes the middle stage the most complex.
The central dogma: DNA (transcription) RNA (translation) protein, and how it generally works
The central dogma is how a gene is "expressed". The order goes from DNA-Transcription-RNA-Translation-Protein. DNA denatures and makes RNA copy called a transcript: In eukaryotes, this involves an RNA polymerase II and transcription factor proteins that work together to initiate the addition of nucleotide to RNA strand. RNA is transported to a ribosome=ribosomal RNA and proteins. tRNAs convert 3 base sequence to amino acid. Amino acid chain folds into proteins.
Principle of deep homology
The comparison of developmental genes and mechanisms between species, merging embryology with evolutionary biology, became the field of Evo-Devo.
Types of DNA mutation as a source of variation
The enzyme lactase and how some portion of the population doesn't have it making them lactose intolerant.
functional constraint
The extent to which a region of DNA is intolerant of mutation, due to a reduction in its ability to carry out the function encoded.
What are the goals of the field of Evo-Devo?
The main goal of evo-devo is to decipher the genetic code and how it translates to form. Also, to understand evolution, how phenotypic variation on which selection acts, can be generated.
Homeodomain
The proteinmotif (60 aa) = homeodomain.
Transcription factor
They are molecules that control the activity of a gene by determining whether the gene's DNA is transcribed into RNA. Transcription factors are gene specific. The way they work is only a small subset of proteins are transcription factors-these are especially important for development. Selective binding affinity to sequence typically of 6-9 bp. Examples include tinman protein binds to TCAAGTG.
How tissue-specific gene transcription happens
Tissue-specific function is not solely regulated by transcription factor expression. Assume bottleneck positions in their corresponding networks. Driven by context-dependent, non-canonical regulatory paths.
atavisms
Traits reappearing that had disappeared generations before
Homeotic genes
Ultrabithorax and Antennape diashare a conserved DNA sequence:the homeobox Mutants where one structure is replaced by another (1894, Bateson). Homeotic genes are responsible.
Posterior prevalence applies to vertebrates
Vertebrae identity match Hox gene boundaries Redundancy - Mutations in multiple copies are needed to create an effect
Patterns of conservation in developmental genes between Drosophila and vertebrates
Vertebrate BMPs - axis determination gradients & tissue growth; homolog dpp involved in dorsal axis and appendage formation FGF genes - axis and limb genes invertebrates; Drosophila - controls formation of branching structures
L/R axis formation: Pitx2; how asymmetry is generated (node, crown cells, cilia)
Vertebrate organ systems are not bilaterally symmetrical (long digestive system) Cilia in nodal cells oriented posterior-ventral and rotation moves in one direction. Cilia in crown cells are immobile and receive differential signal. Deficiencies in dynein cause random L/Rorgan placement and artificially induced reverse flow induces reverse placement
Pleiotropy
When a gene has multiple functions/phenotypic effects
parallel evolution
When shared phenotypes evolve independently using a similar molecular mechanism; a type of trait convergence involving homologous genes, pathways, and/or development
Concept of cell fates becoming determined.
With time, cells become irreversibly committed
How Anterior-Posterior axis is established (generally) in chick embryos. Node as an organizer.
Wnts, BMPs (bone morphogenetic protein family), FGFs (fibroblast growth factor family),RA (retinoic acid) comprise posterior gradient BMP and Wnt antagonists (e.g., chordin, noggin) set up anterior gradients (establish head structures) Gradients induce caudal (Cdx) and together activate Hox genes
polymorphism
a discontinuous genetic variation resulting in the occurrence of several different forms or types of individuals among the members of a single species
fixation
a gene in general or particular nucleotide position in the DNA chain
How the major maternal effect genes work: bicoid, nanos, hunchback, caudal
bicoid, nanos, caudal, hunchback mRNA transported from nurse and follicle cells; mRNA distribution established in egg and dormant until fertilization Bicoid and Nanos proteins diffuse; Bicoid protein binds caudal mRNA (at 3'UTR), preventing translation; Nanos prevents hunchback translation. Gradients established during syncytial cell divisions Bicoid activates hunchback (maternal & gap gene)transcription in a concentration dependent manner
Recapitulation
exhibiting during development the traits of more ancestral taxa before acquiring more derived taxon-specific ones. e.g., humans: "gills", notochord, yolk sac
Selector genes
genes that control cell fate. Can control fields (like eye),compartments, or single cells.
Colinearity
order along chromosome matches order of expression on A-P axis.
Phylotypic stage
pharyngula stage - tail, nervecord, pharyngeal arches
gene flow
the transfer of genetic material from one population to another
