Drosophila Pattern Formation PDF

L20 Pattern Formation: Lessons from Early Drosophila Development Learning objectives: Reading: MBC6 1157-1164, 1169-1171 Understand that the early Drosophila embryo is a syncytium Understand how each of the following classes of genes function to regulat...

L20 Pattern Formation: Lessons from Early Drosophila Development Learning objectives: Reading: MBC6 1157-1164, 1169-1171 Understand that the early Drosophila embryo is a syncytium Understand how each of the following classes of genes function to regulate Drosophila anterior-posterior patterning: egg-polarity genes (maternal genes), gap genes, pair- rule genes, segment polarity genes and Hox genes Understand the mechanisms involved in maintaining the expression of segment polarity genes Appreciate how studying Drosophila Drosophila cleavage divisions How do we form body&tissue patterns during development and how are these patterns maintained throughout life?  Animals share similar body axis and major developmental steps, but early embryo patterning mechanisms differ most Nonpolar egg (mouse/human) Neurulation Dorsal (back) (Vertebrates) Early Embryo Egg -cleavage (most animals) - syncytium (fly) fertilization Gastrulation Organogenesis Animal (external) (P) Posterior (Tail) (A) Anterior (head) Differs most among species Egg Polarity Vegetal (internal) Highly polarized egg (fly) Ventral (belly) Why is Drosophila an important model system for development? Short generation time – 9 days to sexual maturity Sophisticated genetic and molecular biology tools Studies in Drosophila have revealed how genetic control mechanisms govern development Many basic mechanisms, including the functions of specific molecules (Hox genes), are conserved in humans Ed Lewis, Christine Nusslein-Volhard, Eric Wieschaus Won the 1995 Nobel Prize in Physiology or  Segmentation along the A- Fate map of fly P axis patterns the body development 14 plan 0 hrs Fertilization segments Head Gastrulation egg segments (3) 3 hrs Thoracic segments Segmentation (3) 10 hrs (8) Organogenesis 1 day Questions: How is the embryo patterned along the A-P axis? 9 days How does this segmented pattern forms? How does each segment know its identity? How is the pattern maintained throughout development? Approach: genetic screens (looking for mutants with defec  Drosophila early embryo is a syncytium Fertilization nuclear division without cell 13 divisions division prior to rapid, synchronous cellularization (~6000 cells) unfertilized fly egg is already polarized 3-4 hours 0 hours syncytial cellular blastoderm blastoderm rosophila nuclear divisions occur synchronously- > movie  Bristle patterns reveal distinct anterior-posterior (A-P) differences 1-day larvaamong and A within segments micrograph A large mutagenesis screen performed In Drosophila identified 4 classes of genes Important in pattern formation V bristles D P  Egg-polarity and segmentation genes mutant mutant mutant mutant Missing Missing several Missing Polarity defect anterior or contiguous alternating in every posterior segments segments segment Figure 21-19 MCB6 segments N=6 N=8 N=10  A hierarchy of gene regulatory interactions patterns the Drosophila embryo along the A-P axis Egg-polarity genes organize the A-P axis of the early embryo Three (maternal groups ofeffect) genes control segmentation along the A-P axis: (zygotic effect) Gap genes Pair-rule genes Segment-polarity genes Hox genes give identity to each segment MBC6 21-20  Egg-polarity genes encode maternal molecules deposited in the egg Bicoid regulates the anterior system bicoid mutants lack anterior segments Maternal bicoid mRNA is localized to the anterior of the egg prior to fertilization Bicoid protein is made after fertilization and diffuses in the syncytium, forming a gradient from anterior to posterior bicoid mRNA Bicoid is a transcription regulator Bicoid protein Bicoid regulates the expression of gap and pair- rule genes Figure 21-16 MCB6 Bicoid acts as a  Egg-polarity gene products form opposing gradients Gradient Gradient of Bicoid of Nanos in in cytoplas cytoplas m m protein mRNA ? Figure 21-17 MCB6  Egg polarity gene product gradients induce expression of zygotic genes and turn on the ‘gap genes’ Gradient Gradient of Bicoid of Nanos in in cytoplas cytoplas m m  Gap genes Gap genes Mutants lack several contiguous segments Expressed in broad domains Encode transcription regulators Their expression is regulated by egg-polarity genes They regulate the expression of other gap genes and pair-rule genes Expression patterns of two gap genes  Pair-rule genes Pair-rule genes Mutants lack every other segment (either odd or even segments depending on the gene) Expressed in 7 segments (either even or odd) ~ or 7 ‘stripes’ Encode transcription factors Their expression is regulated by gap genes and egg-polarity genes They regulate the expression of other pair- rule genes and segment- polarity genes Expression patterns How is this interesting gene expression pattern regu of two pair-rule and how can we learn about it? genes  Using molecular genetic approaches to determine the regulatory relationship of Example: two Gene gene expressed ‘A’ is specifically products in the two dark stripes (stripe X and stripe X Y). Y Making mutant flies to determine Wildtype which factors regulate embryo expression of gene A Gap-gene Examine gene ‘A’ ‘Z’ deficient expression mutant in mutant flies embryo Gap gene ‘Z’ product Gap gene ‘Z’ product normally activates the normally represses the expression of gene A in expression of gene A in stripe X stripe Y  Expression of pair-rule gene Even-skipped (Eve) stripe 2 You will learn more about the regulation of eve (even-skipped) expression in active learning section 10. Figure 8-14 ECB5  Pair-rule genes Pair-rule genes Mutants lack every other segment (either odd or even segments depending on the gene) Expressed in 7 segments (either even or odd) Encode transcription factors Their expression is regulated by gap genes and egg-polarity genes They regulate the expression of other pair- rule genes and segment- polarity genes Expression patterns of two pair-rule genes  Segment polarity genes Segment polarity genes Mutations show polarity defect in every segment Their expression is regulated by pair-rule genes Expressed in 14 stripes They regulate the expression of other segment polarity genes Control the polarity and boundary of the segments  Segment-polarity genes Engrailed, a segment polarity gene, is expressed in the posterior portion of each segment and maintained Segment polarity genes throughout development They maintain their expression through development Encode transcription factors and signaling pathway 10-hour embryo components  A signaling loop maintains expression of segment-polarity genes one segment Engrailed, Wingless and Hedgehog, are segment polarity genes expressed at posterior end of each segment in neighboring cells Engrailed = transcription factor; Wingless and Hedgehog are secreted proteins and paracrine signaling molecules MUTUALLY REINFORCING SIGNALS between Wingless expressing cells and Hedgehog expressing cells maintains the narrow stripes of expression (cell memory) throughout development gure 21-21 and 21-22 MBC6  When are these genes expressed? cellular syncytial blastoderm blastoderm 13 somatic divisions cells prior to cellularizat ion (~6000 egg-polarity, gap and pair-rule genes segment-polarity genes cells) expressed in syncytial blastoderm expressed in cellular blastoderm encode transcription regulators encode signaling molecules and transiently expressed transcription regulators expressed throughout fly development Figure 21-15 MCB6  A hierarchy of gene regulatory interactions patterns the A- P axis egg-polarity Egg-polarity genes genes Nanos organize the A-P axis of the early embryo Gap genes Kruppel etc gap genes pair-rule genes segment-polarity Pair-rule genes genes control Ftz,etc segmentation along the A-P axis Segment- polarity genes etc Figure 21-20 MBC6 The gap, pair-rule and segment polarity genes define number, spacing, size and polarity of the segments. How do the segments know and remember their identity (i.e. what they should become)?  Hox gene expression is specified by egg polarity & segmentation genes egg-polarity genes organize the A-P axis of the early embryo gap genes pair-rule genes segment-polarity genes control segmentation along the A-P axis Hox genes give identity to each segment Figure 21-20 MCB6 The Hox genes provide identity to each segment Normal Antennapedia Ubx (Ultrabithorax)  Expression pattern and function of Hox proteins Ed Lewis won the Noble prize for discovery of Hox genes Hox proteins are transcription regulators that bind to DNA through their homeodomains act as master regulators by controlling the expression of multiple genes  Hox gene expression is maintained in adults Ed Lewis won the Noble prize for discovery of Hox genes Hox gene expression must be maintained throughout development This occurs via epigenetic inheritance of histone modifications & recruitment of chromatin remodeling complexes The Hox genes are present and play a role in anterior-posterior patterning in all bilaterally symmetric animals, including vertebrates The discovery of the conservation of Hox genes was the first molecular evidence that development of all animals was based on common  Similar expression patterns of Hox genes in flies and mammals Paralogs Gene duplication Redundant Example 1 : Hox10 Figure 21-32 MBC6  Hox10 expression in the lumbar region confers and maintains the identity of this body segment in mice thoracic Hox10 is normally expressed in * the lumbar * region (between the two yellow asterisks) Hox 10 ectopic expression in the thoracic Hox 10 represses thoracic character Figure 21-33 MBC6 region represses thoracic character in the lumbar region  Similar expression patterns of Hox genes in flies and mammals Example 2 : Hox 10, 11, 13 Figure 21-32 MBC6  Hox genes regulate limb development in vertebrates ’S’ region mutant Sr eg mutant ’Z’ region io n affected affected Z re gi Hox n o n o gi re A Wellik and Capecchi. 2003 Science 301: 363–367 control HOXD-13 mutation mutant Hox10Hox11 Hox13 ExpressedExpressed Expressed Fromental-Ramain et al. in S region in Z regionin A region 1996 Development 122: 2997–3011. Wellik and Capecchi. 2003 Science 301: 363–367 Next lecture: early development in the frog