The metameric organisation of the Drosophila embryo is generated early during development, due to the action of maternal effect and zygotic segmentation and homeotic genes. The gap genes participate in the complex process of pattern formation by providing a link between the maternal and the zygotic gene activities. Under the, influence of maternal gene products they become expressed in distinct domains along the anteroposterior axis of the embryo; negative interactions between neighboring gap genes are thought to be involved in establishing the expression domains. The gap gene activities in turn are required for the correct patterning of the pairrule genes; little is known, however, about the underlying mechanisms.

We have monitored the distribution of gap and pairrule genes in wild-type embryos and in embryos in which the anteroposterior body pattern is greatly simplified due to combinations of maternal effect mutations (staufen exuperantia, rusa exuperantia, bicoid oskar, bicoid oskar torsolike, vasa torso exuperantia). We show that the domains of protein distribution of the gap genes hunchback and Krüppel overlap in wild-type embryos. Based on the analysis of the maternal mutant combinations, we suggest an explanation of how this overlap is generated. Furthermore, our data show that different constellations of gap gene activities provide different input for the pair-rule genes, and thus strongly suggest that the overlap of hunchback and Krüppel in wild-type is functional in the formation of the patterns of pair-rule genes.

The metameric organization of the Drosophila embryo is generated early during development. Maternal factors set the frame in which the zygotic segmentation and homeotic genes act. Due to the concerted action of the segmentation genes the appropriate number of metameric units, or segments, is established, and the homeotic selector genes control the diverse pathways by which each of the segments acquires a unique morphology. The gap genes participate in the complex process of pattern formation by providing a link between the maternal and the zygotic gene activities: the gap gene patterns are controlled by the maternal factors, and the gap gene activities in turn control the pattern of zygotic segmentation and homeotic genes (for review see Akam, 1987; Nüsslein-Volhard et al. 1987; Ingham, 1988).

Three groups of maternal effect genes organize the anteroposterior body pattern of the Drosophila embryo. Under their influence the gap genes establish distinct domains of expression along the anteroposterior axis: hunchback (hb) is expressed in the anterior of the embryo as a result of activation by the anterior maternal organizer bicoid (bed) (Tautz, 1988; Driever and Nüsslein-Volhard, 1989). The Krüppel (Kr) domain is established in the middle of the embryo due to repression of an otherwise constitutive expression by all three maternal organizer activities (Gaul and Jackie, 1987, 1989). Apart from the maternal instruction, negative interactions between neighboring gap genes are thought to be involved in the establishment of the expression domains of the gap genes (Meinhardt, 1986; Jackie et al. 1986).

Gap gene activities are necessary for instructing the patterning of the pair-rule genes. Pair-rule genes are expressed in series of 7 (or 8) discrete stripes in wildtype embryos. In gap gene mutant embryos, the patterns of the pair-rule genes show severe perturbations, indicating that the gap genes control pair-rule gene activity (Carroll and Scott, 1986; Frasch and Levine, 1987; Carroll et al. 1988). However, these defects are not suggestive of any straightforward interpretation, and it is not yet well understood how the few gap genes, with their comparatively broad, nonperiodic domains of expression manage to establish the intricate periodic pair-rule gene patterns.

We have monitored the distribution of gap and pairrule genes in wild-type embryos and in embryos in which the anteroposterior body pattern is greatly simplified due to various combinations of maternal effect mutations (staufen exuperantia (stau exu), vasa exuperantia (vas exu), bicoid oskar (bed osk), bicoid oskar torsolike (bed osk tsl), vasa torso exuperantia (vas tor exu))’, in such embryos the patterns of segmentation genes are likewise simplified. We show that the domains of hb and Kr protein distribution, rather than being adjacent, overlap in wild-type embryos. The analysis of maternal effect mutant combinations suggests that the overlap results from differential regulation of hb and Kr by the anterior maternal organizer bed. Furthermore, we show that, in the maternal mutant combinations, the pair-rule genes pattern where the constellation of gap gene activities changes. The data also demonstrate that different constellations of gap gene activities provide qualitatively different input for the pair-rule genes, and thus strongly suggest that the overlap of hb and Kr is functionally relevant for the formation of the patterns of pair-rule genes.

The mutant combinations bcdEIosk166tsl491 and bcdE2osk166 have been described by Nüsslein-Volhard et al. (1987). The mutant combinations vasPDexuPJ and stauHLexuPJ have been described by Schiipbach and Wieschaus (1986). For our study, however, we produced a different allele combination, stauD3exuPJ, for only under such conditions does the ‘bithoracic’ phenotype become fully penetrant. The vasPDtorWKexuPJ combination was kindly provided by Trudi Schüpbach.

To test which promoters are used for the expression of hb in stau exu and vas exu embryos, we crossed males carrying hb promoter-lacZ fusion genes to homozygous maternal effect mutant mothers and monitored the β-galactosidase (/3-gal) distribution with anti--gal-antibodies. We used two different lines: One carries the upstream control elements of the 2.9 kb hb transcript necessary for the expression of the anterior hb domain, and has been shown to contain the bed responsive elements (hb 1.2-lacZ; Schroder et al. 1988). The other carries upstream control elements of the 3.2 kb hb transcript, which is necessary for the early maternal expression and for two stripes of expression in the late blastoderm, at about 50 % and 15% EL, respectively (Schroder, unpublished).

Embryos derived from homozygous mutant mothers (in the following, such embryos will be named according to the genotype of the mother for the sake of brevity) were dechorionated, permeabilized, and fixed using standard procedures. After washing and blocking in BBT (0.15 % crystalline bovine serum albumin, 10mM-Tris-HCl, pH 7.5, 50mM-NaCl, 40mM-MgCl2, 5mM-CaCl2, 20mM-glucose, 50mM-sucrose, 0.1% Tween 20) they were treated with primary antibodies overnight. (Anti-hb antibodies, anti-fushi tarazu (ftz), and anti-engrailed (en) antibodies were kindly provided by D. Tautz, H. Krause, and N. Patel, respectively.) The bound antibodies were detected with biotinylated secondary antibodies and stained with Vectastain ABC-kit (Vectorlabs) using diaminobenzidine following the manufacturer’s instructions except that 0.03% CoCl2 were added to the staining solution. In order to compare expression levels under mutant conditions with those in wild-type, we added wild-type embryos to the batches of mutant embryos and processed them together. For the analysis of the wild-type patterns of hb and Kr, we also used the Vectastain ‘Elite’kit and 0.02-0.1 % NiCl2, which gave improved results.

Overlap of the protein domains o/hb and Kr in wildtype

Due to the maternal transcription of the hb gene, hb RNA is already present in freshly laid eggs, hb protein is first detected prior to pole-cell formation, showing a graded distribution along the anteroposterior axis with high levels in the anterior 1/3 and decreasing levels in the posterior 2/3 (Tautz et al. 1987). The zygotic expression of hb begins at syncytial blastoderm (stages 11/12), with hb protein present in the anterior half of the embryo. At the onset of cellularization, a second domain of expression has emerged posteriorly, forming a cap at the posterior pole (Fig. 1A). As cellularization proceeds, high levels of protein accumulate both in the anterior and in the posterior domain (Fig. IB). At the end of cellular blastoderm, the anterior domain recedes from the anterior tip and becomes inhomogenous in that two stripes of stronger staining appear within the domain. In the posterior, a stripe replaces the cap (Fig. 1C) (see also Tautz, 1988). The borders of the hb domains are not sharp at any time. Fig. IE shows a glancing view of the posterior margin of the anterior hb domain at the middle of stage 14: the nuclear staining reaches background level only gradually, over a range of about 5 cells.

Fig. 1.

Protein expression of hb and Kr in wild-type embryos. Orientation is anterior to the left and dorsal up. (A-C) hb expression. At early stage 14 (A), hb is expressed in the anterior half of the embryo, as well as in a ‘cap’ at the posterior pole. At the middle of stage 14 (B), the accumulation of protein has increased in both domains, the posterior domain becomes non-homogenous in that the terminalmost portion of the domain shows lower levels of expression. At latestage 14 (C), the anterior domain recedes from the tip and the posterior ‘cap’ is replaced by a stripe. (D,E) Embryo at the middle of stage 14 in different focal planes. (D) Sagittal optical section. The arrows mark 50% EL. Compare with I, which shows a Kr staining of an embryo of about the same age. (E) Focus on the periphery of the embryo, in the region of the posterior border of the anterior hb domain. The hb staining fades out gradually toward the posterior margin, over a range of about 5 nuclei. (F-K) Kr expression. At early stage 14 (F), a band of Kr staining is visible, covering about 60 to 33% EL. The staining is quite weak at this stage, and barely above background level at the margins. By the middle of stage 14 (G), the accumulation of protein has strongly increased. At late stage 14 (H), two additional domains of Kr expression are visible, a ‘cap’ at the posterior pole, and a stripe at about 82% EL. (I,K) Embryo at the middle of stage 14 in different focal planes. (I) Sagittal optical section. The arrows mark 50% EL. (K) Focus on the periphery of the embryo, in the region of the Kr domain. The Kr staining fades out toward the margins, attenuating more gradually in the posterior than in the anterior.

Fig. 1.

Protein expression of hb and Kr in wild-type embryos. Orientation is anterior to the left and dorsal up. (A-C) hb expression. At early stage 14 (A), hb is expressed in the anterior half of the embryo, as well as in a ‘cap’ at the posterior pole. At the middle of stage 14 (B), the accumulation of protein has increased in both domains, the posterior domain becomes non-homogenous in that the terminalmost portion of the domain shows lower levels of expression. At latestage 14 (C), the anterior domain recedes from the tip and the posterior ‘cap’ is replaced by a stripe. (D,E) Embryo at the middle of stage 14 in different focal planes. (D) Sagittal optical section. The arrows mark 50% EL. Compare with I, which shows a Kr staining of an embryo of about the same age. (E) Focus on the periphery of the embryo, in the region of the posterior border of the anterior hb domain. The hb staining fades out gradually toward the posterior margin, over a range of about 5 nuclei. (F-K) Kr expression. At early stage 14 (F), a band of Kr staining is visible, covering about 60 to 33% EL. The staining is quite weak at this stage, and barely above background level at the margins. By the middle of stage 14 (G), the accumulation of protein has strongly increased. At late stage 14 (H), two additional domains of Kr expression are visible, a ‘cap’ at the posterior pole, and a stripe at about 82% EL. (I,K) Embryo at the middle of stage 14 in different focal planes. (I) Sagittal optical section. The arrows mark 50% EL. (K) Focus on the periphery of the embryo, in the region of the Kr domain. The Kr staining fades out toward the margins, attenuating more gradually in the posterior than in the anterior.

Kr protein is first detectable at syncytial blastoderm (stage 12) in the middle region of the embryo. At the onset of cellularization, low levels of Kr protein are detectable in a band covering about 60-33% EL (Fig. IF). The accumulation of Kr protein within this band strongly increases as cellularization proceeds (Fig. 1G). The more sensitive staining technique thus reveals that the Kr protein domain is broader than we were able to detect earlier (we reported 54–39 % in Gaul and Jackie (1987)). By the end of cellularization, this central Kr protein domain has narrowed, and two additional domains of Kr expression have been established, a ‘cap’ at the posterior pole, and a stripe at about 85% EL (Fig. 1H). The Kr domain also never shows sharp borders. Rather, the protein staining is most intense in the center of the domain and fades out gradually toward the margins (Fig. IK) (see also Gaul et al. 1987). The fading out of both hb and Kr staining at the margins makes it difficult to define the exact limits of the protein domains; they depend considerably on the sensitivity of detection.

The protein domains of hb (in the anterior) and Kr are not juxtaposed, but overlap substantially. This can be demonstrated by aligning matched embryos of which one is stained for hb and the other for Kr (see Fig. ID,I). Judging from such superpositions, the over lap comprises about 8-12 cells at the middle of stage 14, depending on how strongly the antibody staining has been ‘pushed’ (see Materials and methods). The region of overlap is not homogenous with respect to hb and Kr protein concentration levels. The fact that both hb and Kr staining fade out toward the margins of the domains (Fig. 1E,K) implies that it is only in the center (approximately) of the entire band of overlap that hb and Kr product are both present in relatively high concentrations, while anteriorly high hb levels are accompanied by decreasing concentrations of Kr, and posteriorly high levels of Kr by ceasing levels of hb.

This co-distribution of hb and Kr protein in a band of at least 8 cells in width immediately raises two questions: (I) how does the overlap come about?, and (II) is this overlap functionally relevant for the process of segmentation, or, more specifically, does the simultaneous presence of hb and Kr have an effect on the expression state of the pair-rule genes?

In addressing these questions, it was instructive to analyze the patterns of gap and pair-rule genes in a number of maternal effect mutant combinations: stau exu, vas exu, bed osk tsl, bed osk, vas tor exu. The phenotypes of these mutant combinations lack most of the wild-type pattern elements, and (except in the case of vas tor exu) the residual elements are arranged in mirror-image symmetry. In such embryos, the patterns of gap and pair-rule genes (and similarly of segment polarity and homeotic genes, cf. Gaul and Jäckle, 1989) become much simpler, and the reduction in the number of parameters facilitates the interpretation of the correlations between the relevant patterns.

The patterns of hb and Kr expression in stau exu and vas exu embryos

The maternal effect genes that are involved in the organization of the anteroposterior body pattern have been classified into three groups according to their mutant phenotypes: they are required for the organization of either (1) the anterior, (2) the posterior, or (3) the terminal regions of the embryo (Nüsslein-Volhard et al. 1987).

In the maternal effect mutant combinations stau exu and vas exu, the anterior pattern organizer bed is affected by a reduction in the amount of product and a change in spatial distribution (exu)-, in addition, posterior organizers are defective (stau, vas, respectively). The combination of these mutations leads to a ubiquitous distribution of low amounts of bed activity (Struhl et al. 1989; and see below). The terminal organizers, however, are intact in both mutant combinations.

The phenotype of stau exu embryos is characterized by a lack of head, gnathal and abdominal structures. At the poles of the embryo, two telsons of opposite orientation develop, while, in the middle, segments of thoracic identity form (Fig. 2M) (Schüpbach and Wieschaus, 1986). In contrast, the only cuticular structures formed in vas exu embryos are of gnathal origin (maxillary mouthhooks and cirri); besides, posterior midgut invaginations form at both ends of these embryos during gastrulation, indicating that posterior terminal structures do develop (Fig. 3H) (Schüpbach and Wieschaus, 1986).

Fig. 2.

The effect of stau exu mutations on the expression patterns of hb, Kr, eve, ftz and the segment polarity gene engrailed (en), as visualized by antibody staining, and on the cuticle phenotype. (A-C) hb expression at early (A), late (B) stage 14 and gastrulation (C). hb is expressed throughout the embryo. (D,E) eve expression in early (D) and late (E) stage 14. eve is initially (D) expressed almost throughout the embryo. At late stage 14, two broad eve stripes have formed close to the poles, with the center of the stripes lying at about 20 and 80% EL, respectively (for description of the wild-type pattern of eve, see Frasch et al. (1987)). (F) en expression at gastrulation stage: two stripes are present at about 25 and 75 % EL, respectively. At later stages, two additional stripes of en expression are found in either pole region, which are probably associated with the telson (for description of the en wild-type pattern, see DiNardo et al. (1985)). (G-I) Kr expression at early (G), late (H) stage 14 and gastrulation (I). Kr is expressed in a broad domain covering most of the embryo (about 20-80 % EL). The represssion at the poles is due to the action of terminal organizer genes (Gaul and Jâckle, 1989). High levels of expression persist until after gastrulation. In gastrulating embryos (I) the posterior domain of Kr expression is duplicated anteriorly. (K,L) ftz expression in early (K) and late (L) stage 14. At late stage 14, ftz is expressed in a broad domain in the middle region (about 25-75 % EL), in addition two partial stripes form (at approximately 15 and 85 % EL), which may be associated with the telson (for description of the ftz wild-type pattern, see Carroll and Scott (1985)). (M) Cuticle phenotype, darkfield photograph. Two telsons develop at either end of the embryo. In the middle, segments of thoracic identity form (for a detailed description of the phenotype, see Schüpbach and Wieschaus (1986)).

Fig. 2.

The effect of stau exu mutations on the expression patterns of hb, Kr, eve, ftz and the segment polarity gene engrailed (en), as visualized by antibody staining, and on the cuticle phenotype. (A-C) hb expression at early (A), late (B) stage 14 and gastrulation (C). hb is expressed throughout the embryo. (D,E) eve expression in early (D) and late (E) stage 14. eve is initially (D) expressed almost throughout the embryo. At late stage 14, two broad eve stripes have formed close to the poles, with the center of the stripes lying at about 20 and 80% EL, respectively (for description of the wild-type pattern of eve, see Frasch et al. (1987)). (F) en expression at gastrulation stage: two stripes are present at about 25 and 75 % EL, respectively. At later stages, two additional stripes of en expression are found in either pole region, which are probably associated with the telson (for description of the en wild-type pattern, see DiNardo et al. (1985)). (G-I) Kr expression at early (G), late (H) stage 14 and gastrulation (I). Kr is expressed in a broad domain covering most of the embryo (about 20-80 % EL). The represssion at the poles is due to the action of terminal organizer genes (Gaul and Jâckle, 1989). High levels of expression persist until after gastrulation. In gastrulating embryos (I) the posterior domain of Kr expression is duplicated anteriorly. (K,L) ftz expression in early (K) and late (L) stage 14. At late stage 14, ftz is expressed in a broad domain in the middle region (about 25-75 % EL), in addition two partial stripes form (at approximately 15 and 85 % EL), which may be associated with the telson (for description of the ftz wild-type pattern, see Carroll and Scott (1985)). (M) Cuticle phenotype, darkfield photograph. Two telsons develop at either end of the embryo. In the middle, segments of thoracic identity form (for a detailed description of the phenotype, see Schüpbach and Wieschaus (1986)).

Fig. 3.

The effect of vas exu mutations on the expression patterns of hb, Kr, eve, ftz and en as visualized by antibody staining, and on the cuticle phenotype. (A) hb expression in a mid-stage 14 embryo, hb is expressed throughout the egg. (B,C) eve expression in early (B) and late (C) stage 14. By late stage 14, two stripes close to the poles (at about 20 and 80% EL) have formed. (D) en expression at gastrulation. Two stripes are present at about 25 and 75 % EL, later additional stripes form in the pole regions. (E) Late stage 14 embryo stained against Kr protein. Before the end of cellularization, no Kr expression could be detected. With the onset of gastrulation, Kr protein becomes visible in different regions of the embryo (data not shown). (F,G) ftz expression in early (F) and late (G) stage 14. At late stage 14, ftz is expressed in a broad domain in the middle region (about 20-75 % EL); note that the staining is weaker within the domain than at its margins. (H) Cuticle phenotype, dark-field photograph. Very few cuticular landmarks are discernible; we identify maxillary structures (cirri and mouthhooks) in ‘mirror-image’ orientation (for a detailed description, see Schiipbach and Wieschaus (1986)).

Fig. 3.

The effect of vas exu mutations on the expression patterns of hb, Kr, eve, ftz and en as visualized by antibody staining, and on the cuticle phenotype. (A) hb expression in a mid-stage 14 embryo, hb is expressed throughout the egg. (B,C) eve expression in early (B) and late (C) stage 14. By late stage 14, two stripes close to the poles (at about 20 and 80% EL) have formed. (D) en expression at gastrulation. Two stripes are present at about 25 and 75 % EL, later additional stripes form in the pole regions. (E) Late stage 14 embryo stained against Kr protein. Before the end of cellularization, no Kr expression could be detected. With the onset of gastrulation, Kr protein becomes visible in different regions of the embryo (data not shown). (F,G) ftz expression in early (F) and late (G) stage 14. At late stage 14, ftz is expressed in a broad domain in the middle region (about 20-75 % EL); note that the staining is weaker within the domain than at its margins. (H) Cuticle phenotype, dark-field photograph. Very few cuticular landmarks are discernible; we identify maxillary structures (cirri and mouthhooks) in ‘mirror-image’ orientation (for a detailed description, see Schiipbach and Wieschaus (1986)).

These two mutant combinations exhibit different patterns of hb and Kr expression. In stau exu embryos, hb is expressed along the entire length of the egg, at levels we could not distinguish from wild-type (see Materials and methods) (Fig. 2A-C). The omnipresence of hb protein is caused by the ubiquitous presence of maternal hb transcript that results from the stau mutation (Tautz, 1988), as well as by ubiquitous activation of the zygotic transcription of hb by bed. This could be demonstrated by the following experiment. It has been shown that the zygotic expression of hb in the anterior half of the wild-type embryo is due to the activation by bed (Tautz, 1988). The regulatory sequence regions necessary for the normal expression of hb have been determined by using hb-promoter-lacZ fusion gene constructs in combination with germ-line transformations; they could be delimited to a region of approximately 300 bp upstream of the site of transcription initiation (Schroder et al. 1988). We crossed male flies carrying a hb -lacZ gene fusion construct containing the bed responsive sequences to stau exu homozygous mothers (see Materials and methods) and found that β-gal is distributed uniformly throughout stau exu embryos (data not shown). We conclude that the ubiquitous zygotic expression of hb protein in stau exu embryos is due to activation by bed’, bed therefore has to be present throughout the embryo.

Kr is expressed in a very broad domain in stau exu embryos that covers about 60% of the egg, also at levels that we could not distinguish from wild-type (see Materials and methods) (Fig. 2G-I). Kr is not expressed, however, in either pole region, due to the presence of terminal organizers and their zygotic ‘mediator(s)’, which act to repress Kr expression there (Gaul and Jäckle, 1989). So we find that, in this mutant combination, hb and Kr protein are co-distributed within a very large region, encompassing about 60 % of the embryo.

In vas exu embryos, hb is expressed at wild-type levels in the entire embryo (again due to the ubiquitous presence of maternal hb transcript (vas mutation) and to the ubiquitous activation of zygotic hb expression by bed, as confirmed by vas exu Jih-promoter-lacZ crosses; see also Struhl et al. 1989) (Fig. 3A). However, no expression of Kr is detected during blastoderm (Fig. 3E).

What mechanism generates the overlap of the hb and Kr domains?

The broad co-distribution of hb and Kr product that is observed in stau exu embryos has two important implications. First, it has to be due to genuine co-expression, and therefore suggests that the relatively narrow overlap of the domains of hb and Kr that is observed in wildtype embryos similarly results from an overlap of expression territories rather than from diffusion. In fact, the region of co-distribution of hb and Kr in stau exu embryos can be regarded as a ‘blow up’ of the central portion of the wild-type overlap, i.e. that portion of the overlap that shows higher concentrations of both hb and Kr protein. Second, the co-expression of hb and Kr in stau exu embryos indicates that hb by itself is not sufficient to repress Kr expression (and vice versa). That hb cannot play a crucial role in Kr repression is also suggested by the comparison of stau exu and vas exu embryos: while the hb distribution is the same in both mutants, Kr is expressed in the former and not expressed in the latter. But what is it that causes the difference between the two mutant combinations with respect to Kr expression?

The analysis of bed mutant embryos indicates that bed has a strong negative effect on Kr expression (Gaul and Jäckle, 1987). Moreover, the comparison between the patterns of bed and Kr protein in wild-type embryos clearly shows that this negative effect must be concentration-dependant: bed, showing a graded distribution with its highpoint at the anterior pole, is detectable at lower levels throughout the Kr domain (Driever and Niisslein-Volhard, 1988a). Consequently, it could be a difference in the level of bed activity (or in the activity of some hcd-dependant factor other than hb) that is responsible for the difference in the expression of Kr between vas exu and stau exu embryos.

The marked difference in the phenotypes of vas exu and stau exu embryos indeed supports this interpretation: As has been shown by several independant experiments, bed acts as a morphogen that determines ‘position’ in the anterior half of the embryo. Increases or decreases in bed protein levels in a given region of the embryo cause a corresponding shift of the anterior anlagen to the anterior or to the posterior, respectively (Frohnhôfer and Niisslein-Volhard, 1987; Driever and Niisslein-Volhard, 1988b). The cuticular structures formed in vas exu embryos derive from more anterior positions in the fate map than those formed in stau exu embryos: the middle region produces gnathal elements instead of thorax. Thus, since higher levels of bed determine structures lying more anteriorly in the fate map, we can infer that the level of bed activity is higher in vas exu embryos than it is in stau exu embryos.

The difference regarding the expression of Kr and hb between stau exu and vas exu embryos consequently suggests that the threshold level of bed for repression of Kr is different from the one for activation of hb: in stau exu embryos, the level of bed is too low to repress Kr expression, while in vas exu embryos, with higher levels of bed, Kr expression is repressed. In contrast, hb is activated indiscriminately in both mutants. We conclude that the threshold level of bed for hb activation is lower than that for Kr repression.

This difference in threshold levels provides an explanation for the overlap of the domains of hb and Kr that is observed in wild-type. In wild-type embryos, bed shows a graded distribution with its highpoint at the anterior pole and an exponential decrease in concentration towards posterior (Driever and Niisslein-Volhard, 1988a). Under such conditions, the difference in the threshold levels for hb activation on the one hand, and Kr repression on the other, causes a simultaneous expression of Kr and hb in a defined region: where the level of bed is too low to repress Kr but still high enough to activate hb. Only where bed levels are sufficiently high, will Kr be repressed.

This picture of the regulation of hb and Kr in the anterior of the embryo does not take into account interactions between the two gap genes themselves. Yet, as has been described earlier, the Kr domain extends anteriorly in hbembryos, indicating that hb does have a negative influence on Kr expression (Jäckle et al. 1986; Harding and Levine, 1988); and the hb domain extends posteriorly in Kr embryos (Jäckle et al. 1986). The latter effect seems to be rather weak and occurs too late to be relevant for the establishment of the anterior (blastoderm) hb domain. The effect of hb on Kr, however, while not as strong as the effect of bed on Kr, is quite pronounced and visible as soon as the Kr domain can be detected with antibody staining. Therefore, hb has to play a certain role in the control of Kr expression in the anterior of the embryo, even if it cannot repress Kr expression by itself, as is indicated by the co-expression of hb and Kr in stau exu embryos. We see two possible mechanisms by which hb could be involved in Kr repression along with bed:

  • bed acts as a repressor of Kr, but needs help from hb as a co-factor at lower levels of concentration. It is only at higher levels that bed can repress Kr by itself completely. Such a mechanism implies that bed alone is not a ‘good’ repressor for Kr, that hb acts as an ‘enhancer’, and that it can do so only if bed is already present at a certain threshold level.

  • Another possibility is that hb is only indirectly involved in Kr control. One idea employs the fact that the transcription factor that hb shows homology to, Xenopus transcription factor IIIA, binds not only to DNA but also to RNA (Tautz et al. 1987; Klug and Rhodes, 1987). Thus, hb might be involved in positive post-transcriptional regulation of bed. Under such conditions, bed would be the only repressor of Kr, and an effective one. The lack of hb would cause less bed protein to be produced, and the concentrations necessary for Kr repression would only be reached at a more anterior position.

Gap gene and pair-rule gene expression in maternal mutant combinations

We still have to address the question whether the coexpression of hb and Kr in wild-type embryos is functional. The examination of the correlation between the gap gene and the pair-rule gene patterns in the maternal mutant combinations characterized above suggests an answer to this question, and it also provides some more general information about the way the gap genes control pair-rule gene activity.

To determine the constellations of gap gene activities in the maternal mutant combinations, we monitored the expression of hb and Kr. knirps (kni), the gap gene expressed in the posterior of wild-type embryos (Nauber et al. 1988), is eliminated in all cases. Due to the mutations in posterior organizer genes (osk, vas, stau, respectively) maternal hb protein is expressed uniformly throughout the egg; this in turn causes kni expression to be repressed (Hülskamp et al. 1989). The distribution of terminal organizers (and of the zygotic gene activities that depend on them and mediate their effect), on the other hand, can be inferred from their negative effect on Kr expression (Gaul and Jackie, 1989), as well as from the phenotypes of the maternal mutant combinations. The main mediator of terminal organizer activity is the gap gene tailless (til) (Klingler et al. 1988); however, apart from til at least one more zygotic factor, huckebein, is involved in the organization of the terminal region. This factor apparently acts in the terminalmost portion of the embryo to suppress segmentation gene expression (Weigel, unpublished observation). To monitor pair-rule gene expression, we used ftz and even skipped (eve) antibodies; in wild-type, ftz and eve are initially expressed in the entire region that is to be segmented; later, their patterns resolve into series of stripes that are roughly complementary (Carroll and Scott, 1985; Frasch et al. 1987).

In bed osk tsl embryos, all three maternal organizers are defective, and no cuticle pattern is formed (Nüsslein-Volhard et al. 1987). In such embryos, Kr is the only zygotically active gap gene and is expressed throughout the embryo (Fig. 4A). No patterns of pairrule genes form: ftz and eve are uniformly expressed in the entire embryo (Fig. 4B,C). Thus we find that, if there is no gap gene pattern; that is, if there is no change in the state of expression of gap genes along the anteroposterior axis, no pair-rule gene patterns are formed, either. This result proves that zygotic gene patterns cannot form de novo in the Drosophila blastoderm, but rather depend on a prior pattern. In the absence of patterned instruction from maternal organizers, the gap genes do not pattern, and this causes a subsequent failure of pair-rule genes to resolve spatial patterns.

Fig. 4.

The effect of bed osk tsl, bed osk and vas tor exu mutations on the expression of Kr, hb, eve, ftz and en as visualized by antibody staining. (A-D) bed osk tsl embryos. All of the genes monitored show a ubiquitous expression. (A) Kr expression in a mid-stage 14 embryo. (B) eve expression in a mid-stage 14 embryo. (C) ftz expression in a late stage 14 embryo. (D) en expression in a gastrulating embryo. (E-H) bed osk embryos. (E) Kr expression in a mid-stage 14 embryo, forming a broad domain of expression in the middle region (about 30-70% EL). (F) eve expression in a late stage 14 embryo, eve expression forms a single, broad domain, which shows higher levels of expression at its margins. (G) ftz expression in a late stage 14 embryo, ftz expression forms a broad domain in the middle, flanked by one stripe on either side. (H) en expression in a gastrulating embryo, forming a broad domain in the middle region of the embryo, flanked by one stripe on either side. (1-M) vas tor exu embryos. (1) hb expression in a mid-stage 14 embryo. Like in vas exu embryos (see Fig. 3A), hb is expressed throughout the embryo. (K) eve expression in a mid-stage 14 embryo, forming a cap at the anterior pole. (L) ftz expression in a mid-stage 14 embryo, covering most of the embryo except for a cap at the anterior pole. Later, the expression becomes non-homogenous: a stripe of strong expression is maintained in the anterior, while expression fades out in the posterior (data not shown). (M) en expression in a gastrulating embryo. One single stripe forms close to the anterior pole of the embryo.

Fig. 4.

The effect of bed osk tsl, bed osk and vas tor exu mutations on the expression of Kr, hb, eve, ftz and en as visualized by antibody staining. (A-D) bed osk tsl embryos. All of the genes monitored show a ubiquitous expression. (A) Kr expression in a mid-stage 14 embryo. (B) eve expression in a mid-stage 14 embryo. (C) ftz expression in a late stage 14 embryo. (D) en expression in a gastrulating embryo. (E-H) bed osk embryos. (E) Kr expression in a mid-stage 14 embryo, forming a broad domain of expression in the middle region (about 30-70% EL). (F) eve expression in a late stage 14 embryo, eve expression forms a single, broad domain, which shows higher levels of expression at its margins. (G) ftz expression in a late stage 14 embryo, ftz expression forms a broad domain in the middle, flanked by one stripe on either side. (H) en expression in a gastrulating embryo, forming a broad domain in the middle region of the embryo, flanked by one stripe on either side. (1-M) vas tor exu embryos. (1) hb expression in a mid-stage 14 embryo. Like in vas exu embryos (see Fig. 3A), hb is expressed throughout the embryo. (K) eve expression in a mid-stage 14 embryo, forming a cap at the anterior pole. (L) ftz expression in a mid-stage 14 embryo, covering most of the embryo except for a cap at the anterior pole. Later, the expression becomes non-homogenous: a stripe of strong expression is maintained in the anterior, while expression fades out in the posterior (data not shown). (M) en expression in a gastrulating embryo. One single stripe forms close to the anterior pole of the embryo.

In contrast, in bed osk embryos, in which anterior and posterior organizers are defective, while terminal organizers are intact, patterns do form. The cuticle phenotype of bed osk embryos shows telsons at both ends and naked cuticle in the middle (Nüsslein-Volhard et al. 1987). In these embryos, Kr is expressed in the middle region of the embryo only (Fig. 4E); the repression of Kr expression at the poles is due to the presence of terminal organizers and their zygotic ‘targets’ there (Gaul and Jackie, 1989). Zygotic hb expression is found in the two terminal regions, due to activation by terminal organizers (data not shown). This constellation of gap gene expression is correlated with the following pair-rule gene patterns: eve ‘tries’ to produce two peripheral stripes, but the expression of eve persists in the middle and the stripes do not resolve (Fig. 4F). Meanwhile, ftz forms a broad band in the central region and one peripheral stripe on either side (Fig. 4G). Thus, unlike in wild-type, ftz and eve are coexpressed in the middle region of bed osk embryos. (Frasch et al. (1988) similarly found a broad co-expression of eve and ftz in hb kni double mutants.) We observe that, in the central region, the uniform expression of Kr is accompanied by a uniform pair-rule gene expression; and patterning of the pair-rule genes occurs where the input from the gap genes changes, namely around the borders of the Kr domain.

In stau exu embryos, as we described above, hb and Kr are co-expressed in a broad middle region encompassing about 60 % of the embryo (Fig. 2A-C, G-I). In the pole regions of stau exu embryos, hb and the zygotic terminal genes are present; the latter is implied by the phenotype and by the repression of Kr expression at the termini. In these embryos, eve forms two stripes in the terminal region (Fig. 2E). The distribution of the ftz protein is roughly complementary -a very broad domain is established in the central region of the embryo as well as two narrow stripes near the poles (Fig. 2L; see also Winslow et al. 1988). Thus, in the large region where both Kr and hb are expressed, no further minima or maxima oí ftz or eve expression form. It is in the vicinity of the borders of the Kr domain that their expression patterns resolve. The ftz pattern closely resembles the Kr pattern, whereas the patterns of eve and Kr are approximately complementary.

In vas exu embryos, as we described earlier, Kr is absent, while hb is ubiquitously expressed (Fig. 3A,E). From the phenotype we can infer that maternal terminal organizer activity, and, correspondingly, activity of their zygotic ‘mediator(s)’, is present in the pole regions of these embryos: proctodeal invaginations form at both ends. In such embryos, like in stau exu embryos, the initially uniform expression of eve gradually resolves into two peripheral stripes (Fig. 3B,C), while the ftz expression forms a single broad domain in the middle of the embryo (Fig. 3F,G). However, the two mutant combinations differ in that the peripheral stripes of ftz expression that are observed in stau exu embryos do not form in vas exu embryos. Moreover, as cellularization proceeds, the broad central domain of ftz expression becomes non-homogenous in vas exu embryos, showing a loss of intensity in the middle, while stripes of stronger expression are maintained at the margins (see also Winslow et al. 1988).

In order to test whether the pair-rule gene patterning in vas exu embryos results from terminal organizer activity, we examined vas tor exu triple mutants, in which terminal organizer activity is eliminated in addition. In these embryos, hb is again expressed ubiquitously (Fig. 41) and Kr expression is absent. But eve expression now forms a small cap at the anterior pole (Fig. 4K), and ftz shows a complementary distribution in being expressed in almost the entire egg, except for a small cap at the anterior pole (Fig. 4L). As is observed in vas exu embryos, the ftz domain in vas tor exu embryos becomes non-homogenous at late blastoderm stages, showing a loss of intensity in the posterior while a single stripe of stronger expression is maintained at the anterior margin of the domain (data not shown). These experiments show that, in the posterior of vas exu embryos, it is indeed the terminal organizers (and their zygotic mediators) that are responsible for the patterning of pair-rule genes. In the anterior, however, the situation is more complicated: the terminal organizer activity is involved in pattern formation, for the pairrule gene pattern in the anterior of the triple mutant differs from the one in the double mutant. However, since the anterior and the posterior region in vas tor exu embryos are not identical, the terminal organizer activities cannot be the sole determinant of the patterning of the pair-rule genes.

Correlations between gap gene and pair-rule gene patterns in the maternal mutant combinations

Our experiments show that the patterns of gap genes and the patterns of pair-rule genes are correlated. In all mutant combinations we find that, within an area of uniform gap gene activity, no alteration occurs in the constellation of pair-rule gene expression. In bed osk tsl embryos, ubiquitous expression of Kr and lack of any other zygotic gap gene activity is correlated with ubiquitous expression of ftz and eve. In the middle regions of bed osk and stau exu embryos, which are homogenous with respect to hb and Kr activity, ftz and eve are present or absent in a uniform manner. Conversely, in both mutant combinations, the pair-rule gene expression alters in a region where the gap gene input changes, namely around the borders of the Kr domains. Hence, it appears that unless some differential information is provided by the gap genes, the uniformity of pair-rule gene expression (or non-expression) is not broken down.

The results for vas exu embryos fit in with this analysis. The comparison between vas exu and vas tor exu embryos shows that in both the anterior and the posterior terminal regions of vas exu embryos terminal organizers (and their zygotic mediator(s)) are active in addition to hb, providing differential information for the patterning of the pair-rule genes. The residual pattern of pair-rule genes in vas tor exu embryos (as well as the slight asymmetry in the pair-rule gene patterns of vas exu embryos), however, shows that terminal organizer activities are not the sole determinant of pattern formation in the anterior of vas exu embryos. In fact, the difference between the anterior and the posterior may result from the slightly asymmetric distribution of bed protein in such embryos, with higher levels of concentration in the anterior (Struhl et al. 1989).

The pair-rule gene patterns at the very termini of vas exu, stau exu, and bed osk embryos cannot be explained specifically at present. As pointed out, it is certain that zygotic genes are active in the terminal regions of these embryos, but no probes are currently available, and we therefore know too little about their distribution to determine the relation to the pair-rule gene patterns.

If we now compare the middle regions of bed osk, stau exu, and vas exu embryos, i.e. those regions that are homogenous with respect to the constellation of gap gene activities, we find that the difference between the three mutant combinations in the constellation of gap gene activities is reflected in a difference in the expression of pair-rule genes. The combination of hb and Kr expression in the middle region of stau exu embryos is correlated with the presence of ftz and the absence of eve. In bed osk embryos only Kr is present, and a different constellation is generated at the pair-rule gene level: ftz is on, and eve is on at a low level. In vas exu embryos, presence of hb with absence of Kr is correlated with the absence of eve and the expression of ftz at lower levels. Thus, qualitatively different input from the gap genes leads to different states of expression of pair-rule genes. This means that it does make a difference for the expression of pair-rule genes what kind of gap gene expression a blastoderm cell experiences. In particular, we find that it makes a difference whether a blastoderm cell experiences the expression of Kr alone, of hb without Kr, or of hb and Kr at the same time. This suggests that the overlap of hb and Kr expression that is observed in wild-type embryos is indeed functional and encodes information required for the formation of the normal pair-rule gene patterns.

Possible mechanisms of control of pair-rule gene pattern formation by the gap genes

We have seen that qualitatively different states of gap gene expression are correlated with different states of pair-rule gene expression and that the co-expression of hb and Kr appears to be thus relevant. Based on this observation, one could conceive a combinatorial model of how the gap genes organize the formation of the pairrule gene patterns in wild-type: qualitatively different states of gap gene expression along the anteroposterior axis of the embryo (… hb, hb+Kr, Kr, …) would provide different input for the pair-rule genes and eventually ‘translate’ into different states of pair-rule gene expression. However, the information encoded by the gap genes would not suffice to produce the patterns of the pair-rule genes in this way: within the range of Kr expression, e.g. at least three stripes and two interstripes of ftz expression have to form in wild-type embryos, while at most three qualitatively defined ‘states’ of gap gene expression (hb+Kr, Kr, Kr+knt) would be there to provide the input. Even the overlap of hb and Kr expression in wild-type encompasses not only the second ftz stripe, but also at least part of both the second and the third eve stripe (Gaul, unpublished).

Of course, more genes might be involved in the generation of the pair-rule gene patterns (in particular, the gap gene giant has to be added to the scheme). But, as we have indicated above, the distribution of hb and Kr is graded toward the margins of their domains, and it seems very likely that this, too, is functionally relevant, i.e. that, for the generation of the pair-rule gene patterns, it not only matters which gap genes are expressed in a particular region of the embryo, but also at what level they are expressed. The very fact that the mutant alleles of gap genes differ in the strength of their effect on segmentation and that a graded phenotypic series from wild-type to loss-of-function phenotypes can be established shows that the requirement for gap gene activity is graded, and it therefore rules out simple binary input-output modes in the interaction of gap genes with pair-rule genes. Even under merely heterozygous conditions (at least for hb, Kr, and kni) the segment patterns of hatched larvae show abnormalities (Wieschaus et al. 1984; Lehmann, 1985); these effects are already visible in the pair-rule gene patterns at blastoderm (Frasch and Levine, 1987).

Hence, in addition to the qualitative aspect, i.e. the constellation of gap gene activities, the level of concentration of the gap gene activities appears to be functional in the formation of the pair-rule gene patterns. The gap gene activities, in instructing the pair-rule genes, would thus act as oppositely directed short-range gradients that overlap partially (cf. Odell and Edgar, 1989). Within a band of overlap, accordingly, regions of different ratios of concentration of the two activities would have to be distinguished as having different effects on the pair-rule genes.

With a more sensitive staining technique, the recording of the wild-type distribution of Kr (and of hb) protein has been improved. The Kr protein domain is broader than reported earlier; it covers about 60-33 % EL. This also means that the domain of Kr gene expression is roughly co-extensive with the region of Kr requirement, i.e. the region that is affected in amorphic Kr mutants. The problem that arose from the earlier perception of an incongruity between the phenotypic gap and the domain of expression is thus eliminated.

The overlap of the hb and Kr protein domains in wild-type, and, in particular, our data on the mutant combinations stau exu and vas exu show that negative interactions between hb and Kr do not play the crucial role in the establishment of their domains. We favor the idea that the maternal anterior organizer bed provides the decisive input both for the establishment of the anterior hb domain and for the determination of the anterior border of the Kr domain in wild-type embryos. The overlap of hb and Kr protein could easily be explained mechanistically, if, e.g. bed had a somewhat lower affinity for binding to the Kr control region than for binding to the hb control region.

The overlap of the domains of Kr and hb that is observed in wild-type helps to picture how the few gap genes are able to instruct the patterning of the pair-rule genes. Co-expression of neighboring gap genes provides additional differential information for pair-rule gene pattern formation. However, this does not suffice, and there is evidence that gap gene concentration levels, and ratios between concentration levels, have to be taken into account in addition.

We thank Drs C. Nüsslein-Volhard, T. Schüpbach, and C. Schroder for mutant stocks, M. Frasch, H. Krause, N. Patel, and D. Tautz for providing antibodies, and B. Billi for excellent technical assistance. We also wish to thank G. Jurgens and G. Odell for discussions, and our colleagues in Tübingen and Munich for comments on the manuscript. The work was supported by DFG grants (Leibniz program and JA-312/4-1).

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