Developmental genetics

1. COMSATS INSTITUTE OF INFORMATION TECHNOLOGY


Developmental Genetics


Zulkifal Yousaf






DEVELOPMENTAL GENETICS: -

Every multicellular organism begins life as a unicellular, fertilized egg. This single-
celled zygote undergoes repeated cell divisions, eventually producing millions or trillions
of cells that constitute a complete adult organism. Initially, each cell in the embryo is
totipotent—it has the potential to develop into any cell type. Many cells in plants and
fungi remain totipotent, but animal cells usually become committed to developing into
specific types of cells after just a few early embryonic divisions. This commitment often
comes well before a cell begins to exhibit any characteristics of a particular cell type;
once the cell becomes committed, it cannot reverse its fate and develop into a different
cell type. A cell becomes committed by a process called determination, the mechanism
of which is still unknown. For many years, the work of developmental biologists was
limited to describing the changes that take place in the course of development, because
techniques for probing the intracellular processes behind these changes were unavailable.
But, in recent years, powerful genetic and molecular techniques have had a tremendous

2. influence on the study of development. In a few model systems such as Drosophila, the
molecular mechanisms underlying developmental change are now beginning to be
understood.

The Genetics of Pattern Formation in Drosophila:-
One of the best-studied systems for the genetic control of pattern formation is the
early embryonic development of Drosophila melanogaster. Geneticists have isolated a
large number of mutations in fruit flies that influence all aspects of their development,
and these mutations have been subjected to molecular analysis, providing much
information about how genes control early development

Stages in the development and the genes that control that stages:-
Stages in the early development o in the early development of
fruit flies and Developmental Stage Genes
Establishment of main Egg polarity genes
body axes
Determination of number and Segmentation genes
polarity of body segments
Establishment of identity Homeotic genthe genes that
control
of each segmentage and the genes that control
each stage
The development of the fruit fly :-

An adult fruit fly possesses three basic body parts: head, thorax, and abdomen.The
thorax consists of three segments: the first thoracic segment carries a pair of legs; the
second thoracic segment carries a pair of legs and a pair of wings; and the third thoracic
segment carries a pair of legs and the halteres (rudiments of the second pair of wings
found in most other insects). The abdomen contains nine segments. When a Drosophila
egg has been fertilized, its diploid nucleus immediately divides nine times without
division of the cytoplasm, creating a single, multinucleate cell. These nuclei are scattered
throughout the cytoplasm but later migrate toward the periphery of the embryo and divide
several more times. Next, the cell membrane grows inward and around each nucleus,
creating a layer of approximately 6000 cells at the outer surface of the embryo . Four
nuclei at one end of the embryo develop into pole cells, which eventually give rise to
germ cells. The early embryo then undergoes further development in three distinct stages:

(1) the anterior–posterior axis and the dorsal–ventral axis of the embryo are established
(2) the number and orientation of the body segments are determined
(3) the identity of each individual segment is established
Different sets of genes control each of these three stages

Egg-polarity genes :-

The egg-polarity genes play a crucial role in establishing the two main axes of
development in fruit flies. You can think of these axes as the longitude and latitude of

3. development: any location in the Drosophila embryo can be defined in relation to these
two axes. There are two sets of egg-polarity genes: one set determines the anterior–
posterior axis and the other determines the dorsal–ventral axis. These genes work by
setting up concentration gradients of morphogens within the developing embryo. A
morphogen is a protein whose concentration gradient affects the developmental fate of
the surrounding region. The egg-polarity genes are transcribed into mRNAs during egg
formation in the maternal parent, and these mRNAs become incorporated into the
cytoplasm of the egg. After fertilization, the mRNAs are translated into proteins that play
an important role in determining the anterior–posterior and dorsal–ventral axes of the
embryo.
Because the mRNAs of the polarity genes are produced by the female parent and
influence the phenotype of their offspring, the traits encoded by them are examples of
genetic maternal effects. Egg-polarity genes function by producing proteins that become
asymmetrically distributed in the cytoplasm, giving the egg polarity, or direction. This
asymmetrical distribution may take place in a couple of ways. The mRNA may be
localized to particular regions of the egg cell, leading to an abundance of the protein in
those regions when the mRNA is translated. Alternatively, the mRNA may be randomly
distributed, but the protein that it encodes may become asymmetrically distributed, either
by a transport system that delivers it to particular regions of the cell or by its removal
from particular regions by selective degradation.

Determination of the dorsal–ventral axis:-
The dorsal– ventral axis defines the back (dorsum) and belly (ventrum) of a fly.
At least 12 different genes determine this axis, one of the most important being a gene
called dorsal. The dorsal gene is transcribed and translated in the maternal ovary, and the
resulting mRNA and protein are transferred to the egg during oogenesis. In a newly laid
egg, mRNA and protein encoded by the dorsal gene are uniformly distributed throughout
the cytoplasm but, after the nuclei migrate to the periphery of the embryo Dorsal protein
becomes redistributed. Along one side of the embryo, Dorsal protein remains in the
cytoplasm; this side will become the dorsal surface. Along the other side, Dorsal protein
is taken up into the nuclei; this side will become the ventral surface. At this point, there is
a smooth gradient of increasing nuclear Dorsal concentration from the dorsal to the
ventral side .
The nuclear uptake of Dorsal protein is thought to be governed by a protein called
Cactus, which binds to Dorsal protein and traps it in the cytoplasm. The presence of yet
another protein, called Toll, can alter Dorsal, allowing it to dissociate from Cactus and
move into the nucleus. Together, Cactus and Toll regulate the nuclear distribution of
Dorsal protein, which in turn determines the dorsal–ventral axis of the embryo.
Inside the nucleus, Dorsal protein acts as a transcription factor, binding to regulatory
sites on the DNA and activating or repressing the expression of other genes. High nuclear
concentration of Dorsal protein (as on the ventral side of the embryo) activates a gene
called twist, which causes mesoderm to develop. Low concentrations of Dorsal protein
(as in cells on the dorsal side of the embryo), activates a gene called decapentaplegic,
which specifies dorsal structures. In this way, the ventral and dorsal sides of the embryo
are determined.
0 hours

4. 2 days
3 days
5 days
9 days
5–8 Dorsal protein in the nuclei helps to determine the dorsal–ventral axis
of the Drosophila embryo. (a) Relative concentrations of Dorsal protein in the
cytoplasm and nuclei of cells in
the early Drosophila embryo. (b) Micrograph of a cross section of the embryo
showing the Dorsal protein, darkly stained, in the nuclei along the ventral surface


Determination of the anterior–posterior axis :-
Establishing the anterior–posterior axis of the embryo is a crucial step in early
development.We will consider several genes in this pathway. One important gene is
bicoid, which is first transcribed in the ovary of an adult female during oogenesis. Bicoid
mRNA becomes incorporated into the cytoplasm of the egg and, as it is passes into the
egg, bicoid mRNA becomes anchored to the anterior end of the egg by part of its 3_ end.
This anchoring causes bicoid mRNA to become concentrated at the anterior end (number
of other genes that are active in the ovary are required for proper localization of bicoid
mRNA in the egg.) When the egg has been laid, bicoid mRNA is translated into Bicoid
protein. Because most of the mRNA is at the anterior end of the egg, Bicoid protein is
synthesized there and forms a concentration gradient along the anterior–posterior axis of
the embryo, with a high concentration at the anterior end and a low concentration at
posterior end. This gradient is maintained by the continuous synthesis of Bicoid protein
and its short half-life. The high concentration of Bicoid protein at the anterior end
induces the development of anterior structures such as the head of the fruit fly. Bicoid—
like Dorsal—is a morphogen. It stimulates the development of anterior structures by
binding to regulatory sequences in the DNA and influencing the expression of other
genes. One of the most important of the genes stimulated by Bicoid protein is
hunchback, which is required for the development of the head and thoracic structures of
the fruit fly.
The development of the anterior–posterior axis is also greatly influenced by a gene called
nanos, an egg-polarity gene that acts at the posterior end of the axis. The nanos gene is
transcribed in the adult female, and the resulting mRNA becomes localized at the
posterior end of the egg . After fertilization, nanos mRNA is translated into Nanos
protein, which diffuses slowly toward the anterior end. The Nanos protein gradient is
opposite that of Bicoid protein: Nanos is most concentrated at the posterior end of the

5. embryo and is least concentrated at the anterior end. Nanos protein inhibits the formation
of anterior structures by repressing the translation of hunchback mRNA. The synthesis of
the Hunchback protein is therefore stimulated at the anterior end of the embryo by Bicoid
protein and is repressed at the posterior end by Nanos protein. This combined stimulation
and repression results in a Hunchback protein concentration gradient along the anterior–
posterior axis that, in turn, affects the expression of other genes and helps determine the
anterior and posterior structures.

of the dorsal–ventral axis in
fruit flies
a nd
their actiTn tt anterior–posterior axis in a Drosophila embryo is
determined by concentrations of Bicoid and Nanos proteins


Segmentation genes:-

6. The fruit fly has segmented body plan. When the basic dorsal–ventral and anterior–
posterior axes of the fruit-fly embryo have been established, segmentation genes control
the differentiation of the embryo into individual segments. These genes affect the number
and organization of the segments, and mutations in them usually disrupt whole sets of
segments. The approximately 25 segmentation genes in Drosophila are transcribed after
fertilization; so they don’t exhibit a genetic maternal effect, and their expression is
regulated by the Bicoid and Nanos protein gradients. The segmentation genes fall into
three groups . Gap genes define large sections of the embryo; mutations in these genes
eliminate whole groups of adjacent segments. Mutations in the Krüppel gene, for
example, cause the absence of several adjacent segments. Pair-rule genes define regional
sections of the embryo and affect alternate segments. Mutations in the even-skipped gene
cause the deletion of even-numbered segments, whereas mutations in the fushi tarazu
gene cause the absence of odd-numbered segments. Segment-polarity genes affect the
organization of segments. Mutations in these genes cause part of each segment to be
deleted and replaced by a mirror image of part or all of an adjacent segment. For
example, mutations in the gooseberry gene cause the posterior half of each segment to be
replaced by the anterior half of an adjacent segment. The gap genes, pair-rule genes, and
segment-polarity genes act sequentially, affecting progressively smaller regions of the
embryo. First, the egg-polarity genes activate or repress the gap genes, which divide the
embryo into broad regions. The gap genes, in turn, regulate the pair-rule genes, which
affect the development of pairs of segments. Finally, the pairrule genes influence the
segment-polarity genes, which guide the development of individual segments.

Homeotic genes:-
After the segmentation genes have established the number and orientation of the
segments, homeotic genes become active and determine the identity of individual
segments. Eyes normally arise only on the head segment, whereas legs develop only on

7. the thoracic segments. The products of homeotic genes activate other genes that encode
these segment-specific characteristics. Mutations in the homeotic genes cause body parts
to appear in the wrong segments. Homeotic mutations were first identified in 1894, when
William Bateson noticed that floral parts of plants occasionally appeared in the wrong
place: he found, for example, flowers in which stamens grew in the normal place of
petals. In the late 1940s, Edward Lewis began to study homeotic mutations in
Drosophila, which caused bizarre rearrangements of body parts.
Homeotic genes create addresses for the cells of particular segments, telling the cells
where they are within the regions defined by the segmentation genes. When a homeotic
gene is mutated, the address is wrong and cells in the segment develop as though they
were somewhere else in the embryo. Homeotic genes are expressed after fertilization and
are activated by specific concentrations of the proteins produced by the gap, pair-rule,
and segment-polarity genes. The homeotic genes encode regulatory proteins that
bind to DNA; each gene contains a subset of nucleotides, called a homeobox, that are
similar in all homeotic genes.
The homeobox consists of 180 nucleotides and encodes 60 amino acids that serve as
a DNA-binding domain; this domain is related to the helix-turn-helix motif. Homeoboxes
are also present in segmentation genes and other genes that play a role in spatial
development. There are two major clusters of homeotic genes in Drosophila. One cluster,
the Antennapedia complex, affects the development of the adult fly’s head and anterior
thoracic segments. The other cluster consists of the bithorax complex and includes genes
that influence the adult fly’s posterior thoracic and abdominal segments. Together, the
bithorax and Antennapedia genes are termed the homeotic complex (HOM-C). In
Drosophila, the bithorax complex contains three genes, and the Antennapedia complex
has five; they are all located on the same chromosome. In addition to these eight genes,
HOM-C contains many sequences that regulate the homeotic genes.
Remarkably, the order of the genes in the HOM-C is the same as the order in
which the genes are expressed along the anterior–posterior axis of the body. The genes
that are expressed in the more anterior segments are found at the one end of the complex,
whereas those expressed in the more posterior end of the embryo are found at the other
end of complex. The reason for this correlation is unknown.

Homeobox Genes in Other Organisms:-
After homeotic genes in Drosophila had been isolated and cloned, molecular
geneticists set out to determine if similar genes exist in other animals; probes
complementary to the homeobox of Drosophila genes were used to search for
homologous genes that might play a role in the development of other animals. The search
was hugely successful: homeobox-containing (Hox) genes have been found in all
animals studied so far, including nematodes, beetles, urchins, frogs, birds, and mammals.
They have even been discovered in fungi and plants, indicating that Hox genes arose
early in the evolution of eukaryotes.
In vertebrates, there are four clusters of Hox genes, each of which contains from 9 to 11
genes. Interestingly, the Hox genes of other organisms exhibit the same relation between
order on the chromosome and order of their expression along the anterior–posterior axis
of the embryo as that of Drosophila. Mammalian Hox genes, like those in Drosophila,

8. encode transcription factors that help determine the identity of body regions along an
anterior– posterior axis.

The Control of Development:-
Development is a complex process consisting of numerous events that must take
place in a highly specific sequence. The results of studies in fruit flies and other
organisms reveal that this process is regulated by a large number of genes. In Drosophila,
the dorsal–ventral axis and the anterior–posterior axis are established by maternal genes
these genes encode mRNAs and proteins that are localized to specific regions within the
egg and cause specific genes to be expressed in different regions of the embryo. The
proteins of these genes then stimulate other genes, which in turn stimulate yet other genes
in a cascade of control. As might be expected, most of the gene products in the cascade
are regulatory proteins, which bind to DNA and activate other genes. In the course of
development, successively smaller regions of the embryo are determined .
In Drosophila, first, the major axes and regions of the embryo are established by egg
polarity genes. Next, patterns within each region are determined by the action of
segmentation genes: the gap genes define large sections; the pair-rule genes define
regional sections of the embryo and affect alternate segments; and the segment-polarity
genes affect individual segments. Finally, the homeotic genes provide each segment with
a unique identity. Initial gradients in proteins and mRNA stimulate localized gene
expression, which produces more finely located gradients that stimulate even more
localized gene expression. Developmental regulation thus becomes more and more
narrowly defined. The processes by which limbs, organs, and tissues form (called
morphogenesis) are less well understood, although this pattern of generalized-to-localized
gene expression is encountered frequently.

Programmed Cell Death in Development:-

Cell death is an integral part of multicellular life. Cells in many tissues have a limited
life span, and they die and are replaced continually by new cells. Cell death shapes many
body parts during development: it is responsible for the disappearance of a tadpole’s tail
during metamorphosis and causes the removal of tissue between the digits to produce the
human hand. Cell death is also used to eliminate dangerous cells that have escaped
normal controls.
Cell death in animals is often initiated by the cell itself in a kind of cellular suicide
termed apoptosis. In this process, a cell’s DNA is degraded, its nucleus and cytoplasm
shrink, and the cell undergoes phagocytosis by other cells without any leakage of its
contents . Cells that are injured, on the other hand, die in a relatively uncontrolled manner
called necrosis. In this process, a cell swells and bursts, spilling its contents over
neighboring cells and eliciting an inflammatory response. Apoptosis is essential to
embryogenesis; most multicellular animals cannot complete development if the process is
inhibited. Surprisingly, most cells are programmed to undergo apoptosis and will survive
only if the internal death program is continually held in check. The process of apoptosis
is highly regulated and depends on numerous signals inside and outside the cell.
Geneticists have identified a number of genes having roles in various stages of the
regulation of apoptosis. Some of these genes encode enzymes called caspases, which

9. cleave other proteins at specific sites (after aspartic acid). Each caspase is synthesized as
a large, inactive precursor that is activated by cleavage, ofterv by another caspase.When
one caspase is activated, it cleaves other procaspases that trigger even more caspase
activity. The resulting cascade of caspase activity eventually cleaves proteins essential to
cell function, such as those supporting the nuclear membrane and cytoskeleton. Caspases
also cleave a protein that normally keeps an enzyme that degrades DNA (DNAse) in an
inactive form. Cleavage of this protein activates DNAse and leads to the breakdown of
cellular DNA, which eventually leads to cell death. Procaspases and other proteins
required for cell death are continuously produced by healthy cells, so the potential for cell
suicide is always present. A number of different signals can trigger apoptosis; for
instance, infection by a virus can activate immune cells to secrete substances onto an
infected cell, causing that cell to undergo apoptosis. This process is believed to be a
defense mechanism designed to prevent the reproduction and spread of viruses. Similarly,
DNA damage can induce apoptosis and thus prevent the replication of mutated
sequences. Damage to mitochondria and the accumulation of a misfolded protein in the
endoplasmic reticulum also stimulate programmed cell death.
Apoptosis in animal development is still poorly understood but is believed to be
controlled through cell–cell signaling. The cell death that causes the disappearance of a
tadpole’s tail, for example, is triggered by thyroxin, a hormone produced by the thyroid
gland that increases in concentration during metamorphosis. The elimination of cells
between developing fingers in humans is thought to result from localized signals from
nearby cells. The symptoms of many diseases and disorders are caused by apoptosis or, in
some cases, its absence. In neurodegenerative diseases such as Parkinson disease and
Alzheimer disease, symptoms are caused by a loss of neurons through apoptosis. In heart
attacks and stroke, some cells die through necrosis, but many others undergo apoptosis.
Cancer is often stimulated by mutations in genes that regulate apoptosis, leading to a
failure of apoptosis that would normally eliminate cancer cells.

Evo-Devo: The Study of Evolution and Development:-
―Ontogeny recapitulates phylogeny‖ is a familiar phrase that was coined in the 1860s
by German zoologist Ernst Haeckel to describe his belief—now considered wrong—that
organisms repeat their evolutionary history during development. According to Haeckel’s
theory, a human embryo passes through fish, amphibian, reptilian, and mammalian stages
before developing human traits.
Although ontogeny does not recapitulate phylogeny, many evolutionary biologists
today are turning to the study of development for a better understanding of the processes
and patterns of evolution. Sometimes called ―evo-devo,‖ the study of evolution through
the analysis of development is revealing that the same genes often shape developmental
pathways in distantly related organisms. In humans for example, the same gene controls
the development of eyes, despite the fact that insect and mammalian eyes are thought to
have evolved independently. Similarly, biologists once thought that segmentation in
vertebrates
and invertebrates was only superficially similar, but we now know that, in both
Drosophila and amphioxus (a marine organism closely related to vertebrates). A gene
called distalless, which creates the legs of a fruit fly, has also been found to also play a
role in the development of crustacean branched appendages. This same gene also

10. stimulates body outgrowths of many other organisms, from polycheate worms to starfish.
Similar genes may be part of a developmental pathway
common to two different species but have quite different effects. For example, a Hox
gene called AbdB helps define the posterior end of a Drosophila embryo; a similar group
of genes in birds divides the wing into three segments.
The theme emerging from these studies is that a small, common set of genes may
underlie many basic developmental processes in many different organisms. Evo-devo is
proving that development can reveal much about the process of evolutions.

REFERENCES:-
Genetics a conceptual approach by Benjamin Pierce.