genetic control of development

• Gene expression in a developing fruit-fly embryo
• Organizers

  Responsible for vertebrate body plan development
• Morphogens


  Thought to be responsible for vertebrate body plan development
• Genetic dissection of Drosophila mutants identified the genes responsible for body plan development
• Genetic approach to uncover processes


  • No assumptions made regarding nature of molecules
  • Limited quantity of organizing agents not an issue
  • Can uncover processes for which there is no biochemical assay
• Organizers in animal embryos

  Morphogens were postulated to organize the body over distances via concentration gradients
• Organizers

  • Spemann organizer: dorsal blastopore lip can induce 2nd axis along the D/V axis when transplanted to ventral region
  • Zone of Polarizing Activity (ZPA) could induce extra digits along A/P axis when transplanted from P to A
• Homeotic mutants of Drosophila melanogaster

  "Right structure, wrong place!"
• Mutations affecting the body plan of flies were instrumental for getting to the genes responsible for organizing the body axes (A/P, D/V)
• Drosophila


  Model organism
• Larval denticle bands (referred to as larval exoskeleton in book)


  Key to identifying mutants that disrupt body plan
• Screens carried out by Nusslein-Volhard and Weichaus to identify patterning mutants; both maternal effect and zygotically acting genes
• Mutations in patterning genes


  Can affect the analogous regions in embryos and adult; thus, the same genes pattern both body plans
• Homeotic transformation


  One body part is replaced by another
• Homeotic mutants not just in flies, also in vertebrates
• Hox genes


  Regulate the identity of body parts
• Genes are in the same order on the chromosome as body regions affected in mutants; same is true for expression
• Hox genes are required for identity, not formation
• More posterior Hox genes


  Actually repress more anterior Hox genes, unless "mutant"
• Digoxygenin


  An antigen that is coupled to the RNA nucleotides, can be recognized by antibody coupled to alkaline phosphatase
• Hox genes are expressed in spatially restricted domains
• Hox genes are expressed in structures affected by Hox-gene mutations
• Homeodomain


  Encoded by the homeobox, a helix-turn-helix motif
• Southern blotting used to show that homeotic genes were a part of a family in flies; also shown that homeotic genes were present in vertebrates, including humans
• The order of Hox genes parallels the order of body parts in which they are expressed, just like in flies
• Four clusters of Hox genes, with 9-11 genes in each cluster
• Hox genes regulate the identity of serially repeated structures in Vertebrates; lumbar (gained) vs sacral (lost) vertebrae
• Redundancy is a big issue with vertebrates, sometimes it is more informative to study flies
• Maternal-effect genes


  Genes that were identified (A/P axis): Gap genes, Pair-rule genes, Segment-polarity genes, Homeotic genes
• Genes required for specifying D/V cell fates

  Including mesoderm, neurectoderm, ectoderm
• Most maternal-effect gene products put into egg
• Bicoid mutants


  Missing the anterior region
• Bicoid is one of the elusive "morphogenetic gradients"
• Segmentation-gene mutants


  Missing parts of segments
• Domains of gene expression progressively more refined going from gaps to homeotics
• Gap genes expressed in regions many cells wide, pair-rule 3-4 cells wide, seg-pol 1-2 cells wide
• As for the A/P axis, the D/V axis is specified with "stripes", which in turn specifies tissue layers such as mesoderm, neurectoderm, etc
• Dorsal is expressed in a nuclear-localized gradient
• Most master regulators are transcription factors; they regulate batteries of target genes to carry out their functions
• Gap genes are activated by specific maternally provided proteins