A gene that regulates the bithorax complex differentially in larval and adult cells of Drosophila

Loss-of-function mutations of a new homeotic gene, sxc, in Drosophila cause transformations of body segments, suggesting inappropriate expression of BX-C and ANT-C genes. I present evidence that sxc+ is required during embryogenesis for the selective repression of the BX-C in different larval segments and show that this requirement may be met entirely by maternally derived gene product. sxc+ is also required later in development to ensure the appropriate expression of ANT-C and BX-C genes in adult thoracic and abdominal segments. Absence of sxc+ in the mesothorax apparently results in the ectopic expression of the bx+ (or Ubx+) function in both the anterior and posterior compartments; this suggests that pbx mutations may define a regulatory rather than a structural function.     

hunchback, a gene required for segmentation of an anterior and posterior region of the Drosophila embryo

The locus hunchback (hb) is a member of the gap class of segmentation genes of Drosophila. A number of X-ray-induced deletions locate the hb locus at the chromosomal site 85A3-B1, to the right of the pink locus, which maps in the same interval. A total of 14 EMS and 3 X-ray-induced hb alleles have been studied. Homozygous mutant embryos show deletions of segments in two separate regions. In the six strong alleles, the labium and all three thoracic segments are deleted anteriorly while posteriorly the 8th abdominal segment and adjacent parts of the 7th abdominal segment are lacking. The eight weak alleles show smaller deletions both in the thoracic and posterior abdominal region. In the weakest allele only part of the mesothorax is deleted. Three hb alleles produce a homoeotic transformation: superimposed on a strong or weak deletion phenotype, head or thoracic segments are transformed into abdominal segments, respectively. This suggests that hb might also be involved in the regulation of genes in the Bithorax complex (BX-C). Fate mapping of the normal-appearing segments in strong mutant embryos using the UV-laser beam ablation technique (Lohs-Schardin et al., 1979) shows that these segments arise from the normal blastoderm regions. The mutant phenotype can be recognized soon after the onset of gastrulation in a failure to fully extend the germ band. In 6-hr-old mutant embryos, two clusters of dead cells are observed in the thoracic and posterior abdominal region. These observations indicate region specific requirement of hb gene function. The analysis of germ line chimeras by transplantation of homozygous mutant pole cells shows that hb is already expressed during oogenesis. Homozygous mutant embryos derived from a homozygous mutant germ line have a novel phenotype. The anterior affected region is enlarged, including all three gnathal segments and the anterior three abdominal segments. In addition three abdominal segments with reversed polarity are formed between the remaining head structures and the posterior abdomen. Heterozygous mutant embryos derived from a homozygous mutant germ line develop normally, indicating that maternal gene expression is not required for normal development.     

Effect of Abx, Bx and Pbx Mutations on Expression of Homeotic Genes in Drosophila Larvae

We have examined the patterns of expression of the homeotic gene Ubx in imaginal discs of Drosophila larvae carrying mutations in the abx, bx and pbx regulatory domains. In haltere discs, all five bx insertion mutations examined led to a general reduction in Ubx expression in the anterior compartment; for a given allele, the strength of the adult cuticle phenotype correlated with the degree of Ubx reduction. Deletions mapping near or overlapping the sites of bx insertions, including three abx alleles and the bx34e-prv(bx-prv) allele, showed greatly reduced Ubx expression in parts of the anterior compartment of the haltere disc; however, anterior patches of strong Ubx expression often remained, in highly variable patterns. As expected, the pbx1 mutation led to reduced Ubx expression in the posterior compartment of the haltere disc; surprisingly, pbx1 also led to altered expression of the en protein near the compartment border in the central region of the disc. In the metathoracic leg, all the bx alleles caused extreme reduction in Ubx expression in the anterior regions, with no allele-specific differences. In contrast, abx and bx-prv alleles resulted in patchy anterior reductions in third leg discs. In the larval central nervous system, abx but not bx alleles affected Ubx expression; the bx-prv deletion gave a wild-type phenotype, but it could not fully complement abx mutations. In the posterior wing disc, the bx-prv allele, and to a much lesser extent the bx34e chromosome from which it arose, led to ectopic expression of Ubx. Unlike other grain-of-function mutations in the BX-C, this phenotype appeared to be partially recessive to wild type. Finally, we asked whether the ppx transformation, which results from early lack of Ubx+ function in the mesothorax and is seen in abx animals, is due to ectopic Scr expression. Some mesothoracic leg and wing discs from abx2 larvae displayed ectopic expression of Scr, which was variable in extent but always confined to the posterior compartment.     

Recessive lethal mutations within the bithorax-complex in Drosophila

Summary Genetic deficiencies of the bithorax-complex (BX-C) in Drosophila, have been used to recover recessive lethal mutations in this chromosome region following mutagenesis. Complementation analysis separates these lethal mutations into five groups within a smaller deficiency, though to remove the entire BX-C, and into 20 to the left and 4 to the right of the region. Homozygotes for each of only three groups of lethals, Ubx, abdA and AbdB, produce homoeotic segmental transformations in embryos. The functional domains of abdA and AbdB have been defined by changes in the appearance of larval hypodermal structures and of clones in imaginal tissue. The function abdA is required in all the compartments caudal to the anteroposterior border of abdominal segment 1 up to and including the anterior region of abdominal segment 8, whilst AbdB is required in abdominal segments 5 to 9. One allele of AbdB produces a ninth abdominal setal band and structures characteristic of head segments posterior to A8. Rare adult survivors hemizygous for an AbdB allele have eight abdominal segments in both sexes, and lack genitalia in females. Our findings are discussed in the context of the organisation of genetic functions within the BX-C.     

Segmental determination in Drosophila central nervous system: analysis of the abdominal-A region of the bithorax complex.

The bithorax complex (BX-C) comprises several genes required for the diversification of posterior segments in Drosophila. The BX-C genes control segment differences not only in the epidermis but in other tissues as well, especially in the central nervous system. We have examined the control of one segment-specific neural structure: the lateral dots, a paired structure present in the first abdominal segment of the larval CNS and absent in all following abdominal segments. Our results show that the suppression of lateral dots in segments A3 and A4 requires the presence of two active copies of one of the BX-C genes, abdominal-A (abd-A). We also show that the adjacent BX-C regions, iab-3 and iab-4, can act in trans on abd-A not only when the two copies of BX-C are paired but also, at least to some extent, when pairing is disturbed.     

Genetic interactions between the Polycomb locus and the Antennapedia and Bithorax complexes of Drosophila

Summary We have studied the embryonic and adult phenotypes of genetic combinations between Polycomb (Pc), Regulator of bithorax (Rg-bx) and the genes of the Bithorax complex (BX-C) and the Antennapedia complex (ANT-C). The products of Pc and Rg-bx genes act antagonistically, their mutant combinations leading to the ectopic expression of genes of the BX-C and ANT-C. The genetic analysis of the Pc locus suggests it is a complex gene. Pc⁺ products behave as members of a regulatory set that negatively control the expression of BX-C and ANT-C genes. Genetic combinations between different doses of Pc, Rg-bx and the genes of the BX-C and ANT-C have phenotypes which may be interpreted as resulting from ectopic derepression of posterior selector genes repressing selector genes of anterior segments. The transformation phenotypes of certain genetic combinations differ in embryos and adults. A model of regulation of the BX-C and the ANT-C genes during the imaginal cell proliferation is presented, in which the specification state is maintained by self-activation of a given selector gene and down modulation of other selector genes in the same cell.     

Homoeosis in Drosophila: The Lethal Syndrome of the Regulator of bithorax (or trithorax) Locus and Its Interaction with Other Homoeotic Loci

Regulator of bithorax (Rg-bx)^- [or trithorax (trx)^-] lethal zygotes show anterior transformations of various cuticular features of the larval thorax and abdomen. The Rg-bx^- lethal syndrome depends on the dosage of the bithorax gene complex (BX-C), and lack of Rg-bx⁺ function is antagonistic to posterior transformations displayed by Polycomb ( Pc)^- embryos. Significantly, when the BX-C is deleted, the Rg-bx^- embryos disclose homoeosis of mesothoracic to prothoracic cuticular structures. This homoeotic transformation is due to a reduction in Antennapedia (Antp)⁺ gene activity and is consequently dependent on the dosage of the Antennapedia gene complex (ANT-C), suggesting that the Rg-bx⁺ activity is necessary for proper expression of the Antp⁺ gene. However, the functional relationship between the Rg-bx and Sex combs reduced (Scr) loci in embryogenesis is still to be established.     

Clonal analysis of segmental and compartmental homoeotic transformations in polycomb mutants of Drosophila melanogaster

We describe two new effects of Polycomb mutations on the determination of compartments in the wing of Drosophila. Ventral and posterior wing compartments are transformed partially to their dorsal and anterior counterparts. Although these new phenotypes are most strongly expressed in lethal pharate adults heteroallelic for Pc2 and a new allele PcT2, they are also found regularly but with low expressivity as dominant phenotypes of all the other Pc alleles we tested. Several different intersegmental homoeotic transformations caused by Polycomb have previously been described, leading to the hypothesis that Polycomb regulates the activity of certain selector genes normally active in specific segments. We now show that the degree of expression of the inter- and intrasegmental transformations are highly correlated in a range of different Pc genotypes, and that more than one determinative decision can be affected in a single compartment. This suggests that the wild-type Pc product may act as a general regulator of several different selector genes so as to influence both early embryonic and later determinative decisions in the imaginal discs. To test this idea we used clonal analysis to look at the effects of Pc on clonal restrictions at the dorsoventral and anterioposterior compartment boundaries, and its time of action with respect to each phenotype.     

Head and tail development of the Drosophila embryo involves spalt, a novel homeotic gene

Mutations in spalt (sal), a novel homeotic gene on the second chromosome of Drosophila, cause opposite transformations in two subterminal regions of the embryo: posterior head segments are transformed into anterior thoracic structures and anterior tail segments are transformed into posterior abdominal structures. The embryonic phenotypes of double mutants for sal and various Antennapedia (ANT-C) or bithorax (BX-C) genes indicate that sal acts independently of the hierarchical order of the latter gene complexes. Trans-regulatory gene mutations causing ectopic expression of ANT-C and BX-C genes do not change the realms of sal action. It is proposed that the region-specific action of the sal gene primarily promotes head as opposed to trunk development, while the BX-C gene AbdB distinguishes tail from head.     

Maternal and zygotic requirement for thepolyhomeotic complex genetic locus inDrosophila

Summary The complex genetic locuspolyhomeotic (ph) is a member of thePolycomb (Pc)-group of genes and as such is required for the normal expression of ANT-C and BX-C genes. It also has probably other functions since amorphicph alleles display a cell death phenotype in the ventral epidermis of 12-h-old embryos. Here it is shown that lethal alleles ofph (amorph and strong hypomorph) show transformation of most of their segments towards AB8. Theph ⁺ product is required autonomously in imaginal cells. The total lack ofph ⁺ function prevents viability of the cuticular derivatives of these cells.ph has a strong maternal effect on segmental identity and epidermal development that can not be rescued by one paternally supplied dose ofph ⁺ in the zygote. These phenotypes differ substantially from those of previously describedPc-group genes. AmongPc-group genes,ph seems to be the only one that is strongly required both maternally and zygotically for normal embryonic development.     

Tissue- and stage-specific control of homeotic and segmentation gene expression in Drosophila embryos by the polyhomeotic gene

The distributions of the products of the homeotic genes Sex combs reduced (Scr) and Ultrabithorax (Ubx) and of the segmentation genes, fushi tarazu (ftz), even skipped (eve) and engrailed (en) have been monitored in polyhomeotic (ph) mutant embryos. None of the genes monitored show abnormal expression at the blastoderm stage in the absence of zygotic ph expression. Both Scr and Ubx are ectopically expressed in the epidermis of ph embryos, confirming the earlier proposal, based on genetic analysis, that ph+ acts as a negative regulator of Antennapedia (ANT-C) and bithorax (BX-C) complex genes. At the shortened germ band stage, en is also ectopically expressed, mainly in the anterior region of each segment. In contrast to these effects in the epidermis, the expression of en, Ubx, Scr and ftz is largely or completely suppressed in the central nervous system, whereas eve becomes ectopically expressed in most neurones.     

Polycomblike: A Gene That Appears to Be Required for the Normal Expression of the Bithorax and Antennapedia Gene Complexes of DROSOPHILA MELANOGASTER

A newly identified gene is described that is required for the maintenance of normal identities in many of the body segments of the fly. The effects of mutants in this gene, which is called Polycomblike (Pcl), suggest that its wild-type allele functions in the regulation of the bithorax gene complex (BX-C) and the Antennapedia gene complex (ANT-C). Evidence in favor of this idea derives from (1) the close correspondence between segmental transformations caused by Pcl mutants and those caused by dominant gain-of-function mutants in the BX-C and ANT-C, (2) the interactions observed between Pcl mutants and mutants in these complexes, and (3) the dependence upon BX-C and ANT-C dosage of the severity of at least one of the transformations caused by Pcl mutants. Arguments are presented that the control of the BX-C and ANT-C by Pcl⁺ is negative in nature. The results of clonal analysis experiments indicate that, at least for the BX-C, Pcl⁺ exerts this control until late in development. Since the wild-type allele of another gene, called Polycomb (Pc), has previously been shown to have many of the same properties as Pcl⁺, it appears that the BX-C and perhaps also the ANT-C are continuously regulated during development by at least two and probably several other genes.     

The segment polarity gene costal-2 in Drosophila. I. The organization of both primary and secondary embryonic fields may be affected

A series of loss of function alleles at the costal-2 locus is described. Embryos mutant for lethal alleles that are derived from a mutant female germ line display polarity defects on the larval segments. A posterior part of the segmental denticle belt is missing and in its place is a mirror-image duplication of the anterior part including the segment boundary. Maternally rescued embryos are lethal but have normal morphology. Hypomorphic alleles escape to adults that display pattern duplications on the wings and halteres. Dominant gain of function alleles at the Costal-1 locus are also described and data are presented that argue that these are neomorphic and act in trans to impair functioning of costal-2. Some wild-type isoalleles of costal-2 are particularly sensitive to interference from Costal-1 mutations and different combinations of these alleles with Costal-1 can lead to embryos in which the primary embryonic field is disrupted (bicaudal phenotype) and adults with pattern duplications on the anterior compartment of most body segments.     

Genetic Analysis of the Additional Sex Combs Locus of Drosophila Melanogaster

Additional sex combs (Asx) is a member of the Polycomb group of genes, which are thought to be required for maintenance of chromatin structure. To better understand the function of Asx, we have isolated nine new alleles, each of which acts like a gain of function mutation. Asx is required for normal determination of segment identity. AsxP1 shows an unusual phenotype in that anterior and posterior homeotic transformations are seen in the same individuals, suggesting that AsxP1 might upset chromatin structure in a way that makes both activation and repression of homeotic genes more difficult. Analysis of embryonic and adult phenotypes of Asx alleles suggests that Asx is required zygotically for determination of segment number and polarity. The expression pattern of even-skipped is altered in Asx mutant embryos, suggesting that Asx is required for normal expression of this gene. We have transposon-tagged the Asx gene, and can thus begin molecular analysis of its function.     

Localization of the Antennapedia protein in Drosophila embryos and imaginal discs

Antibodies have been raised against a fusion protein containing the 3' region of the coding sequence of the Antennapedia (Antp) gene fused to β-galactosidase. The distribution of the protein on whole mount embryos and imaginal discs of third instar larvae was examined by immunofluorescence. In young embryos, expression of the Antp protein was limited to the thoracic segments in the epidermis, whereas it was found in all neuromeres of head, thorax and abdomen. At the end of embryogenesis, the Antp protein mainly accumulated in the ventral nervous system in certain parts of the thoracic neuromeres, from posterior T1 to anterior T3, with a gap in posterior T2. Comparison of Antp protein distribution in nervous systems from wild-type and Df P9 embryos, lacking the genes of the Bithorax-complex (BX-C), revealed a pattern of expression which indicated that the BX-C represses Antp in the posterior segments with the exception of the last abdominal neuromeres (A8-9) which are regulated independently. The protein pattern in nervous systems from Sex combs reduced(Scr^xF9) mutant embryos was indistinguishable from that found in wild-type embryos; thus, neurogenic expression of Antp in T1 and the more anterior segments does not appear to be under the control of Scr⁺. All imaginal discs derived from the three thoracic segments express Antp protein. The distribution was distinct in each disc; strongest expression was observed in the proximal parts of the discs. In the leg discs the protein distribution seemed to be compartmentally restricted, whereas in the wing disc this was not the case. Antp protein was not detected in the eye-antennal disc. In embryos, as well as in imaginal discs, the protein is localized in the nucleus.