Regionalisation of segment-forming capacities during early embryogenesis inDrosophila melanogaster

Summary Transverse fragmentation of the egg ofDrosophila melanogaster results in the formation of partial larvae. Anterior and posterior egg fragments develop the respective partial larval patterns. The partial patterns do not add up to the complete pattern.Fragmentation near the middle of the egg during early cleavage causes a gap of 3–4 segments on average in the larva. This gap is reduced to 2 segments on average if operations are performed at the early syncytial blastoderm stage. Fragmentation near the pole regions from early cleavage stages onwards causes a gap of only 2 larval segments on average. When the egg is fragmented at the columnar cellular blastoderm stage or later, the gap at all positions amounts to the size of one segment or less. A gap is also found after incomplete fragmentation, when the ooplasmic bridge between both egg parts was constricted beyond a certain limit.A specific shift of the segment-forming capacities along the egg axis is observed from syncytial blastoderm stages onwards.After partial longitudinal fragmentation no additional structures are observed. In general, the partial transverse patterns add up to the complete pattern, but minor structures like single denticles are missing near the fragmentation site.The results are discussed with respect to current concepts of segment pattern formation during early embryogenesis in dipterans.     

The specification of metameric order in the insectCallosobruchus maculatus Fabr. (Coleoptera)

Summary Mechanically dividing an insect egg into anterior and posterior fragments results in a segment gap (Sander 1976), a loss of non-terminal segments in the constricted region. By altering the stage and duration of constriction, we produced different types of egg fragments in the pea beetleCallosobruchus. The patterns formed by these fragments suggest the existence of interactions between anterior and posterior egg regions that influence segment patterning and placement. Segments in excess of the numbers expected on the basis of permanent constrictions were produced in fragments when: (1) the constriction was released before cellularization occurred and (2) in addition the complementary fragment degenerated. Apparently the degenerating fragment induced the formation of excess segments in the developing fragment. Differences in the time and extent of excess segment formation in anterior versus posterior fragments suggest an asymmetric distribution of prerequisites for segment formation. This conclusion is consistent with our finding that a partial reversal of segment sequence (double abdomen formation) can be induced only in posterior fragments by a degenerating fragment, but not in anterior fragments (see companion paper).The formation of excess segments shows that the segment gap observed after permanent separation cannot be due to non-specific damage, caused by the process of constriction as such, to the egg or to localized putative segment precursors.     

东亚飞蝗胚胎体节的形成过程

体节形成是昆虫胚胎发育过程中的关键问题.东亚飞蝗Locusta migratoria manilensis(Meyen)是一种重要的农业害虫,其体节形成的时序过程尚无详细报道.本研究采用免疫组化和品红染色方法研究了室内人工饲养东亚飞蝗的体节形成过程.结果表明:完成受精后,细胞核开始分裂并向卵表面迁移.细胞核到达卵表面的...     

Development of the embryonic segment pattern in the pea-beetle Callosobruchus maculatus. Supracellular,cellular and molecular aspects

Summary

This review deals with the question of how cells in the early embryo of the pea-beetle differentiate into a sequential pattern of segments. Anterior and posterior fragments of an egg have different options for development depending on whether they are exposed, before cellularization, to decaying ooplasm in the complementary fragment. Without such exposure all fragments produce fewer segments than corresponding fragments obtained at cellularization. With exposure a fraction of anterior and posterior fragments produces considerably more segments than corresponding fragments obtained at cellularization. In addition, posterior fragments are uniquely different from anterior ones in that they also produce reversal of segment sequence which can be restricted to longitudinal strips of the larval cuticle.

The difference in reaction to decaying ooplasm between anterior and posterior fragments suggests an asymmetry in the control of metamerization. Lateral inhibition by an asymmetric gradient of a diffusible morphogen can describe these observations [18] except for the restriction of reversal to longitudinal strips. The latter requires either that morphogen transport be polarized, possibly by a voltage gradient in the egg, or that the interpretation of cell position is polarized. The induction of double abdomens with UV-light and RNase suggests that RNA is part of the control mechanism. This and strip-restricted reversal are features shared by eggs of Coleoptera and Diptera.     

Non-canonical functions of hunchback in segment patterning of the intermediate germ cricket Gryllus bimaculatus

In short and intermediate germ insects, only the anterior segments are specified during the blastoderm stage, leaving the posterior segments to be specified later, during embryogenesis, which differs from the segmentation process in Drosophila, a long germ insect. To elucidate the segmentation mechanisms of short and intermediate germ insects, we have investigated the orthologs of the Drosophila segmentation genes in a phylogenetically basal, intermediate germ insect, Gryllus bimaculatus (Gb). Here, we have focused on its hunchback ortholog (Gb'hb), because Drosophila hb functions as a gap gene during anterior segmentation, referred as a canonical function. Gb'hb is expressed in a gap pattern during the early stages of embryogenesis, and later in the posterior growth zone. By means of embryonic and parental RNA interference for Gb'hb, we found the following: (1) Gb'hb regulates Hox gene expression to specify regional identity in the anterior region, as observed in Drosophila and Oncopeltus; (2) Gb'hb controls germband morphogenesis and segmentation of the anterior region, probably through the pair-rule gene, even-skipped at least; (3) Gb'hb may act as a gap gene in a limited region between the posterior of the prothoracic segment and the anterior of the mesothoracic segment; and (4) Gb'hb is involved in the formation of at least seven abdominal segments, probably through its expression in the posterior growth zone, which is not conserved in Drosophila. These findings suggest that Gb'hb functions in a non-canonical manner in segment patterning. A comparison of our results with the results for other derived species revealed that the canonical hb function may have evolved from the non-canonical hb functions during evolution.     

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.     

Isolation and embryonic expression of an abdominal-A-like gene from the lepidopteran, Manduca sexta.

Using sequence homology to the Drosophila Antennapedia gene, we isolated a homeobox-containing gene from the lepidopteran, Manduca sexta. Sequence analysis and in situ hybridizations to tissue sections suggest that the Manduca gene encodes a lepidopteran homologue of the Drosophila Bithorax complex gene abdominal-A. The predicted amino acid sequence of a 76 amino acid region that includes the homeobox and the regions immediately flanking it are identical between the Manduca and Drosophila genes. Northern blots reveal that the manduca abd-A gene is expressed first in the early embryo and continues to be expressed throughout later embryonic and larval stages. In situ hybridizations show that the posterior half of the first abdominal segment marks the anterior border of the Manduca abd-A expression. This expression pattern demonstrates the conservation of parasegments as domains of gene activity in the lepidopteran embryo. The Manduca abd-A expression extends from the posterior half of the first abdominal segment through the tenth abdominal segment, a domain that is greater than that of the Drosophila abd-A expression, and reflects the difference in visible segment number between the two insects.     

Anterior and posterior centers jointly regulate Bombyx embryo body segmentation

Insect embryo segmentation is largely divided into long and short germ types. In the long germ type, each segment primordium is represented on a large embryonic rudiment of the blastoderm, and segmental patterning occurs nearly simultaneously in the syncytium. In the short germ type, however, only anterior segments are represented in the small embryonic rudiment, usually located on the egg posterior, and the rest of the segments are added sequentially from the posterior growth zone in a cellular context. The long germ type is thought to have evolved from the short germ type. It is proposed that this transition, which appears to have occurred multiple times over the course of evolution, was realized through the acquisition of a localized anterior instruction center. Here, I examined the early segmentation process in the silkmoth Bombyx mori, a lepidopteran insect, in which the mechanisms of anterior-posterior (AP) axis formation have not been well analyzed. In this insect, both the long germ and short germ features have been reported. The mRNAs for two key genes involved in insect AP axis formation, orthodenticle (Bm-otd) and caudal (Bm-cad), are localized maternally in the germ anlage, where they act as anterior and posterior instruction centers, respectively. RNAi studies indicate that, while Bm-cad affects the formation of all the even skipped (Bm-eve) stripes, there is also anterior Bm-eve stripe formation activity that involves Bm-otd. Thus, there is redundancy in Bm-eve stripe formation activity that must be coordinated. Some genetic interactions, identified either experimentally or hypothetically, are also introduced, which might enable robust AP formation in this organism.     

Embryogenesis of the mayfly Ephemera japonica McLachlan (Insecta: Ephemeroptera,Ephemeridae), with special reference to abdominal formation

Embryogenesis of the mayfly Ephemera japonica is described, with special reference to the abdominal morphogenesis. Cleavage is of the typical superficial type. The germ disc is formed by the cell concentration of the embryonic area broadly defined at the posterior half of the blastoderm. The embryo undergoes embryogenesis of the typical short germ type. Blastokinesis in which the extensive and deep invagination of the embryo is involved is similar to those of odonatans and plecopterans. In the longest embryo stage, the abdomen is folded and is divided into four regions (regions I–IV, from anterior to posterior). All the first to eleven segments are derived from regions I and II. Regions III and IV fuse together to form the proctodaeum. This manner of abdominal formation may be regarded as basic in pterygote insects, because a similar manner is found in another palaeopteran group, the odonatans. The caudal filament has been interpreted as the elongation of the eleventh abdominal tergum, but it is now revealed that the caudal filament originates from the posterior extremity of region IV and that its origin is away from the eleventh segment, with regions III and IV or the proctodaeum interposing between. Thus, it is concluded that the caudal filament should be correlated not to the eleventh abdominal segment but to the telson. J. Morphol. 234:97–107, 1997. © 1997 Wiley-Liss, Inc.     

The segmentation cascade in the centipede Strigamia maritima: involvement of the Notch pathway and pair-rule gene homologues

The centipede Strigamia maritima forms all of its segments during embryogenesis. Trunk segments form sequentially from an apparently undifferentiated disk of cells at the posterior of the germ band. We have previously described periodic patterns of gene expression in this posterior disc that precede overt differentiation of segments, and suggested that a segmentation oscillator may be operating in the posterior disc. We now show that genes of the Notch signalling pathway, including the ligand Delta, and homologues of the Drosophila pair-rule genes even-skipped and hairy, show periodic expression in the posterior disc, consistent with their involvement in, or regulation by, such an oscillator. These genes are expressed in a pattern of apparently expanding concentric rings around the proctodeum, which become stripes at the base of the germ band where segments are emerging. In this transition zone, these primary stripes define a double segment periodicity: segmental stripes of engrailed expression, which mark the posterior of each segment, arise at two different phases of the primary pattern. Delta and even-skipped are also activated in secondary stripes that intercalate between primary stripes in this region, further defining the single segment repeat. These data, together with observations that Notch mediated signalling is required for segment pattern formation in other arthropods, suggest that the ancestral arthropod segmentation cascade may have involved a segmentation oscillator that utilised Notch signalling.     

Scanning electron microscopy of Drosophila melanogaster embryogenesis: III. Formation of the head and caudal segments

The formation of both the anterior most and posterior most segments in higher dipteran embryos involves complex movements of primordia which can be best visualized with the scanning electron microscope. During head formation, the gnathocephalic segments partially involute through the stomodeum. The labial segment forms the floor of the mouth, and the fused maxillary and mandibular segments form the lateral sides of the mouth. The involuted clypeolabrum forms the roof of the mouth. Invaginations of cells for segmentally derived sense organs can be found prior to involution on all the gnathocephalic and thoracic segments as well as on the labrum. The antennal sense organ derives from the lateral surface of the procephalic lobe. Following involution of the mouth parts, the dorsal ridge, which arises just anterior to the first thoracic segment, is drawn over the dorsal procephalic lobe producing the deep dorsal sac. The optic lobes of the brain invaginate anterior to the dorsal ridge just prior to the covering over of the head. The formation of the anal segment is similarly complex. Two rudimentary segments are found posterior to the eighth abdominal segment. During shortening of the germ band, the posterior most segment is drawn around the posterior tip of the embryo to lie ventrally. Two large anal pads form lateral to the anus from this segment. The next segment, following dorsal closure, produces a pair of anal sense organs and a central tuft of setae. Finally, the eighth abdominal segment gives rise to the posterior spiracles. Following dorsal closure these three segments fuse to produce the terminal (anal) segment of the larva.     

Different requirements for l(1) giant in two embryonic domains of Drosophila melanogaster

l(1) giant is a zygotic lethal mutation which affects the embryonic development of both the labial/thoracic segments and a subset of posterior abdominal segments. Using antibodies specific for proteins encoded by several Drosophila genes to identify the compartmental origin of the defects, we show that the requirement of giant activity is different in these two embryonic domains. Anteriorly, the posterior compartment of the labial segment is missing at the blastoderm stage. Posteriorly, cells are specifically deleted by cell death within the anterior compartments of abdominal segments 5-7 during germ band elongation. In mature embryos, posterior compartment structures of the peripheral nervous system of A5-7 are fused. In addition to a different pattern of defect in the two parts of the embryo, the kind of action appears different. Anteriorly, giant resembles a gap mutation in that a particular region is missing from the blastoderm fate map, whereas in the abdominal domain, giant affects the development of anterior compartment-specific structures.     

Different requirements for l(1)giant in two embryonic domains of Drosophila melanogaster

L(1)giant is a zygotic lethal mutation which affects the embryonic development of both the labial/thoracic segments and a subset of posterior abdominal segments. Using antibodies specific for proteins encoded by several Drosophila genes to identify the compartmental origin of the defects, we show that the requirement of giant activity is different in these two embryonic domains. Anteriorly, the posterior compartment of the labial segment is missing at the blastoderm stage. Posteriorly, cells are specifically deleted by cell death within the anterior compartments of abdominal segments 5–7 during germ band elongation. In mature embryos, posterior compartment structures of the peripheral nervous system of A5–7 are fused. In addition to a different pattern of defect in the two parts of the embryo, the kind of action appears different. Anteriorly, giant resembles a gap mutation in that a particular region is missing from the blastoderm fate map, whereas in the abdominal domain, giant affects the development of anterior compartment-specific structures.     

A morphogenetic determinant in the anterior pole of an insect egg (Smittia spec., Chironomidae, Diptera): localization by combined centrifugation and ultraviolet irradiation

In chironomid midges, the development of the head and thorax in the embryo requires the function of cytoplasmic determinants localized near the anterior pole of the egg. Experimental inactivation of these determinants causes a dramatic switch in the developmental program of the embryo. Instead of the normal segment pattern, the aberrant pattern “double abdomen” is formed. Head, thorax, and anterior abdominal segments are then replaced by an additional set of posterior abdominal segments jointed with reversed polarity to the original abdomen. To determine the cellular fraction which contains the effective targets for uv induction of double abdomens, Smittia eggs were centrifuged prior to uv irradiation. Accumulation of proteid spheres or lipid droplets in the irradiated anterior pole region caused a considerable decrease in the double abdomen yield. Removal of these components from the target area enhanced double abdomen formation. The maximum yield of double abdomens was obtained after uv irradiation of a cytoplasmic layer in which organelles larger than ribosomes could not be detected. The results of these and other experiments, suggest that ribosomes, ribosomal subunits, or other ribonucleoprotein particles act as effective targets for the uv induction of double abdomens in Smittia eggs.     

Segment polarity gene expression in a myriapod reveals conserved and diverged aspects of early head patterning in arthropods

Arthropods show two kinds of developmental mode. In the so-called long germ developmental mode (as exemplified by the fly Drosophila), all segments are formed almost simultaneously from a preexisting field of cells. In contrast, in the so-called short germ developmental mode (as exemplified by the vast majority of arthropods), only the anterior segments are patterned similarly as in Drosophila, and posterior segments are added in a single or double segmental periodicity from a posterior segment addition zone (SAZ). The addition of segments from the SAZ is controlled by dynamic waves of gene activity. Recent studies on a spider have revealed that a similar dynamic process, involving expression of the segment polarity gene (SPG) hedgehog (hh), is involved in the formation of the anterior head segments. The present study shows that in the myriapod Glomeris marginata the early expression of hh is also in a broad anterior domain, but this domain corresponds only to the ocular and antennal segment. It does not, like in spiders, represent expression in the posterior adjacent segment. In contrast, the anterior hh pattern is conserved in Glomeris and insects. All investigated myriapod SPGs and associated factors are expressed with delay in the premandibular (tritocerebral) segment. This delay is exclusively found in insects and myriapods, but not in chelicerates, crustaceans and onychophorans. Therefore, it may represent a synapomorphy uniting insects and myriapods (Atelocerata hypothesis), contradicting the leading opinion that suggests a sister relationship of crustaceans and insects (Pancrustacea hypothesis). In Glomeris embryos, the SPG engrailed is first expressed in the mandibular segment. This feature is conserved in representatives of all arthropod classes suggesting that the mandibular segment may have a special function in anterior patterning.     

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.