The maternal effect, means a gene product deposited by the mother in the oocyte is sufficient for that oocyte to develop into a normal adult even if it is
mutant for that particular gene. Such gene products function during early embryonic
development. Generally, the maternal products get degraded late in adulthood; therefore, the
phenotype is seen only two generations later: the grandchildren generation. So far, we saw
the anterior-posterior and dorsal-ventral axis formation in the oocyte. Now, we will look at
The initial pattern established by Bicoid and Oskar's asymmetric localization in the oocyte is
insufficient to make a complex organism. So the asymmetry has to be enlarged by involving
various molecules, and that asymmetry has to become localized as well. Only then the
embryo can make multiple tissue types that can form the right organs at the right places. So
how that happens is best understood in the Drosophila embryogenesis, which is why we are
focusing on it. So the first asymmetry starts with the maternal factors that are products of the
maternal germline. They are already deposited either as protein or mRNA into the new
embryo. So, these asymmetrically localized factors determine the next set of genes that need
to be activated. These maternal mRNAs are asymmetrically localized or asymmetrically
translated, which helps in the initial pattern formation. Some of them are translational
regulators as well as transcriptional activators or inhibitors. Like for example, Bicoid is a
translational inhibitor of Caudal, but at the same time, it is the transcriptional activator of
genes that we have not yet discussed; also, it is required for their asymmetric localization.
Finally, those gene products will turn on the transcription of downstream genes called Gap
genes. They are called Gap genes because their mutations led to large gaps in the embryo that
So today, we will see what the Gap is in terms of the embryo. Gap genes are expressed in a
large pattern, as shown in the slide, and this pattern also repeats. So here in the slide, the
orange pattern repeats. So this larger band-like expression in the embryo does have
overlapping distribution as well. These stripes here are not showing any overlap, but they do
overlap like; for example, this orange color may overlap some part of this purple, and purple
may overlap the blue, or they might function independently. These are important in activating
the next set of genes called the Pair-rule genes. They are called so because they form these
stripes like expression patterns, and they usually affect every other segment in the larva that
develops. So here, segments mean the segmented body pattern. If you take any insect and
look at it, you can see a larger abdomen segment and a thoracic segment where you have the
legs and the wings, and then you have the head where you have the antenna, etc. So
Drosophila makes fourteen segments.
These pair-rule genes affect alternative segments. So their expression pattern forms these
precise stripes, as you see in the slide. So from the gradient pattern, now we have gone to this
precise pattern. So the Pair-rule gene products, the Gap gene products, and some of the
maternal products altogether coordinate to activate the next set of genes called Segment
polarity genes. So these genes specify the expression pattern in each segment.
So by the time these Segment polarity genes get activated, the embryo becomes cellularized.
Now the adjacent cells start signaling and influence each other, leading to the activation of
So today, our goal is to go up to segment polarity genes. Homeotic genes we will see in the
(Refer Slide Time: 07:08)
Let us start with maternal effect genes. We are already familiar with Bicoid that gets to the
anterior in the oocyte, which gets translated and forms the protein product. While these are
happening, the dorso-ventral specification is also simultaneously happening. So the Gurken
that has gone to the dorso-anterior suppresses Pipe in the dorsal follicle cells while the ventral
follicles cells get primed to transduce the signal via the Toll receptor. That signal
transduction is required when Dorsal protein is formed a bit later.
The bicoid mRNA that was localized earlier, now gets expressed, and forms a gradient. I
mentioned that the Gurken goes to the dorso-anterior region and inhibits Pipe synthesis in the
adjacent follicle cells during the oocyte stage. The signal transduction at the ventral, where
Dorsal getting into the ventral nuclei, happens in the embryo stage. So the embryo reaches
the blastoderm stage where the nuclear divisions are over, and the nuclei are located
cortically. So this is happening continuously while that process is happening. Similarly, this
anterior-posterior axis formation is happening too. So it would help if you did not disconnect
them; for convenience, we are ignoring all that discussion that we had so far. That does not
mean that all these events happen separately. The asymmetric microtubule formation and, as
a result, the Bicoid hitching a ride on a motor protein and going to the anterior and the Oskar
going to the opposite side, there already the anterior-posterior patterning is forming, and that
is at the oocyte stage. But what I am saying is that now leads to further enlargement by
including many molecules. Therefore the organism can make more specific structures. The
embryo cannot end up making only two domains like the anterior pole and the posterior pole,
and that alone is not enough; it has to make multiple tissue types and structures. Once the
asymmetry is initiated, that needs to be made into more localized asymmetries.
Now the highest concentration of Bicoid at the anterior-most activates certain Gap genes.
These genes are required for the anterior-most segment development, and there are structures
called Acron that form in the anterior segmented part of the body. So the tail-like portion is
called the telson. So these structure formation requires the highest concentration of Bicoid
and other factors, as we will see later.
The Bicoid will suppress the Caudal expression in the anterior portion, allowing the
Hunchback expression to happen there. So Bicoid is a transcription factor as well. So
Hunchback activation does not require a high concentration of Bicoid. Gap genes like empty
spiracles, orthodenticle are the anterior-most, and their activation requires Bicoid, and then
you need a slightly lower concentration of Bicoid for activating Hunchback.
These are all determined primarily by the promoter region's affinity and these transcription
factors rather than the enhancers. So the bicoid is at the anterior, figure (B) is the actual
example of the cartoon we saw in the previous slide, figure (A).
So this particular embryo in figure (B) shows only the protein localized in the anterior region.
Figure (C) is an example of three different gap gene expression patterns. So you can see the
expression of the Hunchback and the Kruppel, another gap gene.
(Refer Slide Time: 14:35)
In this slide, figure(D) shows the expression pattern of a Pair-rule gene called Fushi tarazu,
which was discovered by a Japanese group. Fushi tarazu means fewer segments. This is a late
Pair-rule gene. Pair-rule genes consist of two groups: the early pair-rule gene and the late
pair-rule gene; together, there are eight of them, and we will see them in detail. Figure (E)
shows the expression pattern of Engrailed, a segment polarity gene. Engrailed is seen only in
the anterior part of the segment. If you can count these lines, you will find fourteen of them.
So we will see how each one of these gene’s patterns is established in the coming slides.
(Refer Slide Time: 15:42)
In 1975, Klaus Sanders did a simple experiment, where he took an early embryo of an insect
and tied a thread in the middle of it to create a ligature. This will prevent the movement of
factors between the anterior and posterior regions. Then he observed that the formation
anterior and the posterior structure but not the middle structures. Later he shifted the ligature
timing and found that lesser and lesser of middle segments were missing. So he proposed that
there must be two decision points, one at the anterior and one at the posterior, and they
probably instruct the rest to form different structures in a gradient fashion. So his
observations also suggested the existence of a morphogenetic gradient.
(Refer Slide Time: 17:35)
He also did an experiment, where he irradiated the anterior part of the embryo with UV,
which will affect the RNA present in the anterior part. So he found that the head structure did
not form in that embryo. Instead, it formed two tail-like structures indicating that the RNA
behind this morphogen gradient, at least from the anterior part, is affected by UV, as a result
that disrupt the morphogen required for the head formation. So this observation led Nusslein
Volhard and her group to ask questions like what these morphogens are? How are they
forming the gradient? How these structures are made, and how those asymmetries are
stabilized? And so on. To answer these questions, they decided to mutagenize and look for
mutants where a particular structure was missing. The answers to those questions are the
summary we just saw: involvement of the maternal effect gene, the Gap gene, the Pair-rule,
and so on.
(Refer Slide Time: 18:43)
Let us get into the details of each of these gene types. So the first one is maternal effect
genes; we learned earlier that Bicoid is present in the anterior, and Nanos is present in the
So nanos mRNA is produced by the nurse cells, and they are deposited into the embryo.
Initially, in the egg chamber, the nurse cells are present in the anterior part. So the nanos
mRNA produced by them diffuses in the cytoplasm; they are not carried like bicoid by the
motor proteins. So in the posterior, Oskar binds and protects the nanos mRNA. Oskar also
helps in removing the translation inhibitors and allows the Nanos translation in the posterior.
Generally, nanos 3'UTR is bound by Smaug and Cup proteins in the cytoplasm, which
prevents its translation. Therefore, Nanos is produced only in the posterior.
Now, this Bicoid and Nanos form this concentration gradient that is shown in the slide. This
gradient exists in the syncytium, where the nuclei are nicely arranged in the cortical region.
Upon cellularization, various genes get activated as a downstream product.
(Refer Slide Time: 21:45)
So this is the same experiment that we saw earlier. This is just that it is having more specific
labels like acron, telson, etc. In the wild-type embryo, the acron, head, thoracic and
abdominal segments are formed. But in the bicoid mutant, you see telson, then the abdomen,
and again the telson indicating that the anterior structures like this acron, head, and thoracic
formation requires Bicoid.
(Refer Slide Time: 22:25)
We are familiar with the experiment shown in this slide, where injecting bicoid mRNA into
various regions of the embryo causes anterior structure formation in that region. Here in the
bicoid mutant, if we inject bicoid mRNA in the anterior end, it forms a normal embryo. If
bicoid mRNA is injected into the middle of the embryo, acron is not formed, but it forms a
head with thorax on both sides, behaving like the morphogen gradient. When injected in the
posterior of a wild-type embryo where Bicoid is already present in the anterior region, the
posterior also forms the head structures. We end up making a double-headed insect.
(Refer Slide Time: 23:47)
So this shows an in-situ hybridization for bicoid mRNA, and then you see the gradient here.
(Refer Slide Time: 23:56)
In this slide, figure (B) shows the Bicoid protein gradient in the early drosophila embryo.
This forms a shallow gradient that people believe is because some RNA is present little away
from the anterior region even they are poised for translation. Therefore, in a very short time,
you get this gradient of the Bicoid formed. Figure(C) shows the concentrations of the Bicoid
protein in wild-type and the mutant
(Refer Slide Time: 24:27)
This is how the surface cuticle looks in wild-type and in the bicoid mutant, which is missing
some of the head structures.
(Refer Slide Time: 24:40)
So the Bicoid gradient in the anterior portion suppresses caudal mRNA, and it activates
hunchback transcription. So here, Bicoid acts as a transcriptional factor to activate zygotic
hunchback expression in the nuclei. I want to point out that Bicoid is a maternal factor, but
the hunchback is both maternal and zygotic. So this zygotic hunchback activates some of the
So the highest concentration of the Bicoid in the anterior activates the button head, empty
spiracle, and orthodenticle. Caudal, which is a transcription factor in the posterior, activates
the posterior gap genes knirps and giant. So this is the summary of the anterior and posterior
set up by the maternal effect product gradients. Now let us see about the Nanos in the
So in the posterior, Nanos binds to the hunchback 3'UTR and along with Pumilio. These
proteins bind to hunchback 3'UTR and suppress Hunchback translation in the posterior. If you
can remember, Hunchback comes as a maternal product, so the maternal product distributed
throughout the cytoplasm needs to be inhibited in the posterior. Therefore it will not activate
the thoracic segment related genes in the posterior.
(Refer Slide Time: 28:37)
So this is the Caudal protein gradient. You see, it is a mirror image of the Bicoid expression;
again, it is in the nuclei. So to remind you what is happening developmentally, these are
syncytium, not cellularized. So there are a lot of nuclei in that common cytoplasm. The only
thing is the nuclei are on the cortex. Here you see the surface; therefore, you readily see the
(Refer Slide Time: 29:20)
In the posterior, nanos mRNA is stabilized by Oskar; otherwise, the Smaug and Cup will
bind to the nanos mRNA and suppress it by removing the poly-A tail; as a result, nanos
mRNA gets degraded. Since nanos mRNA is primarily present in the posterior region, it gets
translated and forms a shallow gradient of protein in the posterior, as you see in the slide. So
this Nanos protein produced here collaborates with Pumilio to bind the hunchback mRNA
and suppress the translation of Hunchback. This will prevent the anterior structures forming
in the posterior and allows abdominal structures to form instead.
(Refer Slide Time: 30:51)
This is a summary of anterior-posterior pattern formation by the maternal effect genes. So
hunchback mRNA and caudal mRNA are distributed throughout the oocyte. But bicoid and
nanos mRNA are restricted to the anterior and posterior region. During the early embryonic
cleavage, Bicoid promotes the translation of Hunchback in the anterior and suppresses
Caudal translation. Similarly, in the posterior, Nanos promotes the translation of Caudal and
suppresses the translation of Hunchback. These regulations result in a shallow gradient of
protein expression. In the anterior, Bicoid translation is promoted by proteins mentioned in
figure(C). In the posterior, Nanos translation is enabled by Oskar. Then the Nanos protein,
along with Pumilio and p55, suppresses hunchback mRNA.
(Refer Slide Time: 32:48)
Now let us understand how these extremities such as acron and telson are formed. So this
happens by RTK signaling. The anterior and posterior follicle cells produce a protein called
torso-like. This protein signals another protein called torso, which further activates the MAP
kinase pathway. Even though the torso and RTK receptor are expressed throughout the
oocyte membrane, this MAPK signaling occurs only in the extremes due to these follicles
cells that produce the ligand torso-like protein.
So this signaling translationally suppresses a protein called Groucho. Therefore Groucho
forms a reciprocal gradient to the torso signaling. Groucho prevents the production of
Huckebein and Tailless proteins, which are required for these extremities to form. In the
absence of Groucho, Huckebein and Tailless are produced in the ends. These proteins are
required for these extremities to form, but the acron-telson asymmetry is dictated by the
Bicoid present in the anterior. If Bicoid is present in the anterior, head structures are formed,
and its absence leads to tail structures' formation.
(Refer Slide Time: 35:44)
So far, we saw the maternal effect proteins forming the anterior-posterior embryonic
patterning. Here patterning means the distribution pattern of the molecules. So now, let us
look at the consequences of these gradient setups. So the cartoon in the slide shows the
expression pattern of one of the gap genes called the Kruppel in each stage. It has a broader
expression pattern in the late embryo. So the kruppel mutant larva does not form certain body
parts; the missing parts correspond to the proteins' expression patterns.
(Refer Slide Time: 37:07)
So this slide show a pair-rule gene called Fushi tarazu. Its expression is seen in the very early syncytial embryo. So you see the expression pattern in the late embryo as well. So it is expressed in the posterior region of one segment and anterior region of the adjacent segment.
This region is called parasegment, which will be discussed in the next class. So in the mutant, the anterior part of one segment and posterior part of the adjacent segment will be fused. So wherever this protein is expressed, that parts will be missing in the larva.
Segment polarity genes, on the other hand, form many stripes. So here, the Engrailed forms fourteen stripes in the posterior of each of these segments. Therefore those parts will be missing in the mutant larva.