Morphological and molecular data argue for the labrum being non-apical, articulated, and the appendage of the intercalary segment in the locust

Our analysis of head segmentation in the locust embryo reveals that the labrum is not apical as often interpreted but constitutes the topologically fused appendicular pair of appendages of the third head metamere. Using molecular, immunocytochemical and retrograde axonal staining methods we show that this metamere, the intercalary segment, is innervated by the third brain neuromere-the tritocerebrum. Evidence for the appendicular nature of the labrum is firstly, the presence of an engrailed stripe within its posterior epithelium as is typical of all appendages in the early embryo. Secondly, the labrum is innervated by a segmental nerve originating from the third brain neuromere (the tritocerebrum). Immunocytochemical staining with Lazarillo and horseradish peroxidase antibodies reveal that sensory neurons on the labrum contribute to the segmental (tritocerebral) nerve via the labral nerve in the same way as for the appendages immediately anterior (antenna) and posterior (mandible) on the head. All but one of the adult and embryonic motoneurons innervating the muscles of the labrum have their cell bodies and dendrites located completely within the tritocerebral neuromere and putatively derive from engrailed expressing tritocerebral neuroblasts. Molecular evidence (repo) suggests the labrum is not only appendicular but also articulated, comprising two jointed elements homologous to the coxa and trochanter of the leg.     

Embryonic development of the scorpionfly Panorpa emarginata Cheng with special reference to external morphology (Mecoptera: Panorpidae)

The Mecoptera are thought to be one of the most primitive groups in the Holometabola, but their embryology is rarely studied. By means of scanning electron microscopy, we studied the external features of the embryo of the scorpionfly Panorpa emarginata in middle and late development. The embryo remains in the superficial position until hatching. Embryonic development can be divided into 10 stages along with the first‐instar larva. The external features are described from the germ band to the first‐instar larva, with special reference to the components and segmentation of the head, the segmentation of abdomen and the formation of abdominal prolegs. Our results confirm that the head consists of an anterior‐most acron and six trunk segments: the labral, antennal, intercalary, mandibular, maxillary, and labial segments. The labrum is confirmed to derive from the paired appendages. Our observations also provide additional direct evidence that the abdominal prolegs are not serially homologous with the thoracic legs. The presence of the eleventh abdominal segment is clarified. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Expression patterns of aristaless in developing appendages of Gryllus bimaculatus (cricket)

We report the isolation and expression patterns of aristaless (al), a paired-type homeobox gene, of Gryllus bimaculatus (Gb), a hemimetabola model insect. Gryllus al (Gbal) is expressed in the most distal region of developing labrum, antenna, mandible, maxilla, labium, leg, cercus, and hindgut. Gbal is also expressed in the proximal region, corresponding to the presumptive coxopodite, of the developing antenna, mandible, maxilla, labium, and leg, but not in the developing labrum, cercus, and hindgut. During development of the leg, expression of Gbal changes dynamically with the progress in leg segmentation: Gbal is expressed in order in the presumptive pretarsus, coxa, femur, tibia and tarsus before appearance of morphological segmentation.     

Segmentation of the Vertebrate Head

Historical views of head segmentation are reviewed. The concensusis that the head is segmented essentially in terms of myomeres,and that other organs have responded in varying degrees to this.From the various lines of reasoning a model of the primitivevertebrate is generated. This model denies the tunicate originof the vertebrates—rather it identifies amphioxus as mostlike the ancestral vertebrate. The vertebrate head is made upof a preoral segment plus four other segments. Because of sclerotomites,the head extends through five and a half segments. The nasalorgans and eyes are preoral structures while the ear is locatedbetween segments three and four. The occipital portion of thehead skeleton is formed from the posterior half of the fifthsegment and the anterior half of the sixth; it is vertebra-likein structure. This "segment" is much altered as a result ofthe multiplication of the visceral pouches and is often viewedas the fusion product of several segments. Thus the idea ofcorrespondence between somite and visceral segments posteriorto the second branchial arch is rejected. In some fishes, additionalvertebrae are added to the posterior part of the cranium andthis can be observed in development. The bony cranium of thevertebrate appears to partially reflect segmentation; its componentssuggest a vertebra-like developmental influence in operation.Study of the shark head has contributed much to our knowledgeof this area.     

Genetics,development and composition of the insect head – A beetle’s view

Many questions regarding evolution and ontogeny of the insect head remain open. Likewise, the genetic basis of insect head development is poorly understood. Recently, the investigation of gene expression data and the analysis of patterning gene function have revived interest in insect head development. Here, we argue that the red flour beetle Tribolium castaneum is a well suited model organism to spearhead research with respect to the genetic control of insect head development. We review recent molecular data and discuss its bearing on early development and morphogenesis of the head. We present a novel hypothesis on the ontogenetic origin of insect head sutures and review recent insights into the question on the origin of the labrum. Further, we argue that the study of developmental genes may identify the elusive anterior non-segmental region and present some evidence in favor of its existence. With respect to the question of evolution of patterning we show that the head Anlagen of the fruit fly Drosophila melanogaster and Tribolium differ considerably and we review profound differences of their genetic regulation. Finally, we discuss which insect model species might help us to answer the open questions concerning the genetic regulation of head development and its evolution.     

A new species of Hemicyclops (Copepoda,Poecilostomatoida) from a dredged area in Tokyo Bay,Japan

A new species of poecilostomatoid copepod, Hemicyclops japonicus, is described from a dredged area in Tokyo Bay. The new species can be distinguished from its congeners by a combination of the setation of the first antenna, the segmentation of the antenna and the urosome, the length to width ratio of the caudal ramus and the shape of the genital segment and the labrum.     

Comparative gene expression in the heads of Drosophila melanogaster and Tribolium castaneum and the segmental affinity of the Drosophila hypopharyngeal lobes

SUMMARY Drosophila melanogaster has long played an important role in debates surrounding insect and arthropod head segmentation. It is surprising, therefore, that one important feature of Drosophila head segmentation has remained controversial: namely the position of the boundary between the intercalary and mandibular segments. The Drosophila embryonic head has a pair of structures lying behind the stomodeum known as the hypopharyngeal lobes. Traditionally they have been seen as part of the intercalary segment. More recent work looking at the position of the lobes relative to various marker genes has been somewhat equivocal: segment polarity gene expression has been used to argue for a mandibular affinity of these lobes, while the expression of the anterior-most hox gene labial ( lab ) has supported an intercalary affinity. We have addressed the question of the segmental affinity of the hypopharyngeal lobes by conducting a detailed comparison of gene expression patterns between Drosophila and the red flour beetle Tribolium castaneum , in which the intercalary segment is unambiguously marked out by lab . We demonstrate that there is a large degree of conservation in gene expression patterns between Drosophila and Tribolium , and this argues against an intercalary segment affinity for the hypopharyngeal lobes. The lobes appear to be largely mandibular in origin, although some gene expression attributed to them appears to be associated with the stomodeum. We propose that the difficulties in interpreting the Drosophila head result from a topological shift in the Drosophila embryonic head, associated with the derived process of head involution.     

Expression of engrailed-family genes in the jumping bristletail and discussion on the primitive pattern of insect segmentation

It has been shown that segmentation in the short-germ insects proceeds by a two-step mechanism. The anterior region is simultaneously segmented in a manner similar to that in Drosophila, which is apparently unique to insects, and the rest of the posterior region is segmented sequentially by a mechanism involving a segmentation clock, which is derived from the common ancestor of arthropods. In order to propose the evolutionary scenario of insect segmentation, we examined segmentation in the jumping bristletail, the basalmost extant insect. Using probes for engrailed-family genes for in situ hybridization, we found no sign of simultaneous segmentation in the anterior region of the jumping bristletail embryos. All segments except the anteriormost segment are formed sequentially. This condition shown in the jumping bristletail embryos may represent the primitive pattern of insect segmentation. The intercalating formation of the intercalary segment is assumed to be a synapomorphic trait shared among all insects after the branching of the jumping bristletail.     

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...     

Formation of the insect head involves lateral contribution of the intercalary segment, which depends on Tc-labial function

The insect head is composed of several segments. During embryonic development, the segments fuse to form a rigid head capsule where obvious segmental boundaries are lacking. Hence, the assignment of regions of the insect head to specific segments is hampered, especially with respect to dorsal (vertex) and lateral (gena) parts. We show that upon Tribolium labial (Tc-lab) knock down, the intercalary segment is deleted but not transformed. Furthermore, we find that the intercalary segment contributes to lateral parts of the head cuticle in Tribolium. Based on several additional mutant and RNAi phenotypes that interfere with gnathal segment development, we show that these segments do not contribute to the dorsal head capsule apart from the dorsal ridge. Opposing the classical view but in line with findings in the vinegar fly Drosophila melanogaster and the milkweed bug Oncopeltus fasciatus, we propose a “bend and zipper” model for insect head capsule formation.     

Historical hypotheses regarding segmentation of the vertebrate head

The morphology of the vertebrate head is extremely complex and comprises numerous iterative structures that arise from each of the embryonic germ layers. The search for a fundamental plan uniting all of these serial structures spans ～200 years. The earliest attempt to identify a common plan was J. W. Goethe's vertebral theory of skull organization, in which the skull was interpreted as being formed by a series of trunk vertebrae. This theory was rejected by T. H. Huxley in the 1858 Croonian Lecture and was replaced by the segmented mesodermal model of Francis Balfour, which was elaborated subsequently by A. Marshall, Gavin de Beer, and Edwin Goodrich. This model assumes that the head of the earliest vertebrates consisted of eight segments. It further assumes that each segment contained dorsal muscles arising from the somitic mesoderm, and ventral muscles arising from lateral plate mesoderm, except for the first segment, which lacked ventral muscles derived from the lateral plate mesoderm. The muscles of each head segment were believed to be innervated by two pairs of cranial nerves, homologous to the dorsal and ventral spinal nerves of lampreys. The validity of this theory, known as the Goodrich model, came into question, however, after the discovery that the branchiomeric muscles associated with each pharyngeal arch do not arise from lateral plate mesoderm, as initially proposed by Marshall and subsequently accepted by Goodrich and de Beer, but, rather, arise from paraxial mesoderm. Furthermore, segmentation of the brain into some 14 neuromeres cannot be accommodated by any model involving eight segments. Finally, there is also clear evidence that at least one, if not two, additional series of placodally derived sensory nerves occurs in the head and has no counterpart in the trunk. At present, there is no theory of segmentation that can account for all cephalic iterative structures.     

The myth of intercalary segment in insect head

Embryonic development of the head of Oxyrhachis tarandus (Membracidae) has been investigated in detail to settle the controversy of head segmentation and to refute the occurrence of an intercalary segment. The head is formed from six distinct elements: the prostominal lobe, the paired cephalic lobes, the antennal segment and the three noncontroversial gnathal segments. The prostomial lobe, which possesses a neuromere and a pair of coelomic cavities, represents the first body segment, called the prostomial segment. The tritocerebral lobes of the brain and the stomatogastric nervous system, consisting of a frontal ganglion, clypeolabral nerves, and the recurrent nerve etc., develop from the neuromere of the prostomial lobe. The tritocerebrum thus belongs to the prostomial segment rather than to an imaginary intercalary segment and mainly represents the ganglionic center of the stomatogastric nervous system in the brain. Frons, clypeus, and labrum develop from the outer wall of the prostomial lobulate plate, whereas the epipharyngeal wall, including the cibarial pump, develops from its inner wall. The presence of three coelomic cavities and of three distinct neural masses in the cephalic lobes during the initial stages of development shows that they have developed by the fusion of three distinct segments during the long phylogenetic history of insects. The portion of the germ band presently considered as the intercalary segment is actually the sternal part of the antennal segment. The neural cells located in this region give rise to the deutocerebrum by shifting forward, around the stomodaeum, and always leaving a commissure behind. The intercalary segment is thus a complete illusion. The antennal segment is postoral in the beginning and bears a pair of coelomic cavities, but later on it shifts forward and its sternal part invaginates into the stomodaeum.     

Notch/Delta signalling is not required for segment generation in the basally branching insect Gryllus bimaculatus

Arthropods and vertebrates display a segmental body organisation along all or part of the anterior-posterior axis. Whether this reflects a shared, ancestral developmental genetic mechanism for segmentation is uncertain. In vertebrates, segments are formed sequentially by a segmentation 'clock' of oscillating gene expression involving Notch pathway components. Recent studies in spiders and basal insects have suggested that segmentation in these arthropods also involves Notch-based signalling. These observations have been interpreted as evidence for a shared, ancestral gene network for insect, arthropod and bilaterian segmentation. However, because this pathway can play multiple roles in development, elucidating the specific requirements for Notch signalling is important for understanding the ancestry of segmentation. Here we show that Delta, a ligand of the Notch pathway, is not required for segment formation in the cricket Gryllus bimaculatus, which retains ancestral characteristics of arthropod embryogenesis. Segment patterning genes are expressed before Delta in abdominal segments, and Delta expression does not oscillate in the pre-segmental region or in formed segments. Instead, Delta is required for neuroectoderm and mesectoderm formation; embryos missing these tissues are developmentally delayed and show defects in segment morphology but normal segment number. Thus, what initially appear to be 'segmentation phenotypes' can in fact be due to developmental delays and cell specification errors. Our data do not support an essential or ancestral role of Notch signalling in segment generation across the arthropods, and show that the pleiotropy of the Notch pathway can confound speculation on possible segmentation mechanisms in the last common bilaterian ancestor.     

Head segmentation of trilobites

Although trilobites have provided research subjects for more than two centuries, their head segmentation has remained unresolved. Four glabellar furrows (SO and S1–S3) marking the segmental boundaries are generally present in the cephalic axis, but there are trilobites with one more pair of furrows, the so‐called S4, in the cephalic axis, causing confusion in understanding trilobite head segmentation. Recent advances in developmental biology and palaeontology have shed light on the arthropod head problem, and thus, trilobite head segmentation can be reviewed in the light of this knowledge. Based on the information from the anatomy of exceptionally preserved trilobites and artiopodans closely related to trilobites, it is inferred that trilobite head contains five segments: the anteriormost ocular segment potentially associated with the hypostome, the antennal segment and the following three segments with walking legs. When present, the S4 furrows are situated where the eye ridges meet the cephalic axis of trilobites, indicating that the furrows are incised ‘within’ the anteriormost segment in trilobites with an anteriorly enlarged frontal lobe. Trilobites of the Order Redlichiida, the most primitive stock, show variable conditions in the frontal glabellar conditions, while in other more derived groups, the condition is rather constant. The frontal glabellar condition, therefore, could provide a clue to elucidate the unresolved Cambrian trilobite phylogeny and the Cambrian roots of the post‐Cambrian trilobites.     

Interactions of the Drosophila gap gene giant with maternal and zygotic pattern-forming genes.

The Drosophila gene giant (gt) is a segmentation gene that affects anterior head structures and abdominal segments A5-A7. Immunolocalization of the gt product shows that it is a nuclear protein whose expression is initially activated in an anterior and a posterior domain. Activation of the anterior domain is dependent on the maternal bicoid gradient while activation of the posterior domain requires maternal nanos gene product. Initial expression is not abolished by mutations in any of the zygotic gap genes. By cellular blastoderm, the initial pattern of expression has evolved into one posterior and three anterior stripes of expression. The evolution, position and width of these stripes are dependent on interactions between gt and the other gap genes. In turn, gt activity in these domains affects the expression of the other gap genes. These interactions, typical of the cross-regulation previously observed among gap genes, confirm that gt is a member of the gap gene class whose function is necessary to establish the overall pattern of gap gene expression. After cellular blastoderm, gt protein continues to be expressed in the head region in parts of the maxillary and mandibular segments as well as in the labrum. Expression is never detected in the labial or thoracic segment primordia but persists in certain head structures, including the ring gland, until the end of embryonic development.     

Endonura Cassagnau in Iran,with a key to species of the genus (Collembola,Neanuridae, Neanurinae)

Three new species of Endonura are described from Iran. Endonura dichaeta sp. n. can be recognized by an ogival labrum, head without chaetae O and E, chaeta D connected with tubercle Cl, tubercle Dl with five chaetae on head, absence of tubercles Di on thorax I and tubercle (Di+Di) of thorax V with 2+2 chaetae. Endonura ceratolabralis sp. n. is characterized by large body size, reduction of labral chaetotaxy, ogival labrum, head without chaeta O and fusion of tubercles Di and De on first thoracic segment. Endonura persica sp. n. is distinguished from its congeners by a nonogival labrum, absence of chaeta O, tubercles Dl and (L+So) with five and eight chaetae respectively and claw with inner tooth. The key to all species of the genus is given.     

The appendage role of insect disco genes and possible implications on the evolution of the maggot larval form

Though initially identified as necessary for neural migration, Disconnected and its partially redundant paralog, Disco-related, are required for proper head segment identity during Drosophila embryogenesis. Here, we present evidence that these genes are also required for proper ventral appendage development during development of the adult fly, where they specify medial to distal appendage development. Cells lacking the disco genes cannot contribute to the medial and distal portions of ventral appendages. Further, ectopic disco transforms dorsal appendages toward ventral fates; in wing discs, the medial and distal leg development pathways are activated. Interestingly, this appendage role is conserved in the red flour beetle, Tribolium (where legs develop during embryogenesis), yet in the beetle we found no evidence for a head segmentation role. The lack of an embryonic head specification role in Tribolium could be interpreted as a loss of the head segmentation function in Tribolium or gain of this function during evolution of flies. However, we suggest an alternative explanation. We propose that the disco genes always function as appendage factors, but their appendage nature is masked during Drosophila embryogenesis due to the reduction of limb fields in the maggot style Drosophila larva.