3 OverviewInstructions in the genome establish the developmental fate of cells in multicellular organisms.Developmental pathways consist of sequences of various regulatory steps.The zygote is totipotent, giving rise to all body cells.Gradients of maternally-derived regulatory proteins establish polarity of the body axis and control transcriptional activation of zygotic genes.Transcriptional regulation and cell signaling mediate development in animals and plants.The same set of genes appears to regulate early development in all animals.
4 Development In multicellular organisms, life begins as a single cell. With few exceptions, somatic cells contain the same genetic information as the zygote.In development, cells commit to specific fates and differentially express subsets of genes.Cells identify and respond to their position in developmental fields.Daughter cells may differ with respect to regulatory instructions and developmental fate.
5 Building the embryo Developmental decisions made at specific times during developmentmany are binary, e.g., male or female, germ line or somatic.most are irreversiblemany involve groups of cells rather than single cellsIn animals decisions are made toestablish anterior-posterior and dorsal-ventral axessubdivide anterior-posterior axis into segmentssubdivide dorsal-ventral axis into germ layersproduce various tissues and organsMost decisions involve changes in transcription
6 Sex determinationXX-XY chromosomal systems for sex determination have evolved many timesDifferent molecular pathways for sex determination in different groups of animalsDrosophilaeach cell lineage makes sexual decisionratio of X chromosomes to autosomes determines sexcascade of differential mRNA splicingMammalsTDF gene on Y chromosome determines malenessendocrine hormonal system
9 Sxl toggleRatio of NUM bHLH proteins to DEM bHLH proteins measures X:A ratio by competing for dimer formationDNA binding domain of NUM proteins recognizes Sxl early promotertwice as much NUM protein in females with two X chromosomes as males with one X results in more NUM-NUM homodimersSufficient NUM-NUM homodimers activate Sxl early promoter resulting in SXL protein that alternatively splices larger Sxl transcript from late promotersets up autoregulatory loop in flies with X:A ratio of 1.0in flies with X:A ratio of 0.5, insufficient NUM-NUM homodimers results in no SXL protein and late transcript is normally processed (yields nonfunctional protein)
11 Sxl downstream targetSXL protein activates downstream shunt that leads to female developmentSXL protein binds to primary transcript of tra (transformer) resulting in spliced transcript that produces TRA proteinTRA protein in turn is RNA-binding protein that produces female-specific splicing of dsx (doublesex) transcriptDSX-F transcription factor represses male-specific gene expression resulting in female developmentIn absence of SXL, there is no functional TRA protein, and dsx is spliced to produce DSX-M transcription factor which represses female-specific genes, leading to male development
13 Sex determination in mammals Presence of Y chromosome determines malenessSRY gene in humans encodes transcription factor (testis-determining factor)expression of SRY in developing gonad causes it to develop into testistestis secretes testosterone resulting in male developmentIn XX individuals, absence of SRY protein and subsequent absence of testosterone results in default female shunt pathway
15 Role of cytoskeleton in development Consists of highly organized rods and fibersmicrofilaments (actin)intermediate filamentsmicrotubulesSuch structures are polar, with distinct “+” and “–” endsServe as highway system for intracellular transportAsymmetry of cytoskeletal elements plays fundamental roleslocation of mitotic cleavage planecontrol of cell shapedirected transport of molecules
16 Origin of germ lineIn animals, germ line is set aside from soma in early developmentonly germ cells can undergo meiosissomatic cells form body of organismAsymmetric distribution of cytoplasmic particles (e.g., P granules of Caenorhabditis elegans) by cytoskeletoncells receiving particles develop into germ lineparticles anchored to actin in some organisms, to microtubules in others
18 Drosophila anterior-posterior axis Determined by gradients of BCD (product of bicoid) and HB-M (product of hunchback)mRNA maternally deposited in eggBCD mRNA tethered to “–” ends of microtubules via 3’ UTRHB-M protein gradient depends on NOS proteinnos mRNA tethered to “+” end of microtubule via 3’ UTRNOS protein gradient blocks translation of hb-m mRNA, resulting in HB-M gradientResulting opposite gradients of BCD and Nos determine axis
22 Drosophila dorsal-ventral axis Determined by gradient of transcription factor DL (encoded by dorsal)gradient established by interaction of spz and Toll gene products deposited in oogenesis and released during embryogenesisSPZ-TOLL complex triggers signal transduction pathway in cells that phosphorylates inactive DLPhosphorylated DL migrates to nucleus, activating genes for ventral fates
25 Positional information Localization of mRNAs within cell establishes positional information in cases where developmental fields begin as a single cellFormation of concentration gradients of extracellular diffusible molecules establishes positional information in multicellular developmental fieldsworks by signal transductiondiffusible molecules are known as morphogens
27 Complex pattern: Drosophila Successive interpretation of established, changing, and new gradientsLargely due to changes in transcriptionGenes targeted by gradients of maternal A-P and D-V transcription factors are cardinal genesrespond to these factors at enhancers and silencerssimilar genes in other animals
29 Drosophila development (1) Early syncitial developmentzygotic nucleus divides 9 times with no cell divisionsome nuclei migrate to posterior pole to give rise to germ line4 more mitotic divisions without cell divisionNuclei migrate to surface of egg cytoplasmmembrane forms around them (cellularization)begin responding to positional information in A-P and D-V transcription factor gradients.
33 Drosophila development (3) Developmental fate determined through transcription-factor interactionsA-P cardinal genes = gap genesKruppel and knirps (mutants have gap in normal segmentation)promoters have differential sensitivity to BCD and/or HB-Mestablishes different developmental fields along embryo, roughly defining segmentsBifurcation of development: targets of gap gene encoded transcription factorsone branch to establish correct number of segmentsone branch to assign proper identity to each segment
34 Drosophila development (4) Segment numbergap gene products activate pair-rule genesseveral different pair-rule genesexpression produces repeating pattern of seven stripes, each offsetpair-rule products act combinatorially to regulate transcription of segment-polarity genesexpressed in offset pattern of 14 stripesSegment identitygap gene products target cluster of homeotic gene complexesencode homeodomain transcription factorsmutations alter developmental fate of segmente.g., Bithorax (posterior thorax and abdomen) and Antennapedia (head and anterior thorax)
40 Pattern formationTranscriptional response to gradients (asymmetrical distribution) of transcription factorsMemory of cell fateintracellular and intercellular positive-feedback loopse.g., homeodomain protein binds to enhancer elements of its own gene, ensuring continued transcriptionCell-cell interactionsinductive interaction commits groups of cells to same developmental fatelateral inhibition results in neighboring cells assuming secondary fate
43 GeneralizationsAsymmetry of maternal gene products establishes positional information used for early developmentSuccessive rounds of expression of genes encoding transcription factors establish axes and body part identityPositive feedback loops maintain differentiated stateComponents of one developmental pathway are also found in many othersDifferences in types and concentrations of transcription factors result in different outputs
44 Developmental parallels Early animal development follows fundamentally similar patternRemarkable similarity among homeotic genesone HOM-C cluster in insectsfour HOX clusters in mammalsparalogous to insect clusterexpressed in segmental fashion in early developmentKnockout and genome studies suggest animal development uses same regulatory pathways
47 Development in plantsPlants have different organ systems than animals and plant cells can not migratePlants do not separate soma from germ-line until flower developmentPlants too have hormones and signaling pathwaysArabidopsis thaliana is model systemTranscriptional regulators control fate of four whorls (layers) giving rise to flowerprocess similar action of homeotic genes in animal development
48 Assignment: Concept map, Solved Problems 1 and 2, All Basic and Challenging Problems.