Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Major questions in developmental biology Single genome Diverse cell typesTotipotent zygote Fate refinementDiverse cell fatesCell commitments are largely driven by cell positions within a developmental fieldMajor cellular developmental decisions:• Establish basic body plan coordinates (anterior-posterior, dorsal-ventral)• Subdivision of anterior-posterior axis (segmentation into metameres, specification of fates for each segment)• Subdivision of dorsal-ventral axis (differentiation of primary germ layers: endoderm, mesoderm, ectoderm)• Organ/tissue differentiationFig. 18-7Drosophila syncitial stage embryoFig. 18-8Chapter 18: Genetic basis of development*p. 584Genes controlling early developmentwere discovered in Drosophia mutant screens(Nϋsslein-Volhard, Wieschaus, Lewis)Fig. 18-9A-P axis differentiation by gradients of two proteinsMajor morphogens directing A/P axis formation in Drosophila• BCD (bcd gene): directs anterior development; transcription factor; mRNA is localized; mutations are tail duplications (bicaudal embryos)• HB-M (maternal hb gene): differentiates axial development; transcription factor; mRNA unlocalized • NOS (nos gene): directs posterior development; translation repressor; mRNA is localized; mutations are head duplications• All three are present in gradients in embryosFig. 18-10bcd & nos mRNAs are tightly localized- BCD and NOS proteins form concentration gradientsbcd mutation → double-posterior embryonos mutation → double-anterior embryo• BCD gradient results from diffusion of localized RNA (NOS gradient is similar)• HB-M gradient results from translational repression by NOS protein• Net effect: cells along the A-P axis of the embryo have distinctive combinations of concentrations of BCD and HB-M transcription factors (Experimental perturbations of the gradients demonstrate their roles in determining the A-P axis)Fig. 18-11bcd mRNA is localized to the anterior poleby sequences within its 3’ UTRFig. 18-13The gradient of BCD protein determinesA-P axis cell fates (which cells form cephalic furrow)D-V axis is specified by cell-cell signalling system in Drosophila• DL protein (dl gene): transcription factor; uniform distribution but localization gradient; highest nuclear localization in ventral areas• SPZ protein (spz gene): extracellular ligand for TOLL receptor; secreted assymetrically by follicle cells during embryogenesis; gradient most concentrated in ventral area• TOLL protein (Tl gene): transmembrane receptor activates signal cascade resulting in phosphorylation of CACT protein; uniform distribution• CACT protein (cac t gene): cytosolic protein; uniform distribution; unphosphorylated form binds DL; phosphorylated form releases DL (permitting DL nuclear localization)Fig. 18-15D-V polarity is determined by distributionof the DL protein (transcription factor)DL quantity is similar in all cellsNuclear localization differs in D-V axisNuclear DL activates “ventralizing” genesDL nuclear localization is controlled bya signal transduction cascadeFig. 18-17Loss-of-function mutations thatproduce “dorsalized” embryos(nuclear DL nowhere):•spz•toll•dorsalLoss-of-function mutations thatproduce “ventralized” embryos(nuclear DL everywhere):•cactDL nuclear localization is controlled bya signal transduction cascadeFig. 18-17Fig. 18-19Known types of positional information in embryosA-P and D-V axes are defined by morphogens (BCD, HB-M, DL) encoded by maternal-acting genesThese transcription factors differentially activate a set of zygotic-acting genes – the cardinal genesA-P axis cardinal genes are called gap genes (specify general body regions)Gap genes encode transcription factors and activate the set of pair rule genes (cardinal genes specifying alternating segments – creating segments)Pair rule genes encode transcription factors and activate the set of segment polarity genes (cardinal genes that distinguish anterior/posterior compartments of each segment)Segment polarity genes differentially activate the segment identity genesFig. 18-20Delayed cellularization of the Drosophila embryocompartmentalizes factors and their gradientsFig. 18-21Compartmentalized factors directzone-specific development → segmentsFig. 18-22Loss-of-function mutations of those factorscreate segment-specific changesFig. 18-23Gap gene expression determines zonal identityPair-rule gene expression drive segmentationFig. 18-23Gap gene expression determines zonal identityPair-rule gene expression drive segmentationftz and eve expression patternsA-P and D-V axes are defined by morphogens (BCD, HB-M, DL) encoded by maternal-acting genesThese transcription factors differentially activate a set of zygotic-acting genes – the cardinal genesA-P axis cardinal genes are called gap genes (specify general body regions)Gap genes encode transcription factors and activate the set of pair rule genes (cardinal genes specifying alternating segments – creating segments)Pair rule genes encode transcription factors and activate the set of segment polarity genes (cardinal genes that distinguish anterior/posterior compartments of each segment)Segment polarity genes differentially activate the segment identity genesFig. 18-24Segment identity genes are mostly found in the homeotic gene complexesANT-C (Antennapedia complex): genes for anterior segment identityBX-C (Bithorax complex): genes for posterior segment identityBX-C mutations can transform theidentities of posterior segmentswild-typebithorax mutant (T3 T2)Fig. 18-24Fig. 18-26Embryonic development is driven by ahierachical cascade of transcription factors and signalling systemsFig. 18-30Hox gene clusters are highly similar to Drosophila HOM-C gene clusters…..but, Hox clusters are repeatedFig. 18-30Hox gene clusters are highly similar to Drosophila HOM-C gene clusters…..but, Hox clusters are repeatedHox and HOM-C genes are expressed in similar patterns during developmentFig. 18-30Fig. 18-32Testing the role(s) of Hox genesHox C8