Developmental finite element analysis of cichlid pharyngeal jaws: Quantifying the generation of a key innovation

Advances in imaging and modeling facilitate the calculation of biomechanical forces in biological specimens. These factors play a significant role during ontogenetic development of cichlid pharyngeal jaws, a key innovation responsible for one of the most prolific species diversifications in recent times. MicroCT imaging of radiopaque-stained vertebrate embryos were used to accurately capture the spatial relationships of the pharyngeal jaw apparatus in two cichlid species (Haplochromis elegans and Amatitlania nigrofasciata) for the purpose of creating a time series of developmental stages using finite element models, which can be used to assess the effects of biomechanical forces present in a system at multiple points of its ontogeny. Changes in muscle vector orientations, bite forces, force on the neurocranium where cartilage originates, and stress on upper pharyngeal jaws are analyzed in a comparative context. In addition, microCT scanning revealed the presence of previously unreported cement glands in A. nigrofasciata. The data obtained provide an underrepresented dimension of information on physical forces present in developmental processes and assist in interpreting the role of developmental dynamics in evolution.

Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system

BACKGROUND

The gene regulatory network involved in tooth morphogenesis has been extremely well described in mammals and its modeling has allowed predictions of variations in regulatory pathway that may have led to evolution of tooth shapes. However, very little is known outside of mammals to understand how this regulatory framework may also account for tooth shape evolution at the level of gnathostomes. In this work, we describe expression patterns and proliferation/apoptosis assays to uncover homologous regulatory pathways in the catshark Scyliorhinus canicula.

RESULTS

Because of their similar structural and developmental features, gene expression patterns were described over the four developmental stages of both tooth and scale buds in the catshark. These gene expression patterns differ from mouse tooth development, and discrepancies are also observed between tooth and scale development within the catshark. However, a similar nested expression of Shh and Fgf suggests similar signaling involved in morphogenesis of all structures, although apoptosis assays do not support a strictly equivalent enamel knot system in sharks. Similarities in the topology of gene expression pattern, including Bmp signaling pathway, suggest that mouse molar development is more similar to scale bud development in the catshark.

CONCLUSIONS

These results support the fact that no enamel knot, as described in mammalian teeth, can be described in the morphogenesis of shark teeth or scales. However, homologous signaling pathways are involved in growth and morphogenesis with variations in their respective expression patterns. We speculate that variations in this topology of expression are also a substrate for tooth shape evolution, notably in regulating the growth axis and symmetry of the developing structure.

Dual Role of Jun N-Terminal Kinase Activity in Bone Morphogenetic Protein-Mediated Drosophila Ventral Head Development

The Drosophila bone morphogenetic protein encoded by decapentaplegic (dpp) controls ventral head morphogenesis by expression in the head primordia, eye-antennal imaginal discs. These are epithelial sacs made of two layers: columnar disc proper cells and squamous cells of the peripodial epithelium. dpp expression related to head formation occurs in the peripodial epithelium; cis-regulatory mutations disrupting this expression display defects in sensory vibrissae, rostral membrane, gena, and maxillary palps. Here we document that disruption of this dpp expression causes apoptosis in peripodial cells and underlying disc proper cells. We further show that peripodial Dpp acts directly on the disc proper, indicating that Dpp must cross the disc lumen to act. We demonstrate that palp defects are mechanistically separable from the other mutant phenotypes; both are affected by the c-Jun N-terminal kinase pathway but in opposite ways. Slight reduction of both Jun N-terminal kinase and Dpp activity in peripodial cells causes stronger vibrissae, rostral membrane, and gena defects than Dpp alone; additionally, strong reduction of Jun N-terminal kinase activity alone causes identical defects. A more severe reduction of dpp results in similar vibrissae, rostral membrane, and gena defects, but also causes mutant maxillary palps. This latter defect is correlated with increased peripodial Jun N-terminal kinase activity and can be caused solely by ectopic activation of Jun N-terminal kinase. We conclude that formation of sensory vibrissae, rostral membrane, and gena tissue in head morphogenesis requires the action of Jun N-terminal kinase in peripodial cells, while excessive Jun N-terminal kinase signaling in these same cells inhibits the formation of maxillary palps.

Dvr1 transfers left-right asymmetric signals from Kupffer's vesicle to lateral plate mesoderm in zebrafish

An early step in establishing left-right (LR) symmetry in zebrafish is the generation of asymmetric fluid flow by Kupffer's vesicle (KV). As a result of fluid flow, a signal is generated and propagated from the KV to the left lateral plate mesoderm, activating a transcriptional response of Nodal expression in the left lateral plate mesoderm (LPM). The mechanisms and molecules that aid in this transfer of information from the KV to the left LPM are still not clear. Here we provide several lines of evidence demonstrating a role for a member of the TGFβ family member, Dvr1, a zebrafish Vg1 ortholog. Dvr1 is expressed bilaterally between the KV and the LPM. Knockdown of Dvr1 by morpholino causes dramatically reduced or absent expression of southpaw (spaw, a Nodal homolog), in LPM, and corresponding loss of downstream Lefty (lft1 and lft) expression, and aberrant brain and heart LR patterning. Dvr1 morphant embryos have normal KV morphology and function, normal expression of southpaw (spaw) and charon (cha) in the peri-KV region and normal expression of a variety of LPM markers in LPM. Additionally, Dvr1 knockdown does not alter the capability of LPM to respond to signals that initiate and propagate spaw expression. Co-injection experiments in Xenopus and zebrafish indicate that Dvr1 and Spaw can enhance each other's ability to activate the Nodal response pathway and co-immunoprecipitation experiments reveal differential relationships among activators and inhibitors in this pathway. These results indicate that Dvr1 is responsible for enabling the transfer of a left-right signal from KV to the LPM.

[Effect of exogenous factors on the induction of spicule formation in sea urchin embryonic cell cultures]

The effect of exogenous factors on the realization of the spicule formation program in two sea urchin species, Strongylocentrotus intermedius and S. nudus, has been studied in primary embryonic cell cultures derived from the blastula and gastrula stages. It has been shown that the process of spicule formation depends on the type of substrate and the composition of the medium. An original finding is that calf or horse serum necessary for spicule formation in vitro can be replaced by a complex of factors including insulin, transferrin, and lectins. Methods allowing control over the growth and differentiation of marine invertebrate embryonic cells in vitro open prospects for their application to practical problems such as the establishment of cell cultures producing certain mineral structures.

Emerging properties of animal gene regulatory networks

Gene regulatory networks (GRNs) provide system level explanations of developmental and physiological functions in the terms of the genomic regulatory code. Depending on their developmental functions, GRNs differ in their degree of hierarchy, and also in the types of modular sub-circuit of which they are composed, although there is a commonly employed sub-circuit repertoire. Mathematical modelling of some types of GRN sub-circuit has deepened biological understanding of the functions they mediate. The structural organization of various kinds of GRN reflects their roles in the life process, and causally illuminates both developmental and evolutionary process.

The Hawaiian bobtail squid (Euprymna scolopes): a model to study the molecular basis of eukaryote-prokaryote mutualism and the development and evolution of morphological novelties in cephalopods

The Hawaiian bobtail squid, Euprymna scolopes, is a cephalopod whose small size, short lifespan, rapid growth, and year-round availability make it suitable as a model organism. E. scolopes is studied in three principal contexts: (1) as a model of cephalopod development; (2) as a model of animal-bacterial symbioses; and (3) as a system for studying adaptations of tissues that interact with light. E. scolopes embryos can be obtained continually and can be reared in the laboratory over an entire generation. The embryos and protective chorions are optically clear, facilitating in situ developmental observations, and can be manipulated experimentally. Many molecular protocols have been developed for studying E. scolopes development. This species is best known, however, for its symbiosis with the luminous marine bacterium Vibrio fischeri and has been used to study determinants of symbiont specificity, the influence of symbiosis on development of the squid light organ, and the mechanisms by which a stable association is achieved. Both partners can be grown independently under laboratory conditions, a feature that offers the unusual opportunity to manipulate the symbiosis experimentally. Molecular and genetic tools have been developed for V. fischeri, and a large expressed sequence tag (EST) database is available for the host symbiotic tissues. Additionally, comparisons between light organ form and function to those of the eye can be made. Both types of tissue interact with light, but have divergent embryonic development. As such, they offer an opportunity to study the molecular basis for the evolution of morphological novelties.

Development of the subsoephageal body cells and the pericardiac cells during embryogenesis with diapause in Locusta migratoria (Linnaeus 1758) (Orthoptera: Acrididae)

During Locusta migratoria embryogenesis, the yolk is progressively degraded and the resulting metabolites are released in the haemolymph. We researched the organs possibly involved in the uptake of haemolymphatic proteins. Among organs originated from mesoderm, the SOB (suboesophageal bodies) situated in the embryonic head are remarkable by a very early acquisition of differentiated cytological characters, while most other cells of the embryo are undifferentiated. The SOB quite disappear before hatching. Just before katatrepsis stage, the other organs derived from mesoderm begin to differentiate, including the PC (pericardial cells) which take over from the SOB. These cells, situated in thorax and abdomen, are developed during the dorsal close of embryo. The development and the ultrastructural changes of the SOB cells and of the PC were studied during an embryogenesis with diapause. The morphology of embryos which enter diapause is comparable with that of a continuous development at the beginning of katatrepsis. However, the cells of SOB and PC cells suffer from remarkable changes not only physiologically but cytologically. At the beginning of diapause, the proteosynthetic activity practically disappears in the SOB cells and the lysis areas appear. Nevertheless, the exchanges between these cells and the haemolymph still remain important. For the period of cold, which is necessary to the resumption of development, the aspect of the SOB cells changes and in particular the areas of lysis become less wide. When the embryo reopens its development, the SOB cells show a proteosynthetic activity and the areas of lysis disappear. The changes of the SOB cells and of the PC cells are regularized during the resumption of the development: the SOB cells which had again taken a normal activity start to regress from the stage VII on, while the PC cells take over.

Quantitative analysis of antennal mosaic generation in Drosophila melanogaster by the MARCM system

Mosaics have been used in Drosophila to study development and to generate mutant structures when a mutant allele is homozygous lethal. New approaches of directed somatic recombination based on FRT/FLP methods, have increased mosaicism rates but likewise multiple clones in the same individual appeared more frequently. Production of single clones could be essential for developmental studies; however, for cell-autonomous gene function studies only the presence of homozygous cells for the target recessive allele is relevant. Herein, we report the number and extension of antennal mosaics generated by the MARCM system at different ages. This information is directed to obtain the appropriated mosaic type for the intended application. By applying heat shock at 10 different developmental stages from 0-12 h to 6-7 days after egg laying, more than 50% of mosaics were obtained from 5,028 adults. Single recombinant clones appeared mainly at early stages while massive recombinant areas were observed with late treatments.

[Influence of precocene II on the sensory system of antennae and mouthparts in larvae of the fruit tree tortricid Archips podana Scop. (Lepidoptera: Tortricidae)]

The influence of precocene II, an antijuvenile agent, on morphological characters of the chemoreceptor apparatus of antennae and mouthparts was studied in fifth instar A. podana larvae. Treatment with different doses of precocene was performed at the egg stage. It proved to cause changes in the form and number of basiconic sensilla on the maxillary palps and galea and in the size of basiconic sensilla on the second and third antennal segments. The results are discussed with respect to the influence of precocenes on the insect sensory system and the role of the juvenile hormone in regulation of its development.

Right-elevated expression of charon is regulated by fluid flow in medaka Kupffer's vesicle

Recent studies have revealed that a cilium-generated liquid flow in the node has a crucial role in the establishment of the left-right (LR) axis in the mouse. In fish, Kupffer's vesicle (KV), a teleost-specific spherical organ attached to the tail region, is known to have an equivalent role to the mouse node during LR axis formation. However, at present, there has been no report of an asymmetric gene expressed in KV under the control of fluid flow. Here we report the earliest asymmetric gene in teleost KV, medaka charon, and its regulation. Charon is a member of the Cerberus/DAN family of proteins, first identified in zebrafish. Although zebrafish charon was reported to be symmetrically expressed in KV, medaka charon displays asymmetric expression with more intense expression on the right side. This asymmetric expression was found to be regulated by KV flow because symmetric and up-regulated charon expression was observed in flow-defective embryos with immotile cilia or disrupted KV. Taken together, medaka charon is a reliable gene marker for LR asymmetry in KV and thus, will be useful for the analysis of the early steps downstream of the fluid flow.

Duplicated Abd-B class genes in medaka hoxAa and hoxAb clusters exhibit differential expression patterns in pectoral fin buds

Hox genes form clusters. Invertebrates and Amphioxus have only one hox cluster, but in vertebrates, they are multiple, i.e., four in the basal teleost fish Polyodon and tetrapods (HoxA, B, C, D), but seven or eight in common teleosts. We earlier completely sequenced the entire hox gene loci in medaka fish, showing a total of 46 hox genes to be encoded in seven clusters (hoxAa, Ab, Ba, Bb, Ca, Da, Db). Among them, hoxAa, hoxAb and hoxDa clusters are presumed to be important for fin-to-limb evolution because of their key role in forelimb and pectoral fin development. In the present study, we compared genome organization and nucleotide sequences of the hoxAa and hoxAb clusters to these of tetrapod HoxA clusters, and found greater similarity in hoxAa case. We then analyzed expression of Abd-B family genes in the clusters. In the trunk, those from the hoxAa cluster, i.e., hoxA9a, hoxA10a, hoxA11a and hoxA13a, were expressed in a manner keeping the colinearity rule of the hox expression as those of tetrapods, while those from the hoxAb cluster, i.e., hoxA9b, hoxA10b, hoxA11b and hoxA13b, were not. In the pectoral fins, the hoxAa cluster was expressed in split domains and did not obey the rule. By contrast, those from the hoxAb and hoxDa clusters were expressed in a manner keeping the rule, i.e., an ancestral pattern similar to those of tetrapods. It is plausible that this differential expression of the two clusters is caused by changes occurred in global control regions after cluster duplications.

Function of FGF signaling in the developmental process of the median fin fold in zebrafish

Median fins, unpaired appendages in fish, are fundamental locomotory organs that are believed to have evolved before paired lateral appendages in vertebrates. However, the early process of median fin development remains largely unknown. We investigated the early development of the median fin fold, a rudiment of median fins, and report here the process in zebrafish embryos and the function of FGF signaling in the process. Using expressions of three genes, dlx5a, sp9 and fgf24, as markers of different phases of fold development, our findings suggest that the early process of median fin fold development can be divided into two steps, specification of the median fin fold territory and construction of the fold structure. Both loss-of-function and gain-of-function assays revealed that FGF signaling plays roles in each step, suggesting a common mechanism for the development of median appendages and paired lateral appendages.

Wnt11-R signaling regulates a calcium sensitive EMT event essential for dorsal fin development of Xenopus

In the frog embryo, a sub-population of trunk neural crest (NC) cells undergoes a dorsal route of migration to contribute to the mesenchyme in the core of the dorsal fin. Here we show that a second population of cells, originally located in the dorsomedial region of the somite, also contributes to the fin mesenchyme. We find that the frog orthologue of Wnt11 (Wnt11-R) is expressed in both the NC and somite cell populations that migrate into the fin matrix. Wnt11-R is expressed prior to migration and persists in the mesenchymal cells after they have distributed throughout the fin. Loss of function studies demonstrate that Wnt11-R activity is required for an epithelial to mesenchymal transformation (EMT) event that precedes migration of cells into the fin matrix. In Wnt11-R depleted embryos, the absence of fin core cells leads to defective dorsal fin development and to collapse of the fin structure. Experiments using small molecule inhibitors indicate that dorsal migration of fin core cells depends on calcium signaling through calcium/calmodulin-dependent kinase II (CaMKII). In Wnt11-R depleted embryos, normal migration of NC cells and dorsal somite cells into the fin and normal fin development can be rescued by stimulation of calcium release. These studies are consistent with a model in which Wnt11-R signaling, via a downstream calcium pathway, regulates fin cell migration and, more generally, indicates a role for non-canonical Wnt signaling in regulation of EMT.

Distinct functions of homeodomain-containing and homeodomain-less isoforms encoded by homothorax

The homothorax (hth) gene of Drosophila melanogaster is required for executing Hox functions, for head development, and for forming the proximodistal (PD) axis of the appendages. We show that alternative splicing of hth generates two types of protein isoforms, one that contains a DNA-binding homeodomain (HthFL) and one that does not contain a homeodomain (HDless). Both types of Hth isoforms include the evolutionarily conserved HM domain, which mediates a direct interaction with Extradenticle (Exd), another homeodomain protein. We show that although both HthFL and HDless isoforms of Hth can induce the nuclear localization of Exd, they carry out distinct sets of functions during development. Surprisingly, we find that many of hth's functions, including PD patterning and most Hox-related activities, can be executed by the HDless isoforms. In contrast, antennal development shows an absolute dependency on the HthFL isoform. Thus, alternative splicing of hth results in the generation of multiple transcription factors that execute unique functions in vivo. We further demonstrate that the mouse ortholog of hth, Meis1, also encodes a HDless isoform, suggesting that homeodomain-less variants of this gene family are evolutionarily ancient.

Organ-specific expression of IGF-I during early development of bony fish as revealed in the tilapia, Oreochromis niloticus, by in situ hybridization and immunohistochemistry: indication for the particular importance of local IGF-I

The cellular sites of insulin-like growth factor I (IGF-I) synthesis in the early developing tilapia (0-140 days post fertilization, DPF) were investigated. IGF-I mRNA and peptide appeared in liver as early as 4 DPF and in gastro-intestinal epithelial cells between 5-9 DPF. In exocrine pancreas, the expression of IGF-I started at 4 DPF and continued until 90 DPF. IGF-I production was detected in islets at 6 DPF in non-insulin cells and occurred throughout life. In renal tubules and ducts, IGF-I production started at 8 DPF. IGF-I production in chondrocytes had its onset at 4 DPF, was more pronounced in growing regions and was also found in adults. IGF-I mRNA and peptide appeared in the cytoplasm of skeletal muscle cells at 4 DPF. In gill chloride cells, IGF-I production started at 6 DPF. At 13 DPF, IGF-I was detected in cardiac myocytes. IGF-I-producing epidermal cells appeared at 5 DPF. In brain and ganglia, IGF-I was expressed in virtually all neurones from 6 to 29 DPF, their number decreasing with age. Neurosecretory IGF-I-immunoreactive axons were first seen in the neurohypophysis around 17 DPF. Endocrine cells of the adenohypophysis exhibited IGF-I mRNA at 28 DPF and IGF-I immunoreactivity at 40 DPF. Thus, IGF-I appeared early (4-5 DPF), first in liver, the main source of endocrine IGF-I, and then in organs involved in growth or metabolism. The expression of IGF-I was more pronounced during development than in juvenile and adult life. Local IGF-I therefore seems to have a high functional impact in early growth, metabolism and organogenesis.

Tissue distribution of two field isolates and two vaccine strains of porcine parvovirus in foetal organs after experimental infection of pregnant sows as determined by real-time PCR

The aim of this study was to investigate the tissue distribution of two different field isolates and two vaccine strains of porcine parvoviruses (PPV) in infected piglets after transplacental infection. The viral load in 10 different foetal organs was determined by real-time polymerase chain reaction assays with SYBR Green targeting the viral VP2 gene and the genomic c-myc gene in 12 foetuses. The viral load in foetal tissues differed greatly among the different parvoviruses. Between one virulent field isolate compared with the other field isolate and the vaccine strains, the detected viral copy number differed in an order of magnitude of 10(9). The virulent isolate contained PPV in all 10 organs with viral loads varying between 10(11) and 10(15) per 10(6) cells. Concerning the other field isolate and the two vaccine strains, if PPV was detected, in most of the cases the highest viral load was found in foetal kidneys with a maximum viral load of 10(3) per 10(6) cells. Additionally, PPV was found in the heart of one foetus, in the liver and duodenum of one foetus and in the thymus of one foetus with viral loads varying between 10(2.1) and 10(3.5) per 10(6) cells. In completely mummified foetuses with no discriminable organs of foetuses infected with the vaccine strains and the less virulent isolate, PPV was present in very low amounts or even below the detection limit.

The structure and development of avian lumbosacral specializations of the vertebral canal and the spinal cord with special reference to a possible function as a sense organ of equilibrium

The avian lumbosacral vertebral column and spinal cord show a number of specializations which have recently been interpreted as a sense organ of equilibrium. This sense organ is thought to support balanced walking on the ground. Although most of the peculiar structures have been described previously, there was a need to reevaluate the specializations with regard to the possible function as a sense organ. Specializations were studied in detail in the adult pigeon. The development of the system was studied both in the pigeon (semiprecocial at hatching) and in the chicken (precocial). Specializations in the vertebral canal consist of a considerable enlargement, which is not due to an increase in the size of the spinal nervous tissue, but to a large glycogen body embedded in a dorsal rhomboid sinus. The dorsal wall of the vertebral canal shows segmented bilateral dorsal grooves, which are covered by the meninges towards the lumen of the vertebral canal leaving openings in the midline and laterally. This results in a system of lumbosacral canals which look and may function similar to the semicircular canals in the inner ear. Laterally these canals open above ventrolateral protrusions or accessory lobes of the spinal cord which contain neurons. There are large subarachnoidal cerebrospinal fluid spaces, lateral and ventral to the accessory lobes. Movement of this fluid is thought to stimulate the lobes mechanically. As to the development of avian lumbosacral specializations, main attention was given to the organization of the lobes and the adjacent fluid spaces including the dorsal canals. In the pigeon the system is far from being adult-like at hatching but maturates rapidly after hatching. In the chicken the system looks already adult-like at hatching. The implications derived from the structural findings are discussed with regard to a possible function of the lumbosacral specializations as a sense organ of equilibrium. The adult-like organization in the newly hatched chickens, which walk around immediately after hatching, supports the assumed function as a sense organ involved in the control of locomotion on the ground.

A study of canalization and developmental stability in the sternopleural bristle system of Drosophila melanogaster

Among the explanations for minimizing the effects of extraneous variation has been canalization and developmental stability. However, there is little agreement as to whether these two processes reflect a common set of mechanisms. This confusion is inflated due to the lack of consensus as to a precise definition of canalization. In this study, canalization in the sternopleural bristle system is used to investigate the relationships between measures of canalization and developmental stability by comparing how a panel of naturally derived lines responds to both genetic and environmental perturbations. No evidence for a common mechanism between the different measures of canalization was observed. Furthermore, a hypothesis regarding a common mechanism for environmental and genetic canalization was tested, and no evidence was found to support it. However, there is evidence for a relationship between at least one form of canalization and developmental stability.

Identification of a novel cell-adhesive protein spatiotemporally expressed in the basement membrane of mouse developing hair follicle

We used PCR-based cDNA subtraction to screen for genes up-regulated during mouse hair morphogenesis. One gene selected was predominantly expressed at the tip of developing hair follicles and encoded a protein characterized by the presence of twelve tandem repeats of approximately 120 amino acids and a novel N-terminal domain containing an Arg-Gly-Asp cell-adhesive motif. Immunohistochemistry demonstrated that the protein encoded by this gene, named QBRICK, was localized at the basement membrane zone of embryonic epidermis and hair follicles, in which it was more enriched at the tip rather than the stalk region. Cell adhesion assays showed that QBRICK was active in mediating cell-substratum adhesion through integrins containing alphav or alpha8 chain, but not integrin alpha5beta1. Immunohistochemistry showed that QBRICK colocalized with alphav-containing integrins in the interfollicular region, but with the alpha8-containing integrin at the tip region of developing hair follicles. These results, together, indicate that QBRICK is an adhesive ligand of basement membrane distinctively recognized by cells in the embryonic skin and hair follicles through different types of integrins directed to the Arg-Gly-Asp motif.

Noncanonical Wnt signaling regulates midline convergence of organ primordia during zebrafish development

Several components of noncanonical Wnt signaling pathways are involved in the control of convergence and extension (CE) movements during zebrafish and Xenopus gastrulation. However, the complexity of these pathways and the wide patterns of expression and activity displayed by some of their components immediately suggest additional morphogenetic roles beyond the control of CE. Here we show that the key modular intracellular mediator Dishevelled, through a specific activation of RhoA GTPase, controls the process of convergence of endoderm and organ precursors toward the embryonic midline in the zebrafish embryo. We also show that three Wnt noncanonical ligands wnt4a, silberblick/wnt11, and wnt11-related regulate this process by acting in a largely redundant way. The same ligands are also required, nonredundantly, to control specific aspects of CE that involve interaction of Dishevelled with mediators different from that of RhoA GTPase. Overall, our results uncover a late, previously unexpected role of noncanonical Wnt signaling in the control of midline assembly of organ precursors during vertebrate embryo development.

Duplicate zebrafish pth genes are expressed along the lateral line and in the central nervous system during embryogenesis

PTH plays a critical role in calcium metabolism in tetrapods. The primary site of PTH expression is the parathyroid glands, although it is also detected in the thymus and hypothalamus. Fish lack anatomically distinct parathyroid glands, and the first animals to evolve parathyroid glands were the amphibians. However, fish do have PTH family ligands and receptors, which are functionally similar to their mammalian counterparts. We report the expression patterns of duplicate zebrafish pth genes during embryogenesis. Both zebrafish pth1 and pth2 transcripts are expressed along the lateral line before the migration of the lateral line primordium and later in development Pth protein is detected in lateral line neuromasts by immunohistochemistry. pth1 transcripts are also detected in the central nervous system in the ventral neural tube. These temporally and anatomically restricted expression patterns imply a novel role for PTH family hormones during embryonic development of the zebrafish and allow for the genetic dissection of PTH function in this model organism.

The environmental and genetic regulation of obake expressivity: morphogenetic fields as evolvable systems

The morphogenetic field, a fundamental concept of classical embryology, is once again being invoked to describe developmental processes. Because the evolution of adult structures requires the modification of development, the ways in which morphogenetic fields can change over time may yield insights into evolutionary possibilities. We considered how the duplication/multiplication of a morphogenetic field in fruit flies, caused by the previously described obake (obk) mutation, is regulated by genetic and environmental factors. Mutations of genes in the canonical antenna-producing imaginal disc pathway suppressed duplication as expected, although the results suggested that other pathways might also be involved. Overgrowth mutations, expected to increase duplication, actually suppressed it. Mutations in the heat-shock protein gene Hsp83 did not uniformly enhance obk expressivity as hypothesized. Using third chromosomes extracted from wild-derived lines, natural genetic variation for modifiers of obk function was found to be extensive. Larval crowding suppressed the obk phenotype, but there was no evidence of trade-offs between body or head size and arista number. Our results suggest that a complex interplay of genetic and environmental factors in the regulation of fields may be responsible for ample natural variation in the expressivity of adult phenotypes, affording multiple opportunities for selection and evolutionary modification.

Enlightenment of old ideas from new investigations: more questions regarding the evolution of bacteriogenic light organs in squids

Bioluminescence is widespread among many different types of marine organisms. Metazoans contain two types of luminescence production, bacteriogenic (symbiotic with bacteria) or autogenic, via the production of a luminous secretion or the intrinsic properties of luminous cells. Several species in two families of squids, the Loliginidae and the Sepiolidae (Mollusca: Cephalopoda) harbor bacteriogenic light organs that are found central in the mantle cavity. These light organs are exceptional in function, that is, the morphology and the complexity suggests that the organ has evolved to enhance and direct light emission from bacteria that are harbored inside. Although light organs are widespread among taxa within the Sepiolidae, the origin and development of this important feature is not well studied. We compared light organ morphology from several closely related taxa within the Sepiolidae and combined molecular phylogenetic data using four loci (nuclear ribosomal 28S rRNA and the mitochondrial cytochrome c oxidase subunit I and 12S and 16S rRNA) to determine whether this character was an ancestral trait repeatedly lost among both families or whether it evolved independently as an adaptation to the pelagic and benthic lifestyles. By comparing other closely related extant taxa that do not contain symbiotic light organs, we hypothesized that the ancestral state of sepiolid light organs most likely evolved from part of a separate accessory gland open to the environment that allowed colonization of bacteria to occur and further specialize in the eventual development of the modern light organ.

[The development of the nervous system: from fly to fish, from fish to man....]

The nervous system of vertebrates is more complex and less tractable than that of current model organisms such as the fly and the nematode. Here we present a vertebrate sensory system which is structurally simple, experimentally accessible and genetically suitable: the lateral line of the zebra-fish. We review our recent work on the development of this system, with a particular emphasis on the migration events that shape the pattern of sense organs. Some of the factors involved in these migration events turn out to be similar to the factors that direct the formation of metastases in specific types of human cancers, illustrating once again the remarkable conservation of developmental mechanisms and genes throughout the animal kingdom.

Alpha spectrin is essential for morphogenesis and body wall muscle formation in Caenorhabditis elegans

A common feature of multicellular animals is the ubiquitous presence of the spectrin cytoskeleton. Although discovered over 30 yr ago, the function of spectrin in non-erythrocytes has remained elusive. We have found that the spc-1 gene encodes the only alpha spectrin gene in the Caenorhabditis elegans genome. During embryogenesis, alpha spectrin localizes to the cell membrane in most if not all cells, starting at the first cell stage. Interestingly, this localization is dependent on beta spectrin but not beta(Heavy) spectrin. Furthermore, analysis of spc-1 mutants indicates that beta spectrin requires alpha spectrin to be stably recruited to the cell membrane. Animals lacking functional alpha spectrin fail to complete embryonic elongation and die just after hatching. These mutant animals have defects in the organization of the hypodermal apical actin cytoskeleton that is required for elongation. In addition, we find that the process of elongation is required for the proper differentiation of the body wall muscle. Specifically, when compared with myofilaments in wild-type animals the myofilaments of the body wall muscle in mutant animals are abnormally oriented relative to the longitudinal axis of the embryo, and the body wall muscle cells do not undergo normal cell shape changes.

EGF receptor and Notch signaling act upstream of Eyeless/Pax6 to control eye specification

The Drosophila compound eye is specified by the concerted action of seven nuclear factors that include Eyeless/Pax6. These factors have been called "master control" proteins because loss-of-function mutants lack eyes and ectopic expression can direct ectopic eye development. However, inactivation of these genes does not cause the presumptive eye to change identity. Surprisingly, we find that several of these eye specification genes are not coexpressed in the same embryonic cells-or even in the presumptive eye. We demonstrate that the EGF Receptor and Notch signaling pathways have homeotic functions that are genetically upstream of the eye specification genes, and show that specification occurs much later than previously thought-not during embryonic development but in the second larval stage.

A scanning and transmission electron microscopy study of the parabronchial unit in quail (Coturnix coturnix) and town pigeons (Columba livia)

A combined scanning electron (SEM) and transmission electron microscopy (TEM) investigation was undertaken to gain insight into the complex structural pattern of the atrial compartment and the gas exchange tissue of parabronchial units in quail and town pigeons. The aim was also to depict the changes taking place in the parabronchial unit in the late prehatching and early posthatching periods in quail. The standard SEM and TEM investigation was carried out in 13 mature quail and 8 town pigeons. The developmental study involved embryonic quail (Days 15, 16, 17), newly hatched quail, quail 24 h after hatching, and quail aged 2, 10, 19, and 25 days (3 individuals per group). The luminal relief of the parabronchus is formed by anastomosing interatrial septa delineating the atrial pits, which are thinner and shallower in pigeons. The atrial bottom opens in mature individuals into 3-6 infundibula. The extracellular material represented by trilaminar substance, which does not appear until hatching, veils the surface relief of the parabronchial epithelium, which is consequently hardly accessible to three-dimensional visualization. Only in town pigeons with fewer discontinuous layers of extracellular material was it possible to visualize the surface of the atrial epithelium, that is, of the granular and squamous atrial cells. The SEM analysis has convincingly shown the intricate spatial organization of atria, infundibula, and air and blood capillaries of the gas exchange tissue. The retinacula, that is, parallelly arranged processes of squamous respiratory cells bridging the air-capillary lumina, were evidenced by SEM and TEM. The complex structure of the avian parabronchus has been successfully demonstrated in the present SEM and TEM study.

Embryonic development of the Drosophila brain. I. Pattern of pioneer tracts

The neuropile of the late embryonic Drosophila brain can be subdivided into a vertical component (cervical connective), a transverse component (supraesophageal commissure), and a horizontal component for which we propose the term protocerebral connective. The core of each neuropile component is formed by numerous axon fascicles, the trajectory of which follows an invariant pattern. In the present study we have used an antibody against the adhesion molecule Fasciclin II (FasII) that is expressed in a large number of early differentiating neurons of the Drosophila embryo to follow the development of the axon tracts of the brain. The FasII antigen appears on the surface of clusters of neuronal somata prior to axon outgrowth. These clusters, for which we propose the term fibre tract founder clusters, are laid out in a linear pattern that forms an almost uninterrupted longitudinal track reaching from the ventral nerve cord to the "tip" of the brain. After expressing FasII on their soma, neurons of the fibre tract founder clusters extend axons that grow along the surface of the founder clusters and form a simple system of pioneer tracts for each of the components of the brain neuropile. We have reconstructed the FasII-positive fibre tract founder clusters and their axons from optical sections and generated digital 3-D models that illustrate the spatial relationships of the pioneer tracts. Three fibre tract founder clusters, D/T, P1, and P3m, pioneer the cervical connective. P21 and P2m form a transverse track that pioneers the supraesophageal commissure. P4m and P41/P51/VP5m form two tracts that pioneer a medial and a lateral component of the protocerebral connective, respectively. Because FasII expression continues uninterruptedly into the larval period when the "rudiments" of many parts of the adult neuropile are readily identifiable, it was possible to assign several of the embryonic pioneer tracts to definitive neuropile components, including the median bundle, antennocerebral tract, mushroom body, and posterior optic tract.

Comparison of sequence and function of the Oct-6 genes in zebrafish, chicken and mouse

To examine the role of the Oct-6 gene in Schwann cell differentiation we have cloned and characterized the chicken and zebrafish homologues of the mouse Oct-6 gene. While highly homologous in the Pit1-Oct1/2-Unc86 (POU) domain, sequence similarities are limited outside this domain. Both genes are intronless and both proteins lack the amino acid repeats that are a characteristic feature of the mammalian Oct-6 proteins. However as in mammals, the aminoterminal parts of the chicken and zebrafish Oct-6 proteins are essential for transactivation of octamer containing promoters. By immunohistochemistry we have found that the chicken Oct-6 protein is expressed in late embryonic ensheathing Schwann cells of the sciatic nerve and is rapidly downregulated when myelination proceeds. This expression profile in glial cells is identical to that in the mouse and rat. Furthermore the zebrafish Oct-6 homolog is expressed in the posterior lateral nerve at a time when it contains actively myelinating Schwann cells. Thus despite extensive primary sequence divergence among the vertebrate Oct-6 proteins, the expression of the chicken and zebrafish Oct-6 proteins is consistent with the notion that Oct-6 functions as a 'competence factor' in promyelin cells to execute the myelination program.