BIOLOGICAL OVERVIEW

Pair-rule and segment polarity genes are responsible for determining the uniformity of different segments, in contrast to homeodomain proteins that are responsible for establishing the diversity between segments. Abdominal-B acts in three germ cell layers to fulfill this latter function.

Abdominal-B is the last in linkage order and the most posterior acting of the linked homeodomain proteins of the bithorax. Abdominal-B is unique among the homeotics in that it is transcribed in two forms; a regulatory (r) protein and a morphogenic (m) protein. Regulatory transcripts of Abdominal-B act as repressors, suppressing embryonic ventral epidermal structures in the 8th and 9th segments of the abdomen. Thus ABD-B r and m proteins are critically involved in establishing cell fate in the tail segments of the fly.

Expression is driven by two promoters. The m form is transcribed in parasegments 10-13, corresponding to adult abdominal segments 5-8, while the regulatory protein is transcribed in parasegment 14, corresponding to adult abdominal segment 9. This division of labor does not appear until stage 13, relatively late in development. Earlier, in stage 10, both forms are transcribed in epidermis. In stage 11, both forms are found in epidermis and mesoderm. By stage 12 central nervous system (ventral cord) expression is evident for both forms. In stage 13 m expression becomes restricted to segments 11-13 for all tissues, while r expression becomes restricted to segment 14 for all tissues (De Lorenzi, 1990b). The r protein's designation as regulatory stems from its unique role in segment 14. There it suppresses myogenesis. It is believed that it also represses transcription of the m form.

The distinction between r and m functions was based on the discovery of three classes of regulatory mutations affecting Abdominal-B (Casanova, 1986). One class affects expression in five parasegments (10-14), a second affects expression in only four (parasegments 10-13) and a third class affects expression in just parasegment 14. The regulatory transcript of ABD-B is thought to suppress the proximal morphogenetic (m) function (Casanova, 1986). The smaller r protein differs from its m counterpart in its lack of an M repeat region. This is a particular amino acid segment that lies upstream of the homeobox. The M repeat is rich in glutamines, a classical transcription activation motif (De Lorenzi, 1988).

The lines gene of Drosophila is required for specific functions of the Abdominal-B HOX protein

Genetic evidence shows that lines, a Drosophila segment polarity gene that has yet to be cloned, is required for the function of the Abdominal-B protein. In lines mutantembryos Abdominal-B protein expression is normal but is incapable of promoting its normal function: formation of the posterior spiracles and specification of an eighth abdominal denticle belt. The tail and A8 segment of lin embryos are highly abnormal. The A8 denticle belt is replaced by naked cuticle that occasionally forms a few denticles less pigmented than thenormal ventral denticles. This abnormal A8 cuticle does notresemble the cuticle of any region of the wild-type or of the lin mutantembryo. The absence of anal pads and the abnormal hindgutsuggests abnormal development of abdominal segment 11, however, other aspects of the taildevelopment are normal, such as the formation of an anal tuft. In lin embryos the sensory organs are formed at roughly correct positions but have an abnormal shape (Castelli-Gair, 1998).

The Abd-B gene directs the formation of the posterior spiracles by controlling downstream target genes. The defects associated with lines mutation arise because in linesmutant embryos the Abdominal-B protein cannot activate its direct target empty spiracles (ems) or other downstream genes, such as cut(ct) and spalt(sal), while it can still function as a repressor of Ultrabithorax and abdominal-A. empty spiracles is one gene required for the formation of posterior spiracles. ems expression in the posterior spiracles is regulated by Abd-B. In lin embryos the transcription of ems is not activated in the posterior spiracles, showing that lin is required for Abd-B to activate its direct downstream target. The other putative Abd-B downstream targets cut and spalt are also required for the normal development of the posterior spiracles. The activation of ct and sal in the anlage of the posterior spiracles requires Abd-B function but their activation remains independent of one another and of ems, suggesting that all three genes are independently controlled by Abd-B. In lin mutants neither ct nor sal are activated in the anlage of the posterior spiracles. These results show that in lin mutant embryos, Abd-B is incapable of activating some of its targets. The requirement of lines for Abd-B function is not a specific property of the A8 segment. In wild-type embryos, ectopic Abd-B expression using the GAL4 targeting system results in the formation of ectopic posterior spiracles in segments anterior to A8. In contrast, ectopic Abd-B expression in lin mutants does not form ectopic posterior spiracles showing that no matter where the Abd-B protein is expressed in the embryo it requires lines to be fully functional (Castelli-Gair, 1998).

The effect of lin on Abd-B can be explained at the molecular level if lin is required for protein posttranscriptional modification or as a transcriptional cofactor of Abd-B. Thereis some evidence that the Abd-B protein is posttranslationally modified. If Lin were mediating this process, it would imply that such posttranscriptional modification is functional in vivo. Alternatively if Lines is a transcriptional cofactor of Abd-B, Lines would be interacting with Abd-B in a similar way to that proposed for Extradenticle with Ubx and Abd-A, or Ftz-F1 with Ftz. It is interesting that Exd does not have any effect on Abd-B protein binding or function, and that lin is specific for Abd-B but not for theother Hox genes tested. This suggests that different HOX proteins use different cofactors that contribute to the DNA binding specificity of the HOX proteins (Castelli-Gair, 1998).

The Hox gene Abdominal-B antagonizes appendage development in the genital disc of Drosophila

Abdominal-B is requiredto specify the posterior abdomen and the genitalia.Homologs of Abdominal-B in other species are alsoneeded to determine the posterior part of the body. The function of Abdominal-B in the formation ofDrosophila genitalia has been studied, and the absence ofAbdominal-B in the genital disc of Drosophila has been shown to transformmale and female genitalia into leg or, less frequently, intoantenna. These transformations are accompanied bythe ectopic expression of genes such as Distal-less ordachshund, which are normally required in theseappendages. The extent of wild-type and ectopic Distal-lessexpression depends on the antagonistic activities of theAbdominal-B gene (as a repressor), and of thedecapentaplegic and wingless genes (as activators).Absence of Abdominal-B also changes the expression ofHomothorax, a Hox gene co-factor. These results suggest thatAbdominal-B forms genitalia by modifying an underlyingpositional information and repressing appendagedevelopment. It is proposed that the genital primordia shouldbe subdivided into two regions, one of them competent tobe transformed into an appendage in the absence of Abdominal-B (Estrada, 2001).

Abd-B clones were induced, and they transform posterior abdominal segments into more anterior ones but are normal in the analia. Rare clones transform to distal antennae (second and/or third antennal segment and arista).Transformations to legs or antennae are cell autonomous.The formation of legs requiresthe activity of genes such as homothorax (hth), dac and Dll, which specify the proximal, medial and distal parts of the leg,respectively. Dll expression in wild-type discs is regulated by the combined activities of wingless and dpp in the genitalprimordia, and is confined to two groups of cells inmale and female discs, the female domains being smaller andexpressing lower levels of Dll protein.Since Abd-B is transcribed in the entire genital primordia of thetwo sexes, some cells co-express Abd-B and Dll. Inthe male disc, hth is not expressed in the Dll-expressing cellsand is also excluded from a large group of cells surroundingthem. Levels of antibody signal vary within thedisc, and are higher in the female repressed primordium. Infemales, the hth domain of expression occupies the wholeprimordium. Lower levels of Hth are detected in a regionencompassing the Dll-expressing cells, whereas higher levelsare observed in the male repressed primordium. Inboth sexes, hth expression is absent from the anal primordium.dac is expressed differently in male and female genitalprimordia: in male discs, Dac protein isdetected in two broad lateral bands, while in femalediscs it is found in the central region, almost coincident withthe wg-expressing region. Therefore, the expressionpatterns of hth, dac and Dll differ substantially from thoseobserved in legs (Estrada, 2001).

It is known that expression of Dll is not required to make malegenitalia and that it has only a minor role in the formation ofthe female one. To ascertain the roleof hth in the genitalia, hth minus clones were induced during the third larval period and they were examined in the adult structures. Inthe female genitalia, hth minus clones cause extra growths withadditional vaginal teeth. In males, these clones showoccasionally some abnormalities in the clasper teeth. hth clones in the analia are wild type.Possible interactions between Dll and hth in thegenital disc were sought. In these experiments, unless stated,the results apply both to male and female genital primordia. Dll minusclones in the Dll domain of the male disc have no hth expression. Similarly, in hth minus clones Dll is not ectopicallyexpressed. Dll was also expressed ectopically andthe effect on hth expression was examined. Dll-expressing cells closeto the wild-type Dll domain repress hth expression, although notall the cells do so. By contrast, clones far from the Dll domaindo not affect hth expression (Estrada, 2001).

To characterize the transformation of genitalia into leg or antennal tissues, Abd-B minus clones were examined. Abd-Bminus clones in the genital primordia tend to segregate fromthe rest of the tissue, round up and have smooth borders,suggesting they have acquired different affinities. Bycontrast, clones in the analia have indented borders and donot segregate. Abd-B minus clones in the genitalprimordium close to the normal Dll domain show ectopic,cell-autonomous Dll expression, whereas those farapart do not show such expression. dac is also activated cellautonomously in many Abd-B minus clones.As expected, Dll target genes, such as Bar, also becomeactivated in these clones (Estrada, 2001).

Abd-B minus clones exhibit differential effects on hth, depending on their position: those close to the Dll domain show no hthexpression, whereas those located away from the Dll domainshow a slight increase in hth signal. Clones in intermediatepositions do not significantly change hth levels.This distribution, however, is clearer in females, since in malesthere is a wide region with no hth expression. The repression of hth observed in some Abd-B minus clones may be mediated by the ectopic Dll(Estrada, 2001).

In the genital disc, the transcription of Dll depends, as in theleg disc, on dpp and wg signals. Abd-Brepresses Dll expression. Moreover, increasing Abd-B levelsin the Dll domain suppresses Dll transcription. The antagonistic activities of dpp/wg andAbd-B in determining the Dll distribution was analyzed. Mutations in PKA ectopically activate wg and dpp expression. PKA minus clones in the genital primordia activate Dll, although only in some places. This activation is not mediated by changes in Abd-B levels. Similarly, although Dll is derepressed inmany late Abd-B minus clones, derepressionof either dpp or wg was not observed. It is concluded that there is an antagonism between the activationof Dll by dpp/wg signaling and its repression by Abd-B. Thisis not mediated by changes in the expression of either dpp,wg or Abd-B (Estrada, 2001).

To characterize this antagonism further, Abd-B minusclones that were made were also unable to transduce the dpp signal. Thissignal requires the presence of the type II receptor encoded by the gene punt. In put;Abd-B double mutant clones, Dll is not activated, indicating that, in the absence of Abd-B, Dpp (and possibly Wg) are still required to activate Dll. Abd-B minus clones far from the wild-type Dll domain fail to activate Dll ectopically, suggesting that activation of Dll in the absence of Abd-B depends on the range of diffusion of Dpp and Wg, as in the leg disc and in the anal primordium (Estrada, 2001).

Dll is required for the development of legs and antennae, andinduces these structures when expressed ectopically in the wingor eye-antennal discs. However, although Dll is also expressed in the genital primordia this expression does not lead to the formation of any ofthese appendages. To test if Abd-B prevents Dll function Abd-B was eliminated in Dll-expressing cells; these cells formed leg tissue. However, it is possible that the highlevels of Dll observed in these mutant cells account for the legtransformation. To test this, use was made of the GAL4/UASsystem to increase Dll expression in the genital disc (dpp-GAL4/UAS-Dll flies). Male and female genitalia of thisgenotype are abnormal, but not transformed into leg.To extend these observations, the ability of Dllto promote Bar transcription, a gene expressed in the leg discand activated by Dll, was examined. Bar is not expressedin the female genital primordium and only in a few cells withinthe Dll domain in the male genital primordium;however, Abd-B minus clones show Bar expression in both sexes. When Dll is ectopically expressed in the genital disc,Bar expression is activated in some of the cells that express Dll. These results suggest that, in females, Dll levelsare insufficient to activate Bar when Abd-B is present, but thatincreasing Dll expression or removing Abd-B activates Bartranscription. Abd-B, therefore, prevents some Dll activity infemales. In males, although there is Bar transcription, leg tissueis not formed, probably because Abd-B modifies or preventsthe activation of other Dll target genes. A similar case has beenreported in the wing disc: ectopic Dll activates bric a brac, agene downstream of Dll, both in the wing pouch and the bodywall region of the wing disc; however, legs appear in the wing,but not in the notum (Estrada, 2001).

The Hox gene Antennapedia is involved in legdevelopment. Therefore, an examination was performed to seewhether Antp is derepressed in Abd-B minus clones. Antp is nottranscribed in the wild-type genital disc, but some Abd-B clones show Antp signal. The presence of the Antpproduct, however, is not required to transform the genitalia intoa leg, since Antp:Abd-B double mutant clones still formectopic legs. This result is consistent with the viewthat the role of Antp in leg specification is simply to represshth expression. It seems that Dllalone is able to direct leg development, provided that Hox andhth genes are not transcribed. Under these conditions, leg tissuecan be formed in several appendages: leg, wing, antennal andgenital primordia (Estrada, 2001).

Ubx and abdominal A expression were examinedin Abd-B minus clones. Ubx was not derepressed in these clones,whereas some clones presented weak ectopic abd-Aexpression, but only in some cells (Estrada, 2001).

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc andrequire the activity of the Abdominal-B (Abd-B) Hox gene.This gene encodes two different proteins, Abd-B M and Abd-B R. The embryonic genital disc, like the larval genital disc, is formed bycells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments,which most likely express the Abd-B M, Abd-B R and Caudal products,respectively. Abd-B m is needed for the development of A8 derivativessuch as the external and internal female genitalia, the latter also requiringabdominal-A (abd-A), whereas Abd-B r shapes malegenitalia (A9 in males). Although Abd-B r represses Abd-B min the embryo, in at least part of the male A9 such regulation does not occur.In the male A9, some Abd-B m^–r^– orAbd-B r^– clones activate Distal-less andtransform part of the genitalia into leg or antenna. In the female A8, manyAbd-B m^–r^– mutant clones producesimilar effects, and also downregulate or eliminate abdominal-Aexpression. By contrast, although Abd-B m is the main or onlyAbd-B transcript present in the female A8, Abd-Bm^– clones induced in this primordium do not alterDistal-less or abd-A expression, and transform the A8segment into the A4. The relationship between Abd-B andabd-A in the female genital disc is opposite that of the embryonicepidermis, and contravenes the rule that posteriorly expressed Hox genesdownregulate more anterior ones (Foronda, 2006).

Abd-B is a complex gene: the use of four different promoters and the existence of specific exons give rise to several transcripts that encode two different proteins. The A (m) transcript encodes the Abd-B M (or Abd-B I) protein, and the B, C (r) and gamma RNAs encode theAbd-B R (or Abd-B II) protein. The Abd-B M protein has 221 amino acids more than the Abd-B R product does in its N-terminal domain but both proteins share a commonC-terminal region, which includes the homeodomain.In the embryonic epidermis, the Abd-B M transcript and protein are expressedin parasegments (PS) 10-13 (A5-A8 segments), whereas the Abd-B R transcriptand protein are present in PS14-PS15 (A9-A10) initially, and in PS14 (A9) atlate stages. The gamma RNA is transcribed in just a fewcells of PS14 or PS15 (Foronda, 2006 and references therein).

The role of Abd-B M and Abd-B R products in genital development remainsunclear. Abd-B m mutations transform the A5-A8 segments into the A4segment, both in males and females; the female genitalia are lost whereas malegenitalia remain intact. Significantly, the transformations obtained in either Abd-B m or Abd-B r mutants clearly differ from those observed when all Abd-B functions are eliminated: in some of the clones mutant forAbd-B (m and r), part of the male or femalegenitalia are transformed into leg or antenna. Therefore, the precise role of abd-A, Abd-B m and Abd-B r in genitalia development is not well defined (Foronda, 2006).

This study has analyzed homeotic expression and requirement in terminaliadevelopment. It is proposed that in the embryonic genital disc, as in the larvaldiscs, Abd-B m, Abd-B r and cad are expressed in the A8, A9and A10, respectively. It is also reported that abd-A, Abd-B m andAbd-B r are needed for development of the internal female genitalia,Abd-B m for the development of female external genitalia andAbd-B r for the development of male genitalia. Strikingly,abd-A and Abd-B bear unexpected relationships in maturegenital discs. In the A8 of the female genital disc, Abd-B M maintainsabd-A expression. In Abd-B m mutant clones, however, anotherAbd-B protein maintains abd-A expression but does not preventabd-A function, since these clones transform the A8 segment into the A4.In the male A9, Abd-B r function does not repress the Abd-Bm transcript, at least in part of the primordium, and some Abd-Br mutant clones transform male genitalia into leg or antenna. Theserelationships between Hox genes are different from those reported in theembryonic epidermis and contravene the rule that posteriorly expressed Hoxgenes repress those expressed more anteriorly (Foronda, 2006).

In the third instar genital disc of Drosophila, Abd-B is expressedin the A8 and A9 segments, and cad in the A10. Tostudy whether these expression domains are established early in development,Abd-B and cad transcription were examined in theembryonic genital disc. This disc is identified by the expression of geneslike snail, escargot or headcase (hdc), and the hdc-lacZ B5 line, which reproduces the pattern of hdc RNA expression, was selected to mark the genital disc. At about stage 15, hdc is expressed in three clusters of cells, two anterior ones placed bilaterally, and a third onelocated in a more posterior and central position. The three clustersfuse later in development to form the genital disc. At stage 15, six to seven cells were counted at each of the two anterior groups, and two to three cells in the posterior one, making up a total of 14-17 cells. Doublestaining with anti-Abd-B and anti-ß-galactosidase antibodies (inhdc-lacZ embryos), or with GFP and anti-ß-galactosidase antibody(in cad-Gal4/UAS-GFP; hdc-lacZ/+ embryos), shows thatAbd-B is expressed in the two anterior clusters and cad inthe posterior one (Foronda, 2006).

To ascertain whether the two Abd-B products (Abd-B M and Abd-B R) arepresent in the genital disc primordium, the expression driven byan Abd-B m-Gal4 line was compared with the signaldetected with an antibody that recognizes both Abd-B M and Abd-B R proteins. InUAS-myc-EGFP^F/+;Abd-B-Gal4^LDN/hdc-lacZ embryos, a GFPsignal was seen in about two cells located laterally in each of the two anteriorclusters; these cells most likely express Abd-B m, and, therefore,are also labelled with the anti-Abd-B antibody. There are also8-10 Abd-B-expressing cells not labelled with GFP, and these,probably, correspond to those expressing the Abd-B R protein. Taken together, these results suggest that the embryonic genital primordium includes threegroups of cells that probably express Abd-B m, Abd-B r andcad, respectively (Foronda, 2006).

Study of mutant phenotypes reveals that as in theembryonic cuticle, abd-A and Abd-B m are needed in the A8whereas Abd-B r is required in the A9. The relationship between thesehomeotic products in the mature genital discs, however, clearly differs fromwhat is observed in the embryonic epidermis. The embryonic genital disc has three distinct cell populations at stages 15/16: some anterior-lateral cells transcribe Abd-B m, anterior-central and middle cells express Abd-B r andposterior cells transcribe cad, although the expression of theseproducts may overlap. Because the genital disc is formed by the fusion ofcells coming from the A8, A9 and A10 segments,and by analogy to the expression of these genes in the mature genital discs, it is concluded that Abd-B m, Abd-B r and cad areprobably expressed in the A8, A9 and A10 segments, respectively, of theembryonic genital disc (Foronda, 2006).Abd-B is not only expressed, but also required in the embryonicgenital primordium. In the absence of Abd-B m, the number ofhdc-expressing cells in the disc is reduced, most likely becausethese cells adopt now a more anterior fate, as occurs in the cuticle. When Abd-B r is absent, the genital primordium lacks some cells and is disorganized, and when both Abd-B products are absent, the primordium is reduced to a few, dispersed cells, some of which express Dll ectopically, suggesting a transformation into a leg primordium (Foronda, 2006).

The A8, A9 and A10 primordia of the mature genital discs bear anterior andposterior compartments, with expression of en and wg in eachof these three primordia. Curiously, although three primordia in theembryonic disc can be defined, based on the expression of Abd-B m, Abd-B r and cad, neither en nor wg is expressed in the threeseparate domains at this stage. This may suggest, as was also recently proposed, that new bands of en and wg expression may be formed later indevelopment, in precise concordance with the three primordia definedby the Abd-B m, Abd-B r and cad genes. It is notedthat late en expression is also characteristic of the antennalprimordium of the eye-antennal disc (Foronda, 2006).

abd-A is expressed in the whole internal femalegenitalia except for the parovaria, and this is consistent with experimentsindicating that parovaria derive from the female A9 segment.abd-A has been shown to be required for gonad development, and in theabd-A^iab-3/Df mutant, combinations ovaries are also absent.However, the defects observed in the female internal genitalia are notsimply due to an indirect effect of the lack of gonads, since iab-4mutations prevent the formation of the ovaries but do not alter internalgenitalia formation (Foronda, 2006).

The results indicate that Abd-B m is required for the developmentof female external and internal genitalia, both derived from the female A8.The internal genitalia of Abd-B-Gal4^LDN/UAS-lacZfemales (driving expression only where Abd-B m levels are high)were stained with X-gal except in two structures, the oviducts andparovaria. The absence of oviduct staining in Abd-B-Gal4^LDN/UAS-lacZ females is probably due to theparticular expression driven by this reporter, and does not imply an absenceof Abd-B m transcription in these organs, for two reasons: (1)Abd-B m transcripts are present in the whole A8 segment of the femalegenital disc, and (2) oviduct development is affected in Abd-B mmutant females. Parovaria, by contrast, are not stained in Abd-B-Gal4^LDN/UAS-lacZ or abd-A-lacZ females, andthis agrees with their A9 provenance. This is supported by the observation that in some Abd-B m mutant females parovaria are the only structures that remain in the internal female genitalia (Foronda, 2006).

Abd-B M seems to be the main or only Abd-B product present in thefemale A8, so it was expected that elimination in this segment of just Abd-B Mor of all Abd-B proteins would give similar results. This is not so. SomeAbd-B^– clones transform part of the female genitaliainto leg or antenna, whereas Abd-B m mutant clones convert the eighth tergite, and probably the female genitalia, into an anterior abdominal segment. The differences between Abd-B m^– and AbdB^– clones in the A8 of the female genital disc reveal the existence of unsuspected regulatory interactions between the abd-A and Abd-B genes: whereas Abd-B m^– clones do not affect abd-A, in AbdB^– clones abd-A expression iseliminated. This is a surprising result, because it is contrary to what isobserved in the embryo, where Abd-B represses abd-A (Foronda, 2006).

Abd-B m^– clones induced in the female A8 do notalter abd-A expression but do not change Abd-B expressionlevels either. This is observed with mutations that do not make Abd-B Mprotein, so the Abd-B protein detected is not the Abd-B M product.Surprisingly, although some Abd-B r expression is detected in thefemale A8, uniform Abd-B r expression is not seen throughout thisprimordium and Abd-B r transcripts seem not to be derepressed inAbd-B^M5 mutant clones. No explanation is available for thisconundrum. Perhaps the probe used, although it includes sequences complementary to all of the Abd-B r cDNA sequences that have been published, does not efficiently detect all of the non-Abd-B m transcripts (Foronda, 2006).

The differences in regulatory and functional interactions among geneproducts in the embryo and the genital discs are not limited to those ofAbd-B and abd-A that have been discussed above. Three other possibilities should be considered. (1) Theremay be changes in phenotypic suppression: the transformation of the eighthtergite to the fourth one in Abd-B m^– clones is dueto abd-A. Because in these clones Abd-B protein is still present, this suggests that abd-A may phenotypically suppress Abd-B, differently from what is generally observed in the embryo. (2) Abd-B r represses Abd-B m in the embryo, but some Abd-B r^– clones do not activate Abd-B m in the male disc. (3) abd-A represses Dll in the embryo, but not in the female genital disc, and ectopic Dll can repress abd-A instead. abd-A does not repress Dll in the leg discs either, and this resembles Ubx function, which represses Dll only early in development. By contrast, Abd-B represses Dll in the embryo, in the larval genital disc, and in the leg disc when ectopically expressed (Foronda, 2006).

Abd-B r expression is restricted to the A9 segment in male genitaldiscs, but shows expression in the A9 and in some cells of the A8 in femalegenital discs. In spite of this, Abd-B r^– clones inthe external female genitalia (A8) are phenotypically wild type. In the maleA9, some Abd-B r mutant clones eliminate Abd-B, activateDll and transform part of the genitalia into distal leg or antenna.This is similar to the result obtained in someAbd-B^– clones, and it implies that Abd-B mis not derepressed in these mutant clones. However, Abd-B m isperhaps derepressed in those Abd-B r mutant clones whereAbd-B signal remains (Foronda, 2006).

Although Abd-B r^– clones affect, almostexclusively, male genitalia development, Abd-B r hemizygous ortrans-heterozygous flies lack genitalia and analia in both sexes. Thisprobably reflects the absence of proper interactions between the differentprimordia needed for the growth of the genital disc. InAbd-B r mutant females, the internal genitalia are abnormal, and insome of these females, an absence of parovaria and the presence ofthree or four spermathecae is observed. This phenotype is consistent with asegment-autonomous transformation of A9 derivatives (parovaria) into A8structures (spermathecae), similar to the embryonic cuticular transformationof A9 into A8 observed in Abd-B r mutations. A transformation of parovaria into spermathecae has been described in Polycomblike mutants, and may also indicate a transformation of A9 to A8 (Foronda, 2006).

These results illustrate that there are quite different Hox cross-regulatoryinteractions in the embryo and in the genital disc. The effects in thegenital discs contradict the general rule that genes transcribed moreposteriorly suppress or downregulate the expression of more anterior ones. Thisrule has, nevertheless, some exceptions in genes of the Antennapedia complex.Further, differences in Hox cross-regulation between the embryo and imaginaldiscs are not unprecedented: the proboscipedia (pb) Hox geneis positively regulated by Sex combs reduced in the embryo, butpb activates Sex combs reduced in the labial imaginal disc (Foronda, 2006).

It has been proposed that the primordia of female and male genitalia couldbe subdivided into an 'appendage-like' and a 'trunk-like' region). These two regions of the female A8 can now be defined moreprecisely. The 'appendage-like' region would be that expressing abd-Aand low levels of Abd-B, and corresponds approximately to thepresumptive internal female genitalia. This domain isroughly coincident with the region of expression of a reporter insertion inbuttonhead, the gene that defines ventral appendage development, andthis is also, approximately, the domain where Abd-B^–clones may activate Dll. If this subdivision is correct,the 'appendage' specification defined by buttonhead would berepressed in the wild type by Abd-B, which both limits Dllexpression to a few cells of the A8 primordium and prevents Dllfunction. Abd-B^– clones in this region eliminate abd-A expression and promote leg or antenna development. This subdivision may also apply to the male disc, the penis apparatus presumptive region being the main 'appendage' domain. Similar to what is described in this study, the labial disc possesses a large 'appendage' region that is revealed by Dll derepression in pb mutations. This characteristic, and the changes in Hox gene cross-regulation between the embryo and the imaginal disc, are two features shared by pb/labial disc and Abd-B/genital disc (Foronda, 2006).

GENE STRUCTURE

Four classes of overlapping transcripts are generated from theAbd-B gene The transcriptioninitiation sites for the class A (4.6 kb) and class B (3.4kb) transcripts show that they are generated from separatepromoters. Both of these transcripts are present throughout the period during which the ABD-Bsubfunctions are required. A mutation that inactivates the morphogenetic function is associated witha 411bp deletion of the initiation site for the 4.6 kb RNA. Regulatory function mutationsdisrupt the transcription unit for the 3.4 kb RNA, but not the 4.6 kb RNA. A morphogenetic (m) function is assigned to the 4.6 kb RNA and a regulatory (r) function to the3.4 kb RNA.

A 7.8 kb RNA expressed during embryogenesis may also contribute to theregulatory function. Sequence analysis of cDNAs indicates that the 4.6 kb RNA encodes the 55-kD morphogenetic protein, whereas the 3.4 kb RNA encodes a 30-kD regulatory protein. The m and r proteins share a carboxy-terminal sequence that includes the homeodomain, but the r proteinlacks a glutamine-rich amino-terminal domain found in the m protein (Celniker,1989 and Zavortink, 1989).

Exons - four for class A transcript and five for class B transcript

Bases in 3' UTR - 1604

PROTEIN STRUCTURE

There are two variants to the ABD-B protein. One, a morphogenetic function can be assigned to the 55 KD protein and a second, regulatory function can be assigned to a 30 KD protein (Boulet, 1991).

Amino Acids

The class r 4.6 kb transcript encodes for a protein of 493 amino acids. The class m 3.4 kb transcript encodes for a protein of 270 amino acids (Zavortink, 1989).

See Four paralogous Hox clusters of mammals for homologies of Abd-B with mammalian Hox cluster genes.

date revised: 25 APR 97  

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