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Genetics of Axis Specification
in Drosophila: Segmentation
Genes
Gilbert - Chapter 9
Today’s Goals
• Describe the use of a Genetic screen to identify
functionality of individual gene products
• Identify Eric Weischaus and Christianne NussleinVolhard
• Identify key molecular players in establishing
polarity and segments in various types of embryos
• Define the term “positional information”
• Describe how evolutionarily conserved genes play
similar roles in development of a wide variety of
species
• Define transcription factor
The Segmentation Genes
• Now we have a mechanism for setting up the
Anterior and Posterior ends of the embryo
• BUT- there are many specific segments along
the axis of the embryo
• How can segmentation be achieved?
– We must also examine the formation of
parasegments
– Parasegments make up the segments of the
embryo - embryonic building blocks
Parasegments as defined by
denticle pattern
Segmentation Genes
• Three classes of Segmentation genes
– Hierarchical arrangement based on what they
control
– Now - all are zygotic genes (not maternal-effect)
• Gap genes
– Divide embryo into broad regions (several
parasegments)
• Pair-rule genes
– Divide regions defined by gap genes into smaller
regions (fewer parasegment)
• Segment Polarity genes
– Control patterns within each parasegment
Gap Genes
• Ex. Hunchback, Krüppel, knirps
• Mutations affect broad areas of the body plan
• These genes are activated or repressed by
the maternal effect genes (ex. Bicoid)
• Expressed in broad domains along A-P axis
• Expression patterns correspond very well
with segments that are lost when mutations
occur
Gap Genes
• Example of mutation
– Krüppel
– Expressed in segments 4-6
– Mutant for Krüppel is missing these
segments and those immediately adjacent
to them
Gap Genes
• Expression in the embryo changes over the
course of development
– Begins as low levels across whole embryo
– Becomes consolidated into discrete regions within
the embryo
– Proteins become localized to specific areas along
axis
– Initially controlled by concentration gradients of
maternal effect genes
– Expression patterns eventually defined by
interactions with one another
Maternal Effect Genes
Influence Gap Genes
• High levels of Bicoid and Hunchback
– Induce expression of Giant near the anterior end
– Inhibit expression of knirps
• Slightly lower levels of Hunchback
– Krüppel expression induced
– More complex - several pathways that lead to
activation of genes in multiple places
SO…
• The net result of all of these interactions
– Precise expression patterns of overlapping
mRNAs in specific sub-regions of the embryo
– Overlap is only 8 cells or so
• GAP genes set up sub-regions of the embryo
- not individual segments, but smaller
sections than just Anterior-Posterior
Pair-rule Genes
• Begin to define segments
• Ex. Fushi tarazu (ftz), hairy, even-skipped
(eve)
• Mutation in pair-rule gene deletes every other
parasegment
– Ex. Ftz
• At least eight pair-rule genes act to divide the
embryo into a series of stripes that overlap
Pair-rule genes
• Expressed in stripes along the embryo
that represent the future segmentation
of the body
• Gene is transcribed in a vertical stripe of
cells, not transcribed in next vertical
stripe - continues to alternate along axis
• Divide the embryo into 15 subunits
– PHENOMENALLY specific control of
expression of these genes!
• Within these regions, different
combinations of gene expression
specify both the type of segment that
will form and the proper order of
segments.
Pair-rule Gene Regulation
• How is the transcription of these genes
regulated to appear in alternating stripes of
cells?
– Appears to be under control of Gap genes
– In places where the edges of bands of gap gene
expression overlap (ex. Hunchback with Kruppel)
expression of a pair rule gene is activated
– In places where other bands of gap genes
overlap, a pair rule gene gets turned off
Pair-rule genes: Summary
• Tight control of expression by
enhancer/repressor interactions in the
promoter regions results in 7 stripes of each
gene
• This divides the embryo into the 14
parasegments along the axis
• Each gene is expressed in different
parasegments
• Each parasegment has a different combo of
pair-rule genes