The stable maintenance of expression patterns of homeotic genes depends on the function of a number of negative trans-regulators, termed the Polycomb (Pc) group of genes. We have examined the pattern of expression of the Drosophila segment polarity gene, engrailed (en), in embryos mutant for several different members of the Pc group. Here we report that embryos mutant for two or more Pc group genes show strong ectopic en expression, while only weak derepression of en occurs in embryos mutant for a single Pc group gene. This derepression is independent of two known activators of en expression: en itself and wingless. Additionally, in contrast to the strong ectopic expression of homeotic genes observed in extra sex combs (esc) mutant embryos, the en expression pattern is nearly normal in esc embryos. This suggests that the esc gene product functions in a pathway independent of the other genes in the group. The data indicate that the same group of genes is required for stable restriction of en expression to a striped pattern and for the restriction of expression of homeotic genes along the anterior-posterior axis, and support a global role for the Pc group genes in stable repression of activity of developmental selector genes.

Molecular and genetic studies have uncovered the basis for pattern formation in Drosophila (St Johnson and Nüsslein-Volhard, 1992; Ingham, 1988). Early interactions between maternal and transiently expressed zygotic transcription factors result in the subdivision of the embryo into a series of segments, each composed of an anterior and posterior population of cells. Segmentation requires expression of the engrailed (en) gene in the posterior group of cells in each segment (Lawrence and Morata, 1976; Kornberg et al., 1985), while segmental identity is specified by the selective expression of the homeotic genes of the bithorax and Antennapedia complexes (Lewis, 1978; Kaufman et al., 1980; Sanchez-Herrero et al., 1985; Harding et al., 1985). Unlike the early regulators, that are expressed transiently, en and the homeotic genes are expressed in spatially restricted patterns throughout development. Maintenance of patterned expression involves controls distinct from those that guide pattern establishment (Heemskerk et al., 1991; Struhl and Akam, 1985).

The controls that govern stable expression of selector genes are especially interesting, because they may identify the mechanisms responsible for stable determination of developmental fate. Genetic approaches have identified a group of negative regulators, the Polycomb (Pc) group genes (Duncan, 1982; Jürgens, 1985), that are essential for maintaining spatial restrictions of homeotic gene expression. The Pc group currently includes eleven genes classified according to their similar homeotic phenotypes. In Pc group mutant embryos, the apparently correct initial expression of homeotic genes decays into global expression. This results in transformation of most body segments towards a more posterior segment fate.

If the Pc group genes play a general role in stable determination of cell fate, we might expect them to control the stable pattern of en expression. There is some suggestive evidence that they might do so. Most directly, ectopic expression of en was observed in polyhomeotic (ph ) mutant embryos (Dura and Ingham, 1988; Smouse et al., 1988). Furthermore, Busturia and Morata (1988) observed derepression of an en-Lac-Z fusion gene in Pc clones in mosaic wings of adult animals. Finally, larval cuticle defects seen in embryos lacking both maternal and zygotic Polycomb-like (Pcl) or pleiohomeotic (pho) function are consistent with an effect of these genes on segmentation gene expression (Breen and Duncan, 1986). Nonetheless, an involvement of Pc group genes in en regulation has not been generally accepted because of three factors. (1) ph mutant embryos exhibit pleiotropic phenotypes beyond those observed in embryos mutant for other Pc group genes (Dura et al., 1987); therefore, the observed derepression of en cannot be generalized to other members of this group,(2) Busturia and Morata (1988) failed to detect derepression of en in embryos mutant for Pc itself, and (3) segmentation defects are slight, or nonexistent even in Pc group mutant backgrounds that give severe homeotic transformations. To explore this issue directly, we examined the en expression pattern in embryos mutant for single or multiple Pc group genes. In contrast to a nearly normal expression pattern in most single mutant embryos, we find strong ectopic en expression in double and triple Pc group mutant embryos. The requirement for Pc group function in repression of en supports a global role for Pc group genes in maintaining the repressed state of developmental regulators.

The following mutant alleles were analyzed in this study: Pc3 and Df (3L) Pc-Mk; ScmXF24, ScmD1, and ScmD2; PclE90, and PclD5; Psc1, and Df (2R) vgD; AsxXT129, AsxIIF51, AsxXF23, and Df (2R) trix; E(Pc); pco25; ph503; l(4)29b (pho); esc10 and esc2; and wgcx4. All mutant alleles have been previously described (Duncan, 1982; Jürgens, 1985; Breen and Duncan, 1986; Dura et al., 1987; Phillips and Shearn, 1990; Brunk et al., 1991; Baker, 1987). Recombinant chromosomes and other mutant combinations were made using standard genetic crosses. The triple mutant on the second chromosome was obtained from Jürgens (described by Jürgens, 1985). Psc1 was lethal in combination with Pc3. Instead of Psc1, we used Df (2R) vgD (a deletion of the Psc locus) in combination with Pc3. esc mutant embryos were collected from esc10/esc2 adult flies. esc10/esc2 adults were obtained by crossing esc10 males to females heterozygous for esc2 and a duplication containing the esc locus exactly as described by Struhl (1983). esc10 is a deletion of the esc locus and esc2 is an apparent null allele (for further details see Struhl, 1983). Wild-type embryos were of the sevelen strain.

For immunological and in situ detection embryos were collected and fixed following standard procedures. The en protein was detected using a monoclonal antibody that recognizes the en and invected proteins (Patel et al., 1989). Detailed protocols for reaction of embryos with antibodies and detection using the peroxidase reaction were those described by Kellerman et al. (1990). In situ hybridization using a digoxigenin-labeled Abd-B cDNA was as described previously (Tautz and Pfeifle, 1989). This cDNA probe hybridizes to all of the transcripts of Abd-B (Kuziora and McGinnis, 1988).

We used an antibody to the en protein to examine its expression in embryos mutant for the Pc gene itself, or mutant for other members of the Pc group. In addition to the striped expression found in the wild type (Fig. 1A; DiNardo et al., 1985), in Pc embryos ectopic en-expressing cells are found scattered in the anterior of every segment, mainly in the lateral region of the epidermis (Fig. 1E). It is not surprising that this derepression was previously missed (Busturia and Morata, 1988), as it occurs in relatively late embryos and sensitive staining is required to detect it. Embryos mutant for other Pc group genes also show defects in en expression that range in severity. Exam ples include subtle defects, such as duplication of the lone en-positive cell in the anterior compartments of the first through the seventh abdominal segments (A1-A7) in Sex combs on mid-legs (Scm ; Fig. 1B) and Pcl (Fig. 1C) embryos, as well as patchy expression in the anterior of some segments in Posterior sex combs(Psc, Fig. 1D), pleiohomeotic (pho), and polycombeotic (pco, also called Enhancer of zeste, Jones and Gelbart, 1990) embryos (data not shown).

Fig. 1.

Localization of en (A-J) and Abd-B (K-N) in wild-type and Pc group mutant embryos. The en protein and the Abd-B transcripts were detected as described in Materials and methods. In wild-type embryos (A), en is expressed in a series of stripes about 1-2 cells wide at this stage of development (∼9.5–10.5 hours; DiNardo et al., 1985). Mid-laterally a spur of en expression extends anteriorly. The first through the seventh abdominal segments (A1-A7) also contain a lone en-positive cell in their anterior domains; arrowheads provide a reference point between the third thoracic (T3) and the first abdominal (A1) segments. Mutant embryos (B-J) show different degrees of ectopic en expression (see text). Arrows in B and D point to examples of subtle ectopic expression. The embryo in H is about 20 minutes younger than the rest. Abd-B transcripts are expressed in parasegments 10-14 in wild type (K; Kuziora and McGinnis, 1988) and ectopically in the more anterior parasegments in mutant embryos (L-N). Representative mutant alleles are shown here: Pc3 (E,F,G, H,M,N), ScmXF24 (B,F,L,N), PclE90 (C,G), Psc1 (D), and Df (2R) vgD (H,J); AsxXF23 (J), ph503 (I).

Fig. 1.

Localization of en (A-J) and Abd-B (K-N) in wild-type and Pc group mutant embryos. The en protein and the Abd-B transcripts were detected as described in Materials and methods. In wild-type embryos (A), en is expressed in a series of stripes about 1-2 cells wide at this stage of development (∼9.5–10.5 hours; DiNardo et al., 1985). Mid-laterally a spur of en expression extends anteriorly. The first through the seventh abdominal segments (A1-A7) also contain a lone en-positive cell in their anterior domains; arrowheads provide a reference point between the third thoracic (T3) and the first abdominal (A1) segments. Mutant embryos (B-J) show different degrees of ectopic en expression (see text). Arrows in B and D point to examples of subtle ectopic expression. The embryo in H is about 20 minutes younger than the rest. Abd-B transcripts are expressed in parasegments 10-14 in wild type (K; Kuziora and McGinnis, 1988) and ectopically in the more anterior parasegments in mutant embryos (L-N). Representative mutant alleles are shown here: Pc3 (E,F,G, H,M,N), ScmXF24 (B,F,L,N), PclE90 (C,G), Psc1 (D), and Df (2R) vgD (H,J); AsxXF23 (J), ph503 (I).

The effects of Pc group mutations are synergistic in that embryos homozygous for two or three weak mutations show strong homeotic transformation (Jürgens, 1985). To test whether such synergy might enhance some of the weak effects on en expression, we stained embryos homozygous for two or three different Pc group mutations. All mutant combinations tested gave extensive ectopic en expression (Fig. 1F-H, J). The degree of derepression depends on the particular combination of mutant alleles. The double mutants PcScm and PscPc (Fig. 1F, H) provide dramatic examples of synergy, but in all combinations derepression is invariably stronger than the sum of the effects in single mutants (compare Fig. 1B,C,D, and E with F,G, and H; note that derepression in these backgrounds is not due to homeotic transformations, see Fig. 3 and below). Synergy also appears to apply to phenotype: the double and triple mutant embryos have frequent defects in their larval cuticles (D. M., unpublished). However, segmentation still occurs. We believe that the subtlety of segmental defects is due to the late occurrence of widespread ectopic en expression in these mutant embryos. The earliest ectopic expression, observed in 5-6 hour embryos, occurs with low penetrance and is restricted to a small number of cells primarily in the dorsal ectoderm region. Derepression increases and spreads progressively until it encompasses much of the embryo by 9-10 hours of development.

Fig. 2.

wg-independent derepression of en. (A) wt; (B) wg ; (C) wg ; PcScm. The en protein was detected as described in Materials and methods. Embryos are the progeny of wgcx4/+;Pc3ScmXF24/+ heterozygotes.

Fig. 2.

wg-independent derepression of en. (A) wt; (B) wg ; (C) wg ; PcScm. The en protein was detected as described in Materials and methods. Embryos are the progeny of wgcx4/+;Pc3ScmXF24/+ heterozygotes.

Fig. 3.

Comparison of expression of en (A,B) and Abd-B (C,D) in esc mutant embryos. Note that although en is not derepressed in esc mutant embryos, the morphology of all stripes resembles that of an A8 stripe. esc mutant embryos were collected from esc10/esc2 adult flies constructed as described in Materials and methods.

Fig. 3.

Comparison of expression of en (A,B) and Abd-B (C,D) in esc mutant embryos. Note that although en is not derepressed in esc mutant embryos, the morphology of all stripes resembles that of an A8 stripe. esc mutant embryos were collected from esc10/esc2 adult flies constructed as described in Materials and methods.

As in the case of homeotic genes (Struhl and Akam, 1985; Riley et al., 1987; Kuziora and McGinnis, 1988; Jones and Gelbart, 1990; Simon et al., 1992), the initial en pattern in the Pc group mutant embryos appears normal. Ectopic expression is only detectable after the completion of the early tiers of regulation that establish and refine the en striped pattern (DiNardo et al., 1988; Heemskerk et al., 1991). Thus, Pc group genes encode factors required for maintenance of the expression pattern of en, a segment polarity gene, as well as maintenance of expression patterns of the homeotic genes. This suggests a general role for the Pc group in maintaining expression patterns of developmental regulators in Drosophila.

Following its initial induction by pair-rule gene products, continued en expression in the embryo requires input from at least two other pathways (DiNardo et al., 1988; Martinez-Arias et al., 1988; Heemskerk et al., 1991). These involve autoregulation and cell-cell communication. The latter requires expression of the segment polarity gene wingless (wg) in a row of cells adjacent to the en stripe. We asked whether the ectopic en expression in Pc group mutant backgrounds is also dependent on these pathways for either its initial activation or maintenance. To test this, we examined the pattern of en expression in wg ;Pc Scm mutant embryos. By germ band shortening, in wg embryos ectodermal en expression has completely decayed (Fig. 2B; DiNardo et al. 1985). But in wg ;Pc Scm triple homozygous embryos, we detect a lawn of en-expressing cells throughout the ectoderm (Fig. 2C). Since extensive cell death occurs in wg mutant embryos, the en-positive cells in these embryos probably represent most of the surviving ectodermal cells. Similar results were obtained with en ;Pc Scm mutant embryos (using the encx1 allele and a polyclonal antibody capable of recognizing the truncated en protein produced by encx1; D. M., unpublished). Therefore, in Pc group mutant backgrounds ectopic en expression occurs independently of functions of en and wg, two activators of en expression. Derepression is most likely also independent of pair-rule activators of en, as it occurs at a time when most pair-rule gene products have decayed (for example, see Kellerman et al., 1990). Derepression probably results from the action of positive regulators such as trithorax (trx) that are known to antagonize Pc group mutant phenotypes (Ingham, 1983; Kennison and Tamkun, 1988; Shearn, 1989). However, we note that removal of the zygotic function of trx itself does not suppress en derepression in two Pc group mutant combinations that we have tested so far: ectopic en expression in trxPc and trxPcScm mutant embryos is indistinguishable from the derepression observed in Pc and PcScm embryos, respectively (D. M., unpublished).

If Pc group genes play related roles in the control of en and homeotic genes, we expect a parallel in their effect on expression of these genes. Using in situ hybridization to examine the expression of Abd-B, a homeotic gene that controls the developmental fate of the more posterior abdominal segments (Sanchez-Herrero et al., 1988), we found a general correlation in the degree of its derepression in Pc group mutant backgrounds with that of en. For example, both en and Abd-B are derepressed weakly in Scm (compare Fig. 1B and L), moderately in Pc (compare Fig. 1E and M), and very strongly in PcScm double mutant embryos (compare Fig. 1F and N). A similar relationship also holds for other mutant backgrounds that we have tried (ph, Psc, Asx, Pcl single mutants, and the triple mutant PscAsxPcl ; data not shown). However, there is one exception to the correlation. In embryos in which both the maternal and zygotic functions of esc have been removed, en is unaffected (other than changes due to homeotic transformations, see Fig. 3B and legend), whereas Abd-B is strongly derepressed (Fig. 3D). Thus, the remaining Pc group genes can still restrict en expression in the complete absence of the esc gene product. This supports the hypothesis (Struhl, 1983) that esc functions in a pathway independent of the other genes in the group.

The segmentally repeated expression of en is fundamentally different from the segment-specific expression pattern of homeotic genes. The domain of expression of each homeotic gene is confined by an anterior parasegmental boundary; en on the other hand is expressed in the posterior of every segment. Lewis (1978) suggested that the anterior boundary of the homeotic expression domains could be defined by a gradient of Pc repressor, each homeotic gene having a different sensitivity to this repressor (Fig. 4A). While it was subsequently shown that (1) the Pc transcript is uniformly distributed in the embryo (Paro and Hogness, 1991) and (2) the patterns of homeotic gene expression are initially established correctly in esc and Pc mutant embryos (Struhl and Akam, 1985; Kuziora and McGinnis, 1988), it still remained possible that maintenance of homeotic gene expression patterns relies on a gradient of Pc protein or a gradient in the level of product of one of the other Pc group genes. However, the Pc group gene products are required for repression of en in the anterior domain of every segment. The similar level of this requirement in each segment suggests that Pc group function is uniformly present in the embryo.

Fig. 4.

Requirement for the Pc group in maintaining the off state of en and the homeotic genes suggests that Pc group function is uniformly present in the embryo. Ovals represent anterior (a) or posterior (p) cell nuclei of the second thoracic (T2) through the fifth abdominal (A5) segments in the epidermis along the anterior-posterior body axis. In A, each nucleus is divided into three parts, which designate the different homeotic loci that are simultaneously controlled by the Pc group. B shows the state of expression of en in nuclei corresponding to those in A. Shaded areas denote a locus under Pc group repression and unshaded areas show the regions where a locus is active. A schematic diagram depicting the graded repressor model as envisioned by Lewis (1978)is shown in A. All of the genes in the bithorax complex are inactive in the more anterior segments of the embryo (e.g. anterior T2) where repressor activity is high. Conversely, in the more posterior segments of the embryo (e.g. posterior A4), where repressor activity is low, all of the genes in the complex are active. As shown, the sensitivities of en (B) and the homeotic genes (A;Ubx, abd-A, and Abd-B) are independently controlled in a fashion that cannot be reconciled with genes having a different level of sensitivity to a common graded repressor. Ubx, Ultrabithorax; abd-A, abdominal-A; Abd-B, Abdominal-B.

Fig. 4.

Requirement for the Pc group in maintaining the off state of en and the homeotic genes suggests that Pc group function is uniformly present in the embryo. Ovals represent anterior (a) or posterior (p) cell nuclei of the second thoracic (T2) through the fifth abdominal (A5) segments in the epidermis along the anterior-posterior body axis. In A, each nucleus is divided into three parts, which designate the different homeotic loci that are simultaneously controlled by the Pc group. B shows the state of expression of en in nuclei corresponding to those in A. Shaded areas denote a locus under Pc group repression and unshaded areas show the regions where a locus is active. A schematic diagram depicting the graded repressor model as envisioned by Lewis (1978)is shown in A. All of the genes in the bithorax complex are inactive in the more anterior segments of the embryo (e.g. anterior T2) where repressor activity is high. Conversely, in the more posterior segments of the embryo (e.g. posterior A4), where repressor activity is low, all of the genes in the complex are active. As shown, the sensitivities of en (B) and the homeotic genes (A;Ubx, abd-A, and Abd-B) are independently controlled in a fashion that cannot be reconciled with genes having a different level of sensitivity to a common graded repressor. Ubx, Ultrabithorax; abd-A, abdominal-A; Abd-B, Abdominal-B.

Finally, it is important to note that genes regulated by a common set of Pc group repressors display opposite states of activity within the same cell (Fig. 4). For example, in the anterior of A5, Pc group repression maintains en in the off state but allows Abd-B expression, whereas in the posterior of A3, it maintains Abd-B in the off state but allows en expression (Fig. 4B). If, as we have argued, Pc group function is uniformly present in the embryo, an intriguing question remains: how is repression controlled so as to allow opposite states of activity of the loci it regulates? One possibility is that the Pc group genes encode passive ubiquitous co-factors whose activity is dictated by other, as yet unknown, regulators. These putative regulators would have to be expressed in patterns that complement the expression patterns of en and each of the homeotic genes. However, this hypothesis is not satisfying, because extensive genetic analysis has failed to uncover any such regulators, and because it fails to explain two features of regulation by the Pc group genes: (1) derepression of en in some Pc group mutant embryos (e.g. Pc and Psc embryos, Fig. 1D and E) is stochastic in nature and appears independent of patterned regulators, and (2) Pc group genes maintain the patterns of expression precisely as established by earlier-acting transiently expressed regulators. Even defective patterns occurring in embryos mutant for some of the early regulators are stably maintained (Kellerman et al., 1990), presumably in a Pc group-dependent fashion. We therefore favor another possibility involving imprinting as proposed by Paro and co-workers (Paro, 1990; Franke et al. 1992). Imprinting would provide a means for the stable maintenance of the off states of expression, as set up by the early regulators, in a manner analogous to maintenance of methylation patterns in mammalian cells (Allen et al., 1990).

We are indebted to Hugh Brock, Ian Duncan, Gerd Jürgens, Don Sinclair and Gary Struhl for mutant strains, to Bill McGinnis for Abd-B DNA, to Gary Struhl for advice on making esc flies, to Jill Heemskerk, Steve DiNardo, Bruce Edgar, and J.-P. Vincent for advice on staining procedures, and to Madhu Wahi and Hugh Brock for encouragement and for sharing their unpublished observations on expression of en in some Pc group mutants. We thank Shelagh Campbell, Bob Duronio, Bruce Edgar, Charles Girdham, Pierre Léopold, Tony Shermoen, J.-P. Vincent and Madhu Wahi for comments on the manuscript. Supported by NIH and NSF grants (P. O’F.) and a Helen Hay Whitney fellowship (D. M.).

Allen
,
N. D.
,
Norris
,
M. L.
and
Surani
,
M. A.
(
1990
).
Epigenetic control of transgene expression and imprinting by genetic-specific modifiers
.
Cell
61
,
853
861
.
Baker
,
N. E.
(
1987
).
Molecular cloning of sequences from wingless, a segment polarity gene in Drosophila: the spatial distribution of a transcript in embryos
.
EMBO J
.
6
,
1765
1773
.
Breen
,
T.
and
Duncan
,
I. M.
(
1986
).
Maternal expression of genes that regulate the bithorax complex of Drosophila melanogaster
.
Dev. Biol
.
118
,
442
456
.
Brunk
,
B. P.
,
Martin
,
E. C.
and
Adler
,
P. N.
(
1991
).
Molecular genetics of the Posterior sex combs/Suppressor 2 of zeste region of Drosophila: aberrant expression of the Suppressor 2 of zeste gene results in abnormal bristle development
.
Genetics
128
,
119
132
.
Busturia
,
A.
and
Morata
,
G.
(
1988
).
Ectopic expression of homeotic genes caused by the elimination of the Polycomb gene in Drosophila imaginal epidermis
.
Development
104
,
713
720
.
DiNardo
,
S.
,
Kuner
,
J. M.
,
Theis
,
J.
and
O’Farrell
,
P. H.
(
1985
).
Development of embryonic pattern in D. melanogaster as revealed by accumulation of the nuclear engrailed protein
.
Cell
43
,
59
69
.
DiNardo
,
S.
,
Sher
,
E.
,
Heemskerk-Jongens,
J.
Kassis,
J.A.
and
O’Farrell
,
P. H.
(
1988
).
Two-tiered regulation of spatially pattered engrailed gene expression during Drosophila embyogenesis
.
Nature
332
,
45
53
.
Duncan
,
I. M.
(
1982
).
Polycomblike: A gene that appears to be required for the normal expression of the bithorax and Antennapedia complexes of Drosophila melanogaster
.
Genetics
102
,
49
70
.
Dura
J.-M.
and
Ingham
,
P.
(
1988
).
Tissue- and stage-specific control of homeotic and segmentation gene expression in Drosophila embryos by the polyhomeotic gene
.
Development
103
,
733
741
.
Dura
,
J.-M.
,
Randsholt
,
N. B.
,
Deatrick
,
J.
,
Erk
,
I.
,
Satamaria
,
P.
,
Freeman
,
J. D.
,
Freeman
,
S. J.
,
Weddell
,
D.
and
Brock
,
H. W.
(
1987
).
A complex genetic locus, polyhomeotic, is required for segmental specification and epidermal development in D. melanogaster
.
Cell
51
,
829
839
.
Franke
,
A.
,
DeCamillis
,
M.
,
Zink
,
D.
,
Cheng
,
N.
,
Brock
,
H. W.
and
Paro
,
R.
(
1992
).
Polycomb and polyhomeotic are constituents of a multimeric protein complex in chromatin of Drosophila melanogaster. EMBO J
.
11
,
2941
2950
.
Harding
,
K.
,
Wedeen
,
C.
,
McGinnis
,
W.
and
Levine
,
M.
(
1985
).
Spatially regulated expression of homeotic genes in Drosophila
.
Science
229
,
1236
1242
.
Heemskerk
,
J.
,
DiNardo
,
S.
,
Kostriken
,
R.
and
O’Farrell
,
P. H.
(
1991
).
Multiple modes of engrailed regulation in the progression towards cell fate determination
.
Nature
352
,
404
410
.
Ingham
,
P. W.
(
1983
).
Differential expression of bithorax complex genes in the absence of the extra sex combs and trithorax genes
.
Nature
306
,
591
593
.
Ingham
,
P. W.
(
1988
).
The molecular genetics of embryonic pattern formation in Drosophila
.
Nature
335
,
25
34
.
Jones
,
R. S.
and
Gelbart
,
W. M.
(
1990
).
Genetic analysis of the Enhancer of zeste locus and its role in gene regulation in Drosophila melanogaster
.
Genetics
126
,
185
199
.
Jürgens
,
G.
(
1985
).
A group of genes controlling the spatial expression of the bithorax complex in Drosophila
.
Nature
316
,
153
155
.
Kaufman
,
T. C.
,
Lewis
,
R. A.
and
Wakimoto
,
B. T.
(
1980
).
Cytogenetic analysis of chromosome 3 in Drosophila melanogaster: the homeotic gene complex in polythene chromosome interval 84A-B
.
Genetics
94
,
115
133
.
Kellerman
,
K. A.
,
Mattson
,
D. M.
and
Duncan
,
I.
(
1990
).
Mutations affecting the stability of the fushi tarazu protein of Drosophila
.
Genes Dev
.
4
,
1936
1950
.
Kennison
,
J. A.
and
Tamkun
,
J. W.
(
1988
).
Dosage-dependent modifiers of Polycomb and Antennapedia mutations in Drosophila
.
Proc. Natl. Acad. Sci. USA
85
,
8136
8140
.
Kornberg
,
T.
,
Siden
,
I.
,
O’Farrell
,
P.
and
Simon
,
M.
(
1985
).
The engrailed locus of Drosophila: in situ localization of transcripts reveals compartment-specific expression
.
Cell
40
,
45
53
.
Kuziora
,
M. A.
and
McGinnis
,
W.
(
1988
).
Different transcripts of the Drosophila Abd-B gene correlate with distinct genetic sub-functions
.
EMBO J
.
7
,
3233
3244
.
Lawrence
,
P. A.
and
Morata
,
G.
(
1976
).
Compartments in the wing of Drosophila: a study of the engrailed gene
.
Dev. Biol
.
50
,
321
337
.
Lewis
,
E.
(
1978
).
A gene complex controlling segmentation in Drosophila
.
Nature
276
,
565
570
.
Martinez-Arias
,
A.
,
Baker
,
N. E.
and
Ingham
,
P. W.
(
1988
).
Role of segment polarity genes in the definition and maintenance of cell states in the Drosophila embryo
.
Development
103
,
157
170
.
Paro
,
R.
(
1990
).
Imprinting a determined state into the chromatin of Drosophila
.
Trends Genet
.
6
,
416
421
.
Paro
,
R.
and
Hogness
,
D. S.
(
1991
).
The Polycomb protein shares a homologous domain with a heterochromatin-associated protein of Drosophila
.
Proc. Natl. Acad. Sci. USA
88
,
263
267
.
Patel
,
N. H.
,
Martin-Blanco
,
E.
,
Coleman
,
K. G.
,
Poole
,
S. J.
,
Ellis
,
M. C.
,
Kornberg
,
T. B.
and
Goodman
,
C. S.
(
1989
).
Expression of engrailed proteins in arthropods, annelids, and chordates
.
Cell
58
,
955
968
.
Phillips
,
M. D.
and
Shearn
,
A.
(
1990
).
Mutations in polycombeotic, a Drosophila Polycomb-group gene, cause a wide range of maternal and zygotic phenotypes
.
Genetics
125
,
91
101
.
Riley
,
P. D.
,
Carroll
,
S. B.
and
Scott
,
M. P.
(
1987
).
The expression and regulation of sex combs reduced protein in Drosophila embryos
.
Genes Dev
.
1
,
716
730
.
Sanchez-Herrero
,
E.
,
Vernos,
I.
Marco,
R.
and
Morata
,
G.
(
1985
).
Genetic organization of Drosophila bithorax complex
.
Nature
313
,
108
113
.
Sanchez-Herrero
,
E.
and
Cosby
,
M. A.
(
1988
).
The Abdominal-B gene of Drosophila melanogaster: overlapping transcripts exhibit two different spatial distributions
.
EMBO J
.
7
,
2163
2173
.
Shearn
,
A.
(
1989
).
The ash-1, ash-2, and trithorax genes of Drosophila melanogaster are functionally related
.
Genetics
121
,
517
525
.
Simon
,
J.
,
Chiang
,
A.
and
Bender
,
W.
(
1992
).
Ten different Polycomb group genes are required for spatial control of the abd-A and Abd-B homeotic products
.
Development
114
,
493
505
.
Smouse
,
D.
,
Goodman
,
C.
,
Mahowald
,
A.
and
Perrimon
,
N.
(
1988
).
polyhomeotic: a gene required for the embryonic development of axon pathways in the central nervous system of Drosophila
.
Genes Dev
.
2
,
830
842
.
St Johnson
,
D.
and
Nüsslein-Volhard
,
C.
(
1992
).
The origin of pattern and polarity in the Drosophila embryo
.
Cell
68
,
201
219
.
Struhl
,
G.
(
1983
).
Role of the esc+ gene product in ensuring the selective expression of segment-specific homeotic genes in Drosophila
.
J. Embryol. exp. Morph
.
76
,
297
331
.
Struhl
,
G.
and
Akam
,
M.
(
1985
).
Altered distribution of of Ultrabithorax transcripts in extra sex combsmutant embryos of Drosophila
.
EMBO J
.
4
,
3259
3264
.
Tautz
,
D.
and
Pfeifle
,
C.
(
1989
).
A non-radioactive in situ hybridization method for the localization of specific RNAs in Drosophila embryos reveals translational control of the segmentation gene hunchback
.
Chromosoma
98
,
81
85
.