1. BIO706 Embryology Lecture 2: Developmental Genetics
Faculty of Science, School of Sciences, Natabua Campus Lautoka
BIO706 Embryology Lecture 2: Developmental Genetics

2. Multicellular Organisms
Develop from a single cell – fertilized egg
Egg cell contains a single nucleus
Organism develop from that cell through cell division and cell differentiation
Cells become differentiated by expressing different genes
Cells can be differentiated to a different degree

3. Types of cells according to the level of differentiation
Totipotent cells are undifferentiated and have the potential to develop into all types of cells (eg. egg, embryonic stem cells in animals, meristematic cells and callus in plants)
Pluripotent cells can differentiate into multiple (but not all) types of cells (eg. somatic stem cells in animals)

4. Terminally differentiated cells can no longer assume any other cell type (most animal somatic cells)
Some cells have the capacity to dedifferentiate and become totipotent (eg. some cells in plant cuttings)

5. Cell fate
During development cells divide and differentiate according to a particular pattern
Depending on their location in the embryo cells are destined to become certain type of tissue – this program is called cell fate
E.g. cells from the shoot meristem in plants will develop into leaves, stems and flowers, while the cells from the root meristem develop into roots

6. Development is progressive
Fates of cells are determined at different times,
the specificity of cell fate generally increases as the embryo becomes more developed (can be demonstrated by transplantation experiments)

7. Cell fate control
Cell fate is controlled by the interaction between gene expression and signals from neighbouring cells.
Cell fate is achieved through cascade of differential gene expression. i.e. by different genes being turned on in sequence.

8. Genetic control of development can be determined by studying developmental mutants

9. Developmental mutants
Mutants are generated and screened by observing the phenotype at a particular stage of development.
Mutants defective in their morphology are characterized.

10. Mutants carrying different gene defects can be crossed to determine the genetic pathway involved in a particular developmental stage.
Once the mutants with the desired phenotype have been found the disrupted gene can be mapped and sequenced, and it’s expression pattern determined.

11. Developmental mutants in Arabidopsis
a - wild type, b – j - mutants

12. Frog embryo mutants

13. Embryonic Developmental stages in mouse
Mammals: mouse

14. Developmental pathways follow similar steps in different animal taxa
Model organisms can be used to infer principles for developmental pathways in general
First steps in early development were described using Drosophila mutants

15. Key developmental stages in insects and vertebrates
Fertilization: fusion of sperm and egg
Cleavage: mitotic cleavage divisions without cells growing
Differentiation into 3 germ layers (future endoderm, mesoderm, ectoderm)
Establishment of basic body plan
Organogenesis –differentiation of specialised cells
Growth

16. Embryonic developmental stages in Drosophila

17. cells in each embryo segment in the fly become further differentiated and eventually are used for particular components of the adult fly anatomy

18. Early embryo development

19. Early cell divisions do not involve creation of new plasma membrane i
Early cell divisions do not involve creation of new plasma membrane i.e. the very early embryo consists of a large multi-nucleated cell called the syncytial blastoderm.
This allows cytoplasmic gene products to diffuse freely throughout the entire embryo thus, morphogen gradients play a key role in the first embryonic processes
Nuclei migrate to the periphery of the blastoderm
Later they are compartmentalized by plasma membrane creating a truly multi-cellular embryo

20. Stages in the early development of fly embryo
establishment of the antero-posterior and dorso-ventral body axes
larger-scale patterning along the antero-posterior and dorso-ventral axes
segmentation

21. anterio-posterior (head to tail) axis
Establishment of orientation of body axis is controlled by maternal effects genes deposited into the oocyte before fertilisation.
anterio-posterior (head to tail) axis
dorso – ventral (stomach to back) axis
future head
oocyte development is control by the genotype of the mother

22. Localisation of maternal effects gene mRNA will generate a protein gradient for that gene
Protein gradient for one gene will guide gradients of other proteins generating a body plan of increasing complexity

23. maternal effects genes establish body axis orientation
Gap proteins further subdivide embryo
Pair rule proteins form segments (define positions of 14 periodically spaced parasegments)
Segement polarity proteins fix parasegments and determine boundaries between segments
Homeotic proteins determine segment identity

25. More about early embryo development in fly
The maternal genes expressed during oogenesis obviously play a key role in early patterning. The relationship of the ovarian nurse cells and the follicle cells to the developing ooctye is shown at left.

26. It is the oviarian nurse cells that export bicoid, nanos, hunchback, caudel and torso mRNA into the developing oocyte.
Several other genes of the anterior group and posterior group are involved in localization of bicoid mRNA to the anterior pole and the transport and localization of nanos mRNA to the posterior pole.

27. Maternal hunchback, caudel and torso mRNAs are not localized, but found uniformly in the mature egg.When the egg is fertilized the localized bicoid and nanos mRNA are translated into protein that diffuse towards the opposite poles and set up the morphogen gradients that will regulate the expression of hunchback and caudel.

28. This in turn will lead to the regulated expression of the zygotic GAP, Pair Rule, Segment Polarity and Homeotic genes along the anterior-posterior axis of the embryo and specify the Acron, Head, Thorax, Abdomen, and Telson pattern.

29. Homeobox genes were first identified in Drosophila and subsequently found in other animals
They are transcription factors (regulate other genes) that determine segment identity
eg. antennaepedia, the segment that normally produces antennae is changed into one that produces legs.
wild type antennaepedia

30. Segment identity mutant
Bi-thorax fly wild type fly wingless
Wing building mutant
Segment identity mutant
Have all the right proteins to build structures, but it’s building them in the wrong place
Can tell where to build wings, but don’t have the genetic information to do it.

32. Homeobox genes
Transcription factors that contain a conserved sequence region called homeobox domain (made up of 180 base pairs of DNA)
Regulate transcription of other developmental genes
Arranged on chromosome in the same sequence as the structures that they code for on body axes
More complex in species with more complex development

33. each color-coded box represents one homeobox gene.
boxes are color coded according to the structure that the gene controls

34. Similar genes, in the same order, control the development of the front and back part of the bodies of flies and mice.
These homeobox-containing genes lie on a single chromosome in the fly and on four separate chromosomes in mammals.
In the figure the genes are color coded to match the parts of the body in which they are expressed.  

35. While flies have just one cluster of homeobox-containing genes that lies on a single chromosome, mammals have four similar clusters lying on four separate chromosomes.

36. These clusters arose by duplication, in the course of evolution
These clusters arose by duplication, in the course of evolution. In each cluster, the genes located at one end direct the development of the anterior part of the body, while the genes at the other end control the formation of the posterior part.
The genes in the four clusters work together, "talking" to each other to produce a more complex creature than the fly

37. Plant development

38. After early embryo formation, plants continue to develop structures from meristematic tissues, located at the shoot and root tips.
Because of the rigid cell walls there is no cell movement during plant development.

39. root meristem

41. Shoot apical meristem can develop into leafs or become floral meristem

42. Plant development
Because plant embryos are hidden in seed, they are difficult to study.
We are only beginning to understand early embryogenesis in plants.
Organogenesis, formation of organs (eg. flower, leaf, root) in plants happens from the meristems – available for detailed study and transplant experiments.
Molecular basis of flower development has been described.

43. Development of plant organs is determined by the expression of
MADS box genes in the floral meristem
Cells of the floral meristem form concentric circles or whorls.
Flower organs are positioned according to the localised expression of MADS box genes A, B and C.
Organ Genes expressed
sepal A
petal A + B
stamen B + C
carpel C

44. Flower development mutants in Petunia

46. MADS – box genes -transcription factors
- contain a conserved MADS domain
- present in both monocots and eudicots
CORN (monocot)
Arabidopsis (eudicot)

47. Questions are welcome