Lethal bithorax complex mutations of Drosophila melanogaster show no germ line maternal effects

Three Ultrabithorax (Ubx) alleles and three different deficiencies of the bithorax complex (BX-C) of Drosophila melanogaster have been tested for maternal effects in the germ line. The dominant female sterile technique was used. The Ubx alleles and a deletion of the abdominal region of the BX-C are homozygous viable in germ line clones and show no maternal effects. Two deletions which lack the proximal portion of the BX-C are lethal in the female germ line indicating either that these deficiencies lack genes apart from the BX-C that are necessary for fertility or that there are BX-C genes that are essential for normal maternal germ line function. The significance of the bias in the isolation of only zygotic mutations at the BX-C are discussed with respect to these results.     

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.     

Control of the expression of the bithorax complex genes abdominal-A and abdominal-B by cis-regulatory regions in Drosophila embryos.

The abdominal-A (abd-A) and Abdominal-B (Abd-B) genes of the bithorax complex (BX-C) specify the identity of most of the Drosophila abdomen. Six different classes of infraabdominal (iab) mutations within the BX-C transform a subset of the parasegments affected by the lack of these two genes. It is thought that these mutations define parasegmental cis-regulatory regions that control the expression of abd-A and Abd-B. By staining embryos mutant for different iab mutations with anti-abd-A and anti-Abd-B antibodies I show here that the expression of Abd-B (and probably also abd-A) exhibit a parasegmental regulation. I have also studied the significance of the chromosomal order of parasegmental iab regulatory sequences, and the possible presence of chromosomal 'boundaries' between them, by looking at the expression of abd-A and Abd-B in embryos carrying the Uab and Mcp mutations. These data are discussed in the light of models of parasegmental-specific regulatory regions within the BX-C.     

Metamorphosis of Wing Motor System in the Silk Moth, Bombyx mori: Origin of Wing Motor Neurons

The origin of the peripheral nerve and motor neurons that innervate the adult mesothoracic dorsal longitudinal muscles (DLMs) was examined in the silk moth, Bombyx mori . The anatomical features of the peripheral nerve and motor neurons were investigated by dissection, electron microscopy, and cobalt back-fill staining at different pupal stages. These studies showed that the peripheral nerve (IIN1c) that innervates the adult DLMs originates from a branch (db branch) of the larval mesothoracic dorsal nerve that innervates the larval DLMs. During metamorphosis the larval nerve shortens or lengthens locally without change in its basic branching pattern, and the db branch moves towards the mesothoracic ganglion to become the IIN1c. All the adult DLM motor neurons are from larval ones. Nine of the 14 larval DLM motor neurons survive during metamorphosis to become adult DLM motor neurons, and 5 disappear in early pupal stages.     

Regulatory elements of the bithorax complex that control expression along the anterior-posterior axis

The Drosophila bithorax complex (BX-C) controls segmental development by selectively deploying three protein products, Ubx, abd-A and Abd-B, within specific segments along the body axis. Expression of these products within any one segment (or, more accurately, parasegment) is affected by mutations clustered in a particular region of the BX-C. The regulatory regions defined by this genetic analysis span 20-50 kb and there is one region for each segmental unit. Here we describe regulatory elements from several of these regions, identified by fusion to a Ubx-lacZ gene and analysis in germline transformants. A small DNA fragment from the abx region programs expression with an anterior boundary in the second thoracic segment (parasegment 5). This anterior limit is appropriate, since the abx region normally controls Ubx in parasegment 5. Other regulatory regions of the BX-C that control development of parasegments 6, 7 or 8 contain similar regulatory elements that program expression with anterior limits in parasegments 6, 7 or 8, respectively. These experiments define a class of BX-C regulatory elements that control expression along the anterior-posterior axis. The early appearance of the lacZ patterns in embryos suggests a role for these elements in the initial activation of expression from the BX-C.     

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.     

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.     

Auditory interneurones in the mesothoracic ganglion of crickets

Auditory interneurone responses in the mesothoracic ganglion of the cricket Gryllus bimaculatus were investigated with special regard to temporal features of the calling song. Units representing five response types were found. One type codes verse syllables and intensity. The second codes syllables of highfrequency verses. The third responds as a pulse marker. The fourth shows adaptation and the response pattern depends on the verse frequency. The fifth fires a burst at verse onset.Responses of mesothoracic units recorded in two other cricket species do not differ markedly from those of Gryllus bimaculatus. Particularly, no tuning is found to species-specific differences in their calling songs.The stimulus direction can affect the threshold in different ways: dependence at all frequencies, dependence only between 3 and 6 kHz, and independence are found. The dependence is mainly expressed by a higher threshold for contralateral sounds.The mesothoracic branching of a few neurones was demonstrated by extracellular CoS-staining. These cells pass through the ganglion as connective fibres giving off small branches into the ventro-medial and dorso-medial neuropiles.     

Control of female pheromones in Drosophila melanogaster by homeotic genes

We have investigated the role of the Antennapedia and Bithorax complexes (ANT-C and BX-C) on the production of cuticular hydrocarbons in Drosophila melanogaster. In males, there is little, if any, influence of these complexes on the hydrocarbon pattern. In females, there are large and opposite effects of these complexes on diene production: two ANT-C mutations cause an increase in diene production and a reduction of monoenes, whereas most BX-C mutations result in a decrease in dienes and an increase in monoenes, although their sum remains constant. The effect is the highest in Mcp and iab6 females. It is suggested that a factor originating from the prothorax might activate the conversion of monoenes to dienes in females. The abdomen seems to have a crucial role in the production or control of pheromones: abdominal segments four to seven have the main effects, with a most dramatic effect for segments four and five.     

Initiator elements function to determine the activity state of BX-C enhancers

A >300 kb cis-regulatory region is required for the proper expression of the three bithorax complex (BX-C) homeotic genes. Based on genetic and transgenic analysis, a model has been proposed in which the numerous BX-C cis-regulatory elements are spatially restricted through the activation or repression of parasegment-specific chromatin domains. Particular early embryonic enhancers, called initiators, have been proposed to control this complex process. Here, in order to better understand the process of domain activation, we have undertaken a systematic in situ dissection of the iab-6 cis-regulatory domain using a new method, called InSIRT. Using this method, we create and genetically characterize mutations affecting iab-6 function, including mutations specifically modifying the iab-6 initiator. Through our mutagenesis of the iab-6 initiator, we provide strong evidence that initiators function not to directly control homeotic gene expression but rather as domain control centers to determine the activity state of the enhancers and silencers within a cis-regulatory domain.     

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.     

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.     

Transdetermination in the homeotic eye--antenna imaginal disc of Drosophila melanogaster

The effects of homeotic mutations on transdetermination in eye-antenna imaginal discs of Drosophila melanogaster were studied. After 12 days of culture in vivo, antenna discs transformed to ventral mesothorax by AntpNs or AntpZ, transdetermined to notum and wing structures four to five times more frequently than the corresponding wild-type antenna discs. Likewise, eye discs transformed to dorsal mesothorax by eyopt transdetermined to leg structures, also extremely frequently (90%). It seems that, during culture, homeotic antenna as well as homeotic eye discs tend to complete the structural inventory of the mesothoracic segment. Transdetermination in the homeotic disc parts is interpreted as a regeneration process which reestablishes an entire segment, i.e., the ventral mesothoracic portion (leg) in the antenna disc regenerates dorsal mesothoracic parts, and the dorsal mesothoracic portion in the eye disc (wing) regenerates ventral mesothoracic parts, respectively. This implies that antenna and leg discs (ventral qualities) as well as eye and wing discs (dorsal qualities) are serially homologous. The transdetermination frequency of the untransformed eye disc to notum and wing structures is enhanced by Antp to the same extent as is the transdetermination frequency of the antenna disc. The first allotypic wing disc structure formed by the eye disc is notum, followed by structures of the anterior wing compartment and finally by posterior wing structures. No evidence for such a sequence was found in the transdetermination pattern of the antenna disc.     

Homoeosis in Drosophila: anterior and posterior transformations of Polycomb lethal embryos

Lethal embryos homozygous for Polycomb (Pc) mutations show transformations of segment-specific cuticular features to those of more anterior or posterior segments; the frequency and extent of such changes show differences which depend on the genotype and the region. The mesothorax of Polycomb lethal embryos often shows posterior transformations of the anterior- and posterior-most portions of the segment, and anterior transformations of the medial portion. A comparison of Polycomb embryos also bearing various genetic lesions of the bithorax gene complex (BX-C) shows that the penetrance of anterior transformation and the extent of posterior transformation in the appears independent of posterior transformation, even though cells undergoing each of these changes lie in close proximity in the developing embryo. It has been shown previously that in Polycomb lethal embryos posterior transformations require the normal function of the BX-C. We show here that anterior transformations of the mesothorax and other segments require the normal function of the Sex combs reduced (Scr) locus, also necessary for the normal development of the prothorax and some head segments. Similar observations are also presented for a Polycomblike mutation. We suggest that in Polycomb embryos there are errors in the clonal transmission of determined states resulting in expression of the BX-C and Scr+ loci at abnormal locations, and that such events are probabilistic in nature and show marked regional differences in frequency.     

Chromosomal continuity in the abdominal region of the bithorax complex of Drosophila is not essential for its contribution to metameric identity

We have examined the developmental consequences for larval and imaginal segmental cuticular structure of a chromosomal translocation involving a breakpoint in the abdominal region of the bithorax complex (BX-C). This complex makes an essential contribution to the development of metameric differences in part of the thorax and in all abdominal segments. The breakpoint is proximal to the most distal (iab-7) homeobox, and results in the translocation to the Y chromosome of the Ultrabithorax (Ubx) and abdominal-A (abd-A) domains. The genotype deficient for the distal part of the complex shows normal function for Ubx and abd-A but has a phenotype typical for severe Abd-B mutations. Conversely, the distal fragment retains a segment identity function which must represent a contribution from Abd-B in parasegments 13 and 14; the latter metamere is wild type, indicating that it does not require the contribution of Ubx or abd-A. We also constructed a genotype comprising the proximal fragment of this translocation together with an overlapping distal fragment of the BX-C derived from Df(3R)Ubx109. It therefore contained all sequences of the BX-C though in the abdominal region the abd-A and Abd-B domains were not adjacent to each other in the chromosome. This genotype was phenotypically normal and demonstrates that DNA sequences in the abd-A and Abd-B regions do not require cis-arrangement for their activity.     

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.     

Polyhomeotic: A gene of Drosophila melanogaster required for correct expression of segmental identity

Summary A new locus in Drosophila melanogaster that is required for the correct expression of segmental identity has been discovered. The new locus, termed polyhomeotic (ph), is X-linked and maps cytologically to bands 2D2-3. Homozygous ph flies have homeotic transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes (ANT-C, BX-C), and in addition show loss of the humerus. ph interacts with three other similar mutations: Polycomb (Pc), Polycomblike (Pcl), and extra sex comb (esc), and acts as a dominant enhancer of Pc. The expression of ph depends on the ANT-C and BX-C dosage. ph has no embryonic phenotype, but temperature shift studies on ph ² show that the ph ⁺ product is required during embryogenesis and larval development. We propose that ph mutants in some way disrupt the normal expression of the ANT-C and BX-C, and, therefore, that ph ⁺ is needed for maintenance of segmental identity.     

The Mode of Action of the Bithorax Genes of Drosophila melanogaster

SYNOPSIS. The activity of the genes of the bithorax complex(BX-C) of Drosophila is necessary for the normal developmentof thoracic and abdominal segments. Primordia destined to formthe different segments of the fly are established in differentpositions prior to BX-C function. Different combinations ofBX-C genes are then activated in each position to specify forthe different segment types. In this process some BX-C genesact before others. I propose a temporal binary mechanism whichaccounts for the different combinations of active and inactiveBX-C genes in the thoracic segments.