The document discusses the role of segmentation genes in Drosophila development, explaining their classification into gap genes, pair-rule genes, and segment polarity genes. It outlines how these genes control body plan segmentation through regulation of transcription factors, forming distinct patterns in early embryonic stages. The interactions between cells, mediated by segment polarity genes, determine cell fates within parasegments.

1. By:
Raana
Karimpour
SEGMENTATION
GENES
1
IN GOD WE
TRUST
Raana Karimpour
Bachelor,
Department of
cellular and
molecular
biology, Faculty
of advanced
science and
technology,
Tehran Medical
Sciences,
Islamic Azad
University,
Tehran, Iran

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Cell fate
commitment
Specification capable of
being reversed
Determination irreversible
The transition from specification to determination in
Drosophila is mediated by Segmentation genes
Raana Karimpour

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The term 'segmentation gene' is a classification
given to a broad class of genes that are further
subdivided into three smaller classes of genes.
Within the segmentation gene group, there
are gap genes, pair-rule genes and segment
polarity genes. They control development in this
order.
Gap genes:
subdivide the
embryo.
Mutation of
these gene
create gaps in
the fly’s
segmentations
Pair-rule genes,
subdivides the
embryo into a
series of stripes
and sets the
boundaries of the
parasegments.
Segment polarity
genes establish the
antero-posterior axis of
each segments
Raana Karimpour

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5.  Expression patterns in early embryos are not delineated by
segmental boundaries, but by parasegments: fundamental
units of embryonic gene expression, Cells of adjacent
compartments do not mix.
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SEGMENTS & PARASEGMENTS
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6.  The gap genes are expressed early in development in broad regions.
The products from these genes generally define the different parts
of the embryo that will later develop into segments. While maternal -
effect genes have already defined the anterior and posterior end of
the embryo, these gap genes define the location of the head, thorax
and abdominal region.
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GAP GENES
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 The gap genes encode transcription factors that regulate
the expression of the pair-rule genes.
 The pair-rule genes encode transcription factors that
regulate the expression of the segment polarity genes.
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10.  The transcription patterns of these genes are striking in that
they divide the embryo into the areas that are the precursors
of the segmental body plan
 One vertical band of nuclei expresses a pair-rule gene, then
another band of nuclei does not express it, and then another
band of nuclei expresses it again. The result is a “zebra stripe”
pattern.
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PAIRED-RULE GENE
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13.  Once cells form, interactions take place between the cells
which are mediated by the segment polarity genes
 Through this cell-to-cell signaling, cell fates are established
within each parasegment.
 One row of cells in each parasegment is permitted to express
the Hedgehog protein, while the other expresses the Wingless
protein.
 Activation of Engrailed gene- cells express Hedgehog protein
 In turn, engrailed gene is activated when cells have high levels
of the Even-skipped or Fushi tarazu transcription factors.
 Engrailed transcription marks the anterior boundary of each
parasegments.
 The wingless gene is activated-presence of Sloppy-paired
protein. Marks posterior boundary.
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SEGMENT POLARITY GENES
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Very briefly …
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21.  Developmental Biology by Scott Gilbert translated by Hossein
Baharvand
 Schroeder MD, Pearce M, Fak J, Fan HQ, Unnerstall U, et al.
(2004) Transcriptional control in the segmentation gene network
of Drosophila. PLoS Biol 2(9): e271.
 Role of segment polarity genes in the definition and maintenance
of cell states in the Drosophila embryo A. MARTINEZ ARIAS1'2, N.
E. BAKER1* and P. W. INGHAM3
 NOSSLEIN-VOLHARD, C. & WlESCHAUS, E. (1980).Mutations
affecting segment number and polarity in Drosophila. Nature,
Lond. 287, 795-801.
 NUSSLEIN-VOLHARD, C , WlESCHAUS, E. & JURGENS, G. (1982).
Segmentierung bei Drosophila- eine genetische Analyse. Verh.
Dtsch. Zool. Ges. 1982, 91-104.
 LAWRENCE, P. A. (1981). The cellular basis of segmentation in
insects. Cell 26, 3-10.
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REFERENCES
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THANK YOU
Raana Karimpour