Difference Between Regular Dll And Com Dll
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No, there's a Big difference. COM has a well defined protocols for creating objects, exposing methods, managing memory, publishing type information, managing threading. There is practically no language left that doesn't support using a COM server, no matter what language it was written in.
As to where COM differs from 'C', the main difference is the concept of contracts. COM encourages programmers to think in terms of abstract interfaces between modules rather than a hierarchical, top-down decomposition of functionality. This is one kind of 'OOP', but that term is too loose to be of much use, IMO. Advantages of the contract-oriented approach are manifold for strongly-typed, statically linked languages like C/C++.
COM introduced the concept of interfaces, which are immutable so should not be altered between builds, etc. Every COM object must implement the IUnknown interface which contains the QueryInterface method which is used to ask the object for pointers to other supported interfaces.
When a function within a DLL needs an update or a fix, the deployment and installation of the DLL does not require the program to be relinked with the DLL. Additionally, if multiple programs use the same DLL, the multiple programs will all benefit from the update or the fix. This issue may more frequently occur when you use a third-party DLL that is regularly updated or fixed.
When you create an assembly, all the information that is required for the CLR to run the assembly is contained in the assembly manifest. The assembly manifest contains a list of the dependent assemblies. Therefore, the CLR can maintain a consistent set of assemblies that are used in the application. In Win32 DLLs, you cannot maintain consistency between a set of DLLs that are used in an application when you use shared DLLs.
Although DLLs are the core of the Windows architecture, they have several drawbacks, collectively called "DLL hell".As of 2015[update] Microsoft promotes .NET Framework as one solution to the problems of DLL hell, although they now promote virtualization-based solutions such as Microsoft Virtual PC and Microsoft Application Virtualization, because they offer superior isolation between applications. An alternative mitigating solution to DLL hell has been to implement side-by-side assembly.
You can choose between creating an EXE or a DLL when writing Dot NET code. Both of these include executable code, however, DLL and EXE operate differently from one another. The EXE will create its own thread and reserve resources for it if you run it. A DLL file, on the other hand, is an in-process server, so you cannot run a DLL file on its own. A DLL's code is used by a running application by loading and calling the DLL.
Static links. These are linked earlier in the process and are embedded into the executable. Static libraries are linked to the executable when the program is compiled. Dynamic libraries are linked later, either at runtime or at load time. Static libraries are not shared between programs because they are written into the individual executable.
Listen as Robert Stechuk, an expert in the area of young dual language learners, discusses the use of the term for young children. Next, he describes the distinction between simultaneous and sequential learners, followed by some of the key differences between DLLs and children learning only one language.
The downside of this technique is that the malicious DLL file must be stored on disk, which exposes it to detection by regular security solutions. Nevertheless, this technique is employed by malware developers and is widespread in the wild. For example, Poison Ivy, a popular and long-standing RAT, uses DLL injection. Poison Ivy has been involved in several APT campaigns recommending itself as a tool of choice by APT groups for espionage operations.
Modern systems use synchronous communication to achieve high data transmission rates to and from the DRAMs in the memory system. Systems that communicate synchronously use a clock signal as a timing reference so that data can be transmitted and received with a known relationship to this reference. A difficulty in maintaining this relationship is that process, voltage, and temperature variations can alter the timing relationship between the clock and data signals, resulting in reduced timing margins. This problem gets worse as signaling speeds increase, limiting the ability of systems to communicate data at higher speeds.
Delay Lock Loops (DLLs) and Phase Lock Loops (PLLs) serve similar purposes, and can be used to maintain a fixed timing relationship between signals in environments where process, voltage, and temperature variations cause these relationships to change over time. DLLs and PLLs work by continuously comparing the relationship between two signals and providing feedback to adjust and maintain a fixed relationship between them. Rambus DRAMs were the first DRAMs to incorporate DLLs and PLLs, an important innovation that resulted in increased signaling speeds, compared to alternative DRAM technologies.
A DLL is used to maintain the timing relationship between a clock signal and an output data signal. A critical element of the DLL is the phase detector, which detects phase differences between the clock and output data. The phase detector detects this phase difference, and sends control information through a low pass filter to a variable delay line that adjusts the timing of the internal clock to maintain the desired timing relationship (PLLs use a voltage controlled oscillator to adjust this timing relationship). One of the difficulties of maintaining phase relationships between these two signals is that the loop which provides feedback to the phase detector must account for the timing characteristics of the output logic and output driver. This is important, as it estimates the phase differences between the clock and the data being driven by the output driver. In order to accomplish this, circuits that mimic the behavioral characteristics of the output logic and output driver are inserted into this feedback loop to model timing delays and changes in behavior as process, voltage, and temperature vary. Maintaining the timing relationships between the clock and output data in this manner with DLLs and PLLs results in improved timing margins (as shown in Figure 4), and addresses an important limitation to increasing signaling speeds.
End Users, DRAM manufacturers, designers and integrators can all benefit by incorporating a DLL/PLL on a DRAM. By providing a fixed timing relationship between clock and data signals, DRAM performance is allowed to increase and end users are able to benefit from the overall improvement in system performance. DRAM manufactures are able to reduce production costs and improve DRAM yields with the ability to adjust the timing relationships to compensate for variations in process, voltage and temperature, improving timing margins. By enabling high per-pin transfer rates, DLLs and PLLs allow controller and board designers to reduce IO pin counts, which decreases packaging costs, component count, routing area, and routing complexity. Finally, the ability of DLLs and PLLs to provide fixed timing relationships lets component manufacturers and system integrators relax the specifications. In systems with varying temperature and voltage characteristics, system thermal and power delivery requirements can be relaxed and the DRAMs can still maintain good timing margins, while lowering the costs of the thermal solution, power supply, and system manufacturing.
Distal-less DEVELOPMENTAL BIOLOGY Effects of Mutation or Deletion Since Dll mutations are lethal, it is impossible to observe the effects on adult animals. Larvae however, have rudimentary limbs. In the absence of Dll, these vestigial limbs are deleted. Keilin organs, a distal sensory apparatus of larval appendages, are associated with the developing leg imaginal disc primordia. Thus these sense organs are the rudimentary legs of Drosophila embryos. In Dll mutants, the sensory hairs of Keilin's organs are deleted (S.M. Cohen, 1989).Dll protein can be detected in a central domain in legdiscs throughout most of larval development; in mature discs this domain corresponds to the distal-most regions of the leg: the tarsus and the distal tibia. Clonal analysis reveals that late in development these are the only regions in which Dll function isrequired. Dll3 is the strongest hypomorph in which all of the tarsus is deleted and the tibia and femur are reduced in size. Theexpression of two genes required for the patterning of the tarsus,al and bric à brac (bab) was examined in Dll3leg discs. In wild-type discs, al is expressed in the center of thedisc and bab in the rest of the presumptive tarsus. In Dll3 leg discs no al or babexpression can be detected in the center of the discs.Clonal analysis was performed with a Dll null allele. Clones were generated at various times during development and theresulting adult legs were compared to legs containing wild-typeclones generated at the same time. Dll clones generated early indevelopment fail to be recovered in the region more distalthan the coxa, while later in development phenotypically wild-typeDll clones (but lacking bracts) could be recovered in theproximal tibia and femur but not in more distal regions, wherethey segregate out as cuticular vesicles. The requirement for Dll in thefemur and most of the tibia is lost by about the early thirdinstar. Additional observations reveal that there is a clear difference in thetime at which normally patterned Dll null clones can berecovered in the dorsal femur, as compared to the ventral femur (here'ventral' corresponds only to the ventral third): Dll null clonescan be recovered in the dorsal femur when they are generatedat any stage in development, although earlyin development their frequency is reduced when compared to wild-type.In the trochanter, almost no wild-type Dll clones arerecovered at any stage in development; there is aproximal ring of Dll expression in the third instar leg disc thatprobably corresponds to the trochanter.When a leg is composed almost entirely of Dll null mutanttissue then the region more distalto the coxa is represented only by a small stump of tissue. A marked reduction in the P/D axis can be identified inleg discs consisting almost entirely of Dll null tissue,showing that the leg truncations produced by loss of Dll arenot caused simply by cell death late in development but maybe caused by disruption of normal patterning and growth orcell survival during development. In discs containing largerregions of wild-type tissue, this tissue is generally found in thecenter of the disc surrounded by Dll null tissue, in contrast to wild-type clones that form irregular patternscontributing to any region of the leg. Legs derived from thesetypes of discs develop normal distal regions, but the leg betweenthis region and the coxa is aberrant: there is a marked reductionin growth, the division into segments is disrupted and the sizeand density of bristles is reduced (Campbell, 1998). Proximodistal axis formation in the Drosophila leg: subdivision into proximal and distal domains by Homothorax and Distal-lessThe developing legs of Drosophila are subdivided intoproximal and distal domains by the activity of thehomeodomain proteins Homothorax (Hth) and Distal-less(Dll). The expression domains of Dll and Hth are initiallyreciprocal. In the mature third instar disc, Dll isexpressed in a large central domain that corresponds to thepresumptive tarsus and distal tibia. Dll is also expressedin a secondary ring. X-gal staining of adultlegs carrying a Dll-lacZ reporter gene shows that this ring islocated at the proximal edge of the femur, possibly extendingslightly into the distal trochanter. The central domain of Dll expression is controlled by Wg and Dpp. The proximal ring arises in third instar and does not depend on Wgor Dpp activity. The leg disc is a continuous single-layered epithelial sheetthat forms a series of folds as it grows. The peripheral regionof the disc forms the proximal segments. This region is foldedback over the central region where Dll is expressed. The domain of Hthexpression extends from the peripodial membrane at thetop, through the coxa and trochanter segmentprimordia. The distal-most portion of the Hth domain overlapsthe proximal part of the dac-lacZ domain within theproximal ring of Dll expression in the femur.Dll is expressed alone in the central folds of the disc (whichcorrespond to tarsal segment primordia). In proximal tarsusand tibia, Dll and Dac overlap. Dac is expressedalone in the presumptive femur. Becausethe disc is highly folded, horizontal optical sections makeproximal and distal regions of the disc appear to be closelyapposed, although they are actually far apart along the PD axisin the plane of the disc epithelium. Hth is expressed in the upper layerand around the lateral sides of the epithelial sac. Dll isexpressed in the center of the lower layer. The twoexpression domains abut, but do not overlap. dac-lacZis not detectably expressed at this stage, but can bereliably detected in slightly older discs at the transition from second to third instar. These observations suggest that the primary subdivision of the disc is into two domains: a central Dll-expressing domain and a proximal Hth-expressing domain. Wg and Dpp act together to induce Dll and Dac in the center of the leg disc. Wg and Dpp repress Hth and Teashirt, but not through activation of Dll (Wu, 1999).The expression patterns of Dll and Hth/Exd reflect an earlysubdivision of the disc into proximal and distal domains. Atearly stages of disc development, Dll and Hth/Exd areexpressed in reciprocal domains that account for all cells ofthe disc. At thisstage, Dac is not yet expressed. What is the relationship between Dll and Hth/Exd expression in the early disc? The Dlldomain is defined by Wg and Dpp signaling. The same signals repress nuclear localization ofExd and Hth expression. The reciprocity of Dll and Hthexpression suggests a model in which Wg and Dpp act throughDll to repress Hth in the early disc. However, the analysis ofmarked Dll mutant clones reported here shows that this is not the case.Clones of Dll mutant cells located in the distal region of theleg do not express Hth. This contrasts with recentreports by González-Crespo (1998) and Abu-Shaar (1998) in which evidence is presented for ectopicexpression of Exd and Hth in Dll mutant clones.How can the difference in the results betweenthese reports be reconciled? In both studies, the clones were induced in secondinstar larvae using the same allele of Dll. In the experiments reported here,clones were marked by the absence of Dll protein and by theabsence of a neutral beta-gal marker, which permits definitivegenotyping of the cells independent of Dll expression. In theother reports, clones were marked only by the absence of Dll.The disc epithelium is highly folded and the proximal Hth-expressingepithelium is very close to the distal Dll-expressingepithelium. Unless cells in the clone aredefinitively genotyped, it is difficult to distinguish a genuineclone from a patch of the overlying Hth-expressing proximalepithelium that has been pushed downward into the plane of theoptical section. Serial optical sections of wild-type discs showthat this type of distortion of the disc epithelium can occur indamaged discs as well as in discs that are not obviouslydamaged. How is Hth repressed by Wg and Dpp? Dac is induced byWg and Dpp toward the end of second instar. Hth expands distally, to some extent, in Dacmutant discs. These observations suggest that Dac contributes to Hthrepression. However, Hth is repressedprior to the onset of Dac expression indicating thatDac cannot be the primary repressor. Whether Wg and Dpp actdirectly to repress Hth expression or act via another as yetunidentified repressor remains to be determined (Wu, 1999).In conclusion, Hth and Dll expression appear to definealternative fates in the second instar disc. Under normalcircumstances, there does not appear to be a cell lineagerestriction between these populations (i.e. no compartmentboundary). These results suggest that cells can cross betweenthese territories if they are able to switch between Hth and Dllexpression. This situation appears to be analogous to the DVsubdivision of the leg disc (as opposed to the proximal distal subdivision reported here). DV subdivision is stable at the levelof gene expression in a cell population, but is not a clonallineage restriction boundary. Similarly, the separation of proximal and distal cellpopulations requires Hth function. These results suggest thatcells at the interface between these two territories arespecialized to allow integration of otherwise immisciblepopulations of cells (Wu, 1999 and references).Drosophila terminalia as an appendage-like structureThis study reports the expression pattern of Dll in the genital disc, the requirement of Dll activity for the development of the terminalia and the activation of Dll by the combined action of the morphogenetic signals Wingless (Wg) and Decapentaplegic (Dpp). In Drosophila, the terminalia comprise the entire set of internal and external genitalia (with the exception of thegonads), and includes the hindgut and the anal structures. They arise from a single imaginal disc of ventral origin that has a complex organization and shows bilateral symmetry. The genital disc shows extreme sexual dimorphism. Early in development,the anlage of the genital disc of both sexes consists of threeprimordia: the female genital primordium (FGP); the malegenital primordium (MGP), and the anal primordium (AP).In both sexes, only two of the three primordia develop: thecorresponding genital primordium and the anal primordium.These in turn develop, according to the genetic sex, intofemale or male analia. The undeveloped genital primordiumis the repressed primordium (either RFP or RMP,for the respective female and male genital primordia) (Gorfinkiel, 1999).During the development of the two components of the anal primordium -- the hindgut and the analia -- only the latter is dependent on Dll and hedgehog (hh) function. The hindgut is defined by the expression of the homeobox gene even-skipped. The lack of Dll function in the anal primordia transforms the anal tissue into hindgut by the extension of the eve domain. Meanwhile targeted ectopic Dll represses eve expression and hindgut formation. The Dll requirement for the development of both anal plates in males and only for the dorsal anal plate in females, provides further evidence for the previously held idea that the analia arise from two primordia. In addition, evaluation was made of the requirement for the optomotor-blind (omb) gene which, as in the leg and antenna, is located downstream of Dpp. These results suggest that the terminalia show similar behavior as the leg disc or the antennal part of the eye-antennal disc, consistent with both the proposed ventral origin of the genital disc and the evolutive consideration of the terminalia as an ancestral appendage (Gorfinkiel, 1999). The expression pattern of Dll in the genital disc wasanalyzed. Dll is neither expressed in the embryonic terminalia nor in the embryonic precursor cells ofthe genital disc. In the female third larvalinstar genital disc, Dll shows a localized distribution; itis strongly expressed in a large spot in the central partof the anal primordium and in a faint band of cells in thegenital primordium. It is not detected in the RMP. Similarly, in the male genital disc, Dll is expressed in a largespot both in the anal primordium and in the male genitalprimordium but not in the RFP.Several GAL4 insertions in the Dlllocus were used and these permitted the identifcation of theadult regions where Dll is expressed according to theobserved X-Gal staining. In females, Dll is expressed inthe vaginal plates and in the anal plates. In thedorsal anal plate, Dll is expressed in a generalized manner,while the ventral plate shows fainter Dll expression, whichis stronger in the distal part of the plate. In males, Dll isexpressed in the claspers and anal plates. Theexpression, both in male and female external terminalia,is as predicted by the prospective fate map of the genitaldisc. Theinternal structures are not well defned in term of Dll expression (Gorfinkiel, 1999).To investigate whether there is a functional requirementfor Dll in the terminalia, the phenotype of different viableDll mutant combinations was analyzed. These Dll hypomorphic mutant combinations were initially described bytheir phenotype in the leg and antenna. In the allelic series homozygous Dll3, DllIB/DllMPand Dll3/DllMP, the female dorsal anal plate is reducedwhereas the ventral anal plate is normal. Thevaginal plates are disorganized. In males, theanal plates are strongly reduced. The external genital structures are,however, unaffected. These phenotypes show that the Dllexpression domains do not fully correspond to its requirement. This led to a search for alterations in the internal genital structures. In both males and females, the internalgenitalia appear normal. Surprisingly, under the strongest hypomorphic conditions(Dll3/DllMP), the hindgut is enlarged in both females and males. The few anal structures that remain aresurrounded by hindgut tissue. This result suggests an expansion of hindgut territory at the expense of the anal plates.To further investigate the requirement for Dll, DllSAIclones (marked with yellow) were induced during the larvalstages. Dll2clones do not develop anal plates (males) or dorsal anal plates (females). These clones were recognized since some ofthem still differentiate yellow (y) bristles. Dll2clones donot show detectable alterations in the male external genitalia. Since only a few structures of the genitalia can be analyzedwith the y marker, it is possible that minor phenotypicalterations may go undetected. Dll2clones do not affectthe development of the female ventral anal plates. Thus, although Dll is also expressed in the ventralanal plate in females and the claspers in males, it seemsthat it is not required for the formation of these structures.These results are in agreement with those observed using theviable Dll mutant combinations (Gorfinkiel, 1999).Both the female and male anal primordia give rise to twodifferent adult structures: the hindgut and the anal plates. These territories are well defined by the complementary expression of the homeotic genes Dll and even-skipped(eve). Adult regionsthat express Dll and eve were defined by X-Gal staining ofDll-GAL4/UAS-LacZ and eve-lacZ- flies, respectively.These two genes show a complementary expression pattern.Dll is expressed in the anal plates of both females and males but not in the hindgut. Incontrast, eve is expressed in the hindgut of both females and males. Some residual Dllexpression is detected in the rectal papillae, butthese structures are not derived from the genital disc. Thus,the adult analia and hindgut are defined by Dll and eveexpression patterns, respectively. Also in the genital disc,eve labels the prospective hindgut that occupies the centralpart of the anal primordium while Dll marks the primordiaof the anal plates located at both ends of the primordia inboth females and males. This eve expression both in discs and adults suggests that eve is required forhindgut development (Gorfinkiel, 1999).In the Dll hypomorphiccombinations the hindgut is enlarged and the anal platesare reduced. This phenotype correlates with gene expression, since in Dll2clones, eve expression extends into theanal territory both in females and males. In some Dll2clones eve is only activated in the partof the clone nearest to the normal eve-expression domain. This indicates that there is aregion capable of activating eve that is then transformedto hindgut. This region could correspond to the prospectivedorsal analia in females where Dll is specifically required (Gorfinkiel, 1999).To test if a mutually repressive interaction between thehomeotic genes Dll and eve in the anal primordium canlead to their complementary expression domains, either Dll or eve were ectopically expressed in the presumptive cells of boththe hindgut and analia using the cad-GAL4 line. InUAS-Dll/cad-GAL4 discs, eve is not expressed and thereis a reduction of the whole primordium. The Dlldomain is also reduced in all ventral discsupon Dll ectopic expression because an excess of Dllrepresses its own expression. Theadult flies do not show hindgut structure and the anal platesare also reduced. In UAS-eve/cad-Gal4discs there is a reduction of the Dll domain along with an enlargement of the hindgut primordium, but there are still cells thatco-express Dll and eve. Therefore, it is concludedthat the complementary expression domains of Dll and eve inthe anal primordium is due to eve repression by Dll (Gorfinkiel, 1999).Hh signal is required to form the genital andanal structures but not the hindgut.In the leg and antennal discs, the expression of Dlldepends on the Hh signaling pathway. Using the hhts2allele, it was observed that in the genital disc, Hh is alsorequired for Dll activation: after 4 days at the restrictivetemperature, the genital discs are very small and showno Dll expression. In the same hh ts2 larvae, residual Dll expression can be detected in the trochanter regionof the leg disc.However, eve expression in the anal primordia is maintained and occupies most of the reduced genitaldisc. This result indicates that Dll, but not eve expression,depends on Hh and that all the terminalia with the exceptionof the hindgut require Hh function.To further analyze this hh requirement for Dll activation, the effect of smoothened (smo) lack of function was examined.In smo2 cells, Hh reception is impeded because smo is acomponent of the Hh receptor complex.In the genital disc, Dll expression only disappears in smo2clones when the clone is large enough to cover most of theDll expression domain. Accordingly,eve expression is also ectopically activated in smo2 mutant cells; although in Dll2 cells eve cannot be activated in certain regions of the clones. These results indicate once again that Dll isdependent on Hh function while eve is not (Gorfinkiel, 1999).Large smo2 clones close to the A/P compartment transform some structures of the external genitalia and analia. In the female genitalia, smo2clones duplicate thelong bristle of the vaginal plates and clonesin T8 to produce tissue overgrowth with y2bristles. Large smo2clones reduce the female dorsal analplate, whereas the female ventral anal plate is rarelyaffected. Some clones produce segregated tissuein the female analia labelled with y bristles in the perianalregion. However, small clones or cloneslocated outside the A/P compartment border have no effect. In the male genitalia, smo2clones duplicate the genital arc, part of the claspers and the hypandrium bristle. All these structures are located close to the A/Pcompartment border. As in Dll2 clones, large smo2clonesdelete the anal plate in males. In bothmales and females, only when the clone is large enoughcan Dll expression not be activated in the disc primordia, giving rise to the Dll2phenotype. This result suggests thatonly in large smo2clones both wg and dpp are not activatedand therefore are unable to induce Dll expression (Gorfinkiel, 1999).The hh requirement for the analia but not for the hindguthas also been confirmed by the ectopic expression of Cubitusinterruptus (Ci). ci encodes a transcription factor that actsas an activator of the target genes of the Hh pathway. The overexpression of Ci in theanal primordia of cad-GAL4/UAS-ci flies, leads to theenlargement and fusion of the anal plates. Accordingly, the Dll expression domain in the genital disc is expanded to cover most of the primordia and theeve domain is reduced. This again demonstratesthe complementary and exclusive nature of the eve and Dlldomains in the anal primordia (Gorfinkiel, 1999).The requirement for the Hh signal in Dll activation mightbe mediated by Wg and Dpp signals. This occurs in otherventral discs. Dll expression arises at the juxtaposition ofWg and Dpp expressing cells as revealed by double stainingfor Dll and Dpp, and Dll and Wg. In both genital and analprimordia, Dll expressing cells overlap those thatexpress wg and dpp. It has been previouslyreported that the ectopic expression of both Wg and Dppproduces several phenotypic alterations in both female andmale terminalia. Similar types oftransformations are also induced by the lack of function ofeither patched (ptc) or Protein kinase A (Pka). In thesemutants, the Hh pathway is constitutively active givingrise to the derepression of Wg and Dpp. The lackof Pka function in the genital disc induces ectopic Dll. This Dll induction requires both Wg and Dppsignals in the same cells since Dll is not activated in Pka2;dpp2 and in Pka2;wg2double mutant clones, as occurs in other discs of ventral origin (Gorfinkiel, 1999).In the male repressed primordium (RMP) of the femalegenital disc, wg is expressed but not dpp. Consequently, Dllis not expressed because Dll is only activated in cells thatexpress both dpp and wg. Ectopic Dpp expression in the wgexpression domain driven by the MS248-GAL4 lineinduces Dll 'de novo' in the RMP, whichshows an increase in size. However, these changes do notallow the development of adult structures from this primordium since there is no activation of the male specifc cyto-differentiation genes because the genetic sex has not changed. Dllis not activated in the repressed female primordium (RFP)of the male genital disc despite the fact that, in this primordium, both wg and dpp are normally expressed. This activation does not occur even if the levels of Dpp are increased.These results suggest that specific genes expressed in theRFP can exert a negative control of Dll expression (Gorfinkiel, 1999).In order to find other genes involved in the developmentof the terminal structures, the expressionpattern and the functional requirement for optomotor-blind(omb) were examined. This gene encodes a protein with a DNA-bindingdomain (T domain) and behavesas a downstream gene of the Hh pathway in other imaginaldiscs. In the genital disc,Omb is detected in the dpp expression domains, abuttingthe wg expressing cells. This behaviourof omb expression is similar to that found in the leg andantennal discs. In the genitaldisc, omb is also regulated by the Hh signaling pathwaysince Pka2clones also ectopically express omb.The phenotypes produced due to omb lack offunction using the allele omb282 were examined; homozygous females for this allele could not be obtainedbut some male pharates were analyzed. In males, the dorsalbristles of the claspers and the hypandrium bristles areabsent. Also, the hypandrium isdevoid of hairs and the hypandrium fragma is reduced. Surprisingly, the anal plates are mostlysomewhat enlarged in the ventral region and reduced in thedorsal areas. The structures affected in omb2areduplicated when omb is overexpressed in the dpp domainusing the dpp-GAL4/UAS-omb combination. In males, thedorsal bristles of the clasper and the hypandriumbristles are duplicated. These phenotypes aresimilar to the ones obtained as a result of ectopic Dpp (Gorfinkiel, 1999).The present findings provide further evidence that theterminalia can be considered a ventral appendage. In addition, this is the first evidence for a similar genetic pathway operating in both the ventral and genital/analappendages. From an evolutionary point of view, thislends support to the idea that the terminalia arose as the resultof the modification of a primitive appendage in an ancestor common to all arthropods. The Dll requirement for some of the external genital structures broadens thedefinition of the evolutionarily conserved Dll function tocover a more fundamental role than that of the proximo/distal selector gene in the appendages (Gorfinkiel, 1999).The Dll requirement for the formation of only the dorsal analplate in females and for both anal plates in males lendsfurther support to the idea that the anal plates form from twoprimordia. It is proposed that the anal primordia develop asfollows: at the blastoderm stage, the anal primordiumdivides into two cell populations, one of which will formthe dorsal analia in a female or the complete analia in a male(homologous anal primordium), while the other set of cellswill form the ventral analia in a female and give rise to nostructure in a male (non-homologous primordium). Previous reports using the transformed2ts (tra2ts) mutation are also consistent with this organization. In the switch from male to femaledevelopment, using the tra2tsmutant, the ventral anal platerequires more time to reach a normal female phenotypethan the dorsal anal plate. In the shift from the female-to-male determining temperature, the homologous analprimordium switches into the male program while thenon-homologous primordium is brought into the repressedstate. The functional requirement for Dll only in the homologous anal primordium supports the idea that the analplates originate from two primordia. It also suggests theexistence of other gene(s) responsible for the formation ofthe ventral anal plate in females (Gorfinkiel, 1999).Coexpression of the homeobox genes Distal-less and homothoraxdetermines Drosophila antennal identityThe Distal-less gene is known for its role in proximodistalpatterning of Drosophila limbs. However, Distal-less has asecond critical function during Drosophila limb development,that of distinguishing the antenna from the leg. The antenna-specifyingactivity of Distal-less is genetically separablefrom the proximodistal (PD) patterning function because certainDistal-less allelic combinations exhibit antenna-to-legtransformations without proximodistal truncations. Distal-less has been shown to act in parallel with homothorax (apreviously identified antennal selector gene) to induceantennal differentiation. While mutations in either Distal-lessor homothorax cause antenna-to-leg transformations, neithergene is required for the others expression, and both genes arerequired for antennal expression of spalt. Coexpression ofDistal-less and homothorax activates ectopic spalt expressionand can induce the formation of ectopic antennae at novellocations in the body, including the head, the legs, the wingsand the genital disc derivatives. Ectopic expression ofhomothorax alone is insufficient to induce antennaldifferentiation from most limb fields, including those of thewing. Distal-less therefore is required for more than inductionof a proximodistal axis upon which homothorax superimposesantennal identity. hth encodes a TALE-classhomeodomain protein required for the nuclear localizationof a PBC-class homeodomain protein encoded by extradenticle. Based on their genetic and biochemicalproperties, it is proposed that Homothorax and Extradenticlemay serve as antenna-specific cofactors for Distal-less (Dong, 2000).Animals heterozygous for Dll null alleles exhibit partial antenna-to-leg transformations, indicating that Dll levels may be important forantennal determination. Weak hypomorphic combinations of Dllalleles also lead to partial transformation of the third antennalsegment (a3) and the arista into leg-like structures.Intermediate hypomorphic combinations of Dll alleles transformthe medial antenna toward leg and exhibit distal truncations. Strong combinations of Dll alleles exhibit more severeantennal truncations. These same allelic combinationsresult in progressively more severe truncations of the distal leg. Notably, the antenna-to-leg transformations are nota property of a specific subset of Dll alleles, but are observedwith all Dll alleles when assayed in appropriate combinations.For the transformation phenotype to be apparent, Dll PD function must be largely intact. This is likely due to the fact that the PD axis must be manifest in order for either antennal or leg identity to occur. The fact that transformation is observed without limbtruncation indicates that the antennal selector function is moresensitive to Dll dosage than its PD function. Together, the results of Dll phenotypicanalysis indicate that Dll is required for antennal identity, aswell as for limb outgrowth (Dong, 2000).Antp represses hth, thereby restricting hth expression to theproximal region of the leg. Antpalso represses sal in the leg.Because antennal sal expression is dependent upon both Dlland hth, it was hypothesized that Antp repression of sal might bemediated via Antp repression of hth, which in turn prevents theoverlap of Dll and Hth in the distal leg. Consistent with thispossibility, Sal is expressed in Antp null clones in the Dlldomain where hth is derepressed. It was thereforethought likely that Antp may be repressing sal expressionindirectly by preventing hth from being expressed in the Dlldomain of the leg. Since both Dll and Hth are required forantennal differentiation, by preventing the coexpression of Dlland Hth, Antp can preclude antennal development.The explanation for this favored by the authors is that when Hth isectopically expressed using the GAL4/UAS system, Dllexpression is downregulated in the cells producing Hth. Thesecells would then have Hth, but insufficient Dll. If both arerequired for antennal differentiation, antennal differentiationwould not be possible. Consistent with this idea, a decrease in Dllexpression in leg cells ectopically expressing Hth is seen (Dong, 2000).Could Dll form a functional complex with Hth andExd in the antenna? Given that Dll and Hth cooperate to regulate antennaldifferentiation, it is of interest to elucidate the molecular basisof this synergy. Exd and its vertebrate counterpart, Pbx, areknown cofactors for a variety of homeodomain proteins,including Labial, Engrailed and Ultrabithorax. Hth is required for retention of Exd inthe nucleus and may form part of the functional Exd/Hoxcomplex. Vertebrate homologs of Hth,the Meis and Prep proteins, have been shown to form trimericcomplexes with Hox and Pbx proteins.Several lines of evidence now support the idea that Exd andHth are cofactors for the Dll homeodomain protein in thedeveloping Drosophila antenna. These include: (1) the similarantenna-to-leg transformation phenotypes of Dll, hth and exdmutants; (2) the known physical interactions of Exd and Hthwith other homeodomain proteins; (3) the fact that Dll and hthfunction in parallel to regulate antennal development, and (4)the fact that ectopically expressing Hth can mimic loss of Dllfunction in the antenna. Testing whether Dll, Hth and Exdinteract physically and whether such a complex activatesantennal enhancers will be important steps towardunderstanding limb development and tissue-specific generegulation (Dong, 2000).Proximodistal domain specification and interactions in developing Drosophila appendages The morphological diversification of appendages represents a crucial aspect of animal body plan evolution. Thearthropod antenna and leg are homologous appendages, thought to have arisen via duplication and divergence of anancestral structure. To gaininsight into how variations between the antenna and the leg may have arisen, the epistaticrelationships among three major proximodistal patterning genes, Distal-less, dachshund and homothorax, have been compared in theantenna and leg of Drosophila. Drosophila appendages are subdivided into different proximodistaldomains specified by specific genes, and limb-specific interactions between genes and the functions of these genes are crucial for antenna-legdifferences. In particular, in the leg, but not in the antenna, mutually antagonistic interactions exist between the proximal and medial domains, as wellas between medial and distal domains. The lack of such antagonism in the antenna leads to extensive coexpression of Distal-less and homothorax,which in turn is essential for differentiation of antennal morphology. Furthermore, a fundamental difference between the twoappendages is the presence in the leg and absence in the antenna of a functional medial domain specified by dachshund. These results lead to aproposal that the acquisition of particular proximodistal subdomains and the evolution of their interactions has been essential for the diversification oflimb morphology (Dong, 2001).Each segment in the Drosophila leg is considered to be homologous to part or all of a segment in the antenna. The correspondences are based on reproducible homeotic transformations that can occur between parts of the two limbs. Such correlation enables a comparison of the expression domains of Dll, dac and hth between the antenna and the leg. The relative wild-type expression of these three important PD patterning genes of the leg differs in the antenna, indicating that their PD axes are differentially subdivided (Dong, 2001).For example, at late third instar, Dll expression extends more proximally in the antenna into regions homologous to the leg trochanter. In addition, dac is expressed at lower levels and is expressed in fewer segments in the antenna than in the leg. The dac expression domain in the antenna lies completely within the Dll expression domain. In contrast, the dac and Dll domains in the leg are exclusive when dac expression is activated and remain largely non-overlapping at late third instar. hth is expressed only proximally in the leg, but is expressed throughout the antenna disc until early larval stages when it is lost from distal cells. Because Dpp and Wg, which regulate Dll, dac and hth in the leg, are similarly expressed in the antenna, it is thought unlikely that the differences in Dll, dac and hth expression could be accounted for by variations in Dpp and Wg expression. Instead, it is hypothesized that the differences are due to limb type-specific interactions between Dll, dac and hth. The results of experiments described here confirm that this is the case (Dong, 2001).Gradients of the morphogens, Wg and Dpp, initiate the PD organization of the Drosophila leg by activating Dll and repressing dac distally and by repressing hth in the distal and medial leg. This creates three domains, distal, medial and proximal, that are specified by the expression Dll, dac and hth, respectively. The expression of dac is derepressed in clones of Dll-null cells in the presumptive distal region of the leg disc. The reciprocal is observed in dac null clones, where Dll expression expands into the medial domain. Mutually repressive interactions between the distal and medial domains therefore are required to keep these domains distinct from one another (Dong, 2001).If the antenna is homologous to the leg, one might expect to find many genetic parallels, particularly with respect to the three major PD patterning genes of the leg, Dll, dac and hth. As in the leg, Dll and hth are required to specify the distal and proximal domains of the antenna. However, dac has a different function in the antenna. No deletions of antennal segments are observed in dac-null flies. In addition, the genetic relationships between Dll, dac and hth are different in the developing antenna. Specifically, the extensive overlap in expression of these three genes in the antenna indicates that domains are not kept separated as they are in the leg. The normal expression domain of dac in the antenna lies completely within an area of hth and Dll coexpression, making it unlikely that dac represses either gene. Nonetheless, because Dll and hth appear to have slightly lower levels of expression where dac is normally expressed, a test was performed to see whether either Hth or Dll levels would be elevated if dac were removed. No detectable changes in the levels of either Dll or Hth were observed in clones of cells that lack Dac. Therefore unlike the situation in the leg, Dac is insufficient to antagonize the expression of either Dll or hth in the antenna. Taken together, these data indicate that mutual antagonism is not a universal feature of appendage development (Dong, 2001).The antennal regulation of dac by Dll also differs from that of the leg. The regulation of dac by Dll in the antenna varies depending on the proximodistal location. Dll can be a dac repressor or activator, or exert no effect on dac. Dac expression is not activated in Dll-null clones in the presumptive arista, whereas Dll-null clones in the presumptive base of the arista (segments a4 and a5) exhibit non-cell-autonomous dac activation, and Dll-null clones in the presumptive third antennal segment (a3), where dac is normally expressed, result in loss of dac. These data indicate that the regulation of dac by Dll in the antenna is different from that in the leg. They also indicate that the normal antennal expression of dac both requires Dll and has PD regional specificities. Because both Dll and Hth are required for antennal identity and are coexpressed with dac, Hth may also be required for the antennal expression of dac. Consistent with this view, ectopic expression of either Dll in antennal cells expressing Hth or of Hth in antennal cells expressing Dll can activate dac, as can ectopic coexpression of Dll and Hth in the wing disc. Furthermore, antennal dac expression, is not efficiently repressed by ectopic Hth (Dong, 2001).Unlike Dll-null clones, both Dll hypomorphs and hth-null clones exhibit antenna-to-leg transformations. Examination of Dll hypomorphs and hth-null clones therefore reveals their homeotic functions. One such function may be the repression of leg dac. Leg expression of dac encompasses more segments and occurs at higher levels compared with the antenna. As in Dll hypomorphic leg discs, in Dll hypomorphic antenna discs, dac expression expands distally. hth-null clones exhibit derepression of dac in a1, a2 and a4 and elevation of Dac levels in a3. It is therefore proposed that the derepression of dac in Dll hypomorphs and in hth-null clones may represent leg-specific dac expression. Conclusive evidence for this awaits identification of dac enhancer elements and analysis of their regulatory inputs. Nonetheless, taken together, these data support the view that the regulation of leg and antennal dac expression occurs via distinct mechanisms and that the homeotic functions of Dll and hth are mediated not only through activation of antenna-specific genes such as spalt, but also through the active repression of leg development (Dong, 2001).Appendages are subdivided by mutually antagonistic domains.Gradients of the morphogens Dpp and Wg initiate the PD organization of the Drosophila leg by activating Dll and repressing dac and hth distally, and by allowing the activation of dac while repressing hth medially. This creates three domains, distal, medial and proximal, that are specified respectively by expression of Dll, dac and hth. Further refinement and maintenance of the borders between domains requires mutually antagonistic interactions between proximal and medial domains as well as between medial and distal domains. Specifically, Dll and dac are mutually repressive. Also, mutually repressive interactions between the proximal and medial domains do exist via Tsh repression of dac and Dac repression of hth. Thus, pattern formation in the leg requires mutually antagonistic interactions among all three domains in order to refine and maintain borders that initially were set up by morphogens (Dong, 2001).In contrast to the situation in the Drosophila leg, Dll, dac and hth are expressed in largely overlapping patterns in the antenna. This suggests that there is not mutual antagonism between Dll and hth in the antenna. Furthermore, that the entire antennal expression domain of dac lies within an area of Dll and hth coexpression indicates that Dac was unlikely to repress the antennal expression of either Dll or hth. Analysis of dac mutants confirms that Dac does not antagonize either proximal or distal development in the antenna but it does so in the leg. Therefore mutual antagonism is not a universal feature of appendage development (Dong, 2001).Interestingly, in more basal insects like the cricket, Acheta domesticus, Dll and n-Exd expression are exclusive in the antenna. Since n-Exd is normally coincident with hth expression, it is inferred that Dll and Hth expression are exclusive in the cricket antenna. If exclusion reflects mutual antagonism, this in turn could indicate that mutual antagonism between proximal and distal domains is lost in the antenna within the insect lineage during the course of dipteran evolution (Dong, 2001).It is noted that the absence of antagonism of any single PD domain towards another leads to overlap of otherwise exclusively expressed transcription factors. This, in turn, may permit the coexpressed factors to execute additional functions. Indeed, while Hth is required for proximal patterning of both antenna and leg, and Dll is required for distal patterning of both antenna and leg, their coexpression leads to the differentiation of antenna-specific cell fates. Thus, expression of distinct combinations of transcription factors such as Dll, Dac and n-Exd/Hth both in specific domains along the PD axis and between appendage types is likely to activate and repress particular suites of target genes, thereby contributing to differences in appendage morphologies (Dong, 2001).The ability of Dll, Dac and n-Exd/Hth to repress the expression of one another undoubtedly is context-dependent. However, the only known factor involved in context specification is the Hox protein Antp. In the presence of Antp in the antenna, Dll and Hth are no longer coexpressed. Conversely, when Antp is removed from the leg, hth is derepressed in cells expressing Dll. Thus Antp appears to play a role in some aspects of domain antagonism. It remains unclear whether Antp directly modulates interactions among Dll, Dac and n-Exd/Hth or whether there are other molecules that intervene (Dong, 2001).n-Exd/hth and Dll, and their homologs are expressed respectively in the proximal and distal domains in the appendages of animals as diverse as arthropods and vertebrates, and are required for the proximal and distal development in many Drosophila appendages. It is therefore suggested that the existence of both proximal and distal domains in appendages pre-dates the evolution of the arthropods. However, with the available information, it cannot be said whether these domains in the ancestral appendage were distinct, as they are in the modern Drosophila leg, or overlapping, as they are in the Drosophila antenna. It is speculated that n-Exd and hth, and their vertebrate homologs, the Pbx and Meis genes, were ancestrally expressed in the body wall because they are in modern animals and that as limbs evolved, they were originally expressed throughout the entire outgrowth. Subsequent antagonism by distal factors such as Dll could have allowed for the evolution of additional domains within different appendages (Dong, 2001).This comparison of the Drosophila antenna and leg leads to the conclusion that a fundamental difference between these homologous appendages is the presence of a functional medial domain in the leg, specified by dac. The antenna has fewer segments, with dac expressed at relatively low levels and in only one of the segments, whereas dac is expressed in at least four leg segments. Loss of dac results in medial deletions in the leg but not in the antenna. Repression of proximal and distal genes by dac is not observed in the antenna, as it is in the leg. Consequently, the antennal expression of n-Exd/hth and Dll are not separated in the antenna by a medial domain that expresses dac. For these reasons, it is proposed that the acquisition of a medial domain, possibly through the use of dac, may have been a distinct step in appendage evolution. Consistent with this, increasing the territory and levels of dac expression in the antenna leads to repression of hth and Dll and to the differentiation of medial leg structures (Dong, 2001).Two scenarios by which the existing Drosophila domain organizations may have arisen can be envisioned, given primitive appendages that had only proximal and distal domains. One possibility is that the medial domains were initially acquired by both the antenna and leg, but lost from the antenna sometime prior to the evolution of Drosophila. A second possibility is that the medial domain is an innovation of only the leg and may never have existed in the antenna. The expression of dac in the legs and its absence in the antennae of other arthropods may provide support for the latter scenario. Comparison of the relative domains of expression and the functions of Dll, dac and hth in other organisms will undoubtedly lead to further insights into how distinct PD domains were acquired and became patterned during the course of appendage evolution (Dong, 2001).Ectopic expression of DREF induces DNA synthesis, apoptosis, and unusual morphogenesis in the Drosophila eye imaginal disc: possible interaction with Polycomb and trithorax group proteinsThe promoters of Drosophila genes encoding DNAreplication-related proteins contain transcriptionregulatory element DRE (5'-TATCGATA) in addition toE2F recognition sites. A specific DRE-binding factor, DNA replication-related element factor or DREF, positively regulates DRE-containing genes. In addition, it has beenreported that DREF can bind to a sequence in the hsp70 scs'chromatin boundary element that is also recognized by boundary element-associated factor, and thus DREF may participate in regulating insulator activity. To examine DREF function in vivo, transgenic flies wereestablished in which ectopic expression of DREF wastargeted to the eye imaginal discs. Adult flies expressing DREFexhibited a severe rough eye phenotype. Expression of DREF inducesectopic DNA synthesis in the cells behind the morphogeneticfurrow that are normally postmitotic, and abolishesphotoreceptor specifications of R1, R6, and R7.Furthermore, DREF expression caused apoptosis in the imaginaldisc cells in the region where commitment to R1/R6 cells takes place,suggesting that failure of differentiation of R1/R6 photoreceptor cellsmight cause apoptosis. The DREF-induced rough eye phenotype issuppressed by a half-dose reduction of the E2F gene, one ofthe genes regulated by DREF, indicating that the DREFoverexpression phenotype is useful to screen for modifiers of DREFactivity. Among Polycomb/trithorax group genes, it was found that a half-dose reduction of some of the trithorax groupgenes involved in determining chromatin structure or chromatinremodeling (brahma, moira, and osa)significantly suppresses and that reduction of Distal-lessenhances the DREF-induced rough eye phenotype. The results suggest apossibility that DREF activity might be regulated by proteincomplexes that play a role in modulating chromatin structure. Geneticcrosses of transgenic flies expressing DREF to a collection ofDrosophila deficiency stocks allowed identification of severalgenomic regions, deletions of which caused enhancement or suppressionof the DREF-induced rough eye phenotype. These deletions should be useful to identify novel targets of DREF and its positive or negative regulators (Hirose, 2001). Distal-less: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | References 2b1af7f3a8