Voltage sensor-trapping: enhanced activation of sodium channels by beta-scorpion toxin bound to the S3-S4 loop in domain II

Polypeptide neurotoxins alter ion channel gating by binding to extracellular receptor sites, even though the voltage sensors are in their S4 transmembrane segments. By analysis of sodium channel chimeras, a beta-scorpion toxin is shown here to negatively shift voltage dependence of activation and enhance closed state inactivation by binding to a receptor site that requires glycine 845 (Gly-845) in the S3-S4 loop at the extracellular end of the S4 segment in domain II of the alpha subunit. Toxin action requires prior depolarization to drive the S4 voltage sensors outward, but these effects are lost in the mutant G845N. The results reveal a voltage sensor-trapping model of toxin action in which the IIS4 voltage sensor is trapped in its outward, activated position by toxin binding.

Scara5 is a ferritin receptor mediating non-transferrin iron delivery

Developing organs require iron for a myriad of functions, but embryos deleted of the major adult transport proteins, transferrin or its receptor transferrin receptor1 (TfR1(-/-)), still initiate organogenesis, suggesting that non-transferrin pathways are important. To examine these pathways, we developed chimeras composed of fluorescence-tagged TfR1(-/-) cells and untagged wild-type cells. In the kidney, TfR1(-/-) cells populated capsule and stroma, mesenchyme and nephron, but were underrepresented in ureteric bud tips. Consistently, TfR1 provided transferrin to the ureteric bud, but not to the capsule or the stroma. Instead of transferrin, we found that the capsule internalized ferritin. Since the capsule expressed a novel receptor called Scara5, we tested its role in ferritin uptake and found that Scara5 bound serum ferritin and then stimulated its endocytosis from the cell surface with consequent iron delivery. These data implicate cell type-specific mechanisms of iron traffic in organogenesis, which alternatively utilize transferrin or non-transferrin iron delivery pathways.

Wild bees enhance honey bees' pollination of hybrid sunflower

Pollinators are required for producing 15-30% of the human food supply, and farmers rely on managed honey bees throughout the world to provide these services. Yet honey bees are not always the most efficient pollinators of all crops and are declining in various parts of the world. Crop pollination shortages are becoming increasingly common. We found that behavioral interactions between wild and honey bees increase the pollination efficiency of honey bees on hybrid sunflower up to 5-fold, effectively doubling honey bee pollination services on the average field. These indirect contributions caused by interspecific interactions between wild and honey bees were more than five times more important than the contributions wild bees make to sunflower pollination directly. Both proximity to natural habitat and crop planting practices were significantly correlated with pollination services provided directly and indirectly by wild bees. Our results suggest that conserving wild habitat at the landscape scale and altering selected farm management techniques could increase hybrid sunflower production. These findings also demonstrate the economic importance of interspecific interactions for ecosystem services and suggest that protecting wild bee populations can help buffer the human food supply from honey bee shortages.

The role of selective transport in neuronal protein sorting

To assess whether selective microtubule-based vesicle transport underlies the polarized distribution of neuronal proteins, we expressed green fluorescent protein- (GFP-) tagged chimeras of representative axonal and dendritic membrane proteins in cultured hippocampal neurons and visualized the transport of carrier vesicles containing these proteins in living cells. Vesicles containing a dendritic protein, transferrin receptor (TfR), were preferentially transported into dendrites and excluded from axons. In contrast, vesicles containing the axonal protein NgCAM (neuron-glia cell adhesion molecule) were transported into both dendrites and axons. These data demonstrate that neurons utilize two distinct mechanisms for the targeting of polarized membrane proteins, one (for dendritic proteins) based on selective transport, the other (for axonal proteins) based on a selectivity "filter" that occurs downstream of transport.

TRPM8 voltage sensor mutants reveal a mechanism for integrating thermal and chemical stimuli

TRPM8, a member of the transient receptor potential (TRP) channel superfamily, is expressed in thermosensitive neurons, in which it functions as a cold and menthol sensor. TRPM8 and most other temperature-sensitive TRP channels (thermoTRPs) are voltage gated; temperature and ligands regulate channel opening by shifting the voltage dependence of activation. The mechanisms and structures underlying gating of thermoTRPs are currently poorly understood. Here we show that charge-neutralizing mutations in transmembrane segment 4 (S4) and the S4-S5 linker of human TRPM8 reduce the channel's gating charge, which indicates that this region is part of the voltage sensor. Mutagenesis-induced changes in voltage sensitivity translated into altered thermal sensitivity, thereby establishing the strict coupling between voltage and temperature sensing. Specific mutations in this region also affected menthol affinity, which indicates a direct interaction between menthol and the TRPM8 voltage sensor. Based on these findings, we present a Monod-Wyman-Changeux-type model explaining the combined effects of voltage, temperature and menthol on TRPM8 gating.

Chimaeric mice deficient in dystroglycans develop muscular dystrophy and have disrupted myoneural synapses

Mutations in the dystrophin gene (DMD) and in genes encoding several dystrophin-associated proteins result in Duchenne and other forms of muscular dystrophy. alpha-Dystroglycan (Dg) binds to laminins in the basement membrane surrounding each myofibre and docks with beta-Dg, a transmembrane protein, which in turn interacts with dystrophin or utrophin in the subplasmalemmal cytoskeleton. alpha- and beta-Dgs are thought to form the functional core of a larger complex of proteins extending from the basement membrane to the intracellular cytoskeleton, which serves as a superstructure necessary for sarcolemmal integrity. Dgs have also been implicated in the formation of synaptic densities of acetylcholine receptors (AChRs) on skeletal muscle. Here we report that chimaeric mice generated with ES cells targeted for both Dg alleles have skeletal muscles essentially devoid of Dgs and develop a progressive muscle pathology with changes emblematic of muscular dystrophies in humans. In addition, many neuromuscular junctions are disrupted in these mice. The ultrastructure of basement membranes and the deposition of laminin within them, however, appears unaffected in Dg-deficient muscles. We conclude that Dgs are necessary for myofibre survival and synapse differentiation or stability, but not for the formation of the muscle basement membrane, and that Dgs may have more than a purely structural function in maintaining muscle integrity.

Organization and development of the tail bud analyzed with the quail-chick chimaera system

After closure of the posterior neuropore, the caudal part of the embryo designated as the 'tail bud' forms a mass of undifferentiated cells from which the lumbosacral and caudal parts of the body develop. It has been proposed that the tail bud is a homogeneous structure comparable to a blastema (Holmdahl, 1925; Griffith et al., 1992). Another view is that morphogenesis of the tail bud is merely the continuation of the gastrulation process (Pasteels, 1937, 1943). In order to try to solve this controversy, we have studied the fate of definite and discrete regions of the tail bud at the 25-somite stage by using the quail-chick marker system. We found that the tail bud is composed of different domains endowed with a definite fate. A ventro-rostral region equivalent to the chordo-neural hinge defined by Pasteels gives rise to the notochord and floor plate and thus corresponds to the Hensen's node which in the tail bud pursues its rostrocaudal movement. The presumptive territory of the lateral walls of the lumbo-sacro-caudal neural tube is located caudally to the Hensen's node as it stands at the 25-somite stage. Material destined to form the sacral and caudal somites is still located in the dorsal midline in the caudalmost part of the tail bud. We thus show that the movements of invagination and divergence which characterize gastrulation are still going on in the tail bud after the 25-somite stage. Thus the somitic material located medio-dorsally diverges laterally and contributes by apposition to the growth of the trunco-caudal part of the body. The parallel between tail bud development in Amniotes and Amphibians as described recently by Gont et al. (1993) is striking and points to the unity in the development mechanisms within the Vertebrate phylum.

Photosynthesis limitations during water stress acclimation and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandierixV. rupestris)

The hybrid Richter-110 (Vitis berlandierixVitis rupestris) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to sustained water-withholding to induce acclimation to two different levels of water stress, followed by rewatering to induce recovery. The goal was to analyse how photosynthesis is regulated during acclimation to water stress and recovery. In particular, the regulation of stomatal conductance (g(s)), mesophyll conductance to CO(2) (g(m)), leaf photochemistry (chlorophyll fluorescence and thermoluminescence), and biochemistry (V(c,max)) were assessed. During water stress, g(s) declined to 0.1 and less than 0.05 mol CO(2) m(-2) s(-1) in moderately and severely water-stressed plants, respectively, and was kept quite constant during an acclimation period of 1-week. Leaf photochemistry proved to be very resistant to the applied water-stress conditions. By contrast, g(m) and V(c,max) were affected by water stress, but they were not kept constant during the acclimation period. g(m) was initially unaffected by water stress, and V(c,max) even increased above control values. However, after several days of acclimation to water stress, both parameters declined below (g(m)) or at (V(c,max)) control values. For the latter two parameters there seemed to be an interaction between water stress and cumulative irradiance, since both recovered to control values after several cloudy days despite water stress. A photosynthesis limitation analysis revealed that diffusional limitations and not biochemical limitations accounted for the observed decline in photosynthesis during water stress and slow recovery after rewatering, both in moderately and severely stressed plants. However, the relative contribution of stomatal (SL) and mesophyll conductance (MCL) limitations changes during acclimation to water stress, from predominant SL early during water stress to similar SL and MCL after acclimation. Finally, photosynthesis recovery after rewatering was mostly limited by SL, since stomatal closure recovered much more slowly than g(m).

Somatic sex identity is cell autonomous in the chicken

In the mammalian model of sex determination, embryos are considered to be sexually indifferent until the transient action of a sex-determining gene initiates gonadal differentiation. Although this model is thought to apply to all vertebrates, this has yet to be established. Here we have examined three lateral gynandromorph chickens (a rare, naturally occurring phenomenon in which one side of the animal appears male and the other female) to investigate the sex-determining mechanism in birds. These studies demonstrated that gynandromorph birds are genuine male:female chimaeras, and indicated that male and female avian somatic cells may have an inherent sex identity. To test this hypothesis, we transplanted presumptive mesoderm between embryos of reciprocal sexes to generate embryos containing male:female chimaeric gonads. In contrast to the outcome for mammalian mixed-sex chimaeras, in chicken mixed-sex chimaeras the donor cells were excluded from the functional structures of the host gonad. In an example where female tissue was transplanted into a male host, donor cells contributing to the developing testis retained a female identity and expressed a marker of female function. Our study demonstrates that avian somatic cells possess an inherent sex identity and that, in birds, sexual differentiation is substantively cell autonomous.

Temporal and cellular requirements for Fms signaling during zebrafish adult pigment pattern development

Ectothermic vertebrates exhibit a diverse array of adult pigment patterns. A common element of these patterns is alternating dark and light stripes each comprising different classes of neural crest-derived pigment cells. In the zebrafish, Danio rerio, alternating horizontal stripes of black melanophores and yellow xanthophores are a prominent feature of the adult pigment pattern. In fms mutant zebrafish, however, xanthophores fail to develop and melanophore stripes are severely disrupted. fms encodes a type III receptor tyrosine kinase expressed by xanthophores and their precursors and is the closest known homologue of kit, which has long been studied for roles in pigment pattern development in amniotes. In this study we assess the cellular and temporal requirements for Fms activity in promoting adult pigment pattern development. By transplanting cells between fms mutants and either wild-type or nacre mutant zebrafish, we show that fms acts autonomously to the xanthophore lineage in promoting the striped arrangement of adult melanophores. To identify critical periods for fms activity, we isolated temperature sensitive alleles of fms and performed reciprocal temperature shift experiments at a range of stages from embryo to adult. These analyses demonstrate that Fms is essential for maintaining cells of the xanthophore lineage as well as maintaining the organization of melanophore stripes throughout development. Finally, we show that restoring Fms activity even at late larval stages allows essentially complete recovery of xanthophores and the development of a normal melanophore stripe pattern. Our findings suggest that fms is not required for establishing a population of precursor cells during embryogenesis but is required for recruiting pigment cell precursors to xanthophore fates, with concomitant effects on melanophore organization.

A homing receptor-IgG chimera as a probe for adhesive ligands of lymph node high endothelial venules

The binding of lymphocytes to high endothelial venules (HEV) within peripheral lymph nodes (pln) is thought to be mediated by a lectinlike adhesion molecule termed the pln homing receptor (pln HR). The cloning and sequencing of cDNAs encoding both murine and human pln HR revealed that these adhesion molecules contain protein motifs that are homologous to C-type or calcium dependent lectin domains as well as to epidermal growth factor (egf) and complement-regulatory protein domains. We have produced a novel, antibody-like form of the murine HR by joining the extracellular region of the receptor to a human IgG heavy chain. This antibody-like molecule is capable of recognizing carbohydrates, blocking the binding of lymphocytes to pln HEV, and serving as a histochemical reagent for the staining of pln HEV. This murine HR-IgG chimera should prove useful in analyzing the distribution of the HR ligand(s) in normal as well as in inflammatory states.

Chimera analysis of the Clock mutation in mice shows that complex cellular integration determines circadian behavior

The Clock mutation lengthens periodicity and reduces amplitude of circadian rhythms in mice. The effects of Clock are cell intrinsic and can be observed at the level of single neurons in the suprachiasmatic nucleus. To address how cells of contrasting genotype functionally interact in vivo to control circadian behavior, we have analyzed a series of Clock mutant mouse aggregation chimeras. Circadian behavior in Clock/Clock <--> wild-type chimeric individuals was determined by the proportion of mutant versus normal cells. Significantly, a number of intermediate phenotypes, including Clock/+ phenocopies and novel combinations of the parental behavioral characteristics, were seen in balanced chimeras. Multivariate statistical techniques were used to quantitatively analyze relationships among circadian period, amplitude, and suprachiasmatic nucleus composition. Together, our results demonstrate that complex integration of cellular phenotypes determines the generation and expression of coherent circadian rhythms at the organismal level.

Germline chimeric chickens from dispersed donor blastodermal cells and compromised recipient embryos

Stage-X blastoderms, within intact eggs from White Leghorn hens, were exposed to 500–700 rads of gamma radiation from a 60Co source prior to injection, into the subgerminal cavity, of approximately 100 or 200–400 dispersed cells from stage-X blastoderms isolated from eggs laid by Barred Plymouth Rock hens. Embryos developing past day 14 of incubation and hatched chicks were assessed for donor and recipient cell contribution to the melanocyte population through examination of black and yellow down pigmentation, respectively (Barred Plymouth Rocks have a recessive allele at the I locus while the White Leghorns have a dominant allele at the I locus). Of the 809 embryos injected with approximately 100 cells, 192 developed past day 14 and black pigmentation, indicating somatic chimerism, was observed on 118 of the 192 (58%) embryos and chicks. Of the 296 embryos injected with 200–400 donor cells, 86 developed past day 14 of incubation. Somatic chimerism was observed on 55 of the 86 (64%) embryos and chicks. To test for germline chimerism, birds surviving to maturity were mated to Barred Plymouth Rocks. Five somatically chimeric females were produced when approximately 100 cells were injected, and one was a germline chimera. Six somatic female chimeras were produced following the injection of 200–400 cells, three of which proved to be germline chimeras by the presence of Barred Rock chicks among their offspring. Two of the nine males produced by injecting approximately 100 cells were germline chimeras.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular interactions determine neuronal phenotypes in rodent retinal cultures

Progenitor cells isolated from early rat embryo retinas differentiate into phenotypes normally generated early in retinal development (e.g., ganglion cells), whereas progenitors isolated from postnatal retinas differentiate into later-generated retinal cell types (e.g., rod photoreceptors; Reh and Kljavin, J. Neurosci. 9:4179-4189; 1989; Adler and Hatlee, 1989; Science 243:391-393; Sparrow, Hicks, and Barnstable, 1990, Dev. Brain Res. 51:69-84). To determine whether this change in committment is intrinsic to the progenitor cells, or alternatively can be modified by interactions with their developing environment, I co-cultured mouse and rat retinal cells, from different developmental stages, and identified the resulting phenotypes with species-specific and cell class-specific antibodies. I found that the phenotypes into which mouse neuroepithelial cells differentiate depends on the phenotypes of the rat cells that surround them. Retinal precursor cells from embryonic day (E) 10-12 will adopt the rod photoreceptor phenotype only when close to cells expressing this phenotype. By contrast, when the E10-12 retinal progenitor cells are cultured with cells from the cerebral cortex, they differentiate primarily into large multipolar neurons, similar in their morphology and antigen expression to retinal ganglion cells. These results indicate that interactions among the cells of the developing retina are important in the determination of cell fate.

Human iPSC-derived mature microglia retain their identity and functionally integrate in the chimeric mouse brain

Microglia, the brain-resident macrophages, exhibit highly dynamic functions in neurodevelopment and neurodegeneration. Human microglia possess unique features as compared to mouse microglia, but our understanding of human microglial functions is largely limited by an inability to obtain human microglia under homeostatic states. Here, we develop a human pluripotent stem cell (hPSC)-based microglial chimeric mouse brain model by transplanting hPSC-derived primitive macrophage progenitors into neonatal mouse brains. Single-cell RNA-sequencing of the microglial chimeric mouse brains reveals that xenografted hPSC-derived microglia largely retain human microglial identity, as they exhibit signature gene expression patterns consistent with physiological human microglia and recapitulate heterogeneity of adult human microglia. Importantly, the engrafted hPSC-derived microglia exhibit dynamic response to cuprizone-induced demyelination and species-specific transcriptomic differences in the expression of neurological disease-risk genes in microglia. This model will serve as a tool to study the role of human microglia in brain development and degeneration.

The neural tube patterns vessels developmentally using the VEGF signaling pathway

Embryonic blood vessels form in a reproducible pattern that interfaces with other embryonic structures and tissues, but the sources and identities of signals that pattern vessels are not well characterized. We hypothesized that the neural tube provides vascular patterning signal(s) that direct formation of the perineural vascular plexus (PNVP) that encompasses the neural tube at mid-gestation. Both surgically placed ectopic neural tubes and ectopic neural tubes engineered genetically were able to recruit a vascular plexus, showing that the neural tube is the source of a vascular patterning signal. In mouse-quail chimeras with the graft separated from the neural tube by a buffer of host cells, graft-derived vascular cells contributed to the PNVP,indicating that the neural tube signal(s) can act at a distance. Murine neural tube vascular endothelial growth factor A (VEGFA) expression was temporally and spatially correlated with PNVP formation, suggesting it is a component of the neural tube signal. A collagen explant model was developed in which presomitic mesoderm explants formed a vascular plexus in the presence of added VEGFA. Co-cultures between presomitic mesoderm and neural tube also supported vascular plexus formation, indicating that the neural tube could replace the requirement for VEGFA. Moreover, a combination of pharmacological and genetic perturbations showed that VEGFA signaling through FLK1 is a required component of the neural tube vascular patterning signal. Thus, the neural tube is the first structure identified as a midline signaling center for embryonic vascular pattern formation in higher vertebrates, and VEGFA is a necessary component of the neural tube vascular patterning signal. These data suggest a model whereby embryonic structures with little or no capacity for angioblast generation act as a nexus for vessel patterning.

Facultative cheater mutants reveal the genetic complexity of cooperation in social amoebae

Cooperation is central to many major transitions in evolution, including the emergence of eukaryotic cells, multicellularity and eusociality. Cooperation can be destroyed by the spread of cheater mutants that do not cooperate but gain the benefits of cooperation from others. However, cooperation can be preserved if cheaters are facultative, cheating others but cooperating among themselves. Several cheater mutants have been studied before, but no study has attempted a genome-scale investigation of the genetic opportunities for cheating. Here we describe such a screen in a social amoeba and show that cheating is multifaceted by revealing cheater mutations in well over 100 genes of diverse types. Many of these mutants cheat facultatively, producing more than their fair share of spores in chimaeras, but cooperating normally when clonal. These findings indicate that phenotypically stable cooperative systems may nevertheless harbour genetic conflicts. The opportunities for evolutionary moves and countermoves in such conflicts may select for the involvement of multiple pathways and numerous genes.

Chick/quail chimeras with partial cerebellar grafts: an analysis of the origin and migration of cerebellar cells

Chick/quail chimeras with partial cerebellar grafts have been performed to obtain further information about the origin and migratory movements of cerebellar cortical neurons. The grafts were performed by exchanging between these two species a precise, small portion of the E2 cerebellar primordium, as defined in Martinez and Alvarado-Mallart (Eur. J. Neurosci. 1:549-560, 1989). All grafts were done unilaterally. The chimeric cerebella, fixed at various developmental stages, were analyzed in serial Feulgen-stained preparations to map the distribution of donor and host cells in the ependymal layer (considered to be reminiscent of the primary germinative neuroepithelium) and in the various cortical layers. In some of the oldest cases, we also used antiquail immunostaining to recognize quail cells. In the ependymal layer, it has been possible to conclude that each hemicerebellar primordium undergoes a morphogenetic rotation that changes its rostrocaudal axis to a rostromedio-caudolateral direction. However, important individual variations were observed among the chimeric embryos with respect to the ependymal area expected to be formed by donor cells. These variations cannot be explained solely on the basis of microsurgical procedure; however, they suggest the existence of important reciprocal interaction between host and grafted neuroepithelia. Therefore, it was not possible to draw a precise fate map of the E2 cerebellar primordium. Nevertheless, the dispersion of grafted cells in the cerebellar cortex, when compared to the real extent of the ependymal grafted area in each particular case, provided important data: (1) The external granular layer (EGL), the secondary germinative epithelium, seems not to originate exclusively from the "germinative trigone," as is usually considered the case. It emerges from a larger but restricted portion of the primary cerebellar matrix extending about the caudal fourth or third of the ventricular epithelium, as defined after its morphogenetic rotation. (2) The Purkinje cells (PCs) develop from all areas of the cerebellar epithelium. Although the distribution of donor PCs parallels the grafted ventricular layer mediolaterally, donor PCs extend more in the rostrocaudal dimension. The PC layer is formed mainly by donor cells in the lobules underlain by the grafted ependymal layer. However, donor PCs are also observed in cortical lobules surmounting the host ventricular layer. In these lobules, the donor PCs form clusters of various widths interrupting the host PCs. Reciprocally, clusters of host PCs are also found in the lobules formed mainly by donor PCs. The alternate small clusters of donor or host PCs are surrounded by Bergmann fibers of the other species' origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation of pluripotent stem cells from cultured porcine primordial germ cells

Embryonic germ (EG) cells are undifferentiated stem cells isolated from cultured primordial germ cells (PGC). To date, EG cells have been isolated only in the mouse. Murine EG cells share several characteristics with embryonic stem (ES) cells, including morphology, pluripotency, and the capacity for germline transmission. We report here the isolation of porcine EG cells. PGC collected from Day 24 or 25 porcine embryos were cultured on mitotically inactivated murine fibroblasts. Four EG cell lines were isolated from repeated subculture of porcine PGC. Porcine EG cells morphologically resembled murine ES cells and consistently expressed alkaline phosphatase activity. These cell lines maintained a normal diploid karyotype and survived after cryopreservation. Porcine EG cells were capable of differentiating into a wide range of cell types in culture, including endodermal, trophoblast-like, epithelial-like, fibroblast-like, and neuron-like cells. In suspension culture, porcine EG cells formed embryoid bodies. When injected into host blastocysts, the EG cells were able to differentiate and contribute to tissues of a chimeric piglet. Both in vitro and in vivo evidence demonstrates that the isolated EG cells were pluripotent. These cells are potentially useful for genetic manipulation in pigs.

Staggered cell-intrinsic timing of ath5 expression underlies the wave of ganglion cell neurogenesis in the zebrafish retina

In the developing nervous system, progenitor cells must decide when to withdraw from the cell cycle and commence differentiation. There is considerable debate whether cell-extrinsic or cell-intrinsic factors are most important for triggering this switch. In the vertebrate retina, initiation of neurogenesis has recently been explained by a 'sequential-induction' model--signals from newly differentiated neurons are thought to trigger neurogenesis in adjacent progenitors, creating a wave of neurogenesis that spreads across the retina in a stereotypical manner. We show here, however, that the wave of neurogenesis in the zebrafish retina can emerge through the independent action of progenitor cells--progenitors in different parts of the retina appear pre-specified to initiate neurogenesis at different times. We provide evidence that midline Sonic hedgehog signals, acting before the onset of neurogenesis, are part of the mechanism that sets the neurogenic timer in these cells. Our results highlight the importance of intrinsic factors for triggering neurogenesis, but they also suggest that early signals can modulate these intrinsic factors to influence the timing of neurogenesis many cell cycles later, thereby potentially coordinating axial patterning with control of neuron number and cell fate.

Selective dispersal of avian rhombomere cells in orthotopic and heterotopic grafts

During hindbrain development, cells become segregated into segmental groups, rhombomeres, by mechanisms that are presently unknown. One contributory factor early in development may be an alternating periodicity in cell surface properties down the neuraxis. This possibility was previously suggested by experiments in which tissue from different segmental levels was apposed in the absence of a boundary. New boundaries were regenerated only when rhombomeres from adjacent positions or positions three rhombomeres distant from one another were apposed. Combinations of two odd-numbered or two even-numbered rhombomeres usually failed to generate a boundary. In order to pursue this phenomenon to the cellular level, we have used two approaches, both involving donor-to-host transplantation. First, quail rhombomeres were grafted at various hindbrain levels of a chick host. Apposition of rhombomere 4 (r4) with r3 was concomitant with negligible cell mixing across the interface. By contrast, combinations of r3 with r5 or with r3 tissue led to cell mixing that was more extensive in combinations of identical rhombomeres (r3 with r3) than between two alternate ones (r3 with r5). Secondly, we grafted small pieces of fluorescently prelabelled chick rhombomere tissue at various hindbrain levels of chick hosts. In most cases, cells dispersed widely when transplanted orthopically or two segments distant from that of their origin. Cells transplanted into an adjacent segment, however, showed a tendency to remain undispersed. Among the different graft combinations, furthermore, there was a variation in the extent of dispersal that showed an additional level of complexity not revealed in boundary regeneration experiments. The possibility is raised that the early partitioning of rhombomeres involves a hierarchy in the adhesive preferences of cell-cell interactions along the neuraxis.

Derivation and characterization of pluripotent embryonic germ cells in chicken

Embryonic germ (EG) cell lines established from primordial germ cells (PGCs) are undifferentiated and pluripotent stem cells. To date, EG cells with proven germ-line transmission have been completely established only in the mouse with embryonic stem (ES) cells. We isolated PGCs from 5.5-day-old (stage 28) chicken embryonic gonads and established a putative chicken EG cell line with EG culture medium supplemented with stem cell factor (SCF), leukemia inhibitory factor (LIF), basic fibroblast growth factor (bFGF), interleukin-11 (IL-11), and insulin-like growth factor-I (IGF-I). These cells grew continuously for ten passages (4 months) on a feeder layer of mitotically active chicken embryonic fibroblasts. After several passages, these cells were characterized by screening with the periodic acid-Schiff reaction, anti-SSEA-1 antibody, and a proliferation assay. The chicken EG cells maintained characteristics of gonadal PGCs and undifferentiated stem cells. When cultured in suspension, the chicken EG cells successfully formed an embryoid body and differentiated into a variety of cell types. The chicken EG cells were injected into stage X blastodermal layer and produced chimeric chickens with various differentiated tissues derived from the EG cells. Chicken EG cells will be useful for the production of transgenic chickens and for studies of germ cell differentiation and genomic imprinting.

The costs and benefits of being a chimera

Most multicellular organisms are uniclonal. This is hypothesized to be because uniclonal organisms function better than chimeras (non-clonal organisms), owing to reduced levels of internal genetic conflict. We tested this idea using the social amoeba or slime mold Dictyostelium discoideum. When starving, the normally solitary amoebae aggregate to form a differentiated multicellular slug that migrates towards light and forms a fruiting body, facilitating the dispersal of spores. We added 10(7) amoebae to Petri plates containing 1, 2, 5 or 10 clones mixed together. We found an intrinsic cost to chimerism: chimeric slugs moved significantly less far than uniclonal slugs of the same size. However, in nature, joining with other clones to form a chimera should increase slug size, and larger slugs travel further. We incorporated this size effect into a second experiment by giving chimeras more cells than single clones (single clones had 10(6) cells, two-clone chimeras had 2 x 10(6) cells and so on). The uniclonal treatments then simulated a clone in a mixture that refuses to form chimeras. In this experiment, chimeras moved significantly further than the uniclonal slugs, in spite of the intrinsic cost. Thus, chimerism is costly, which may be why it evolves so seldom, but in D. discoideum the benefits of large size appear to compensate.

Establishment of rostrocaudal polarity in tectal primordium: engrailed expression and subsequent tectal polarity

In the E4 (embryonic day 4) chick tectal primordium, engrailed expression is strong at the caudal end and gradually weakens toward the rostral end. We used quail-chick chimeric tecta to investigate how the caudorostral gradient of engrailed expression is established and whether it is correlated with the subsequent rostrocaudal polarity of tectal development. To examine the positional value of the tectal primordium, we produced ectopic tecta in the diencephalon by transplanting a part of the mesencephalic alar plate heterotopically. In the ectopic tectum, the gradient of the engrailed expression reversed and the strength of the expression was dependent on the distance from the mes-diencephalon junction; the nearer the ectopic tectum was to the junction, the weaker the expression was. Consequently, the pattern of the engrailed expression in the host and ectopic tecta was nearly a mirror image, suggesting the existence of a repressive influence around the mes-diencephalon junction on the engrailed expression. We examined cytoarchitectonic development in the ectopic tecta, which normally proceeds in a gradient along the rostrocaudal axis; the rostral shows more advanced lamination than the caudal. In contrast, the caudal part of the ectopic tecta (near to the mes-diencephalon junction) showed more advanced lamination than the rostral. In both the host and ectopic tecta, advanced lamination was observed where the engrailed expression was repressed, and vice versa. Next we studied the correlation between engrailed expression and retinotectal projection from a view of plasticity and rigidity of rostrocaudal polarity in the tectum. We produced ectopic tecta by anisochronal transplantations between E3 host and E2 donor, and showed that there is little repressive influence at E3 around the mes-diencephalon junction. We then made chimeric double-rostral tectum (caudal half of it was replaced by rostral half of the donor tectum) or double-caudal tectum at E3. The transplants kept their original staining pattern in hosts. Consequently, the chimeric tecta showed wholly negative or positive staining of engrailed protein on the grafted side. In such tecta retinotectal projection pattern was disturbed as if the transplants retained their original position-specific characters. We propose from these heterotopic and anisochronal experiments that the engrailed expression can be a marker for subsequent rostrocaudal polarity in the tectum, both as regards cytoarchitectonic development and retinotectal projection.

A cell autonomous function of Brachyury in T/T embryonic stem cell chimaeras

Developmental genetics has shown that the Brachyury (T) gene has a key role in mesoderm formation during gastrulation in the mouse. Homozygous embryos have a defective allantois, degenerate or absent notochord and disrupted primitive streak and node. The neural tube is kinked and somite formation interrupted. The T gene has been cloned and is expressed during the early stages of gastrulation, being restricted to the primitive streak region, nascent mesoderm and notochord. Neither the sequence of the gene nor its expression pattern define its developmental function. To study the cell autonomy of the T mutation we have isolated and genetically characterized embryonic stem cell lines and studied their behaviour in chimaeras. T/+ embryonic stem cells form normal chimaeras, whereas T/T in equilibrium with +/+ chimaeras mimic the T/T mutant phenotype. The results indicate that the T gene acts cell autonomously in the primitive streak and notochord but may activate a signalling pathway involved in the specification of other mesodermal tissues.