1. CHAPTER 19 DEVELOPMENTAL GENETICS Brenda Leady, University of Toledo
Prepared by
Brenda Leady, University of Toledo

2. Development
Refers to a series of changes in the state of the cell, tissue, organ, or organism
Underlying process that gives rise to the structure and function of living organisms
Developmental genetics aimed at understanding how gene expression controls this process

3. Sperm and egg unite to produce a zygote
That diploid cell divides and develops into the embryo
Cells divide and begin to arrange themselves
Each cell becomes determined – destined to become a particular cell type
Commitment occurs before differentiation – cell’s function and morphology have permanently changed into a highly specialized cell type

4. Genome is a set of genes that constitute the program of development
Unicellular organisms – controls structure and function of the single cell
Multicellular – controls cellular features and the arrangement of cells

5. Model organisms Fruit fly Drosophila melanogaster
Advanced techniques for generating and analyzing mutants
Large enough for easy study but small enough to determine where genes are expressed
Nematode worm Caenorhabditis elegans
Simplicity – only about a thousand somatic cells
Pattern of cell division and fate of each cell known

6. House mouse Mus musculus Zebrafish Brachydanio rerio
Thale cress Arabidopsis thaliana
Wild mustard family
Short generation time, small genome

7. Pattern formation
Coordination of events leading to the formation of a body with a particular pattern
Formation of an adult body with 3 axes
Dorsoventral, anteroposterior, and right-left
May also be segmented
Plant bodies are formed along a root-shoot axis in a radial pattern

9. Positional information
Each cell in the body must become the appropriate cell type based on its relative position
Each cell receives positional information that tells it where to go and what to become
Cell may respond by
Cell division, cell migration, cell differentiation or cell death (apoptosis)

12. 2 main mechanisms used to communicate positional information
Morphogens
Cell adhesion

13. Morphogens Give positional information and promote cellular changes
Act in a concentration dependent manner with a critical threshold concentration
Distributed asymmetrically
In the oocyte or egg precursor
In the embryo by secretion and transport
Induction – cells govern fate of neighboring cells

15. Cell adhesion Each cell makes its own cell adhesion molecules (CAMs)
Positioning of a cell within a multicellular organism is strongly influenced by the combination of contacts it makes with other cells and with the extracellular matrix

17. Hierarchy of transcription factors
Four general phases for body formation
Organize body along major axes
Organize into smaller regions (organs, legs)
Cells organize to produce body parts
Cells themselves change morphologies and become differentiated
Differential gene regulation – certain genes expressed at specific phase of development in a particular cell type
Parallel between phases and expression of specific transcription factors

19. Animal development Drosophila model
Oocyte establishes pattern for adult
Elongated cell with positional information
After fertilization, zygote develops into blastoderm
Series of nuclear divisions without cytoplasmic division (produces many free nuclei) synctial blastoderm
Individual cells are created after nuclei line up along cell membrane (cellular blastoderm)

21. Gastrulation involves cells migrating to the interior
3 cell layers formed- ectoderm, mesoderm and endoderm
Segmented body plan develops
Head, thorax and abdomen
Larva – free living
Pupa – undergoes metamorphosis
Adult
Egg to adult in 10 days

23. Phase 1 Pattern development
First phase is establishment of body axes
Morphogens are distributed prior to fertilization
Bicoid, example morphogen
Mutation results in larva with 2 posterior ends
Nurse cells are located near anterior end of oocyte
Bicoid gene transcribed in nurse cells and mRNA transported into anterior end of oocyte
Maternal effect
Transcription factor that activates particular genes at specific times
Asymmetrical distribution means activated only in certain regions

25. Phase 2 Segments Normal Drosophila embryo divided into 15 segments
3 head, 3 thoracic and 9 abdominal
Each will give rise to unique morphological features in adult

26. Bithorax gene complex
Normal – wings on 2nd thoracic segment and 2 halteres on 3rd thoracic segment
Mutant – 3rd segment has wings so 2 sets of wings and no halteres

27. 3 classes of segmentation genes
Gap genes
Mutation – several adjacent segments are missing
Pair-rule genes
Mutation – alternating segments or parts of segments deleted
Segment-polarity genes
Mutation – portions of segments to be missing either anterior or posterior region and adjacent regions to mirror each other

29. To create a segment in phase 2, a group of genes acts sequentially to govern the fate of a given body region
Maternal effect genes, which promote phase 1 pattern development, activate gap genes
Seen as broad bands of gap gene expression in the embryo
Gap genes and maternal effect genes then activate the pair-rule genes in alternating stripes in the embryo
Once the pair-rule genes are activated, their gene products then regulate the segment-polarity genes
Expression of a segment-polarity gene corresponds to portions of segments in the adult fly

31. Phase 3 Segment characteristics
Each segment begins to develop its own unique characteristics
Cell fate – ultimate morphological features that a cell or group of cells will adopt
Mutations in homeotic genes alter cell fate
Bithorax is an example of a homeotic mutation
Order of homeotic genes along chromosome corresponds to their expression along the anteroposterior axis of body
Colinearity rule

33. Role of homeotic genes to determine identity of particular segments

34. Homeobox – coding sequence of homeotic genes contains 180-bp sequence
Homeotic genes encode homeotic proteins that function as transcription factors
Activate transcription of specific genes that promote developmental changes
Homeobox – coding sequence of homeotic genes contains 180-bp sequence
Encodes homeodomain for DNA binding

35. A Homologous Group of Homeotic Genes Is Found in All Animals
Identify vertebrate genes that are homologous to those that control development in simpler organisms such as Drosophila
Hybridization techniques
Homologous genes are evolutionarily derived from the same ancestral gene and have similar DNA sequences
Hox genes in mice
Follow colinearity rule
Key role in patterning anteroposterior axis

37. Phase 4 Cell differentiation
Emphasis shifts to cell differentiation
Studied in mammalian cell culture lines
Differential gene expression underlies cell differentiation
Stem cell characteristics
Capacity to divide
Daughter cells can differentiate into 1 or more cell types

39. Stem cell categories Totipotent Pluripotent
Ultimate stem cell is fertilized egg
Can produce all adult cell types
Pluripotent
Embryonic stem cells (ES cells)
Embryonic germ cells (EG cells)
Can differentiate into almost any cell but a single cell has lost the ability to produce an entire individual

41. Multipotent Unipotent Can differentiate far fewer types of cells
Hematopoietic stem cells (HScs)
Unipotent
Daughter cells become only one cell type
Stem cells in testis produce only sperm

43. Robert Davis, Harold Weintraub, and Andrew Lasser Identified Genes Encoding Transcription Factors That Promote Muscle Cell Differentiation
What causes stem cells to differentiate into a particular cell type?
Certain proteins function as “master transcription factors”
Initial experimental strategy to identify genes expressed only in differentiating muscle cells
Narrowed down to 3 genes
Would any of these 3 genes cause nonmuscle cells to differentiate into muscle cells?

45. Belongs to myogenic bHLH genes
MyoD was the only one to cause fibroblasts to differentiate into muscle cells
Belongs to myogenic bHLH genes
Found in all vertebrates and activated during skeletal muscle development
Features promoting muscle cell differentiation
Basic domain binds specifically to an enhancer DNA sequence that is adjacent to genes that are expressed only in muscle cells
Protein contains an activation domain that stimulates the ability of RNA polymerase to initiate transcription
Interacts with other cellular proteins
Id – prevents muscle differentiation too soon
E – activates gene expression

47. Plant development 2 key features of complex plant morphology
Root-shoot axis
Radial pattern

48. Differs from animal development
No cell migration
No morphogens
An entirely new individual can be regenerated from somatic cells (totipotent)
Similarities to animal development
Use differential gene expression
Use of transcription factors

49. Arabidopsis model for development
After fertilization, the first cellular division is asymmetrical and produces a smaller apical cell and a larger basal cell
Apical cell – gives rise to most of embryo and shoot
Basal cell – root and suspensor cell for seed formation
Heart stage – about 100 cells
Basic organization established
Root meristem – gives rise only to root
Shoot meristem – all aerial parts of plant – stem, leaves, flowers
2 cotyledons to store nutrients

51. Seedling 3 main regions Shoot meristem organizing center
Apical region – leaves and flowers
Central region – stem
Basal region – roots
Shoot meristem organizing center
Central zone – maintains undifferentiated stem cells
Peripheral zone – dividing cells that will differentiate

52. Apical, central and basal regions express different sets of genes
Apical-basal patterning genes
Defects can cause dramatic effects

53. Plant homeotic genes First known homeotic genes discovered in plants
Normal Arabidopsis flower composed of 4 concentric whorls
Sepals – outer whorl, protects bud
Petals
Stamens – make pollen (male gametophyte)
Carpel – produces female gametophyte

54. ABC model for flower development
3 gene classes A, B, C and E
Whorl 1 – gene A product = sepals
Whorl 2 – gene A, B and E products = petals
Whorl 3 – gene B, C and E products = stamens
Whorl 4 – gene C and E products = carpel
Leaf structure is default pathway
If A,B,C and E are defective, leaves result