The egg of Drosophila is centrolecithal in nature. In Drosophila embryo superficial cleavage of the pattern occurs due to the centrolecithal egg. In a fertilized egg, during division, only karyokinesis or nuclear division occurs without cytokinesis, which exhibits syncytium stage. Holometabolous type of development takes place in Drosophila.

After fertilization egg is converted into embryo and embryo show transition into larva through hatching within one day then pupa get form within 5 days after fertilization, like that, along with continuous hatching process and radical remodeling of body pupa get converted into adult or imago, this process of conversation from pupa to adult known as metamorphosis. Adult contains three region

Head region, which posses mouth, eyes and antennae, 2. Thoracic segment:- which is three in number (T1 contain one pair of the leg, T2 contain one pair of leg along with one pair of wings, T3 contain one pair of legs along with one pair of halteres), 3. Abdominal segment (A1 to A8).

Early mitotic nuclear division occurs at every 8 minutes through excessive rapid and synchronous manner within the central portion of fertilizing the egg, up to eight nuclear division nuclei present in the central portion, during the consequent nuclear division nuclei migrate to periphery and form monolayer and now known as syncytial blastoderm. During ninth division ascend 15 nuclei migrates to the extreme posterior and form pole cell (primordial germ cell). Pole cells give rise to gametes (sperm and eggs).

After thirteen round of nuclear division fly, the plasma membrane of embryo starts the inward movement to surround each nucleus. After the 14th cell cycle, the syncytial blastoderm gets converted into cellular blastoderm. Later during gastrulation, all nuclei become part of the cellular blastoderm. The morphogenetic movement takes place and this transformed the monolayer embryo into multilayer embryo. In late gastrulation, a series of movements take place within fly embryo known as germ band extension.

Up to 10th cycle, cell follows only synthesis and division phase of the cell cycle. The G1 and G2 stage is entirely absent. There is no transcription prior to the tenth cycle, transcription get started from 11th cycle thus the interphase for 11th cycle get lengthen and onset of zygotic gene transcription takes place, this is known as midblastula transition, this cycle is an important regulator for the formation and initiation of full-length transcript. Protein gets required for the transcription of the zygotic gene provided maternally. 

The ratio between the number of nuclei and the volume of cytoplasm affects the activity of the cell cycle regulator CDK1 and CDK1 are required for entry into mitosis, CDK1 is constitutive expression through 10th cell division cycle. The activity of CDK1 is blocked by the activation of checkpoint factor genes, which cause phosphorylation of CDK1, which also cause slow down of DNA replication, and those checkpoint factors get titrate by the nuclear material mass generated through nuclear division. TBP and RNA polymerase present in cytoplasm up to 10th division and its presence in cytoplasm mediate by CDK1 and phosphorylation of CDK1responsible for their subcellular localization.

The ratio of nuclei to cytoplasm also control the transcription by the titration of the maternal transcriptional repressor,  Zygotic gene transcription depending on the nuclear density and its timing of transcription is also critical. After the transcription of the gene that regulates the midblastula transition various event takes place, like maternal transcript degradation, cell cycle reorganization, cellularization, sex determination associated regulatory event and patterning of dorsal-ventral  & anterior-posterior axis.

4.1.         Anterior-posterior axis

The patterning of the anterior-posterior axis, which form near the completion of gastrulation. The anterior-posterior axis defines by two classes of gene, 1. maternal effector gene (MEG) or egg polarity gene or coordinates gene, 2. Zygotic gene or segmentation gene. Zygotic gene further divides into four gene subclasses, 1. Gap gene, 2. Pair rule gene, 3. Segment polarity gene, 4. Homeotic gene.

Maternal effector genes (MEG) act as a transcription factor for gap gene, gap gene act as a transcription factor for pair rule gene, pair-rule gene act as a transcriptional factor for segment polarity gene and segment polarity gene act as a transcription factor for the homeotic gene.

4.2.         Maternal effector genes (MEG):

Maternal effector genes are responsible for the anterior-posterior polarity of oocyte through intercellular communication between the oocyte and the somatic follicle cells of the surrounding epithelium. Localization of maternal effector genes transcript within oocytes carried out by microtubule. Microtubule array oriented within oocyte in such a way that positive end remain towards the posterior site and negative end towards the anterior site. Maternal effector genes are bicoid, Nanos, hunchback, caudal, torso, toll. mRNA of all those genes get the form by follicle cells or nurse cell and get transported through dynein protein into the oocyte before fertilization.

Translation products of maternal effector genes act as a transcription factor for next downstream genes and get arranged in a gradient, which gives rise to the anterior-posterior axis of the fertilized embryo. Syncytial environment of early embryo facilitates the formation of this gradient, in turn, these gradients get combine to supply the zygotic genome along with the positional information, required for further patterning of the embryo.

4.2.1.     Bicoid gene:

mRNA of bicoid is localized at the anterior end through exuperantia protein, exuperantia-like protein, swallow protein and staufen protein along with the help of microtubule motor protein dynein. This protein complex binds to 3’UTR of bicoid mRNA require for the specification of anterior cell fates. Some additional protein GRIP75, GRIP128, γ tubulin ring complex and dynein light chain also required to localize bicoid mRNA after 10th nuclear division. 

Poly A tail is not present on bicoid mRNA, after fertilization poly A tail get format 3’end of bicoid mRNA then translation of bicoid protein occur and bicoid protein diffuse from its site of synthesis and arrange in a gradient manner anterior to posterior, high at anterior region. Bicoid is a homeodomain contain transcription factor and also act as a morphogen. The half-life of bicoid protein is 30 minutes. This bicoid protein from anterior to posterior gradient through diffusion from its site of synthesis. Bicoid binds with caudal mRNA at the anterior end and prevents the translation of caudal mRNA at the anterior end.

4.2.2.     Hunchback (hb):

Hunchback mRNA (maternal) uniformly distributed throughout the early embryo. After fertilization hunchback mRNA get express to form hunchback protein. Hunchback protein is express in a gradient manner from anterior to posterior. At posterior portion expression of hunchback mRNA block by Nanos protein, here Nanos protein act as translation regulator. Hunchback mRNA contain Nanos response element and another protein pumilo which is encoded by the posterior group of the gene also present on hunchback mRNA.

At posterior part of embryo pumilo protein and Nanos binds to recruit Nanos response element of hunchback mRNA. The interaction between pumilo and Nanos causes deadenylation of hunchback mRNA. Deadenylation decreases the transability of hunchback mRNA. Hunchback (HB) is a zinc finger transcription factor which contains zinc protein domain. Both bicoid and hunchback were known as the first set of the maternal gene.

4.2.3.     Nanos gene :

Nanos maternal genes are responsible for the specification of posterior end. Posterior cell fate gets specified by Nanos and another factor Oskar also play an important role in posterior cell fate. mRNA of Nanos is localized at posterior end mediate by Oskar, Staufen, Tudor, Vasa, Valois, all these factors involve a complex repertoire of the maternal gene. SHE protein anchor nonos mRNA at posterior end with 3’UTR of Nanos mRNA.

A translation inhibitor protein Smaug binds to 3’UTR of Nanos mRNA. Smaug recruits another protein CUP that prevents the association of Nanos mRNA with ribosome machinery thus inhibit the translation of Nanos. After fertilization, translation of Nanos requires the assistance of both Oskar and Staufen protein thus their mRNA should be located at the posterior end. Oskar and Staufen mRNA are localized by kinesin I motor protein. Staufen requires for translation of Oskar mRNA at posterior end and then Oskar protein binds to Nanos mRNA at a specific region of 3’UTR which also contain Nanos-Smaug-CUP complex, in turn, cause the disassociation of CUP, as a result, Nanos now interact with ribosome machinery and proceeds to translation, thus mediate posterior gradient. The posterior gradients are responsible for the formation of germline and specification of abdominal cell fate. Nanos protein, an RNA binding protein diffuse from its site of synthesis and arrange in a gradient manner posterior to anterior. Nanos is high in the posterior region. Two types of Oskar protein get express from Oskar mRNA. One is long Oskar and second is short Oskar. Short Oskar nucleates the formation of pole plasm and long Oskar is using in anchoring of pole plasm component at posterior side.

4.2.4.     Caudal gene :

Caudal mRNA uniformly distributed throughout the early embryo. After fertilization caudal mRNA get express to form caudal protein. Caudal protein is express in a gradient manner from posterior to anterior. At anterior end, the expression of caudal mRNA is blocked by bicoid protein, here bicoid protein act as translation regulator. Caudal mRNA contain bicoid response element and at the anterior part of embryo bicoid binds to its response element and block the binding of eIF4G, thus inhibiting the circularization of mRNA in turn translation of caudal mRNA. CAD is a transcription factor which also contains homeodomain.

Both caudal and Nanos was known as the second set of the maternal gene.

4.2.5.     Torso gene:

Torso gene is known as the third set of maternal effector gene, which is responsible to form extremities of anterior side known as acorn and posterior known as telson. Torso mRNA form by ovarian nurse cell which is transported to the developing oocyte. After fertilization torso mRNA get express to form torso protein. Torso protein is a tyrosine kinase receptor and the receptor gets inserted into the entire plasma membrane of egg but the activation of receptor restricted to the extremities only by the availability of its active form of ligand. The trunk protein secreted by egg act as a ligand of torso receptor, but trunk protein secreted in an inactive form and get activated by the proteolytic cleavage result in the generation of C-terminal fragment cause the activation of torso receptor.

Proteolytic cleavage is done by torso-like protein formed by the expression of the trunk-like gene present in follicle cell of the anterior and posterior border of the embryo, thus the activation of torso restricted to extremities. Once the torso receptor gets activated at both pole then it elicits a signalling cascade, which causes the activation of zygotic gene tailless and Huckebein. Now tailless and Huckebein cause the specification of acorn and telson. Formation of acorn depends on the action of bicoid, torso and Huckebein, like that formation of telson depend on the action of nonos, torso and tailless.

Toll gene: a fourth set of MEG, responsible for the formation of the dorsal and ventral axis.

4.3.         Zygotic gene

The transition from the syncytial blastoderm stage of the fly embryo to the segmented body direct by the expression of the zygotic gene. Maternal effector gene (MEG) product act as a transcription factor for the expression of the zygotic gap gene. Which are first zygotic gene get to express in the embryo and this stage is known as midblastula transition occur after the 10th cycle of nuclear division, this also suggests that MEG gets translated up to 10th cycle.

4.3.1.     Gap gene: gap gene expression in the overlapping domain from the rim of the nuclei present in syncytial blastoderm after 10th nuclear cycle. Those genes are called gap gene because the mutant embryo for gap gene display the absence of several contiguous segments and leaving the gap between segments. The mutation does not affect the polarity of the embryo.

Expression of each gap gene takes place one or two broad strips that govern several future segments. Major gap genes are a hunchback, kruppel, giant, knirps, tailless, huckebein (zygotic), orthodenticle, buttoned, empty spiracles. All of them are zinc finger transcriptional factor. gapgene giant is leucine zipper transcriptional factor and orthodenticle & spiracles, both of them are homeodomain transcriptional factor. Expression of all gap gene protein products takes place in an overlapping manner within segments of the embryo.

The anterior portion of embryo governs by hunchback (express in up to mid-T3 segment or five parasegments in gradient form, get the decrease in two parasegments then increase in three and get completely decrease up to last of 5 parasegments), ocelliless, empty spiracles, buttoned and buttonhead responsible for head and thorax. Expression of all of them governs by the gradient of bicoid and hunchback protein. Kruppel (Kr) is responsible for a thoracic region and get express from T1 to T3 segment or middle third to middle sixth parasegment and its expression depend on bicoid (high) and hunchback (low) gradient. giant express in first and second parasegment and completely decrease in the third parasegment and its expression governs by the gradient of bicoid and hunchback protein. The interactive gradient of huckebein, bicoid and hunchback responsible for acorn formation at the anterior extreme.

The posterior portion of embryo governs by Knirps, giant, tailless. Knirps starts to express where bicoid concentration very low, from mid-sixth parasegment onwards and responsible for the abdomen. giant express in the posterior segment of the abdomen and depends on the gradient of caudal and Nanos. The interactive gradient of tailless, Nanos and caudal responsible for telson formation at the posterior extreme.

Borders of gap gene expression also define by repressive and activated interaction among these gap gene thus also called transcriptional activator and regulator, for example, giant and kruppel repress each other expression as well as a hunchback and knirps repress each other expression. Kruppel strip border refines through negative regulation takes place anteriorly by hunchback (HB), Tailless (TLL) and giant (GT) and posteriorly by giant, knirps (KNI) and tailless. These interactions are critical to sustaining the gap gene expression along the anterior-posterior axis. In the case of the hunchback, protein act as a negative regulator and work in a concentration-dependent manner. High level represses kruppel, low level repress posterior giant, intermediate level repress knirps, thus hunchback act as a long-range repressor. Short range repressor is known as kruppel, knirps, giant, all of them mostly interact with dCtBP (maternally expressed corepressor) through PxDLSxK/R/H motif, which makes interaction with Polycomb and histone deacetylase protein.

Gap gene promoter organized to response the graded concentration of maternal transcription factor because it has different binding sites to respond to different concentration of maternal transcriptional factor. For example, in case of bicoid, (BCD), which is transcriptional factor for hunchback promoter and hunchback promoter have two binding sites for bicoid, one respond in high concentration of bicoid called low-affinity binding site of bicoid at anterior region and another respond in low concentration of bicoid called high-affinity binding site of bicoid at posterior region, this site also work at mid or centre region of embryo.

As the number of bicoid binding site increase, the strength of response also increases but the affinity factor of binding sites also work along with them. Sharper strip border due to multiple binding sites of bicoid and those sites responsible for increasing the transition take place rate between off and on the state as the concentration of bicoid vary. If cooperative binding on these sites then sigmoid transition between off state and on the state. bicoid molecule show cooperative binding at the hanchback promoter, CHIP act as a cofactor for bicoid in this process.

4.3.2.     Pair rule gene: Transcription of pair-rule gene start at the end of the syncytial blastoderm stage. Thus just before cellularization periodicity emerges within embryonic body plan show the start of expression of the pair-rule gene. Now aperiodic gap gene pattern gets resolve into seven evenly placed gene strips.

Pair rule genes are two type

Primary pair rule gene- even-skipped (eve-a homeodomain transcriptional factor), runt (runta noval transcriptional factor), hairy (h-HLH transcriptional factor)

Secondary pair rule gene- Fushi tarazu (ftz-a homeodomain transcriptional factor), odd paired (opa-zinc finger transcriptional factor), sloppy paired (slp-forkhead transcriptional factor), paired (prd-homeodomain transcriptional factor), odd skipped (odd- zinc finger transcriptional factor),

This class of gene expression firstly mark the indication of the segmentation, in turn, demarcate the 14 parasegments known as the developmental compartment in the early embryo, each governs a population of cell give rise to individual segments through development. Organization of parasegment with respect to the segment is out of phase so that each segment start as of the posterior domain of one parasegment and the anterior domain of next parasegment. This also shows that each parasegment also divides in the anterior and posterior segment. The segmental arrangement is lost in the head region due to the fusion of anterior parasegments. Pair rule protein responsible for allocation of the cell into a parasegmental unit called parasegment and cell of adjacent parasegment never mix. The domain of cell lineage restriction known as compartments. Imaginal disc of anterior-posterior boundaries, those are formed by the portion of the parasegmental border in the future give rise to limb.

For determining parasegmental boundaries, size and identity gene even-skipped and gene Fushi tarazu play a very important role. Fushi tarazu and even stripes possess overlapping boundaries prior to cellularization and later those overlap resolve and convert within ensuing borders. Thus parasegment has the capability to sense and modulate their comparative sizes. If changes in parasegment more than 20% than the ability of parasegment to monitor and correct size does not occur. For evens kipped and Fushi tarazu runt and hairy act as repressor respectively. The concentration maintained by Odd paired and sloppy paired (SLP). The action of pair-rule gene bordered each parasegment and each pair rule gene expressed in a series of seven transverse strips, which are responsible for the alternative fashion of expression of pair-rule gene within 14 parasegment means each strip equivalent to every second parasegment. For example, even skipped which is primary pair rule gene express in odd parasegment, like 1, 3, 5, 7, 9, 11, 13 parasegment and some other secondary pair rule gene express in even parasegment in turn show alternative fashion of expression, thus pair rule gene show “zebra” striped expression pattern.

The expression of particular pair rule gene in particular parasegment depends on the concentration of transcription factor for that pair-rule gene. Gap gene and MEG product are a transcription factor for primary pair rule gene and bind to one strip from seven strips, which is responsible for expression of that pair rule gene into that particular parasegment. Thus the strips are themselves enhance the expression of governing pair rule gene in particular segment to which that strip belong to express that pair rule gene due to binding of the respective transcriptional factor of that parasegment.

Expression of the strip also depends on some other enhancer. Product or primary pair rule gene act as a transcriptional factor for secondary pair rule gene. For example hairy repress the expression of Fushi tarazu into broadband and create an interstrip region between them, those stripes do not show expression of Fushi tarazu. Then Fushi tarazu, which also a transcription factor autoregulate its transcription. Odd-skipped (ODD) ectopic expression show it plays an early role for the establishment of the runt, even skipped and hairy, thus show that both pair-rule gene expressions depend on the Material effect gene (MEG) and gap gene product along with cross-regulatory interaction between primary and secondary pair rule gene.

Now LET'S TALK an example of strip second of Fushi tarazu along with its enhancer. The enhancer contains five binding sites for bicoid, three for giant, one for hunchback and three for kruppel. Here bicoid and hunchback act as activator and kruppel along with giant act as a repressor and the concentration of activator and repressor responsible for activation and repression of Fushi tarazu. Repressor act by two methods, first, competing for overlapping binding site and second by local quenching.

Pair rule gene also acts as the transcription factor, but most of them act as a repressor ( ex., even skipped (EVE), hairy (H), Odd-skipped sloppy pair runt, but some time hairy also act as an activator. Fushi tarazu, odd paired and paired act as an activator, Fushi tarazu found to repress two of its target. Groucho (GRO) act as co-repressor for hairy and runt. RPF3 a histone deacetylase help even skipped in its repressive function.

4.3.3.     Segment polarity gene: Segment polarity gene gives polarity to each segment. Pair rule gene act as transcription factor along with regulator for segment polarity gene expression and we already TALK cellularization get completed during pair-rule gene expression, thus segment polarity gene are acting in cellular rather then syncytial environment. In cellular blastoderm embryo get organize into a series of compartment due to allotment of cells into non-overlapping cell population and here cell of adjacent compartment does not get the mix, thus lineage-restricted devolvement takes place. Less segment polarity gene act as a transcription factor and much of them not act as a transcription factor.

Example of segment polarity gene are as follows: engrailed (en-homeodomain transcription factor), wingless (wnt or wingless-signalling ligand), cubitus interruptus (ci-zinc finger transcription factor), hedgehog (hg-N-terminus signalling ligand), fused (serine-threonine kinase), dishevelled (cytoplasmic transducer of wnt signalling), zeste white 3 (serine/threonine kinase), patched (transmembrane receptor of hedgehog), frizzled (transmembrane receptor of wingless), gooseberry (gsb-paired homeodomain transcriptional factor), pangolin (pan-HMGtranscriptional factor), armadillo (arm-transcriptional regulator).

One segment gets to start from mid of one parasegment and end in the mid of other parasegments, and thus posterior of the first parasegment become anterior of the first segment and anterior of the second parasegment become posterior of the first segment, in turn, parasegmental division in embryo defines by segment polarity gene. Polarity error occurs in the segment due to the mutation in segment polarity gene, as a result, deletion, mirror-image or duplication and segment orientation reversal take place.

Odd number engrailed stripes require gene paired along with even skipped activity and even number engrailed stripes require Fushi tarazu along with the odd paired activity. All of them act by their respective response element on the engrailed gene promoter. In the case of wingless, it is positively regulated by paired along with odd paired and negatively regulate by Fushi tarazu and even skipped. Due to their action wingless express posterior in parasegment. The maintenance of anterior border of wingless through negative regulation is done by odd and the action of odd is negatively regulated by paired. Sloppy paired also responsible for wingless expression after its establishment and sloppy paired negatively regulate Fushi tarazu, even skipped and engrailed. Expression of engrailed and wingless also done by an autoregulatory loop. Segment polarity gene promoter possesses two types of enhancer one for odd no strip and second for even number strip.

Engrailed having an important role to establish a regional identity within parasegment along with future segment. Thus engrailed known as selector gene for the process of segmentation provide the molecular signal by its expression in descendants, in turn, informing cells to their regarding the position, as a part of the anterior compartment of parasegment in early stage and later to become posterior compartment of parasegment. Engrailed play an important role to define anterior-posterior compartment boundaries to be as a part of the regulatory loop involving the wingless and hedgehog cell-cell signalling pathway. Wingless express anterior compartment of segment and hedgehog express the posterior compartment of the segment.

4.4.         Wingless signalling:

In absence of wingless: wingless receptor 'frizzled' is not active in turn disheveled and LRP5/6 indicative, thus β-catenine/armadillo inactive by binding of axin, gsk (cause phosphorylation of armadillo), ck1 and APC, thus leads to polyubiquitination and get degraded, so it can not act as the transcription factor for wingless responsive gene like engrailed.

In presence of wingless: wingless binds to its receptor and then LRP5/6 binds to axin, gsk, thus β catenine get free, go to the nucleus and activate wingless responsive gene like engrailed. The engrailed in turn act as a transcription factor for hedgehog, then hedgehog protein get secreted outside the cell. The hedgehog protein binds to its receptor present on cell present at posterior compartment of the segment.

4.5.         Hedgehog signalling:

In absence of hedgehog: in the absence of hedgehog its receptor patch binds to smoothened get become inactive and the ci activator of hedgehog signalling bind to PKA, Fu, Sufu, gsk-1, ck1, cos and SCF, thus get degraded and convert in to ci repressor, go to the nucleus, binds to the response element of hedgehog-responsive gene and repress the transcription of  hedgehog-responsive gene  (wingless).

In the presence of hedgehog: hedgehog binds to its patch receptor, so that smoothened not able to block the patch then signal transduction occur and ci receptor remain free to go to the nucleus and activate the transcription of hedgehog-responsive gene, thus wingless also get express and wingless protein secreted out of the cell through co-translation transport and move to its receptor present at anterior end of the compartment.

4.6.         Homeotic genes: Responsibility of the homeotic gene to assign a unique identifier to each segment and thus patterning along the anterior-posterior (AP) axis. Mutation in homeotic gene result in homeosis, which governs the transformation of structure (one or more) or segment (one or more) into the resemblance of other, but never responsible for the loss of segment and we can say mutation result in ectopic expression of the body part. Gap gene and pair-rule gene in conjugation act as a transcription factor for homeotic gene and act on cis-regulatory elements called initiator enhancer elements. Homeotic gene themselves act as the transcriptional factor and posses MADS domain, in which M denotes MCM, A denotes agamous, D denote deficiently, S denotes SRF).

In Drosophila homeotic gene cluster as HOM-c Box, which divides into two complexes:

Bithorax (BX-C), which themselves contain Ultrabithorax, Abdominal-A and Abdominal B. Fifth to thirteen parasegment identity is controlled by these genes and called a posterior homeotic gene.

Antennapedia (ANT-C), which themselves contain labial (lab), proboscipedia (pb), Deformed (Dfd), Sex comb reduced (Scr) and Antennapedia (Antp). The identity of parasegment present prior to five parasegments get controlled by these genes, called an anterior homeotic gene.

Anterior and posterior homeotic gene represses the expression of each other in their expression region. For eg, Antennapedia represses in ultra biothorax (Ubx) region. If ultra biothorax absent in turn result in one wing now express from T3 just like T2 and homeotic transformation occur. In other cases when Antennapedia mutated or absent, expression of leg occur in place of the antenna. If the mutation in bithorax then T3 show similarity to T2 having a pair of wings along with two abdomens.

HOX is the homologous cluster of the homeotic gene present in the worm to man. There is a precise correlation between the genes expressed along the anterior-posterior axis of the developing embryo and the relative position of each homologue in each of these cluster of the homeotic gene (between HOX and HOM). Only one homeotic gene cluster HOM-C present in primitive insects and the split between Ant-C along with BX-C taken place during insect evolution, thus it shows original cluster form by duplication and the ancestral complex has 8 genes. This complex themselves duplicated in higher metazoans and in duplicated twicely in mammals thus had four clusters (HOX-A to HOX-D). All homeotic gene are the homeodomain-containing transcriptional factor but all homeobox-containing gene are the hot homeotic gene. Homeobox is 60 amino acid or 180 basspair containing conserve box.

Extradenticle (EXD) is a TALE homeodomain protein act as a cofactor for all homeotic gene and both bind to each other through their homeodomain. On double-stranded DNA homeodomain of both protein bind at the opposite side and interact with each other by conserve motif of six amino acid (hexapeptide) located at HOM protein homeodomain N-terminal and EXD homeodomain C-terminal. This interactive model comes in the selective binding model of the homeodomain and reported in fork head (fkh) promoter, between sex comb reduce and extradenticle. One other example of Distal-less gene (Dll) responsible for leg formation in thoracic segments and ultra biothorax along with Aba A control its expression in the abdominal segment, in turn, help the Dll repressor cooperative binding to dsDNA with its cofactor EXD and Homothorax.

According to activity regulation model of the homeodomain, the cofactor is not necessary for the activity of the homeotic gene, this reported in case of Deformed (DFD) and Fushi tarazu when make the association with the powerful transcription activation domain of VP16 protein. HOX gene also possesses cofactor, which includes genes from MEIS and PBC family, thus also show cooperative binding to dsDNA with its cofactor. Mutation in HOXD13 result in synpolydactyly (dominant disorder, which affects digit formation) and mutation in HOXA13 result in hand-foot-genital syndrome.

Mostly HOM gene initially show parasegmental gene expression, sometimes more than one HOM gene also express in a parasegment in combination pattern, in 5-12 parasegment ultra biothorax expression takes place and this is area of gap gene kruppel expression, act as activator for ultra biothorax through categorize the broad central region to which ultra biothorax expression occur. Hunchback and even shipped act as a negative regulator for ultra biothorax. But Fushi tarazu shows positive regulation, by generating pair rule periodicity through strip border sharpening. Hunchback and even shipped make interaction with dMI-2 and RPD-3 respectively, dMI-2 and RPD-3 belongs to histone deacetylase complex in turn help hunchback and even shipped to perform their function. Ultra biothorax promoter contains upstream binding sites for ultra biothorax, even shipped, Fushi tarazu which are found to be closely associated with PREs, TREs, PRE and TRE are those are response element for Polycomb and trithorax gene respectively. Polycomb complex thought to be recruit by even shipped. Largely Fushi tarazu target makes interaction with Fushi tarazu-F1 (orphan nuclear receptor), which recruit histone acetyltransferase.

Autoregulation and interaction through cross-regulation cause additional refinement in the HOM gene after the disappearance of the gap and pair-rule protein. Protein encoded by trithorax (trxG) gene responsible for maintaining the state of transcriptional activation of HOM gene by interacting activator and in turn coactivator which adds acetyl group, thus regulate chromatin remodeling, as well as themselves regulate chromatin remodelling because trithorax possess histone acetyltransferase activity and Polycomb (PcG)  responsible for maintaining the state of transcriptional repression of HOM gene by interacting repressor and in turn corepressor which remove acetyl group thus regulate chromatin remodelling, as well as themselves, regulate chromatin remodelling because polycomb possess histone deacetylase activity and chromodomain, Both genes are two additional classes of genes, responsible for maintaining the transcriptional state of the HOM gene. Chromodomain also present in MOF and MSL-3 (a protein involved in dosage compensation) required to bind roX (structural protein), this RNA target the RNA/protein complex to the male chromosome.

 4.7.        Dorsal-Ventral axis formation:

After fertilization, the nucleus of zygote moves to future dorsal surface. The nucleus transcribe the Gurken mRNA in the dorsal side. Gurken mRNA translate into Gurken protein. This Gurken protein is secreted by oocytes. Gurken protein work as the ligand for torpedo receptor present on the dorsal follicular cell. Torpedo receptor block the pipe protein synthesis after binding with Gurken protein. This pipe is not formed at the dorsal surface. Pipe gene expression gets inhibited by torpedo protein at dorsal side, pipe gene play important role in ventralization of the oocyte surface. Ventralization of the embryo occurs, if the mutation or maternal deficiencies take place within either the gurken or the torpedo gene. Through the transplantation experiment, two results come. First, the torpedo mutant eggs were able to produce normal embryos if they transplant in to wild type ovary. Second, wild type egg develops mutant, ventralized embryos if they transplanted into the mother's egg chamber, which is torpedo mutant.

4.8.         Ventral axis formation:

Only the ventral follicle cells were able to make the pipe. A protein torpedo does not block pipe synthesis because gurken protein is absent in ventral surface. Slightly later stage of development unknown protein (x) get modified by pipe protein and both of them get the bind  and get secreted from the ventral follicle cells. Pipe activates the Nudel protein. Nudel protein is secreted by ventral embryonic cells and present in the perivitelline fluid.

Three serine proteases which are the product of gastrulation defective (gd), snake (snk), and easter (en) genes and these are secreted within inactive form into the perivitelline fluid, get activated in a cascade manner by nudel.

Activated Nudel activates the Gastrulation-defective protease. The cleaved Gd works as protease and cleaved the snake protein. The cleaved snake works as protease and become active. The cleaved snake cleaved the ester protein. Nudel cleaved the inactive Gd. Cleaved Easter protease cause the cleavage of the Spatzle protein. Ventral portion of the embryo is the place where the cleavage of these three proteases be limited.

Spatzle protein is now able to bind to its receptor, which is toll protein (maternal product) present in the oocyte cell membrane throughout and it is the product of the toll gene. Thus, Toll signal only get receive by the ventral cell. When Spatzle binds to and activates the Toll protein, Toll cause the activation of Pelle, which is a protein kinase. Pelle with the help of Tube, get bring on to the plasma membrane and Tube also get activated here. In the cytoplasm of the syncytial blastoderm, Dorsal-Cactus complex is present. Pelle a protein Kinase cause the phosphorylation of Cactus, as a result, Cactus undergo ubiquitinylation and get degraded. Dorsal this causes the release of ventral side and Dorsal protein moves to nuclei of the ventral part of the embryo. It is a transcription factor, cause the transcription of the gene specifying ventral cell types, as a result, surface get ventralizes.

About 90 minutes after fertilization synthesis of Dorsal protein takes place and present throughout the syncytial blastoderm of Drosophila’s early embryo, as dorsal protein produce instantly form complex with cactus, which is also present in the cytoplasm of the syncytial blastoderm.

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