Example Of Development - Drosophila Flashcards
Advantages of using drosophila in the lab and Nobel prices
Advantages
• Easy to maintain
• High reproductive rate (life cycle 9-10 days)
• Genome fully sequenced (~140 Mb, ~16,000 genes)
• 60% of genes have human homologs
• Very amenable to genetics
Nobel prizes
• 1933 Morgan “for his discoveries concerning
the role played by the chromosome in heredity”
• 1995 Lewis, Nüsslein-Volhard, and Wieschaus
“for their discoveries concerning the genetic
control of early embryonic development”
• 2017 Hall, Rosbash and Young “for their
discoveries of the molecular mechanisms
controlling the circadian rhythm”
Superficial Meroblastic cleavage
- eight nuclear division (8 mins each) in the central portion of the egg produce 256 nuclei
- during the ninth division cycle - five nuclei reach the posterior surface, cellular is and generate the pole cell - give the gametes of the adult
- during these division cycles the embryo is called a syncytial blastoderm
- the nuclei reach the periphery of the egg during the tenth division cycle
- following division cycles thirteen the cell egg cell membrane field in between the nuclei partitioning each nucleus into an individual cell
- this process generates the cellular blastoderm
Drosophila: gastrulation
• The mesoderm undergoes invagination at the ventral furrow
• The endoderm invaginates at both anterior and posterior locations, fusing to form the gut
• Pole (germ) cells enter the embryo through the posterior midgut invagination
• The embryo divides into head (x3), thorax (x3) and abdominal (x8) segments
Drosophila: larval development
• The ventral side of the larvae has denticle belts, alternating patches of hairs and cuticle
• Larvae feed and grow rapidly, twice replacing the cuticle
• Growth is due to an increase in cell size and polyploidy, not cell division
• The third instar larvae pupates and undergoes metamorphosis
Drosophila: genetics
Morgan pioneered the use of Drosophila as a genetic model system :
• He was the first to test the chromosome theory of inheritance
• He isolated the first mutation, white and using
genetic crosses demonstrated it was linked to the
X-chromosome
• Along with his graduate student Alfred
Sturtevant demonstrated linkage of several
genes and produced the first genetic maps
• Awarded the Nobel prize in 1933 “for his
discoveries concerning the role played by the
chromosomes in heredity
Drosophila: forward genetic screens
Nüsslein-Volhard and Weischaus forward genetic
screen(s):
• Aimed to identify all genes required for
Drosophila embryogenesis in an unbiased
manner
• Screened 26,980 mutagenised lines of flies
• Identified 580 embryonic lethal mutations that
disrupted segmentation
• Complementation analysis identified 139
different genes that could be placed into three
groups - gap genes, pair-rule genes, segment, polarity genes
• Subsequent screens identified maternal effect
genes that also affect embryo patterning
Drosophila: pair-rule genes
- disrupt ever other segment - either odd or even
- are expressed in 7 stripes
- include: even skipped, fushi-Tarazu, hairy, odd paired, odd skipped, paired, runt, sloppy paired
Drosophila: segment polarity genes
• Disrupt either the anterior or posterior
part of each segment
• Are expressed in 14 segment-wide
stripes
• Include: wingless, hedgehog, armadillo, cubitus interruptus, engrailed, fused, gooseberry, invected, pangolin, patched
Drosophila: a genetic hierarchy of segmentation genes
• Maternal genes: mRNA is maternal deposited
and asymmetrically localised. They are transcribed during oogenesis and along with
hunchback (a gap gene) the proteins establish
a morphogen gradient which activates
different gap genes at different concentrations
• Gap genes: activate specific pair-rule genes
based on combinatorial enhancer binding
• Pair-rule genes: are expressed in 7 stripes and
activate segment polarity genes (to specify the
14 segments of the embryo)
• Segment polarity genes: are expressed in 14
stripes and act as morphogens to pattern each
segment along its A-P axis determining the
denticle belts (alternating pattern of hairs and
cuticle)
Drosophila: the screen identified all major conserved components of the Wnt/wingless and hedgehog signalling pathways
• In humans mutations in in the sonic hedgehog pathway are responsible for cyclopia, and polydactyly and has been linked, along with Wnt signalling to a number of different cancers
Drosophila: homeotic genes
- Homeotic genes: pair-rule genes combine with gap genes to activate the expression of the homeotic genes
• Homeotic mutations transform one body segment into another
• There are 8 homeotic genes in Drosophila
• Lewis showed that Ultrabithorax (Ubx), abdominal A (abd-A), and Abdominal-B (Abd-B) were closely linked to each other on chromosome
3 and he called this region the Bithorax Complex.
• Inpsired by Lewis, other workers showed that labial (lab), proboscipedia (pb), Deformed (Dfd),
Sex combs reduced (Scr), and Antennapedia (Antp) were also closely linked on Chromosome 3, but some distance form Ubx. This was called the Antennapedia Complex.
• The segments affected by loss-of- function mutations in these genes and the expression of the genes themselves are colinear with the genes position on the chromosome, with mutations affecting more posterior segments as we move from left to right. In this respect the two complexes behave as one.
• All 8 genes encode transcription
factors
• All homeotic genes contain a highly conserved DNA sequence called the homeobox, which encodes a sequence specific DNA binding domain called the homeodomain
All animals have homeotic (HOX) genes
• HOX genes have been found in most multicellular organisms but not in bacteria. They are usually found in Homeotic complexes (except plants), with similar organization to the
Homeotic Complex of insects
- Most animals have only a single
complex but vertebrates usually have
four
