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Summary NWI-BB084B - Molecular Principles of Development
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Molecular Principles of Development Abstract
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Molecular Principles of developmentBasic Concepts
Drosophila development
From fertilized egg to adult fly only takes 9 days. After fertilization, the zygote nucleus
undergoes a series of rapid mitotic divisions, one about every 9 minutes. It takes 3 hours until
gastrulation starts.
o Syncytium, blastoderm
Drospohila egg is elongated , already had 2 sides (already has polarity). One side: Sperm
entry through micropyle (anterior). On the other side, the pole cells are located this is were
the germ line (gametes) will form. At one point the syncytium will cellularize and become a
blastoderm.
Syncytium: Early on development the nuclei divide but the cells don’t that is called a
syncytium. Bag of cytoplasm containing multiple nuclei. Nuclei division without
cytokinesis(without actual cell division, that would separate the nuclei with a cellular
membrane). The nuclei that are in the center will move to the periphery of the cytoplasm and
then form cellular membranes around them. The syncytium allows diffusion of proteins
Book definition: a cell with many nuclei in a common cytoplasm.
Blastoderm: in the syncytical blastoderm the nuclei have migrated to the periphery of
cytoplasm but have not yet formed cellular membranes. In the cellular blastoderm these
nuclei have formed cell membranes around them.
,Book definition: a post-cleavage embryo composed of a solid layer of cell rather than a
spherical blastula, as found in early chick and zebrafish.
→ Role of RNA transport, regulation of translation and protein diffusion
→ Maternal, gap, pair-rule, segmentation and selector genes
Left: Some RNA are localized in the oocyte (drosophila) and when they get translated they
form a local source and the proteins diffuse. Bicoid inhibit the translation of caudal protein
mRNA.
Right: (upper panel) after fertilization, maternal gene products laid down in the egg, such as
bicoid mRNA (red) are translated. When translated, the bicoid protein diffuses and its
concentration along the main axis provides positional information that activates the zygotic
genes. (lower panel) the four main classes of zygotic genes acting along the antero-posterior
axis are the gap genes, the pair- rule genes, the segmentation genes and the selector or
homeotic, genes. The gap genes define regional differences, which result in the expression of
a periodic pattern of gene activity by pair-rule genes, which define the parasegments and
foreshadow segmentation. The segmentation genes elaborate patterning within the
parasegments, and thus the future segments, and the selector genes determine segment
identity ( e.g whether a segment is thoracic of abdominal , and which specific segment it is).
Maternal genes: genes that are expressed in the oocyte and were already present in the
oocyte before fertilization. In drosophila embryo a lot of maternal genes (bicoid (anterior)
and caudal) are expressed in gradients. (examples: bicoid, nanos and caudel)
When the oocyte starts to express its own genes (half from mother and half from father), it
starts to respond to the activity of the maternal proteins.
,Gap genes: usually there is a gap in their expression pattern across the embryo.(examples:
hunchback)
Book definition: any number of zygotic genes coding for transcription factors expressed in
early Drosophila development that subdivide the embryo in to regions along the antero-
posterior axis.
The activity of both maternal and gap genes produces a stripe pattern in drosophila embryo.
This pattern looks like the segments that are seen later in life.
Pair-rule genes: any number of genes in Drosophila that are involved in delimiting
parasegments. They are expressed in transverse stripes in the blastoderm, each pair-rule gene
being expressed in alternate parasegments. (examples : even-skipped and Fushi tarazu)
Segmentation genes: any number of genes in Drosophila involved in patterning the
parasegments and segments. (Examples: engrailed and gooseberry)
Selector gene: a gene whose expression determines the behavior or properties of a group of
cells, and whose continued expression is required to maintain that behavior. (hox genes)
• Forward genetics
Start with mutations, look what they are all doing, investigate the most interesting ones by
phenotype. Random mutagenesis, screen for phenotypes (Different from reverse genetics:
Make mutation in gene of interest, then analyze phenotype)
Forward genetics (book) type of genetic analysis in which a mutant organism is first
identified by its unusual phenotype and then genetic experiments are done to discover which
gene is responsible for the mutant phenotype
Reverse genetics (book) an approach to genetic analysis that starts with the nucleotide
sequence of a gene or amino acid sequence of a protein and then uses that information to
determine the gene’s function
• Drosophila versus vertebrate life cycle
Micropyles: in Drosophila and zebrafish, not Xenopus and mammals
Germline: set apart earl, pole cells in Drosophila and germ plasm in zebrafish
Maternal axes and symmetry (cf. lectures germ layers)
Drosophila: Anterior-posterior (A-P), Dorsal-ventral (D-V)
Zebrafish, Xenopus: Animal-vegetal (radial symmetry)
Mammals: No polarity (point-symmetry)
Syncytium and cleavages
Drosophila: Nuclear divisions, syncytium
Zebrafish: Meroblastic divisions, yolk syncytial layer
Xenopus, mouse: Holoblastic divisions
, Early embryonic cell division time
Drosophila: 9 minutes (genome 140 MBp)
Zebrafish: 15 minutes (genome 1,700 MBp)
Xenopus: 25 minutes (genome 1,560 MBp)
Mouse: 15-20 hours (genome 2,720 MBp)
• Blastula, gastrula, neurula
Blastula: early stage in the development of some embryos (e.g. amphibians, sea urchin),
which is the outcome of cleavage. It is a hollow ball of cells, composed of an epithelial layer
of cells enclosing a fluid-filled cavity – the blastocoel.
Gastrula: the stage in animal development at which prospective endodermal an mesodermal
cells of the blastula or blastoderm move inside the embryo
Neurula: the stage of vertebrate embryonic development at the end of gastrulation when the
neural tube is forming.
• Patterning, body axes (A-P, D-V)
How does a cell know where it is, where it going or what cell it is becoming → patterning.
The process of establishing positional information at the molecular level among similar cells.
Patterning is not the same as differential gene expression.
Pattern formation initially involves laying down the overall body plan – defining the main
body axes of the embryo from head (anterior) to tail (posterior), and from back (dorsal) to
underside (ventral).
Patterning: Process which establishes differential gene expression (among
otherwise similar cells) that is directly related to position within the embryo
Establishes body axes
Dorsal-Ventral (D-V) (back - underside)
Anterior-Posterior (A-P) (head - tail)
Medial-lateral / left-right (L-R)
Starts with polarity / symmetry-breaking
Asymmetric cell divisions
Molecular gradients
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