Sequential Regulation of Gene Expression During Cellular Differentiation Determination commits a cell to its final fate Determination precedes differentiation Cell differentiation is marked by the production of tissue-specific proteins

Myoblasts produce muscle-specific proteins and form skeletal muscle cells MyoD is one of several “master regulatory genes” that produce proteins that commit the cell to becoming skeletal muscle The MyoD protein is a transcription factor that binds to enhancers of various target genes

Master regulatory gene myoD Other muscle-specific genes Fig. 18-16-1 Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Figure 18.16 Determination and differentiation of muscle cells

MyoD protein (transcription Myoblast factor) (determined) Nucleus Fig. 18-16-2 Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Figure 18.16 Determination and differentiation of muscle cells

(fully differentiated cell) Fig. 18-16-3 Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Figure 18.16 Determination and differentiation of muscle cells mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins MyoD Another transcription factor Part of a muscle fiber (fully differentiated cell)

Pattern Formation: Setting Up the Body Plan Pattern formation is the development of a spatial organization of tissues and organs In animals, pattern formation begins with the establishment of the major axes Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells

Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans

The Life Cycle of Drosophila In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization After fertilization, the embryo develops into a segmented larva with three larval stages

Figure 18.17 Key developmental events in the life cycle of Drosophila Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Left Ventral (a) Adult Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell 2 Unfertilized egg Egg shell Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Figure 18.17 Key developmental events in the life cycle of Drosophila Embryonic development 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva

Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Fig. 18-17a Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Figure 18.17 Key developmental events in the life cycle of Drosophila Left Ventral (a) Adult

(b) Development from egg to larva Fig. 18-17b Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell 2 Unfertilized egg Egg shell Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Embryonic development Figure 18.17 Key developmental events in the life cycle of Drosophila 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva

Genetic Analysis of Early Development: Scientific Inquiry Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus won a Nobel 1995 Prize for decoding pattern formation in Drosophila Lewis demonstrated that genes direct the developmental process

Eye Leg Antenna Wild type Mutant Fig. 18-18 Figure 18.18 Abnormal pattern formation in Drosophila Wild type Mutant

Eye Antenna Wild type Fig. 18-18a Figure 18.18 Abnormal pattern formation in Drosophila Antenna Wild type

Fig. 18-18b Figure 18.18 Abnormal pattern formation in Drosophila Leg Mutant

Nüsslein-Volhard and Wieschaus studied segment formation They created mutants, conducted breeding experiments, and looked for corresponding genes Breeding experiments were complicated by embryonic lethals, embryos with lethal mutations They found 120 genes essential for normal segmentation

Animation: Development of Head-Tail Axis in Fruit Flies Axis Establishment Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila These maternal effect genes are also called egg-polarity genes because they control orientation of the egg and consequently the fly Animation: Development of Head-Tail Axis in Fruit Flies

Bicoid: A Morphogen Determining Head Structures One maternal effect gene, the bicoid gene, affects the front half of the body An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends

Fig. 18-19 EXPERIMENT Tail Head T1 T2 A8 T3 A7 A1 A2 A3 A4 A5 A6 Wild-type larva Tail Tail A8 A8 A7 A6 A7 Mutant larva (bicoid) RESULTS Figure 18.19 Is Bicoid a morphogen that determines the anterior end of a fruit fly? Fertilization, translation of bicoid mRNA 100 µm Anterior end Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo

EXPERIMENT Tail Head Wild-type larva Tail Tail Mutant larva (bicoid) Fig. 18-19a EXPERIMENT Tail Head A8 T1 T2 T3 A7 A1 A6 A2 A3 A4 A5 Wild-type larva Tail Tail Figure 18.19 Is Bicoid a morphogen that determines the anterior end of a fruit fly? A8 A8 A7 A7 A6 Mutant larva (bicoid)

RESULTS Anterior end Bicoid mRNA in mature unfertilized egg Fig. 18-19b RESULTS Fertilization, translation of bicoid mRNA 100 µm Anterior end Figure 18.19 Is Bicoid a morphogen that determines the anterior end of a fruit fly? Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo

CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Fig. 18-19c CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Figure 18.19 Is Bicoid a morphogen that determines the anterior end of a fruit fly? Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo

This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features

The bicoid research is important for three reasons: – It identified a specific protein required for some early steps in pattern formation – It increased understanding of the mother’s role in embryo development – It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo