Chapters 19 - Genetic Analysis of Development:
Development
Development refers to interaction of then genome with the cytoplasm
and external environment to produce a programmed sequence of
typically irreversible events.
Differentiation
Differentiation refers to the formation of cell types, tissues, and organs
through specific gene regulation. A single cell with one genotype
produces a variety of specialized tissues and organs.
Development and differentiation can be studied at many levels:
1.
Morphology
2.
Biochemistry
3.
Activation-repression patterns
Model systems for the study of development and differentiation and why
they are studied:
•
Drosophila melanogaster (fruit fly)
•
•
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Caenorhabditis elegans (nematode)
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•
•
•
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Genome is small (97 Mb)
# and lineage of body cells are known
Genetic crosses and selfings are easy
Body is transparent
Brachydanio rerio (zebrafish)
•
•
•
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Long tradition of study.
Lots of mutations affecting development identified.
Embryos are transparent.
Large #s of fish can be bred.
Screening techniques are well-developed.
Arabidopsis thaliana (plant)
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Small, easy to cross and analyze large numbers of progeny.
Many developmental mutations identified.
Fruit Fly (Drosophila melanogaster)
Nematode
Caenorhabditis elegans
Fig. 19.2
Showing development from
the two-cell stage to the
adult.
Fig. 19.4, Zebrafish (Brachydanio rerio)
Fig. 19.3, Arabidopsis thaliana
Genetic regulation of development in Drosophila:
Fig. 19.18,
Developmental stages of Drosophila
(10-12 days)
Egg

Larva (3 instars)

Pupa

Adult

Embryonic development in Drosophila:
•
Development begins with fertilization.
•
Prior to fertilization, molecular gradients exist within the eggs.
Polar cytoplasm occurs at the posterior end---example of maternal
effect.
•
2 nuclei fuse after fertilization to form a zygote.
•
9 mitotic divisions occur without cell division, and after 7 divisions,
some nuclei migrate to the polar cytoplasm (posterior) creating
germ-line precursors.
•
Other nuclei migrate to the cell surface and form blastoderm
precursor.
•
4 more mitotic divisions occur and all nuclei are separated by cell
membranes.
Fig. 19.19, Embryonic development in Drosophila.
Subsequent development depends on two processes:
1.
Anterior-posterior and dorsalventral molecular gradients exist in
the egg---mRNAs and proteins
placed in egg by mother confer
maternal effect.
2.
Formation of (1) parasegments
and (2)embryonic segments, which
give rise to (3) adult segments.
Fig. 19.20,
Adult segmentation reflect
Embryo segmentation
Three major classes of genes control development and differentation
*Mutations identified by presence lethal or abnormal structures during
development.
1.
Maternal effect genes
2.
Segmentation genes
3.
Homeotic genes
1. Maternal effect genes
Expressed by the mother during egg production; they control polarity of
the egg and the thus embryo.
bicoid gene
•
Regulates formation of anterior structures (mutants possess
posterior structures at each end).
•
Gene is transcribed during egg production, and expressed after
fertilization.
nanos gene
•
Regulates abdomen formation (mRNAs collect in posterior of
the egg).
torso gene
•
Transcription and translation occur during egg production.
•
Occurs throughout the eggs, but is only active at the poles.
Fig. 19.24
Distribution of bicoid mRNA and
protein in the egg
A = Anterior
P = Posterior
2. Segmentation genes:
Determine the segments of the embryo and adult, and thus divide the
embryo into regions that correspond to the adult segmentation
patterns.
1.
Gap genes
 Subdivide the embryo along the anterior-posterior axis.

2.
Pair rule genes
 Divide the the embryo into regions, each containing
parasegments.

3.
Mutation results in the deletion of several adjacent
segments.
Mutations cause deletions of the same part of a pattern in
every other segment.
Segment polarity genes
 Determine regions that become segments of larvae and
adults

Mutants possess parts of segments replaced by mirror
images of adjacent half segments.
Fig. 19.25, Functions for segmentation genes defined by mutations.
3. Homeotic genes:
•
Homeotic genes specify the body part to develop at each segment.
•
Adult body parts develop from undifferentiated larval tissues called
imaginal discs.
•
Homeotic mutants develop a different body part at a particular
segment (imaginal disc) than the usual body part.
•
Different homeotic gene groups share similar sequences of ~180 bp
called homeoboxes that code proteins.
•
Homeoboxes regulate development and produce proteins that bind
upstream of the gene units.
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Homeotic gene complexes are abbreviated Hox.
•
Hox genes also specify body plans in vertebrates and plants.
Fig. 19.21, Locations of homologous imaginal discs in larva and adult.
Fig. 19.26
Examples of homeotic
Drosophila mutant with the
bithorax mutation
Fig. 21.27, 2nd edition
Antennapedia and aristapedia mutants
Fig. 19.1, Second set of eyes in place of antennae:
Fig. 19.28,
Organization of bithorax homeotic genes in a 300kb region of the
Drosophila genome.
T = thoracic
A = abdominal
Fig. 19.29
Homologous Hox gene
clusters occur in Drosophila
and the mouse.
How do development biologists study differential expression of genes
during development and differentiation?
Immunofluorescene assays that bind to specific mRNAs and proteins.
How do development biologists study differential expression of genes
during development and differentiation?
Quantitative real-time RT-PCR of cDNA from mRNA transcripts.
How do development biologists study differential expression of genes
during development and differentiation?
Gene knockout using a transformation, or other gene silencing
techniques like RNAi.