Drosophila development csir net part 2 | Drosophila development cleavage and gastrulation

Added: 2026-05-24

[00:00:00]So that's a big problem. That's a big change because if the dorsal and ventral identity is misplaced, it's not done properly, then attachment of internal organs and different systems will be wrong.

[00:00:12]>> Sorry, I didn't understand the question I heard.

[00:00:17]>> Okay. So now what's about the dorsal signal? So we have talked about the vententral identity and the vententral identity is done by dorsal protein and the concentration and morphogenic gradient of dorsal protein but the dorsal site is also determined as I mentioned earlier by a signaling molecule and a receptor that is girkin and torpedo. Torpedo is the receptor and girkin is the signaling molecule.

[00:00:46]Okay. Now you may ask a question that why do you need girkin and torpedo? Why do you need a separate system? Because dorsal protein itself uh establishes the ventral dorsal identity. That's true.

[00:00:56]That's fine. But still we have a one more extra step to mark dorsal sight separately. Dorsal protein marks ventral side like ventral site in a proper way.

[00:01:08]Along with that dorsal protein also helps into maintain that. But beyond that idea some extra accessory proteins needed for dorsal identity separately that will be done. Final confirmation of the dorsal identity will be done by girkin and torpedo. How it's done? So again this is torpedo. Torpedo is a receptor EGFR epidermal growth factor receptor EGFR. So there will be expression of torpedo in the dorsal follicle cell. Basically uh we know that vententral side of the follical cell there is the movement of dorsal protein while in the dorsal side of the follical cell there is a movement of torpedo. So torpedo moves there and cause dorso vententral patterning. Okay. So we found out that there's torpedo mucan they cause ventralized embryos. So no dorsal side is generally formed without torpedo. So torpedo has some role. What is exactly the role is not clear yet but torpedo has some role in determining the dorsal side of the epide.

[00:02:12]So what happen is that gurkin is the signaling molecule expression in the germline cells oite or ner cell has follicle to adopt a dorsal identity. So what happen is you can see clearly that this is the terminal follicle cells and they have receptors in the red colored receptors like this right. So these are the receptors. These are known as torpedo or girkin receptor. Torpedo itself is known as girkin receptor. And this uh green dot signaling molecules are girkin proteins. This girkin proteins will move towards the torpedo or girkin receptor binds to it and that signals uh that part of the embryo to be dorsal.

[00:02:55]Okay. And if we if we take this garin and torpedo out then dorsal part of the embry is not formed.

[00:03:04]Generally it found a ventralized embryo.

[00:03:06]probably there's some role related to the how dorsal protein uh changes and alters the expression profile of DPC and twist but it's not properly understood.

[00:03:18]So till this point we've been talking about body axis determination and there are uh three different axis we talked about anterior posterior axis and this terminal axis right and dorso vententral axis. So what we are going to talk here is basically we're going to discuss about the different morphogens which are actually helping to mark a part of the embryo to be anterior, posterior, dorsal or ventral or terminal. For example, bocoid vcoid uh helps in in the middle state. Bcoid concentration is only restricted to one side. Again it's restricted to anterior side but right after fertilization it has a concentration gradient maximum to the anterior slowly decreasing throughout the embryo to the opposite side of the anterior. Then what what else we have? We have a posterior nanos is present in the posterior side concentrated before fertilization. After fertilization nanos gradient will be formed. Okay. And once the gradient is formed then obviously this nanosporting gradient will help expression of lips joint or other RNAs and the products to finally make that part posterior.

[00:04:30]And third one is torso where basically the torso proteins are transl transferred to the terminal side the active torso. Basically the torso proteins torso like proteins are present before fertilization. Right after fertilization the torso proteins are activated and spread across the terminal side. They helps in determining the terminal side of the embryo. So these are the different morphogens we talked about. Anterior decoded by pcoid, posterior by nanos and terminal by torso. Okay.

[00:05:02]So till this point we've been talking about the body axis determination.

[00:05:06]That's one thing to understand about body access determination. The second thing that you need to know is the gastrolation. I think that the most important event of any development is the gastrolation where different layers of tissue generate ectodormodorm endoderm. From these different layers of tissue different uh different body organ tissue structures are generated. So the process of gastrolation in drosophila is known as germ band extension. What is germ band extension?

[00:05:34]Germ band extension is simply uh what we have some pole cells. Remember these are the pole cells here. The pole cells pushes the posterior end over this is the posterior end over the dorsal side over the dorsal side. Pole cells pushes the posterior end over dorsal side and that originates segments that originate segments. The segments that we can see from outside right. The segments are visible. Now try to understand one simple concept here. When we talk about a segment which is visible segment visible segmentation of the body and it's kind of true you know we we see this sort of segmentation after this even earlier it's normal but now there are segments you if if I draw properly it will look the segmentation will look something like this right this is how the segmentation looks like now this segmentation is visible from outside but inside of which there are some other imaginary segmentation ation known as paras segmentation. Okay. So there are this two type of segmentation.

[00:06:36]Segmentation which is visible from outside and another one is the paras segments formation of parasg. Now what are these parasgments? The paras segments are basically uh fundamental units of gene expression in drosophila development. So basically there is no physical boundary. So you one may imagine that maybe as per this picture uh this is one segment inside. If if you cross if you see inside they may be separated with boundaries but actually that's not the case inside they are not separated with any physical boundaries no separation with physical boundaries what happens here inside there is a separation based on gene expression profile okay so gene expression patterns are different so let's say this is an area uh this is where x and y is being expressed in this segment only there's no physical difference but till this portion x and Y is expressing the second place only Y is expressing the third place Z YX is expressing the fourth place Z and X is expressing. So based on the expression profile expression pattern we can separate the total length of the embryo. We call them parasgments and there are 14 such paras. Three parase segments form mouth, three parase segments form thorax and eight parase segments form tur abdominal part of the uh adult ply. Remember this always and in this particular picture you can clearly see the movement of jump band and extension and you can see the formation of three different layers.

[00:08:09]Epidermis with blue color, gut yellow color, misoderm with the red color.

[00:08:14]Okay. And the nervous system again with blue color.

[00:08:19]Epidermis with the gray color. So now this epidermis is very very important right to understand here. This blue is nervous system. So that is kind of ectoderm. The red is kind of messoderm and the yellow is kind of endoderm here which forms the gut. And the rest of this gray colored region is epidermis.

[00:08:34]And this epidermis is an important uh part because this epidermis skin they fold. The folding of epidermis will be done and this folding will create what is known as imaginal imaginal discs.

[00:08:50]forms the imaginal discs. From those imagininal discs, different organ structures will be generated like eye, antenna, okay, legs and genitals and all these different structures will be generated.

[00:09:04]So what are these parasgments? We've been talking about the parase segments, right? So parasg fundamental units of embryionic gene expression. So this is a picture showing the segments from outside you can clearly see the segments right there are different body segments and this is maybe this is the segment for the mouth this is for the thorax and rest of them for the abdomen but inside of which they are not the same way how the gene expression is going on the segments can be saying that this is ma mx lb these three are making the head t1 t2 t3 making thorax and A1 to A8 par segments making the abdomen part. Now they have compartments and individual compartments. Individual compartments if you look at this body compartments in individual compartment there's a posterior anterior axis anterior posterior axis in this case posterior in the left side and the right side. They they put it that way whatever way you can to this is how the expression is done. And you see the expression pattern of different genes are different in these paras. It's very difficult to understand with this picture. So I I'll show you one picture to make you understand this process with much clear understanding.

[00:10:29]Where is it? Yeah. So here you can clearly see how this paras work and how the different gene expression works in this paras. You see this is what we have anterior site. This is a posterior site and what we clearly see is the expression of different gene products.

[00:10:45]Remember we started with a gene product that is maternal effect genes. Maternal effect genes the gene uh the product of which the mRNA is already present in the egg. After fertilization it's translated to protein and then after that rest of them are segmentation genes. So first kind of segmentation gene is a gap gene.

[00:11:05]First segmentation gene is gap gene.

[00:11:07]Second segmentation gene is pair rule gene. Third segmentation gene is segment polarity gene and fourth segmentation gene is homeotic gene. Now here you can see that the expression profile for them try to understand. So primarily what we have we have bigoid and hunchback gradient to the anterior side nanos and cordal to the posterior side. Right?

[00:11:27]Then we have gap gene expression. What are the gap genes? Tailless giant nips Right? So and then in the pair rule genes what we have we have even skipped this blue one is even skipped the name of the gene the red one is known as fussu. So what we can clearly see if I break it like this then this part of the segment this part of the segment received what received becoid it receives hunchback high concentration of them it receives taillessness.

[00:11:58]The second segment if you look it receives again medium bocoid a moderate amount of becoid hunchback in high amount then joint. So you can see that in the first paras it may have becoid, hunchback and tailless. In second parase segment we have hunchback, bocoid and giant. Giant is extra a new one in this par segment and the expression of becoid is medium concentration but earlier it was very high concentration. So based on this idea in each individual parase segment the expression profile will be different and based on that we have this parase segment and formation of this par segment. So let's move on and talk about uh this idea now. So once the parase segments are prepared the different gene interactions are done and based on the gene interaction the fragmentation is created and kept. Remember one thing there are body segments visible from outside and there are this paras which is not visible which is kind of imaginary inside but this segment and parase segment are not in sync. That means the area of the segment that is visible from outside and the parase segment is not in sync with each other.

[00:13:08]So parase segment produces imaginal discs. I told you what is imaginal disc.

[00:13:13]It is nothing but a thickening of the epidermis of an insect larva which on pupation develops into a particular organ of an adult insect that is the imaginal disc. And there are total 10 types. Nine pairs of imaginal disc and one fused pair of imaginal disc. Okay.

[00:13:34]So imaginal discs are already predetermined for the developmental fate of epidermis. So epidermis particular region of the epidermis because the body axis has already been determined right they know what is anterior posterior. So in anterior side head is there head means eyes will be there antenna will be there. If it's thorax in the middle, legs will be there, wings will be there, wings in the dorsal side, legs in the vententral side. So those things are determined. So nine pairs, 18 total, nine pairs and one fused pair. Fused pair contains antenna and eye and the rest of the pairs they have lip mouth parts leg pairs for leg wing one pair, wood trim being another pair, generators another pair. All these different imaginal discs are created from what?

[00:14:20]They are created from the epidermis.

[00:14:23]Okay. And this is very very important.

[00:14:26]They create they are created from the epidermis. And there are two classes of genes that we generally talk about. Till this point we've been talking about all the different types of genes. And we know that the body access is determined particularly when we talk about the drosophila genetics particularly when we talk about the different gene products and how they interact with themselves.

[00:14:43]There's a hierarchy of gene expression.

[00:14:44]Remember I told you at the beginning and this starts with the maternal effect genes then moves to the segmentation gene. So there are two major classes maternal effect and zygotic or segmentation you can say zygotic gene or segmentation gene. So maternal aector genes or meg egg polarity they are also known as egg polarity gene because these genes are already marking the polarity for eggs because these gene products are already present in the egg. Okay anterior side posterior side and they are already localized localized to the anterior nanos localized to the posterior even before fertilization. That's why they are called as maternal aector genes because they are present in the egg. The source is mother's source. And the second thing they are known as egg polarity gene because they determine the polarity of the egg as well. Then comes the zygotic genes also known as segmentation genes. The zygotic genes have several four different subclasses.

[00:15:46]Gap genes, pair genes, segment polarity genes and homeotic genes. They function in that order. So gap genes regulate the activity of pad rule. Peru regulates the activity of segment polarity. Segment polarity regulates the activity of homeotic genes. And this gap gene act as a transcription factor for paradule.

[00:16:05]Peru act as a transcription factor for segmentation. Segment polarity gene and segment polarity gene act as a transcription factor for homeotic genes.

[00:16:14]Now the segment polarity gene and so this parallel genes are of two different types early and a late per gene. So early per gene will be activated by gap gene first then early per gene will activate late perule gene and late per gene will activate segment polarity. Early perule can also activate segment polarity. So this is a sequential event hierarchial gene expression that is very important to maintain the development of embra. Now I'm going to show you a table which explains a lot of information in this one single table regarding the hierarchy of drosophila genetics in the development.

[00:16:56]Where we start with this maternal aector genes or MEG. The maternal effect genes what are the function establishment the gradient of anterior and posterior poles of the egg. So they are present in the egg even before fertilization.

[00:17:10]The representative genes becoid oscar cordal torso trunk are also examples.

[00:17:16]What happens to the mutation if you mutate them? Major disturbances in anterior posterior organization because primary organization or primary access of organization in drosophila is anterior posterior organization which is done by bcoid and nano gradient both are part of metal aector genes. Then we have segmentation genes and first kind of segmentation gene is a gap gene. Gap gene defines broad areas in the egg. Earlier the egg was fine. What we are trying to do is segmenting the egg physically and also basically segmentation we are seeing not physically always but the imaginable segmentation of gene expression. So earlier and nanos then segmentation of three segments head thorax abdomen that is by gap genes. Example hunchback crooles ribs tailless these are the four most important type of segmentation gene that is a gap gene effect. If you mutate them, adjacent segment will be missing and major part of the body may be missing. Maybe head, maybe thorax, maybe abdomen missing. Then second type of segmentation gene, pair rule gene. A pair rule gene. What is the role of pair rule gene? Per rule gene express itself skipping one of the body parase segments. So if there are total 14 paras, pair rule genes will only express in seven segments. Okay. So let's imagine there are 1 2 3 4 5 6 7 8 9 10 11 12 13 and 14. So pair rule can express either skipping the odd one or skipping the even one. If it's skipped odd then is known as odd skipped or even skipped. Okay based on that idea. But if it skip odd or even number of parasgments, still it will express in seven paras. Okay. So pair rule gene express in the seven parase segments.

[00:19:14]Skipping one whether either it can skip odd number of pargments or even numbers.

[00:19:19]Example of pair rule genes even skipped runu odd skipped odd paired paired.

[00:19:27]These are all examples of pair rule gene.

[00:19:31]What is the effect if we mutate per gene part or any pattern in that embryo will be deleted.

[00:19:39]Then comes the third kind of segmentation genes that is segment polarity gene. Segment polarity gene.

[00:19:45]What is the role of segment polarity gene? Segment polarity gene acts in all 14 segments and what they do is in individual segment anterior posterior identity. It's very important that even in the individual parase segments, individual body segments, physical segments, anterior posterior axis is determined otherwise addition or attachment of organs is not possible. So the whole map is there the whole embryo what is anterior posterior dorsal vententral terminal. Now individual body segments and they need to mark anterior posterior and individual body segments.

[00:20:20]The example of segment polarity gene is engraved, gooseberry, hedgehog, patch, smoon, wingless.

[00:20:26]Okay. And if so basically in each paras there is anterior and posterior side determined by the segment polarity gene expressing in all 14 paras. So the difference is that in pair rule genes are expressing on seven body segments skipping one uh in between while segment polarity gene is expressing in all 14 par segments.

[00:20:48]Now if they fail to recognize anterior or posterior it can either make two anterior or two posterior structures or two posterior sides. That's another problem.

[00:21:02]Homeotic genes. Finally what homeotic gene does determine the regional characteristics. Means once the body access is determined body patterning is done. So we know what is anterior what is posterior. We know in individual segment what is antenna posterior. Then the organ must be added in that respective area. That attachment is done by homeotic genes. And there are three types antennopedia complex. Two types antennopedia complex and bthorax complex. Antennopedia complex and bthorax complex. Now if there is any mutation then obviously there won't be addition of organ structures properly like the eye like the antenna like the uh wings okay this problems may be originated from there so this particular table is very very important I want all of you to remember this table remember the type of gene know the name of some example of the genes their functions and what happens if you mutate them this is the list of examples for all the different kinds of genes. You see gap jeans, nip, hunchback, giant, tailless examples. Pair rule hairy even skip run which are primary pair rule genes. You can easily remember it by her hairy event skipped and runt. Secondary pair rule fussu odd pair odd skip slloppy pair paired all this segment polarity hedgehog wingless engrailed armadillo gooseberry and pangoline all the names all the type are segment polarity genes okay simply know their names because sometimes in the exam they will ask question with these names or they'll ask question with xyz different name of the protein you need to understand only one thing what you need to understand here is basically that what class of gene family they should belong. Once you understand what class of gene they belong then you can easily answer that question.

[00:23:00]Okay. So this is the table which we already discussed where we have maternal effect genes starting then we have gap genes, pair rule genes, segment polarity genes and how they are expressing. So maternal effect gene expression is clear. We know the gap gene expression has different expression pattern.

[00:23:16]Tailless, giant rips, giant, tailless and the expression profile is different. So if you look at here we have little bit giant here we have only giant a little bit of rips all this pu gene expression you can see there are two pu genes are listed here even skipped with the green uh blue color fitharu with red color the expression is in a uh repetitive manner.

[00:23:40]So even skip skipping even number of body par segments on the other hand segment polarity pattern you can see engraed and wingless polarity pattern in individual body segments there's expression of engra and wingless okay so this body pattern is clearly so what these genes are doing gap genes there are three genes that are very important hunchback joint and These are the primary three gap genes that you need to know. Perhold genes.

[00:24:14]The primary pair genes you need to know even skipped and run. These three you need to know. All of them are DNA binding protein activator or activator.

[00:24:24]They act as a transcription factors. So they bind to the DNA and allow the transcription of particular genes.

[00:24:33]So basically when we see the expression profile of gap pay rule how it's done the gap genes expression is broad. So this is hunchback expression and then this there is segmentation further segmentation will be done. So from gap gene onward gap gene then pair rule segment polarity the expression is being discrete and more concentrated. Okay more concentrated expression we can easily observe. So this is a gap gene expression. You can clearly see that this is the hunchback's expression to the first half or anterior part of the embryo. And we have the different is like giant expression here. Then we have croo expression, ribs expression and again giant expression. So different expression profile. So here only you can see along with hunchback we have giant.

[00:25:19]This is giant. Then you have along with hunchback. Although hunchback concentration is decaying or decreasing, we have little bit of giant, little bit of hunchback, but more of a here. Here mostly we have ribs.

[00:25:33]Okay.

[00:25:34]And a little bit of as well. And here we have little bit of and little bit of giant. So you can see if we look at three different sections.

[00:25:44]This sections giant and hunchback only.

[00:25:47]This section hunchback This section ribs. This section joint. So if you look at four different sections, section 1 2 3 4 in four different sections. Four different se paras. The expression of genes are different. First para segment hunchback with giant. Second one hunchback low, high. Third one cpple and rips.

[00:26:10]Okay. Fourth one croopple and rips and giant. So this is how the expression profile is there. And this this kind of expression profile is unique. And this expression profile dictates the expression of further uh downstream gene productions.

[00:26:26]So the the pair rule genes like primary pair rule genes like even skip rant and hairy there are three easy to remember her secondary pair gene fitzu paired sloppy paired odds. Okay these are all different types. These are all transcription factors. What are they?

[00:26:44]They are nothing but a transcription factors. So either they can have zinc finger motive.

[00:26:51]Okay, they either have zinc finger motive or open zinc motive. Based on that motive they can bind to the DNA and cause the rapid expression of target genes. Okay, these are the job of pair rule genes. And the one more thing that you need to understand here is how the pair rule gene is expressing. Remember pair rule genes are expressing skipping skipping paras. Okay.

[00:27:18]So they skip pair segments how they skip? So expressing in two then four then six then 8 then 10 like that. So in this particular example they are skipping odd number of parasgments. In some other case they may skip even number of parasgments but they express only seven pargments out of 14. And segment polarity gene as I mentioned earlier their job is to provide polarity to each segments. Individual segment must have anterior posterior polarity and that must be governed by the segment polarity gene. The pair gene act as a transcription factor along with regulator for segment polarity gene expression. Example of segment polarity genes remember engrail short form en wingless. Okay. WG and hedgehog HG.

[00:28:12]These are the three different types.

[00:28:14]Engrad, windless and hedgehog are the three primary segment polarity G. There are some other example like dig patch, frizzle, gooseberry, armadopangoline.

[00:28:24]But these are the three most common type. So engra is a homeodain transcription factor. Wingless signaling liant and hedgehog is also a signaling liant. Windless and hedgehog both act as a signaling liant, a signaling molecule that elicitates a signaling response that is necessary for individual segment polarity and engrails act as a transcription factor for homeotic genes. The genes which are under the revelation of segment polarity gene. This is what you can see how exactly the hedgehog signaling works. The distribution of hedgehog works. You see the hedgehog is present in in individual if you look at an individual segment there is a high concentration of hedgehog somewhere and very low concentration of hedgehog on the other side. Okay. So in in absence of hedgehog and in presence of hedgehog what happens you can clearly see here if the hedgehog concentration is high at this side and this is where the wingless concentration is high. So hedgehog and wingless expression were by inhibiting themselves in opposite uh like by the contact manner inhibition manner. So wingless allow wingless to be expressed in one side of the embryo. So this side of the embryo has more wingless and this side of the embryo has more hedgehog more hedgehog in that side more wingless this side. So where more hedgehog is present this side will have an anterior identity and where more wingless is present is going to have a posterior identity. So in this particular embryo we are not looking at individual body segments here we're looking at only one segment but the hedgehog concentration is more that side of that segment is going to be anterior and the concentration of wingless is more that side is going to be posterior.

[00:30:18]So wingless with posterior and hedgehog with anterior identity. And here also you can see there's a gradient formation in that particular segment of hedgehog and of uh wingless. Okay. So this is posterior side. This is anterior side as per this idea. So wingless will posterior identity hedgehog with anterior identity and hedgehog will inhibit the expression of wingless wing will inhibit the expression of hog at the moment. Okay. So that's how the signaling is done for a segment polarity gene and finally the segment polarity gene regulates homeotic genes regulated by segment polarity gene. Segment polarity gene act as a transcription factor for homeotic genes. Now these homeotic genes are responsible to assign unique identity to each segment thus patterning along the anterior posterior axis. So basically it's a patterning agent of anterior posterior axis. The body axis is determined individual body fragment axis is determined. So now the job is to attach the organ structure individual body paras that is done by homeotic genes. Bythorax is one type of homeotic gene known as BXC and antennopedia NTC is known as another type of homeotic gene. Bythorax type and antennopedia type. Okay. So bthorax type can be divided into three different types. Ultravythorax abdominal A and abdominal B. You can see this is ultravythorax. Here we have ultraviother ubi abdominal ABDA, abdominal B, ABD, B that is that side, back side of the body and front side of the body we have antenna PDR which consists of lab, PB, DFD, STR, NTP all this NTP is very famous for antenna PDR complexes and rest of them are different. So Lab labial and deformed DFD STR is six comb reduced structure STR. Okay, these are the three different. So these structures earlier they are responsible for making different body sections. You can see as as you can see six cone structure antennopedia and all of the head and all the structures in the back. Ultra bthorax is make the thorax region.

[00:32:38]Abdominal A and abdominal B makes the abdomen part and it makes the abdomen part mean basically causes the abdominal organs to be associated and attached to the abdomen of the insect. That's how the homeotic gene works. See the homeotic genes finally have a discrete pattern of expression based on the map that is laid by the pair gene segment polarity gene earlier. Okay, that's how the rosophila development completes.