Segmental organisation of the head in the embryo of Drosophila melanogaster

Summary Embryos of Drosophila melanogaster were irradiated in the presumptive head region with a UV-laser microbeam of 20 m diameter at two developmental stages, the cellular blastoderm and the extended germ band. The ensuing defects were scored in the cuticle pattern of the head of the first-instar larva, which is described in detail in this paper. The defects caused by irradiating germ band embryos when morphologically recognisable lobes appear in the head region were used to establish the segmental origin of various head structures. This information enabled us to translate the spatial distribution of blastoderm defects into a fate map of segment anlagen. The gnathal segments derive from a region of the blastoderm between 60% and 70% egg length (EL) dorsally and 60% and 80% ventrally. The area anterior to the mandibular anlage and posterior to the stomodaeum is occupied by the small anlagen of the intercalary and antennal segments ventrally and dorsally, respectively. The labrum, which originates from a paired anlage dorsally at 90% EL, is separated from the remaining head segments by an area for which we did not observe cuticle defects following blastoderm irradiation, presumably because those cells give rise to the brain. The dorsal and lateral parts of the cephalo-pharyngeal skeleton appear to be the only cuticle derivatives of the non-segmental acron. These structures derive from a dorso-lateral area just behind the putative brain anlage and may overlap the latter. In addition to the segment anlagen, the regions of the presumptive dorsal pouch, anterior lobe and post-oral epithelium, whose morphogenetic movements during head involution result in the characteristic acephalic appearance of the larva, have been projected onto the blastoderm fate map. The results suggest that initially the head of the Drosophila embryo does not differ substantially from the generalised insect head as judged by comparison of fate map and segmental organisation.     

Mutations affecting the pattern of the larval cuticle inDrosophila melanogaster

Summary The present report describes the recovery and genetic characterization of mutant alleles at zygotic loci on the third chromosome ofDrosophila melanogaster which alter the morphology of the larval cuticle. We derived 12600 single lines from ethyl methane sulfonate (EMS)-treatedst e orrucuca chromosomes and assayed them for embryonic lethal mutations by estimating hatch rates of egg collections. About 7100 of these lines yielded at least a quarter of unhatched eggs and were then scored for embryonic phenotypes. Through microscopic examination of unhatched eggs 1772 lines corresponding to 24% of all lethal hits were classified as embryonic lethal. In 198 lines (2.7% of all lethal hits), mutant embryos showed distinct abnormalities of the larval cuticle. These embryonic visible mutants define 45 loci by complementation analysis. For 32 loci, more than one mutant allele was recovered, with an average of 5.8 alleles per locus. Complementation of all other mutants was shown by 13 mutants. The genes were localized on the genetic map by recombination analysis, as well as cytologically by complementation analysis with deficiencies. They appear to be randomly distributed along the chromosome. Allele frequencies and comparisons with deficiency phenotypes indicate that the 45 loci represent most, if not all, zygotic loci on the third chromosome, where lack of function recognizably affects the morphology of the larval cuticle.     

A small genetic region that controls dihydroorotate dehydrogenase in Drosophila melanogaster

A locus is described that controls levels of mitochondrial dihydroorotate dehydrogenase (EC 1.3.3.1) in Drosophila melanogaster. The effects of alleles of the locus, Dhod, are manifest in preparations from whole organisms as well as in partially purified mitochondrial preparations; however, other mitochondrial functions do not appear to be appreciably affected by Dhod genotypes. The locus maps near p in the proximal portion of the right arm of chromosome 3. Flies trisomic for a chromosome segment including that region display elevated enzyme levels, implying that an enzyme structural gene is in that vicinity. Furthermore, Dhod alleles are semidominant in heterozygotes, suggesting that the dosage-sensitive element detected in the trisomics is actually the Dhod locus. These findings are discussed relative to the role of dihydroorotate dehydrogenase in the de novo pyrimidine biosynthetic pathway and relative to other pathway mutants that have been described in Drosophila.This work was supported by NSF Grants PCM 76-17214 to W. Cohen and PCM 78-14164 To J. Rawls, as well as NIH Research Career Development Award 1 KO4 AM00676 to J. Rawls.     

Nucleotide divergence of the rp49 gene region between Drosophila melanogaster and two species of the Obscura group of Drosophila

Summary A 2.1-kb SStI fragment including the rp49 gene and the 3 end of the -serendipity gene has been cloned and sequenced in Drosophila pseudoobscura. rp49 maps at region 62 on the tip of chromosome II of this species. Both the coding and flanking regions have been aligned and compared with those of D. subobscura. There is no evidence for heterogeneity in the rate of silent substitution between the rp49 coding region and the rate of substitutions in flanking regions, the overall silent divergence per site being 0.19. Noncoding regions also differ between both species by different insertions/deletions, some of which are related to repeated sequences. The rp49 region of D. pseudoobscura shows a strong codon bias similar to those of D. subobscura and D. melanogaster. Comparison of the rates of silent (K _S ) and nonsilent (K _a ) substitutions of the rp49 gene and other genes completely sequenced in D. pseudoobscura and D. melanogaster confirms previous results indicating that rp49 is evolving slowly both at silent and nonsilent sites. According to the data for the rp49 region, D. pseudoobscura and D. subobscura lineages would have diverged some 9 Myr ago, if one assumes a divergence time of 30 Myr for the melanogaster and obscura groups.Offprint requests to: C. Segarra     

Ether-induced segmentation disturbances in Drosophila melanogaster

Summary Drosophila embryos, exposed to ether between 1 and 4 h after oviposition, develop defects ranging from the complete lack of segmentation to isolated gaps in single segments. Between these extremes are varying extents of incomplete and abnormal segmentation. On the basis of both their temporal and spatial characteristics, five major phenotype classes may be distinguished: headless — unsegmented or incompletely segmented anteriorly; gap — interruptions of segmentation not obviously periodic; alternating segment gaps — interruptions with double segment periodicities; fused segments; and short segments — truncations with single segment periodicities. Many defects resemble known mutant phenotypes. The disturbances in segmentation are predominantly global and frequently accompanied by alterations in segment specification, such that the segments obtained show no resemblance to the normal homologues. These features, together with the distinctive spatiotemporal characteristics of the defects, all point to segmentation as a dynamic process. The regular spacing of the segments and the fact that the entire range of defects is inducible by ether are further consistent with the hypothesis that at least part of the segmentation process may consist of physicochemical reactions coordinated over the whole body. The relationship between our data and data from genetic and other analyses are briefly discussed.     

Embryonic cAMP and developmental potential in Drosophila melanogaster

Summary Measurements of cAMP in early embryos of Drosophila melanogaster demonstrate that the dunce gene plays a major role, and the rutabaga gene a secondary role, in maternal regulation of embryonic cAMP content. Studying the double mutant combination, we find that variability in elevated cAMP content between individual embryos is associated with a wide variability in developmental potential. Embryos with about five times the normal cAMP content define a threshold between apparently normal and abnormal development. Measurements of cAMP content in anterior and posterior halves of embryos indicate that the posterior embryonic region, which is developmentally more sensitive to the effects of elevated cAMP than the anterior region, does not contain more cAMP than the anterior region. The variety of developmental defects observed is discussed in relation to possible targets of cAMP action.Offprint requests to: J.A. Kiger, Jr     

Basal relationships in the Drosophila melanogaster species group

The Drosophila melanogaster species group is a popular model for evolutionary studies due to its morphological and ecological diversity and its inclusion of the model species D. melanogaster. However, phylogenetic relationships among major lineages within this species group remain controversial. In this report, the phylogeny of 10 species representing each of the well-supported monophyletic clades in the melanogaster group was studied using the sequences of 14 loci that together comprise 9493 nucleotide positions. Combined Bayesian analysis using gene-specific substitution models produced a 100% credible set of two trees. In the strict consensus of these trees, the ananassae subgroup branches first in the melanogaster species group, followed by the montium subgroup. The remaining lineages form a monophyletic clade in which D. ficusphila and D. elegans branch first, followed by D. biarmipes, D. eugracilis, and the melanogaster subgroup. This strongly supported phylogeny resolves most basal relationships in the melanogaster species group, and provides a framework that can be extended in the future to encompass more species.     

Ontogeny of sorbitol dehydrogenases in Drosophila melanogaster

It has been shown that crude extracts of Drosophila melanogaster adults contain three distinctly different enzymes which catalyze the oxidation of d-sorbitol into d-fructose. These include (1) a soluble NAD-dependent sorbitol dehydrogenase (NAD-SoDH^s), (2) a mitochondrial NAD-dependent sorbitol dehydrogenase (NAD-SoDH^m), and (3) a soluble NADP-dependent sorbitol dehydrogenase (NADP-SoDH). Developmental studies have shown that the activities of all three of these enzymes are lowest during the larval stages while highest levels are seen during or shortly prior to the adult period. With respect to NAD-SoDH^s, studies of tissue distribution in adults have shown that highest activity is associated with thoracic musculature in both sexes and with organs of the male reproductive system. The developmental profile of this enzyme reveals a significant increase in activity at between 40 and 60 hr after hatching. This time interval corresponds closely to that during which the paternally derived NAD-SoDH^s gene is expressed. An additional increase in activity is seen in male pupae at 160 hr and in female adults at 210 hr. The rapid increase in males takes place immediately following the developmental period during which the testes attach to their respective duct systems. NADP-SoDH activity is concentrated among organs of the thorax and abdomen in both sexes. Males show significantly higher levels of this enzyme during the late pupal and early adult periods. In contrast to the patterns of distribution seen for NAD-SoDH^s and NADP-SoDH, 91–92% of the total NAD-SoDH^m activity in adults is localized to the thoracic musculature. The developmental profile of this enzyme reveals a significant increase in activity during the late pupal and early adult periods, when flight muscle mitochondria are known to be proliferating and undergoing structural maturation.This work was supported in part by Grant No. GY-10830 from the National Science Foundation and a faculty research fellowship from The University of Toledo Board of Trustees.     

Second-site modifiers of the Delta wing phenotype in Drosophila melanogaster

Summary We have screened for dominant enhancers and suppressors of the wing phenotype associated with two Delta alleles: Dl ^9P39, an amorphic allele, and Dl ^FE32, an antimorphic allele. The interactions of some of the modifiers with Delta are due to haplo-insufficient expression of the corresponding genes. Although not explicitly shown for the remaining cases, we assume that haploin-sufficiency is also the basis for the relationships of these genes to Delta, since no allele specific interactions were observed. The modifiers found define 22 genes with pleiotropic expression, which can be classified into two groups: genes required for wing vein pattern formation and for neurogenesis, and genes which are not required for neurogenesis. Among the genes of the first group, Hairless and Star were previously known to participate in neural development. One further modifier was found which may correspond to a new neurogenic gene. The second group of genes is larger and includes already known loci, e.g., Plexate, blistered, plexus, etc, as well as other previously unidentified genes, which function during wing morphogenesis. Correspondence to: J.A. Campos-Ortega     

Dynamics of spermiogenesis in Drosophila melanogaster

Summary A morphogenetic process that transforms spermatids from a syncytial state to a state in which each spermatid is invested in its own membrane, is initiated at the head region of the spermatid bundle and traverses through the entire length of the bundle in the testis of Drosophila melanogaster. This process not only eliminates the syncytial bridges between spermatids but also removes unneeded organelles and the excess parts of the nuclear membrane, nucleoplasm and cytoplasm. It also brings about structural modifications to flagellar elements. The propagation of this process is seen as the caudal movement of a fusiform swelling of the spermatid bundle, 100 or more in length. Spermatids are individualized in the basal half of the swelling, whereas they remain syncytial in the apical half. The swelling increases its volume as it accumulates cytoplasmic debris while traversing the sperm bundle, from about 15 in maximum diameter in the basal testicular region to as large as 30 at the apical end where it becomes a bag of wastes. A variation of the process in a mutant stock which is known to inactivate up to half of the products of meiosis is briefly described. The morphological change of interspermatid bridges prior to the individualization is also reported.This work was supported by grants from the National Institutes of Health (USPHS-HD 03015 and GM-15971) and a contract from the Atomic Energy Commission, AT(04-3)-34, P.A. 150.Graduate training grant USPHS GM 00702.     

Mesoderm induction in the future tail region of Xenopus

Summary We have used interspecific grafts between Xenopus borealis and Xenopus laevis to study the signalling system that produces tail mesoderm. Early gastrula ectoderm grafted into the posterior neural plate region of neurulae responds to a mesodermal inducing signal in this region and forms mainly tail somites; this signal persists until at least the early tail bud stage. Ventral ectoderm grafted into the posterior neural plate loses its competence to respond to this signal after stage 10 1/2. We have established the specification of anterior and posterior neural plate ectoderm. In ectodermal sandwiches or when grafted into unusual positions, anterior regions gave rise to mainly nervous system and posterior regions to large amounts of muscle, together with some nervous system. Thus it was impossible to assess the competence of posterior neural plate ectoderm to form further mesoderm and hence to establish if mesodermal induction continues during neurulation in unmanipulated embryos.     

Developmental regulation of phenylalanine hydroxylase activity in Drosophila melanogaster

Herein we demonstrate that Drosophila larvae possess a synthetic activity capable of converting phenylalanine to tyrosine. This system is readily extractable and displays many characteristics of phenylalanine hydroxylase systems described in other organisms, the most notable being that a tetrahydropteridine is required for full expression of activity. The level of phenylalanine hydroxylase activity present in the organism varies with the stage of development: from an undetected level of activity at the first larval instar, there is a rapid increase in phenylalanine hydroxylase activity which reaches a peak at the time of puparium formation, after which there is a rapid decrease again to an undetected level.     

Studies on transvection at the bithorax complex in Drosophila melanogaster

Summary We have studied the influence of some mutations in the bithorax complex on the observed synapsis dependent phenotype of the genotypes Cbx ¹Ubx¹/+ and bx ^34e/Ubx¹. The effect of these mutations is similar to that introduced by disruption of pairing or by the z ^a mutation. Among the bx mutations, we find that bx ⁸ behaves differently from most other bx mutations in its influence on the synapsis dependent phenotype. This observation induced us to map the position of bx ⁸ with respect to other bx mutations; we find that it maps between bx ^34e and bx ³. We show how some of the observations reported here can be fitted into a model of activation of the bithorax complex proposed by one of us.     

The role of DE-cadherin during cellularization, germ layer formation and early neurogenesis in the Drosophila embryo

The Drosophila E-cadherin homolog, DE-cadherin, is expressed and required in all epithelial tissues throughout embryogenesis. Due to a strong maternal component of DE-cadherin, its early function during embryogenesis has remained elusive. The expression of a dominant negative DE-cadherin construct (UAS-DE-cad(ex)) using maternally active driver lines allowed us to analyze the requirements for DE-cadherin during this early phase of development. Maternally expressed DE-cad(ex) result in phenotype with variable expressivity. Most severely affected embryos have abnormalities in epithelialization of the blastoderm, resulting in loss of the blastodermal cells' apico-basal polarity and monolayered structure. Another phenotypic class forms a rather normal blastoderm, but shows abnormalities in proliferation and morphogenetic movements during gastrulation and neurulation. Mitosis of the mesoderm occurs prematurely before invagination, and proliferation in the ectoderm, normally a highly ordered process, occurs in a random pattern. Mitotic spindles of ectodermal cells, normally aligned horizontally, frequently occurred vertically or at an oblique angle. This finding further supports recent findings indicating that, in the wild-type ectoderm, the zonula adherens is required for the horizontal orientation of mitotic spindles. Proliferation defects in DE-cad(ex)-expressing embryos are accompanied by the loss of epithelial structure of ectoderm and neuroectoderm. These germ layers form irregular double or triple layers of rounded cells that lack zonula adherens. In the multilayered neuroectoderm, epidermal precursors, neuroblasts and ganglion mother cells occurred intermingled, attesting to the pivotal role of DE-cadherin in delamination and polarized division of neuroblasts. By contrast, the overall number and spacing of neuroblasts was grossly normal, indicating that DE-cadherin-mediated adhesion is less important for cell-cell interaction controlling the ratio of epidermal vs. neural progenitors.     

Heterosis in crosses between geographically separated populations of Drosophila melanogaster

Summary An experiment was performed to test the hypothesis that the genetic distance between populations estimated from enzyme loci could be used to predict the amount of heterosis that would occur in crosses between these populations. A partial diallel cross using 11 populations of Drosophila melanogaster from the AustralianPacific region and from England was carried out. Heterosis for larval viability, fecundity, cold shock mortality, and an index of these three traits was recorded. When two populations originating from the same location were crossed, no heterosis occurred, but otherwise heterosis was significant for all traits. For larval viability, a similar low level of heterosis occurred in all crosses. For cold shock mortality, the level of heterosis varied widely and fecundity showed a pattern intermediate between these two. The geographic distance between the sites from which populations originated was not correlated with the amount of heterosis in their crosses. There was a tendency for populations from ecologically different environments to show heterosis in crosses. Genetic distance based on ten enzyme loci was correlated with heterosis for cold shock mortality and the combined trait index. These results can be explained by the hypothesis that genes affecting larval viability are subject to strong, uniform selection in all populations, which limits the extent to which gene frequencies can drift apart. However, genes affecting cold shock mortality and the enzyme loci are subject to different selection pressures in different environments. This divergent selection combined with genetic drift causes divergence in gene frequency and heterosis.     

Vital genes in the heterochromatin of chromosomes 2 and 3 of Drosophila melanogaster

Heterochromatin has been traditionally regarded as a genomic wasteland, but in the last three decades extensive genetic and molecular studies have shown that this ubiquitous component of eukaryotic chromosomes may perform important biological functions. In D. melanogaster, about 30 genes that are essential for viability and/or fertility have been mapped to the heterochromatin of the major autosomes. Thus far, the known essential genes exhibit a peculiar molecular organization. They consist of single-copy exons, while their introns are comprised mainly of degenerate transposons. Moreover, about one hundred predicted genes that escaped previous genetic analyses have been associated with the proximal regions of chromosome arms but it remains to be determined how many of these genes are actually located within the heterochromatin. In this overview, we present available data on the mapping, molecular organization and function of known vital genes embedded in the heterochromatin of chromosomes 2 and 3. Repetitive loci, such as Responder and the ABO elements, which are also located in the heterochromatin of chromosome 2, are not discussed here because they have been reviewed in detail elsewhere.     

Evolution of the transposable element mariner in the Drosophila melanogaster species group

The population biology and molecular evolution of the transposable element mariner has been studied in the eight species of the melanogaster subgroup of the Drosophila subgenus Sophophora. The element occurs in D. simulans, D. mauritiana, D. sechellia, D. teissieri, and D. yakuba, but is not found in D. melanogaster, D. erecta, or D. orena. Sequence comparisons suggest that the mariner element was present in the ancestor of the species subgroup and was lost in some of the lineages. Most species contain both active and inactive mariner elements. A deletion of most of the 3 end characterizes many elements in D. teissieri, but in other species the inactive elements differ from active ones only by simple nucleotide substitutions or small additions/deletions. Active mariner elements from all species are quite similar in nucleotide sequence, although there are some-species-specific differences. Many, but not all, of the inactive elements are also quite closely related. The genome of D. mauritiana contains 20–30 copies of mariner, that of D. simulans 0–10, and that of D. sechellia only two copies (at fixed positions in the genome). The mariner situation in D. sechellia may reflect a reduced effective population size owing to the restricted geographical range of this species and its ecological specialization to the fruit of Morinda citrifolia.     

Genetic,ontogenetic, and tissue-specific variation of dipeptidases in Drosophila melanogaster

Three dipeptidases in Drosophila melanogaster are under independent genetic control and their structural genes have been localized, Dip-A to 2R and Dip-B and Dip-C to 3R (Voelker and Langley, 1978; Ohnishi and Voelker, 1981). These enzymes were characterized with respect to their substrate specificities, genetic variability (electrophoretic mobility and quantitative activity level), ontogeny (activity and isozyme pattern), and tissue localization. The dipeptide substrate specificities of DIP-A and DIP-B overlap each other considerably, but do not overlap with DIP-C. In natural populations, DIP-B and DIP-C are essentially monomorphic electrophoretically whereas DIP-A is polymorphic for three allozymes. Both DIP-A and DIP-B show quantitative genetic variation of activity level within an allozyme class. All three enzymes are expressed at all stages in the life cycle, but DIP-A and DIP-B activities vary considerably according to developmental stage and sex of adult. The tissue localizations of DIP-A and DIP-B activities show similar patterns and a nearly ubiquitous occurrence of both enzymes, but with particularly high values in larval and adult midguts and in the adult female reproductive system. These results suggest a general metabolic role for the enzymes, such as regulation of the concentrated pools of amino acids and oligopeptides found in Drosophila tissues.This work was supported by Public Health Service Grant GM 11546.Paper No. 7066 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh.     

Embryonic origin of the imaginal discs of the head of Drosophila melanogaster

The embryonic development of the primordia of the Drosophila head was studied by using an enhancer trap line expressed in these structures from embryonic stage 13 onward. Particular attention was given to the question of how the adult head primordia relate to the larval head segments. The clypeo-labral bud to the stage 13 embryo is located at a lateral position in the labrum adjacent to the labral sensory complex (epiphysis). Both clypeo-labral bud and sensory complex are located anterior to the engrailed-expression domain of the labrum. Throughout late embryogenesis and the larval period, the clypeo-labral bud forms integral part of the epithelium lining the roof of the atrium. The labial disc originates from the lateral labial segment adjacent to the labial sensory complex (hypophysis). It partially overlaps with the labial en-domain. After head involution, the labial disc forms a small pocket in the ventro-lateral wall of the atrium. The eye-antenna disc develops from a relatively large territory occupying the dorso-posterior part of the procephalic lobe, as well as parts of the dorsal gnathal segments. Cells in this territory are greatly reduced in number by cell death during stages 12–14. After head involution, the presumptive eye-antenna disc occupies a position in the lateral-posterior part of the dorsal pouch. Evagination of this tissue occurs during the first hours after hatching. In the embryo, no en-expression is present in the presumptive eye-antenna disc. en-expression starts in three separate regions in the third instar larva.     

Phylogenetic incongruence in the Drosophila melanogaster species group

Drosophila melanogaster and its close relatives are used extensively in comparative biology. Despite the importance of phylogenetic information for such studies, relationships between some melanogaster species group members are unclear due to conflicting phylogenetic signals at different loci. In this study, we use twelve nuclear loci (eleven coding and one non-coding) to assess the degree of phylogenetic incongruence in this model system. We focus on two nodes: (1) the node joining the Drosophila erecta-Drosophila orena, Drosophila melanogaster-Drosophila simulans, and Drosophila yakuba-Drosophila teissieri lineages, and (2) the node joining the lineages leading to the melanogaster, takahashii, and eugracilis subgroups. We find limited evidence for incongruence at the first node; our data, as well as those of several previous studies, strongly support monophyly of a clade consisting of D. erecta-D. orena and D. yakuba-D. teissieri. By contrast, using likelihood based tests of congruence, we find robust evidence for topological incongruence at the second node. Different loci support different relationships among the melanogaster, takahashii, and eugracilis subgroups, and the observed incongruence is not easily attributable to homoplasy, non-equilibrium base composition, or positive selection on a subset of loci. We argue that lineage sorting in the common ancestor of these three subgroups is the most plausible explanation for our observations. Such lineage sorting may lead to biased estimation of tree topology and evolutionary rates, and may confound inferences of positive selection.