Segmental organisation of the tail region in the embryo of Drosophila melanogaster: A blastoderm fate map of the cuticle structures of the larval tail region

An anatomical atlas of Drosophila melanogaster-the wild-type

Scanning electron microscopy is the method of choice to visualize the surface structures of animals, fungi, plants, or inorganic objects at the highest resolution and often with impressive appeal. Numerous scanning electron microscope (SEM) images exist of Drosophila melanogaster, one of the most important model organisms in genetics and developmental biology, which have been taken partly for esthetics and often to solve scientific questions. Our work presents a collection of images comprising many prominent anatomical details of D. melanogaster in excellent quality to create a research and teaching resource for all Drosophilists.

A timer gene network is spatially regulated by the terminal system in the Drosophila embryo

In insect embryos, anteroposterior patterning is coordinated by the sequential expression of the 'timer' genes caudal, Dichaete, and odd-paired, whose expression dynamics correlate with the mode of segmentation. In Drosophila, the timer genes are expressed broadly across much of the blastoderm, which segments simultaneously, but their expression is delayed in a small 'tail' region, just anterior to the hindgut, which segments during germband extension. Specification of the tail and the hindgut depends on the terminal gap gene tailless, but beyond this the regulation of the timer genes is poorly understood. We used a combination of multiplexed imaging, mutant analysis, and gene network modelling to resolve the regulation of the timer genes, identifying 11 new regulatory interactions and clarifying the mechanism of posterior terminal patterning. We propose that a dynamic Tailless expression gradient modulates the intrinsic dynamics of a timer gene cross-regulatory module, delineating the tail region and delaying its developmental maturation.

Programmed cell death acts at different stages of Drosophila neurodevelopment to shape the central nervous system

Nervous system development is a process that integrates cell proliferation, differentiation, and programmed cell death (PCD). PCD is an evolutionary conserved mechanism and a fundamental developmental process by which the final cell number in a nervous system is established. In vertebrates and invertebrates, PCD can be determined intrinsically by cell lineage and age, as well as extrinsically by nutritional, metabolic, and hormonal states. Drosophila has been an instrumental model for understanding how this mechanism is regulated. We review the role of PCD in Drosophila central nervous system development from neural progenitors to neurons, its molecular mechanism and function, how it is regulated and implemented, and how it ultimately shapes the fly central nervous system from the embryo to the adult. Finally, we discuss ideas that emerged while integrating this information.

TALEN-induced gene knock out in Drosophila

We report here a case study of TALEN-induced gene knock out of the trachealess gene of Drosophila. Two pairs of TALEN constructs caused targeted mutation in the germ line of 39% and 17% of injected animals, respectively. In the extreme case 100% of the progeny of TALEN-injected fly was mutated, suggesting that highly efficient biallelic germ line mutagenesis was achieved. The mutagenic efficiency of the TALEN pairs paralleled their activity of single strand annealing (SSA) assay in cultured cells. All mutations were deletion of 1 to 20 base pairs. Merit and demerit of TALEN-based gene knockout approach compared to other genome editing technologies is discussed.

Abdominal-B and caudal inhibit the formation of specific neuroblasts in the Drosophila tail region

The central nervous system of Drosophila melanogaster consists of fused segmental units (neuromeres), each generated by a characteristic number of neural stem cells (neuroblasts). In the embryo, thoracic and anterior abdominal neuromeres are almost equally sized and formed by repetitive sets of neuroblasts, whereas the terminal abdominal neuromeres are generated by significantly smaller populations of progenitor cells. Here we investigated the role of the Hox gene Abdominal-B in shaping the terminal neuromeres. We show that the regulatory isoform of Abdominal-B (Abd-B.r) not only confers abdominal fate to specific neuroblasts (e.g. NB6-4) and regulates programmed cell death of several progeny cells within certain neuroblast lineages (e.g. NB3-3) in parasegment 14, but also inhibits the formation of a specific set of neuroblasts in parasegment 15 (including NB7-3). We further show that Abd-B.r requires cooperation of the ParaHox gene caudal to unfold its full competence concerning neuroblast inhibition and specification. Thus, our findings demonstrate that combined action of Abdominal-B and caudal contributes to the size and composition of the terminal neuromeres by regulating both the number and lineages of specific neuroblasts.

Neuroblast pattern and identity in the Drosophila tail region and role of doublesex in the survival of sex-specific precursors

The central nervous system is composed of segmental units (neuromeres), the size and complexity of which evolved in correspondence to their functional requirements. In Drosophila, neuromeres develop from populations of neural stem cells (neuroblasts) that delaminate from the early embryonic neuroectoderm in a stereotyped spatial and temporal pattern. Pattern units closely resemble the ground state and are rather invariant in thoracic (T1-T3) and anterior abdominal (A1-A7) segments of the embryonic ventral nerve cord. Here, we provide a comprehensive neuroblast map of the terminal abdominal neuromeres A8-A10, which exhibit a progressively derived character. Compared with thoracic and anterior abdominal segments, neuroblast numbers are reduced by 28% in A9 and 66% in A10 and are almost entirely absent in the posterior compartments of these segments. However, all neuroblasts formed exhibit serial homology to their counterparts in more anterior segments and are individually identifiable based on their combinatorial code of marker gene expression, position, delamination time point and the presence of characteristic progeny cells. Furthermore, we traced the embryonic origin and characterised the postembryonic lineages of a set of terminal neuroblasts, which have been previously reported to exhibit sex-specific proliferation behaviour during postembryonic development. We show that the respective sex-specific product of the gene doublesex promotes programmed cell death of these neuroblasts in females, and is needed for their survival, but not proliferation, in males. These data establish the terminal neuromeres as a model for further investigations into the mechanisms controlling segment- and sex-specific patterning in the central nervous system.

A combinatorial enhancer recognized by Mad, TCF and Brinker first activates then represses dpp expression in the posterior spiracles of Drosophila

A previous genetic analysis of a reporter gene carrying a 375-bp region from a dpp intron (dppMX-lacZ) revealed that the Wingless and Dpp pathways are required to activate dpp expression in posterior spiracle formation. Here we report that within the dppMX region there is an enhancer with binding sites for TCF and Mad that are essential for activating dppMX expression in posterior spiracles. There is also a binding site for Brinker likely employed to repress dppMX expression. This combinatorial enhancer may be the first identified with the ability to integrate temporally distinct positive (TCF and Mad) and negative (Brinker) inputs in the same cells. Cuticle studies on a unique dpp mutant lacking this enhancer showed that it is required for viability and that the Filzkorper are U-shaped rather than straight. Together with gene expression data from these mutants and from brk mutants, our results suggest that there are two rounds of Dpp signaling in posterior spiracle development. The first round is associated with dorsal-ventral patterning and is necessary for designating the posterior spiracle field. The second is governed by the combinatorial enhancer and begins during germ band retraction. The second round appears necessary for proper spiracle internal morphology and fusion with the remainder of the tracheal system. Intriguingly, several aspects of dpp posterior spiracle expression and function are similar to demonstrated roles for Wnt and BMP signaling in proximal-distal outgrowth of the mammalian embryonic lung.

Wingless signaling in a large insect, the blowflyLucilia sericata: A beautiful example of evolutionary developmental biology

Blowflies are the primary facultative agent in causing myiasis of domestic sheep in the whole world and, at the same time, it is an important tool for forensic medicine. Surprisingly, and in contrast to its importance, almost no data regarding the embryology and molecular markers are known for this insect. In this report, we present a detailed description of the blowfly Lucilia sericata embryogenesis and of imaginal disc development. The embryogenesis of Lucilia strongly resembles that of Drosophila, despite their apparent size difference. Moreover, imaginal disc development appears to be equally well conserved. Through cloning, expression, and functional studies, we show that the Lucilia Wingless (Wg) protein is highly conserved between the two species. We further show that parasegments are established in Lucilia, however, engrailed expression shows a more dynamic expression pattern than expected in comparison to Drosophila. Over-expression of Lucilia Wingless in Drosophila shows wingless-like wing phenotypes, suggesting that Lucilia Wingless blocks the signalling activity of Drosophila Wingless. Upon injection of wg dsRNA, we observe a "lawn of denticle" phenotype, closely resembling that of Drosophila. Due to the large size of the insect, the distance over which Wingless exerts signalling activity is up to three times larger than in Drosophila, yet the consequences are very similar. Our data demonstrate long-range wingless signaling mechanisms adapted for patterning large domains of naked cuticle and suggest signaling properties of Lucilia Wingless that are distinct from those of Drosophila Wingless.

Development of the genitalia in Drosophila melanogaster

The imaginal discs of Drosophila melanogaster are an excellent material with which to analyze how signaling pathways and Hox genes control growth and pattern formation. The study of one of these discs, the genital disc, offers, in addition, the possibility of integrating the sex determination pathway into this analysis. This disc, whose growth and shape are sexually dimorphic, gives rise to the genitalia and analia, the more posterior structures of the fruit fly. Male genitalia, which develop from the ninth abdominal segment, and female genitalia, which develop mostly from the eighth one, display a characteristic array of structures. We will review here some recent findings about the development of these organs. As in other discs, different signaling pathways establish the positional information in the genital primordia. The Hox and sex determination genes modify these signaling routes at different levels to specify the particular growth and differentiation of male and female genitalia.

Function of bicoid and hunchback homologs in the basal cyclorrhaphan fly Megaselia (Phoridae)

The Drosophila gene bicoid functions at the beginning of a gene cascade that specifies anterior structures in the embryo. Its transcripts are localized at the anterior pole of the oocyte, giving rise to a Bicoid protein gradient, which regulates the spatially restricted expression of target genes along the anterior-posterior axis of the embryo in a concentration-dependent manner. The morphogen function of Bicoid requires the coactivity of the zinc finger transcription factor Hunchback, which is expressed in a Bicoid-dependent fashion in the anterior half of the embryo. Whereas hunchback is conserved throughout insects, bicoid homologs are known only from cyclorrhaphan flies. Thus far, identification of hunchback and bicoid homologs rests only on sequence comparison. In this study, we used double-stranded RNA interference (RNAi) to address the function of bicoid and hunchback homologs in embryos of the lower cyclorrhaphan fly Megaselia abdita (Phoridae). Megaselia-hunchback RNAi causes hunchback-like phenotypes as observed in Drosophila, but Megaselia-bicoid RNAi causes phenotypes different from corresponding RNAi experiments in Drosophila and bicoid mutant embryos. Megaselia-bicoid is required not only for the head and thorax but also for the development of four abdominal segments. This difference between Megaselia and Drosophila suggests that the range of functional bicoid activity has been reduced in higher flies.

Analysis of the genes involved in organizing the tail segments of the Drosophila melanogaster embryo

The metameric organization of the Drosophila melanogaster tail is obscured by developmental events that partially suppress or fuse some of its regions. To better define the developmental origins and segmental identities in the tail of the Drosophila embryo, we documented expression patterns and mutant phenotypes of several genes that play important roles in its morphogenesis. We documented the domains of engrailed (en), Abdominal-B (Abd-B) and caudal (cad) expression in the tail region. The staining pattern of cut (ct) was used to correlate the embryonic sense organs with their respective positions on the larval cuticle. The en patterns in different Bithorax-Complex (BX-C) Abd-B morphogenetic (m) and regulatory (r) mutants demonstrated that Abd-B functions to, among other things, suppress embryonic ventral epidermal structures on the posterior side of A8 to A9. Ventral epidermal structures were not added back into the en pattern in r- or BX-C- mutants, indicating that although the BX-C functions extend through A10, other non-BX-C genes must be required for development of this segment.

The screw gene encodes a ubiquitously expressed member of the TGF-beta family required for specification of dorsal cell fates in the Drosophila embryo

The decapentaplegic (dpp) gene product, a TGF-beta related ligand, acts as an extracellular morphogen to establish at least two cellular response thresholds within the dorsal half of the Drosophila embryo. Null mutations in the screw (scw) gene are phenotypically similar to moderate dpp mutants and cause dorsal cells to adopt ventral fates. We show that scw encodes a novel TGF-beta protein and is an integral part of the signal that specifies dorsal pattern. Although scw is expressed uniformly during blastoderm stages, its effect on development appears graded and is restricted to the dorsal side of the embryo. Our results indicate that DPP activity alone is insufficient to specify different dorsal cell fates. We propose that SCW and DPP act together to establish distinct response boundaries within the dorsal half of the embryo, perhaps by forming heterodimers that have higher activity than homodimers of either molecule alone.

Dorsal midline fate in Drosophila embryos requires twisted gastrulation, a gene encoding a secreted protein related to human connective tissue growth factor

The twisted gastrulation (tsg) gene is one of seven known zygotic genes that specify the fate of dorsal cells in Drosophila embryos. Mutations in these genes cause at least some of the cells on the dorsal half of the embryo to adopt more ventral cell fates leading to the proposal that most of these genes participate in establishing, maintaining, or modulating a gradient of a single signaling molecule DECAPENTAPLEGIC (DPP). We have examined the effects of tsg mutations on the development of cuticule elements, expression of a region specific enhancer trap, and patterns of mitotic domains. Mutations of tsg only affect the fate of a narrow strip of dorsal midline cells and do not affect dorsal ectoderm cells. However, the pattern of tsg expression is not coincident with the territories affected by tsg mutations. Structural analysis of the tsg gene reveals features of a secreted protein suggesting an extracellular site of action. The TSG protein bears a weak resemblance to human connective tissue growth factor (CTGF), a TGF-beta-induced protein. We propose that dorsal midline cell fate is specified by the combination of both a TSG and a DPP signal to which the dorsal midline cells are uniquely competent to respond.

Multiple HOM-C gene interactions specify cell fates in the nematode central nervous system

Intricate patterns of overlapping HOM-C gene expression along the A/P axis have been observed in many organisms; however, the significance of these patterns in establishing the ultimate fates of individual cells is not well understood. We have examined the expression of the Caenorhabditis elegans Antennapedia homolog mab-5 and its role in specifying cell fates in the posterior of the ventral nerve cord. We find that the pattern of fates specified by mab-5 not only depends on mab-5 expression but also on post-translational interactions with the neighboring HOM-C gene lin-39 and a second, inferred gene activity. Where mab-5 expression overlaps with lin-39 activity, they can interact in two different ways depending on the cell type: They can either effectively neutralize one another where they are both expressed or lin-39 can predominate over mab-5. As observed for Antennapedia in Drosophila, expression of mab-5 itself is repressed by the next most posterior HOM-C gene, egl-5. Thus, a surprising diversity in HOM-C regulatory mechanisms exists within a small set of cells even in a simple organism.

Abdominal-B protein isoforms exhibit distinct cuticular transformations and regulatory activities when ectopically expressed in Drosophila embryos

The Drosophila homeotic gene Abdominal-B includes two genetically distinct elements, a morphogenetic (m) activity and a regulatory (r) activity. The proteins responsible for these activities were ectopically expressed in fly embryos. The larval cuticular transformations which result are consistent with the genetically defined role of each protein during normal embryogenesis. Both ABD-B proteins activate ectopic expression of transcripts encoding the m protein, but the levels of Antennapedia, Ultrabithorax and abdominal-A transcripts are differentially repressed. A structural and functional comparison of the ABD-B proteins and a chimeric DFD/ABD-B protein reaffirms that target specificity is largely determined by the homeodomain region and suggests protein domains outside of the homeodomain influence the activation or repression of target gene expression.

The regulation of empty spiracles by Abdominal-B mediates an abdominal segment identity function

The empty spiracles (ems) homeo box gene is required for the development of the Drosophila larval filzkörper, which are structural specializations of the eighth abdominal segment. Filzkörper development is also dependent on the function of the homeotic selector gene Abdominal-B (Abd-B). Here, we show that ems is a downstream gene that is transcriptionally regulated by Abd-B proteins. This regulation is mediated by an Abd-B-dependent ems cis-regulatory element that in early- to mid-stage embryos is activated only in the eighth abdominal segment. Genetic epistasis tests suggest that both ems and Abd-B are required in combination for the specification of the filzkörper primordia. In a general sense, these results also provide evidence that the hierarchical level immediately downstream of the homeotic genes contains additional homeo domain transcription factors that define subsegmental domain identities.

Molecular genetics of aristaless, a prd-type homeo box gene involved in the morphogenesis of proximal and distal pattern elements in a subset of appendages in Drosophila

Viable aristaless (al) mutations of Drosophila affect pattern elements at both ends of the proximodistal axis in a subset of adult appendages. The al gene has been cloned and identified by P-element-mediated germ-line transformation with a genomic DNA fragment, which rescues a lethal mutation of al as well as aspects of the adult al phenotype. The al gene contains a prd-type homeo domain and a Pro/Gln-rich domain and, hence, probably encodes a transcription factor. Its transcript distribution in third-instar imaginal discs closely corresponds to the anlagen of the tissues that later become visibly affected in adult al mutants. The striking similarity of a bimodal al expression in different imaginal discs indicates that al is under the control of a "prepattern," which is shared at least among antennal, leg, and wing discs. The al gene is also transcribed during embryogenesis. Apart from a function in the ontogeny of specific larval head and tail organs, its embryonic transcript pattern suggests a possible role in early imaginal disc development.

The Drosophila dorsal-ventral patterning gene tolloid is related to human bone morphogenetic protein 1

Mutations in the Drosophila tolloid (tld) gene lead to a partial transformation of dorsal ectoderm into ventral ectoderm. The null phenotype of tld is similar to, but less severe than decapentaplegic (dpp), a TGF-beta family member required for the formation of all dorsal structures. We have cloned the tld locus by P element tagging. At the blastoderm stage, tld RNA is expressed dorsally, similar to that described for dpp. Analysis of a tld cDNA reveals three sequence motifs: an N terminal region of similarity to a metalloprotease, two EGF-like repeats, and five copies of a repeat found in human complement proteins C1r and C1s. tld sequence is 41% identical to human bone morphogenetic protein 1 (BMP-1); the closest members to dpp within the TGF-beta superfamily are BMP-2 and BMP-4, two other bone morphogenetic proteins. These findings suggest that these genes are members of a signal generating pathway that has been conserved between insects and mammals.

The gene teashirt is required for the development of Drosophila embryonic trunk segments and encodes a protein with widely spaced zinc finger motifs

We have discovered a reporter gene insertion that is expressed in the trunk region of Drosophila embryos. Genetic and molecular details of a new regulatory gene neighboring the reporter gene insertion, which we call teashirt (tsh), are described. In situ hybridization of a tsh probe to embryos shows that this gene is expressed in a way similar to the reporter gene. Mutations of tsh show that the gene is required for normal development of the ventral trunk region of embryos, which correlates with the spatial expression of the gene in the anteroposterior axis but not in the dorsoventral axis. Sequencing of a tsh cDNA shows that the putative protein possesses three distantly spaced CX2CX12HX5H zinc finger motifs.

Phenotypic consequences and genetic interactions of a null mutation in the Drosophila Posterior Sex Combs gene

The Posterior Sex Combs (Psc) gene of Drosophila is a member of the Polycomb (Pc) group of transregulatory genes. Previous analyses of the function of this gene in Drosophila embryogenesis have been hampered by the lack of a null mutation. We recently isolated a mutation that deletes the 5' end of the Psc gene. This allele appears to be a null mutation, and we have used it to determine the Psc zygotic null phenotype and to look at the interactions of a null allele of Psc with five other Pc group mutations. We find evidence for transformations along both the anterior-posterior and dorsal-ventral axes in embryos of a variety of genotypes that include a null mutation in Psc. The phenotypes of embryos that are doubly mutant for a null allele of Psc and a mutation in a second Pc group gene show dramatic synergistic effects, but in their specifics they are dependent on the identity of the second Pc group gene. This is different from the relatively uniform phenotypes seen among double mutants that contained the allele Psc1, which has both gain and loss of function properties. The differences in the phenotypes of the doubly mutant embryos allow us to eliminate one class of molecular models to explain the dramatic synergism seen with mutations in this group of genes.

The Drosophila gene tailless is expressed at the embryonic termini and is a member of the steroid receptor superfamily

The zygotically active tailless (tll) gene plays a key role in the establishment of nonmetameric domains at the anterior and posterior poles of the Drosophila embryo. We have cloned the tll gene and show that it encodes a protein with striking similarity to steroid hormone receptors in both the DNA binding "finger" and ligand binding domains. tll RNA is initially expressed in embryos in two mirror-image symmetrical domains; this pattern then quickly resolves into a pattern consistent with the mutant phenotype: a posterior cap and an anterior dorsal stripe. That the tll gene may also play a role in the nervous system is suggested by its strong expression in the forming brain and transient expression in the peripheral nervous system.

The Drosophila patched gene encodes a putative membrane protein required for segmental patterning

The patched (ptc) gene is one of several segment polarity genes required for correct patterning within every segment of Drosophila. The absence of ptc gene function causes a transformation of the fate of cells in the middle part of each segment so that they form pattern elements characteristic of cells positioned around the segment border. Analysis of the mutant phenotype demonstrates that both segment and parasegment borders are included in the duplicated pattern of ptc mutants. We have cloned the ptc gene and deduced that the product is a 1286 amino acid protein with at least seven putative transmembrane alpha helices. ptc RNA is expressed in embryos in broad stripes of segmental periodicity that later split into two stripes per segment primordium. The pattern of expression does not directly predict the transformation seen in ptc mutant embryos, suggesting that ptc participates in cell interactions that establish pattern within the segment.

Rescue of bicoid mutant Drosophila embryos by bicoid fusion proteins containing heterologous activating sequences

The maternal gene bicoid (bcd) determines pattern in the anterior half of the Drosophila embryo. It is reported here that the injection of bcd mutant embryos with messenger RNAs that encode proteins consisting of heterologous acidic transcriptional activating sequences fused to the DNA-binding portion of the bcd gene product, can completely restore the anterior pattern of the embryo.

The homeotic gene fork head encodes a nuclear protein and is expressed in the terminal regions of the Drosophila embryo

The region-specific homeotic gene fork head (fkh) promotes terminal as opposed to segmental development in the Drosophila embryo. We have cloned the fkh region by chromosomal walking. P element-mediated germ-line transformation and sequence comparison of wild-type and mutant alleles identify the fkh gene within the cloned region. fkh is expressed in the early embryo in the two terminal domains that are homeotically transformed in fkh mutant embryos. The nuclear localization of the fkh protein suggests that fkh regulates the transcription of other, subordinate, genes. The fkh gene product, however, does not contain a known protein motif, such as the homeodomain or the zinc fingers, nor is it similar in sequence to any other known protein.

Isolation, structure, and expression of labial, a homeotic gene of the Antennapedia Complex involved in Drosophila head development

The labial (lab) gene of Drosophila melanogaster is necessary for the proper development of the embryonic (larval) and adult head. We have identified the lab transcription unit within the proximal portion of the Antennapedia Complex (ANT-C) by mapping the molecular lesions associated with chromosomally rearranged lab alleles. We present its molecular structure, nucleotide sequence, and temporal pattern of expression. In addition, using antibodies generated against a fusion protein, we show that in the embryo the lab protein is distributed in neural and epidermal cells of the procephalic lobe; in a discrete loop of the midgut; and in specific progenitor sensory cells of the clypeolabrum, thoracic segments, and tail region. The regions of lab expression in the developing cephalon represent nonsegmented domains that are anterior to and largely nonoverlapping with the domains of expression of the Deformed (Dfd) and proboscipedia (pb) genes, two other homeotic loci of the ANT-C that also function to direct the development of head structures. Furthermore, lab head expression is associated with the complex cellular movements of head involution, a process that not only is defective in lab embryos, but the failure of which appears to be largely responsible for the defects observed in mutant embryos. Finally, we suggest that lab head expression provides a molecular marker for an intercalary segment, an ancestral segment that has become morphologically indistinct during the evolution of the insect head.

Reciprocal effects of hyper- and hypoactivity mutations in the Drosophila pattern gene torso

In Drosophila, five "terminal" polarity genes must be active in females in order for them to produce embryos with normal anterior and posterior ends. Hypoactivity mutations in one such gene, torso, result in the loss of the most posterior domain of fushi tarazu expression and the terminal cuticular structures. In contrast, a torso hyperactivity mutation causes the loss of central fushi tarazu expression and central cuticular structures. Cytoplasmic leakage, transplantation, and temperature-shift experiments suggest that the latter effect is caused by abnormal persistence of the torso product in the central region of the embryo during early development. Thus, the amount and timing of torso activity is key to distinguishing the central and terminal regions of the embryo. Mutations in the tailless terminal gene act as dominant maternal suppressors of the hyperactive torso allele, indicating that the torso product acts through, or in concert with, the tailless product.

The segmentation gene Krüppel of Drosophila melanogaster has homeotic properties

In Drosophila hindgut, Malpighian tubules and posterior midgut develop from the most posterior region of the blastoderm. One of the genes that influences the differentiation of the Malpighian tubules is Krüppel (Kr), a segmentation gene of the gap class. Kr homozygous embryos lack thoracic and abdominal segments and, depending on the allele, develop nearly normal Malpighian tubules or do not differentiate them at all. In the wild type, injection of horseradish peroxidase (HRP) into cells of the early gastrula at various posterior positions results in labeling of hindgut (93%), Malpighian tubules (46%), and posterior midgut (20%). Malpighian tubules were labeled only in combination with hindgut. In Kr1 homozygous embryos that lack Malpighian tubules, the label was restricted to hindgut (84%) and posterior midgut (24%). Because we could not find significant cell death in the posterior region of Kr1 embryos, we counted the cell nuclei in the hindguts of wild-type and mutant embryos. The results show that the hindgut in Kr1 embryos contains those cells that would differentiate into Malpighian tubules in wild type. Therefore, we conclude that the Krüppel gene exhibits a homeotic function in addition to its role as a segmentation gene and is involved in separating hindgut and Malpighian tubule cells and in the elongation process as well.

A group of genes required for pattern formation in the ventral ectoderm of the Drosophila embryo

Mutations in the genes spitz (spi), Star (S), single-minded (sim), pointed (pnt), rhomboid (rho) (all zygotic), and sichel (sic) (maternal), collectively called the spitz group, cause similar pattern alterations in ventral ectodermal derivatives of the Drosophila embryo. The cuticle structures lacking in mutant embryos normally derive from longitudinal strips of the ventro-lateral blastoderm. Defects were found in the median part of the central nervous system in whole-mount embryos stained with anti-HRP (horseradish peroxidase) antibodies. In addition, the nerve cells expressing the even-skipped protein appeared abnormally arranged. These results suggest that groups of cells from the same region, including both epidermal and neural precursor cells, require spitz-group gene activity for normal development. The members of the spitz group differ from one another: sim affects a more median strip of the ventral ectoderm than the other zygotic genes and pnt causes separation rather than deletion of pattern elements. As shown by pole cell transplantations, spi and S are also required for normal development of the female germ line, while sim, rho, and pnt appear to be exclusively zygotically expressed, and the maternal gene sic acts in the germ line autonomously. Some embryos produced by sic-homozygous females differentiate the spitz phenotype, others develop normally or die early. Of all the spitz-group genes, sim appears to have the most specific effect on the embryonic pattern. The significance of the spitz-group phenotypes for the dorso-ventral pattern formation is discussed.

Function of torso in determining the terminal anlagen of the Drosophila embryo

The formation of the unsegmented terminal regions of the Drosophila larva, acron and telson requires the function of at least five maternal genes (terminal genes class). In their absence, the telson and acron are not formed. One of them, torso (tor), has gain-of-function alleles which have an opposite phenotype to the lack-of-function (tor-) alleles: the segmented regions of the larval body, thorax and abdomen, are missing, whereas the acron is not affected and the telson is enlarged. In strong gain-of-function mutants, the pair-rule gene fushi tarazu (ftz) is not expressed, demonstrating the suppression of the segmentation process in an early stage of development. The tor gain-of-function effect is neutralized, and segmentation is restored in double mutants with the zygotic gene tailless (tll), which has a phenotype similar (but not identical) to that of tor-. This suggests that tor acts through tll, and that in the gain-of-function alleles of tor, the tll gene product is ectopically expressed at middle positions of the embryo, where it inhibits the expression of segmentation genes like ftz.

Maternal effects of general and regional specificity on embryos of Drosophila melanogaster caused by dunce and rutabaga mutant combinations

The developmental patterns of embryos produced by female germ line cells homozygous for null-enzyme mutations of dunce and for dunce in combination with each of three different rutabaga mutations are compared with the normal pattern. At least three discrete developmental defects at progressive stages following fertilization can be identified and correlated with the loss of adenylate cyclase activity caused by rutabaga mutations, suggesting that the defects are caused by elevated cyclic AMP levels in female germ line cells. The earliest defect occurs soon after fertilization and affects DNA replication and mitosis, prevents nuclear migration, and leads to large polyploid nuclei. A later defect prevents cleavage nuclei from migrating into, or dividing in, the posterior region of the egg. The last affects the developmental behavior or fate of blastoderm cells. Some of these defects mimic those produced by previously described maternal-effect mutations.

Determination of anteroposterior polarity in Drosophila

The principles of pattern formation in embryogenesis can be studied in Drosophila by means of a powerful combination of genetic and transplantation experiments. The segmented pattern of the Drosophila embryo is organized by two activities localized at the anterior and posterior egg poles. Both activities exert inducing and polarizing effects on the pattern when transplanted to other egg regions. A small set of maternal genes have been identified that are required for these activities. Mutants in these genes lack either the anterior or posterior part of the segmented pattern. The unsegmented terminal embryonic regions require a third class of genes and form independently of the anterior and posterior centers.

Structure of two genes at the gooseberry locus related to the paired gene and their spatial expression during Drosophila embryogenesis

The gooseberry (gsb) locus contains two closely linked genes, BSH9 and BSH4, which are structurally related to each other and to the paired (prd) gene. Sequence analysis of genomic DNA and cDNA shows that BSH9 and BSH4 can encode proteins of 427 and 452 amino acids, respectively. The structural homology between these two putative proteins and the prd protein consists essentially of two domains forming most of the amino-terminal halves of the proteins: the prd domain of 128 amino acids and a prd-type homeo domain of 60 amino acids, which is extended by 18 amino acids at its amino-terminal end. The temporal profiles of BSH9 and BSH4 transcripts, as characterized by Northern analysis, show a peak shortly after the peak of prd transcripts. The spatial distributions of BSH9 and BSH4 transcripts have been analyzed by in situ hybridization to whole-mount and sectioned embryos. BSH9 transcripts appear in the posterior ventrolateral part of each primordial segment throughout the embryo, including head and tail segments. Transcripts are initially restricted to the ectoderm, in which they arise as two spatially shifted and temporally delayed waves exhibiting double-segment periodicity and anteroposterior polarity. During germ-band extension, BSH9 is induced in the mesoderm in register with the ectoderm and neurectoderm and in the tail segments A9-A11. In contrast, BSH4 transcripts appear with a single-segment repeat, first, in the neurectoderm during germ-band extension and, later, in single neurons during neuronal differentiation. BSH9, BSH4, and prd are activated in cells that are in register along the anteroposterior axis of the embryo in the posterior parts of primordial segments comprising the posterior compartments of engrailed expression.