Developmental Biology of Drosophila

DROSOPHILA
DEVELOPMENT
Arun Kumar Pradhan
ONA
Odisha NET Academy ONA 09337727724

DEVELOPMENT
 An organism arises from a fertilized egg as the
result of three related processes
• Cell division
• Cell differentiation
• Morphogenesis

DIFFERENTIATION
 Cells may initially remain undifferentiated
• Embryonic stem cells
 Cells ultimately
differentiate
• Become specialized in
structure and function

DIFFERENTIATION
 Virtually all cells within a multicellular
organism are genetically identical
 Differences between cells are due to differences
in gene expression
• Different subsets of genes are “on” and “off”
• Different cell types make different proteins

GENE EXPRESSION
 Much of the regulation of gene expression
occurs at the level of transcription
 Transcriptional regulation
of gene expression is
directed by
• Maternal molecules in the
cell’s cytoplasm
• Signals from other cells

PATTERN FORMATION
 The development of a spatial organization in
which an organism’s tissues and organs are all in
their characteristic places
 In animals, it begins in early embryo
• Basic body plan is established
• Major axes are established early
 Studied most extensively in Drosophila
melanogaster

Drosophila melanogaster
 Bilaterally symmetric segmented body
• Head
• Thorax
• Abdomen

 Cytoplasmic determinants are present in the
unfertilized egg
• Provide positional information for placement of axes
prior to fertilization
• Establishes number and orientation of segments
• Ultimately trigger formation of specific structures
within each segment

 Egg develops in mother’s ovary
• Surrounding cells with nutrients, etc.
 Mitosis begins following fertilization
• First ten divisions include no growth, cytokinesis
• Single multinucleate cell results

 Nuclei migrate to periphery of embryo at tenth
division
 Plasma membranes finally partition ~6,000
nuclei into separate cells at the thirteenth
division
• Basic body plan already
determined at this time
 Body axes and segment
boundaries

 Subsequent embryonic events create clearly
visible segments
• Initially look very similar
 Some cells move to new
positions
• Organs form
 Wormlike larva hatches
• Eats, grows, & molts

 Larva eventually forms a
pupa
• Enclosed in a case
 Metamorphosis occurs
• Change from larva to adult fly
 Adult fly emerges from case
• Each segment is anatomically
distinct

 Each segment in the adult fly is anatomically
distinct
• Characteristic appendages

 Gradients of maternal molecules in the early
embryo control axis formation
• Cytoplasmic determinants already present in
unfertilized egg
• Encoded by mother’s maternal effect genes
 a.k.a., “Egg-polarity genes”
 Encode proteins or mRNAs that are placed into the egg
while still in the mother’s ovary

 One group of maternal effect genes establishes
the anterior-posterior axis of the embryo
 Another set of maternal effect genes establishes
the dorsal-ventral axis
 Female flies possessing mutations in maternal
effect genes appear phenotypically normal, but
produce offspring with mutant phenotypes

 Bicoid is an egg-polarity gene
• “Two-tailed”
 Mothers defective in bicoid produce embryos
lacking the front half of their body
• Duplicate posterior structure at both ends

 Bicoid gene product is concentrated at anterior
end of fly embryo
• Gradient of gene product
• Essential for setting
up anterior end of fly
 Gradients of other proteins
determine the posterior end
and the dorsal-ventral axis

The bicoid gradient regulates the
expression of segmentation genes in a
concentration-dependent fashion.
There are peak levels of the
Bicoid protein in anterior
regions, intermediate levels in
certral regions, and low levels
in posterior regions.
Orthodenticle is activated only
by high levels of the Bicoid
gradient in the head;
hunchback is activated by both
high and intermediate levels in
the head and thorax.

In central
regions of the
embryo, the
orthodenticle
gene is off. In
contrast,
hunchback is
on. These is
control by the
levels of
Bicoid protein.Odisha NET Academy ONA 09337727724

Hunchback expression is
also regulated at the level
of translation
The translation is blocked in posterior regions by
RNA-binding protein called Nanos. After the
Nanos mRNA is translated, the protein diffuses from
posterior regions to form a gradient. The translation of
the maternal hunchback mRNA is arrested by the
Nanos protein. The Nanos gradient thereby leads to the
formation of a reciprocal Hunchback gradient in
anterior regions.

Hunchback and gap
proteins produce segmentation stripes of
gene expression.

 The bicoid protein and the products of other egg-
polarity genes are
transcription factors
• Regulate the expression
of some of the embryo’s
genes

 Segmentation genes
• Genes of embryo
• Expression regulated by products of egg-polarity
genes
• Direct the actual formation of segments after the
embryo’s major axes are defined

 Three sets of segmentation genes are activated
sequentially
• Gap genes
• Pair-rule genes
• Segment polarity genes
 The activation of these sets of genes defines the
animal’s body plan
• Each sequential set regulates increasingly fine details

 Gap genes
• Map out basic subdivisions along the embryo’s
anterior-posterior axis
• Mutations cause “gaps” in the animal’s segmentation

 Pair-rule genes
• Define pattern in terms of pairs of segments
• Mutations result in embryos having half the normal
number of segments

 Segment polarity genes
• Set the anterior-posterior
axis of each segment
• Mutations produce
segments where part of the
segment mirrors another
part of the same segment

 The products of many of the segmentation genes
are transcription factors
• Directly activate the next set of genes
 Summary
• Products of the egg-polarity genes regulate the
regional expression of the gap genes
• Gap genes control the localized expression of the
pair-rule genes
• Pair rule genes activate specific segment polarity
genes in different parts of each segment
• Segment polarity genes activate homeotic genes

HOMEOTIC GENES
 Master regulatory genes
 Specify the types of appendages and other
structures that each segment will form
 Mutations produce flies with structures in
incorrect places

HOMEOTIC GENES
 Encode transcription factors
 Control the expression of genes responsible for
specific anatomical structures
• e.g., “Antennae go here”
• e.g., “Legs go here”

HOMEOTIC GENES
 Homeotic genes are
master genes that
regulate the
expression of
numerous other genes
• Some of the regulated
genes are regulatory
themselves

Drosophila DEVELOPMENT
Hierarchy of Gene Activity
 Maternal genes
 Segmentation genes of embryo
• Gap genes
• Pair-rule genes
• Segment polarity genes
 Homeotic genes of the embryo
 Other genes of the embryo

HOMEOTIC GENES
 Homeotic genes of Drosophila all possess
homologous segments
• 180-nucleotide sequence = homeobox
• Encodes 60-amino-acid homeodomain
 Homologous sequences have been found in
many other animals
• e.g., Insects, nematodes, mollusks, fish, frogs, birds,
humans, etc.
• Related genes are even found in yeast, etc.
• Hox genes Odisha NET Academy ONA 09337727724

HOMEOTIC GENES
 Vertebrate genes
homologous to the
homeotic genes of
Drosophila have
maintained their
chromosomal
arrangement

Thank you

Developmental Biology of Drosophila

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