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PubMed2005

Asymmetrical competition and patterns of abundance of Aedes albopictus and Culex pipiens (Diptera: Culicidae).

We tested for competitive advantage among larvae of Aedes albopictus (Skuse) and Culex pipiens L. in a laboratory experiment and determined the frequency and spatial and temporal patterns of co-occurrence in the field in East St. Louis, IL. In a laboratory competition experiment at multiple combined densities of Ae. albopictus and Cx. pipiens larvae, Ae. albopictus survivorship and developmental times were significantly affected by conspecific densities but not by Cx. pipiens densities. In contrast, Cx. pipiens survivorship and developmental times were significantly affected by both conspecific and Ae. albopictus densities. Per capita rate of increase (r') for Ae. albopictus cohorts declined significantly due to density of conspecifics, but not density of Cx. pipiens. Interspecific competition between Ae. albopictus and Cx. pipiens under these laboratory conditions was strong and asymmetrical, with the effect of Ae. albopictus on Cx. pipiens much stronger than the reverse. In monthly samples from tire sites in East St. Louis, Ae. albopictus was highly seasonal, occurring in relatively low abundance from early May to July and increasing in abundance in August and September. Co-occurrence corresponded to the seasonality of Ae. albopictus, with Cx. pipiens encountering Ae. albopictus in more tires and at higher numbers within a tire, in August and September. Abundance of both species was high in residential areas and was unrelated to overstory cover, total nitrogen, and total phosphorus. Abundance of Ae. albopictus, but not of Cx. pipiens, was positively associated with conductivity. We expect Cx. pipiens to suffer from the effects of interspecific competition in tires in which it encounters Ae. albopictus. Interspecific competition between these species may be of both ecological and medical importance.

Aedes↗
PubMed1999

Mannose receptor is expressed in normal and dystrophic retinal pigment epithelium.

In normal retinas, the phagocytosis of shed photoreceptor outer segments is mediated in part through a mannose receptor protein located in the apical retinal pigment epithelium membrane. As dystrophic rats of the Royal College of Surgeons have a defect in which the retinal pigment epithelium (RPE) is unable to phagocytize the shed outer segments, it is hypothesized that mannose receptor expression will be lost with the progression of photoreceptor degeneration. Immunohistochemical and molecular techniques have been used to study the developmental expression of the mannose receptor in normal and dystrophic retinal pigment epithelium. By immunofluorescence, the mannose receptor is localized to the retinal pigment epithelium, apical membrane region, beginning around 5 days postnatally in both normal and dystrophic retinas. In immunoblots, bands at 175 kDa are labelled by an anti-mannose receptor antibody in apical membrane samples from both normal and dystrophic RPE at all developmental times sampled. RT-PCR analysis reveals that mannose receptor message is present in normal and dystrophic RPE samples at all developmental time points examined. The present study demonstrates that the expression of the mannose receptor begins prior to outer segment differentiation and the initiation of phagocytosis in both normal and dystrophic RPE. Expression of the mannose receptor continues to be unchanged during the progression of photoreceptor degeneration in the dystrophic retina.

Animals↗
PubMed2004

Octodon degus: a diurnal, social, and long-lived rodent.

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.

Age Factors↗
PubMed1985

Perinatal development of the liver in rat and spiny mouse. Its relation to altricial and precocial timing of birth.

Rat (Rattus norvegicus) and spiny mouse (Acomys cahirinus) are closely related murinoid species that mainly differ in the developmental timing of birth. A comparison between the developmental profiles of some characteristic enzymes of the liver of both species was carried out to elucidate the question to what extent are these enzymic profiles and hence the maturation of the liver related to the timing of birth? It was found that these organotypic enzymes first become detectable at the same developmental stage in both species. Likewise, the weaning phase of the enzymic profiles occurs at the same developmental time point in both species. It is argued that both the first appearance and the weaning increase in enzyme activity levels occur at endogenously programmed timepoints with only superimposed effects of hormones. In contrast, the perinatal phase of the enzymic profile is completely dependent on the developmental timing of birth and therefore appears not to be anchored to a particular developmental timepoint, but rather to be dependent on birth-associated (hormonal) adaptation. In accordance with this hypothesis it was found that the morphological development of the liver proceeded independent of the timing of birth. Furthermore, the hormonal regulation of the investigated enzymes was found to be the same in both species. Despite the more advanced state of morphological development of the liver in the spiny mouse at birth, it was found that the inducibility of organotypic gene expression by hormones in spiny mouse fetuses was as limited as in rat fetuses. This observation therefore suggests that the intra-uterine environment is responsible for the limited inducibility of enzymes before birth.

Animals↗
PubMed2003

Cloning, developmental regulation and neural localization of rat epsilon-sarcoglycan.

Mutations in the gene for epsilon sarcoglycan (epsilon-SG) are associated with a disorder of the central nervous system, the myoclonus-dystonia syndrome (MDS; DYT11). In contrast, mutations of other sarcoglycan family members lead to limb-girdle muscular dystrophies. To establish the framework for functional studies of epsilon-SG, we cloned rat epsilon-SG cDNA, quantified epsilon-SG mRNA levels in neural and non-neural tissues at different developmental time points with relative quantitative multiplex real-time reverse transcriptase PCR (RT-PCR), and characterized the distribution of epsilon-SG mRNA in brain with in situ hybridization. Rat epsilon-SG cDNA contains an open reading frame (ORF) of 1311 bp that encodes a 437-amino acid (aa) protein with 95.9% and 98.2% identity to human and mouse epsilon-SG amino acid sequences, respectively. Using real-time RT-PCR, epsilon-SG was detected in both neural (cerebellar cortex, striatum, cerebral cortex, thalamus, hippocampus) and non-neural (muscle, liver, kidney, heart) tissues at each developmental time point tested [Embryonic Day 20 (E20), Postnatal Day 1 (P1), P7, P14, P36, 6 months, 1.5 years). Levels of epsilon-SG mRNA were highest at E20 in all tissues. The developmental regulation of epsilon-SG mRNA expression was most striking in muscle with E20 and early postnatal epsilon-SG mRNA levels over 10 times higher than those seen in adult rats. In adult rats, epsilon-SG mRNA levels were several-fold higher in brain, particularly cerebellar cortex, than in muscle. Radioactive in situ hybridization showed that epsilon-SG mRNA was widely distributed in rat brain. Robust hybridization signal was obtained from regions with dense neuronal packing such as the hippocampus, cerebellar molecular layer, and cerebral cortex. Our results suggest that epsilon-SG participates in the development of both neural and non-neural tissues and contributes to neuronal structure in the adult central nervous system.

Animals↗
PubMed2005

[Effects of glyphosate on life histroy characteristics of freshwater rotifer Brachionus calyciflorus].

The life table study on the life history characteristics of freshwater rotifer Brachionus calyciflorus under effects of different concentrations glyphosate showed that glyphosate had significant effects on the embryonic developmental time, duration of juvenile and reproductive periods, average lifespan, net reproductive rate, and intrinsic population increasing rate of the rotifer. The embryonic developmental time was significantly lengthened when exposed to 3 and 8 mg x L(-1) glyphosate, the juvenile period began to be lengthened significantly when exposed to 3 mg x L(-1) glyphosate, while the net reproductive rate and intrinsic population increasing rate decreased significantly when the rotifer was exposed to 8.00 and 10.50 mg x L(-1) glyphosate, respectively. Among all the parameters, intrinsic population increasing rate was the most sensitive parameter which could be used in monitoring the effects of glyphosate on the life history characteristics of B. calyciflorus.

Animals↗
PubMed2002

Evaluation of turfgrass species and cultivars for potential resistance to twolined spittlebug (Hemiptera: Cercopidae).

Potential resistance to the twolined spittlebug, Prosapia bicincta (Say), was evaluated among 56 turfgrass genotypes. Greenhouse, laboratory, and field bioassays identified differences in spittlebug survival and development, host preference and damage levels, and turfgrass tolerance to and ability to recover from pest induced injury. All centipede grasses demonstrated high levels of susceptibility, followed by bermudagrasses, seashore paspalums, and zoysiagrasses. Average nymphal survival to the adult stage ranged from 1.5 to 78.1%. Development required 38.1-62.0 d under greenhouse conditions, depending on plant taxa. Among seashore paspalums, nymphal survival to the adult stage was lowest and duration of development was longest on HI-1, 'Sea Isle 2000', 561-79, and 'Mauna Kea'. Reduced spittlebug survival and increased developmental times were also observed on the bermudagrasses BERPC 91-15 and 'Tifway'. Although zoysiagrasses supported spittlebug development and survival to the adult stage, developmental times were extended on the zoysiagrass cultivars 'Emerald' and 'El Toro'. Spittlebug preference varied with generation evaluated. First-generation spittlebugs inflicted the greatest damage on TC201 (centipede grass), 'Primavera' (bermudagrass), and 'Emerald' (zoysiagrass) in choice tests. In the fall, second-generation spittlebugs damaged TC201 (centipedegrass) and 'Sea Isle 1' (paspalum) most severely, whereas 561-79 (paspalum) and 'Emerald'(zoysiagrass) were less severely affected. Among taxa included in field trials, HI-1, 'Mauna Kea', 'Sea Isle 2000',and AP-14 paspalums, 'Tifway' bermudagrass, and 'Emerald' zoysiagrass were most tolerant (demonstrated the best regrowth potential following twolined spittlebug feeding).

Animals↗
PubMed2006

Effect of corn hybrids expressing the coleopteran-specific cry3Bb1 protein for corn rootworm control on aboveground insect predators.

Field and laboratory studies were conducted to determine the effect of transgenic Bacillus thuringiensis (Bt) corn, Zea mays L. (YieldGard Rootworm), expressing the Cry3Bb1 protein on aboveground nontarget insect predators (minute pirate bug, ladybird beetles, and carabids). Visual counts of adult and immature Orius insidiosus (Say), Coleomegilla maculata (DeGeer), Hippodamia convergens Gurin-Meneville, and Scymnus spp. occurring in Bt corn and its non-Bt isoline were made at Manhattan, KS, in 2002 and at Manhattan and Scandia, KS, in 2003. No significant differences were found between the Bt corn and non-Bt isoline plots in the abundance (number per plant) of O. insidiosus, C. maculata, H. convergens, and Scymnus spp. Field predation on Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) egg masses was also observed during the silking stage of corn at Manhattan and Scandia in 2003. No significant differences were observed among treatments in predation rate for predators with chewing versus sucking mouthparts. Two laboratory studies determined the effect of Cry3Bb1 protein expressed in Bt corn pollen on C. maculata and carabids. The larvae of C. maculata were reared on Bt pollen, non-Bt pollen, or greenbugs, Schizaphis graminum (Rondani). The duration of larval and pupal stages, developmental time from egg hatch to adult emergence, percentage of survival, and elytra length were compared among treatments. There were no significant differences in developmental time of larvae fed pollen or greenbugs during their first two instars. However, significantly prolonged development of the third (1 d) and fourth instars (2 d) was observed for larvae fed greenbugs only. Total time for larval development was significantly longer for larvae that fed on greenbugs versus larvae fed on pollen. No significant differences were observed among treatments in the percentage of larvae that pupated or pupal stage duration. Larvae that fed on greenbugs had higher pupal and adult weights compared with pollen-fed larvae. However, pupal and adult weights did not vary between the Bt and non-Bt pollen treatments. No significant differences occurred in longevity and elytra length of beetles among all treatments. Two carabid species, Harpalus caliginosus F. and Harpalus pensylvanicus DeGeer, were reared on moistened dog food sprinkled with Bt or non-Bt corn pollen. No significant differences in mortality of H. caliginosus and H. pensylvanicus were detected among any of the treatments. There was no significant effect of Bt pollen on fecundity and egg viability of H. caliginosus. Our studies showed that YieldGard Rootworm had no effect on the selected coleopteran predators; therefore, this Bt corn hybrid could be used in an integrated pest management system.

Animals↗
PubMed2003

Effects of GNA transgenic sugarcane on life history parameters of Parallorhogas pyralophagus (Marsh) (Hymenoptera: Braconidae), a parasitoid of Mexican rice borer.

Parallorhogas pyralophagus (Marsh) is the principal parasitoid of the Mexican rice borer [Eoreuma loftini (Dyar)], the primary pest of sugarcane in south Texas. Insect resistant transgenic sugarcane producing Galanthus nivalis agglutinin (GNA) was developed to improve control of E. loftini. The present laboratory study addressed whether over two consecutive generations transgenic sugarcane delivered via artificial diet has adverse host-mediated effects on P. pyralophagus life history parameters. The results of this study showed that a number of life history parameters were affected by transgenic sugarcane, whereas others were not affected, and that effects varied between generations. In the first generation, adult longevity was increased by approximately 2 d, and cocoon to adult and egg to adult developmental times were prolonged by approximately 1 d in parasitoids exposed to transgenic sugarcane, whereas effects were not evident on adult size; egg load; egg to cocoon developmental time; rates of gain of longevity and egg load with adult size; and egg, larval, and pupal mortality. However, in the second generation, adult longevity was reduced by approximately 3 d, adult size by approximately 5%, egg load by approximately 24%, and rate of gain of longevity with adult size by approximately 21%, whereas effects were not evident on the rate of gain of egg load with adult size. It was concluded that although GNA transgenic sugarcane, ingested via E. loftini tissues, was not acutely toxic to P. pyralophagus, the sublethal effects on life history parameters measured in this study must be considered in a broader context to determine their possible ecological significance.

Animals↗
PubMed2006

Cadherin-based resistance to Bacillus thuringiensis cotton in hybrid strains of pink bollworm: fitness costs and incomplete resistance.

Recessive resistance to Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., in laboratory-selected strains of pink bollworm, Pectinophora gossypiella (Saunders), is associated with three resistance alleles (r1, r2, and r3) of a cadherin gene. Previous experiments based on measurement of fitness components in Bt-resistant and Bt-susceptible strains revealed that fitness costs and incomplete resistance are associated with resistance. Here, we used two hybrid strains of pink bollworm, each containing a mixture of susceptible and resistant individuals, and polymerase chain reaction (PCR) amplifications to test the association between cadherin genotype and fitness components for individuals sharing a common genetic background. All survivors on Bt cotton had two r alleles, confirming that recessive cadherin alleles are tightly linked with resistance to Bt cotton. On non-Bt cotton, significantly greater developmental time for rr than ss larvae indicated a recessive fitness cost, but costs did not affect survival or pupal weight. Incomplete resistance was manifested as longer developmental time, lower survival, and smaller pupal weight in rr individuals developing on Bt cotton compared with non-Bt cotton. As in previous experiments, no significant variation in performance on Bt cotton was detected among rr genotypes. However, a meta-analysis of data from seven experiments revealed that survival on Bt cotton relative to non-Bt cotton was lower in r2r3 and higher in r1r2 compared with the other rr genotypes. Assessment of fitness components associated with cadherin genotypes in hybrid strains of pink bollworm confirms that recessive resistance to Bt cotton is associated with recessive fitness costs and incomplete resistance.

Animals↗
PubMed1994

Development rate and mortality of immature Parasarcophaga (Liopygia) ruficornis (Diptera: Sarcophagidae) at constant laboratory temperatures.

Developmental time of Parasarcophaga ruficornis (F.) was determined at constant temperatures of 13, 16, 19, 22, 25, 28, 31, 34, and 37 degrees C. Estimates of the lower developmental threshold temperatures (tL) were 10.5, -7.0, 6.3, 8.8, and 7.7 degrees C and the thermal constants (K) were 65.15, 111.93, 152.13, 229.66, and 388.22 degree-days (DD) for feeding larvae, wandering larvae, larvae (feeding + wandering), pupae, and total developmental time, respectively. Flies reached their maximum weight in pupal and adult stages at 25 and 28 degrees C, whereas minimal weights were recorded at 16 and 37 degrees C. The optimal temperature in terms of rapid development, low mortality, and greatest weight ranged between 22 and 28 degrees C.

Animals↗
PubMed1992

Developmental expression of the proenkephalin and prosomatostatin genes in cultured cortical and cerebellar astrocytes.

Astrocytes were prepared from rats of 4 ages, embryonic day 20, postnatal days 3 and 8, and adult, in order to study the developmental time course of expression of enkephalin and somatostatin (SS). Glial fibrillary acidic protein (GFAP) content was constant in both cortical and cerebellar astrocytes prepared from all ages. SS mRNA and peptide decreased over this developmental time course in cerebellar astrocytes; the time course of changes in SS mRNA paralleled that for rat cerebellum. Proenkephalin (PE) mRNA increased about 3-fold in cerebellar astrocytes from embryonic day 20 to adult but remained constant in cortical astrocytes; in contrast, PE mRNA showed a 10- to 12-fold increase in rat cerebellum and cortex developmentally. For both cerebellar and cortical astrocytes, free met-enkephalin decreased from embryonic day 20 to adult, whereas total met-enkephalin (measured following trypsin-carboxypeptidase B digestion of the extracts) increased. These results suggest (1) that there is a developmental regulation of the expression of both enkephalin and SS peptides in astrocytes, and (2) that the regulation occurs at the level of transcription for SS but at the level of precursor processing for PE. Possible trophic functions for astrocyte-derived peptides early in CNS development are discussed.

Aging↗
PubMed2005

Effects of Bt maize on the herbivore Spodoptera littoralis (Lepidoptera: Noctuidae) and the parasitoid Cotesia marginiventris (Hymenoptera: Braconidae).

Recent studies have shown that transgenic insect resistant plants can have negative effects on non-target herbivores as well as on beneficial insects. The study of tritrophic interactions gives insight into the complex mechanisms of food webs in the field and can easily be incorporated into a tiered risk assessment framework. We investigated the effects of transgenic maize (Zea mays) expressing insecticidal proteins derived from Bacillus thuringiensis (Bt maize) on Spodoptera littoralis, a non-target herbivore, and on the hymenopteran parasitoid Cotesia marginiventris. In a laboratory study, S. littoralis larvae were reared for their whole lifespan on a mixture of leaves and stems from 2-4-week old Bt maize plants. S. littoralis survival, developmental times and larval weights were significantly affected by Bt maize diet. However, adult moths, which survived development on Bt maize, were the same size as the adults from the control group. C. marginiventris survival, developmental times and cocoon weights were significantly negatively affected if their S. littoralis host larva had been fed Bt maize. ELISA tests confirmed that S. littoralis larvae ingest high amounts of CrylA(b) toxin while feeding on Bt maize. In S. littoralis pupae and in C. marginiventris cocoon silk, only traces of the toxin could be detected. No toxin was found in S. littoralis and C. marginiventris adults. Thus the toxin is not accumulating in the trophic levels and in fact appears to be excreted. Our results suggest that the effects on C. marginiventris when developing in susceptible S. littoralis larvae are indirect (host mediated). The biological relevance of those results and the significance of this study in risk assessment are discussed.

Animals↗
PubMed2005

Temperature effects on development and survival of two stable flies, Stomoxys calcitrans and Stomoxys niger niger (Diptera: muscidae), in La Réunion Island.

Two stable fly species, Stomoxys calcitrans (L., 1758) and Stomoxys niger niger Macquart, 1851, co-occur in La Reunion, where they are important pests of cattle. The survival and developmental rate of the immature stages were compared at five constant temperatures from 15 to 35 degrees C. In both species, immature survival was highest at 20-25 degrees C and markedly decreased at 15 and 35 degrees C. At the lower temperatures, mortality was observed mainly for S. calcitrans larvae and S. niger eggs. At the higher temperatures, mainly pupae of both species died. At all temperatures, S. calcitrans survived better than S. niger. Developmental time was highly similar in both species, decreasing from 71 d at 15 degrees C to 13 d at 30 degrees C in S. calcitrans and from 69 d at 15 degrees C to 14 d at 30 degrees C in S. niger. Developmental times increased slightly at 35 degrees C. Surprisingly, the tropical S. niger developed slightly faster than the cosmopolitan S. calcitrans at 15-20 degrees C; the reverse was found at higher temperatures. Temperature summation models confirmed that S. niger had a lower developmental threshold than S. calcitrans (11.3 versus 12.2 degrees C) and higher day-degree (DD) requirements to complete development (251 versus 225 DD). Overall, the results suggest that S. calcitrans is better adapted than S. niger, in terms of adult production from eggs, in the temperature range of La Reunion.

Animals↗
PubMed1993

The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. V. Female phenotypic expression on natural genetic backgrounds and in natural environments.

The abnormal abdomen (aa) syndrome in Drosophila mercatorum depends on the presence of R1 inserts in a third or more of the X-linked 28S rDNA genes and the absence of selective underreplication of inserted repeats in polytene tissues that is controlled by an X-linked locus (ur) half a map unit from the rDNA complex. This syndrome affects both life history and morphology in the laboratory. Because abnormal morphologies are rarely encountered in nature, the purpose of this study is to see if the female life history traits are still affected under more natural genetic backgrounds and environmental conditions. Two outbred stocks were extracted from the natural population living near Kamuela, Hawaii: KaaX that has only X chromosomes with uraa alleles, and K+X that has only ur+ alleles. These two stocks have nonoverlapping distributions of insert proportions, indicating strong disequilibrium between the ur locus and the rDNA complex. The KaaX stock had almost no morphological penetrance of uraa, indicating that genetic background is important. KaaX expressed longer female egg-to-adult developmental times, increased early adult female fecundity, and decreased female adult longevity compared with K+X. By bagging natural rots of the cactus Opuntia megacantha near Kamuela, Hawaii, it was shown that egg-to-adult developmental time is slowed down by 0.92 days in females bearing uraa alleles in nature, with no detectable slowdown in uraa males. The bagged rot data also indicate that females bearing uraa alleles have a strong fecundity advantage in nature under some ecological conditions but not others.

Abdomen↗
PubMed1974

A quantitative study of synapses on motor neuron dendritic growth cones in developing mouse spinal cord.

The proportion of synaptic contacts occurring on dendrites as well as on dendritic growth cones and filopodia was determined from electron micrographs of developing mouse (C57BL/6J) spinal cord. Comparable areas of the marginal zone adjacent to the lateral motor nucleus were sampled from specimens on the 13th-16th days of embryonic development (E13-E16). At the beginning of this period, synapses upon growth cones and filopodia comprise about 80% of the observed synaptic junctions, but this proportion decreases with developmental time so that in E16 specimens growth cone synapses account for slightly less than 30% of the synaptic population. Conversely, at E13, synapses upon dendrites comprise less than 20% of the total number of synapses, but increase with developmental time so that they account for about 65% of the synaptic population of E16 specimens. From these data, we suggest the following temporal sequence for the formation of synaptic junctions on motor neuron dendrites growing into the marginal zone. New synapses are initially made upon the filopodia of dendritic growth cones. A synaptically contacted filopodium expands to become a growth cone while the original growth cone begins to differentiate into a dendrite. This process is repeated as the dendrite grows farther into the marginal zone so that synapses originally made with filopodia come to be located upon dendrites. This speculation is briefly discussed in relation to the work and ideas of others concerning synaptogenesis and dendritic development.

Aging↗
PubMed2002

Induction of thymocyte positive selection does not convey immediate resistance to negative selection.

The acquisition of functional competence represents a critical phase during intrathymic development of T cells. Thymocytes reaching this stage represent cells which have been positively selected on the basis of major histocompatibility complex reactivity, but which have also been purged of potentially autoreactive T-cell receptor specificities by negative selection. While the developmental window in which thymocytes are subjected to positive selection is now well defined, the precise developmental timing of negative selection, in relation to positive selection events, is less clear. Moreover, the underlying mechanism allowing single-positive thymocytes to respond to T-cell receptor ligation by activation rather than death, remains controversial. Here we have analysed the developmental timing of negative selection in relation to positive selection, using measurement of thymocyte susceptibility to dendritic cell presentation of the superantigen staphylococcal enterotoxin B (SEB). We show that thymocytes which have received initial positive selection signals, namely CD4+ CD8+ CD69+ thymocytes, like their CD4+ CD8+ CD69minus sign precursors, are susceptible to negative selection, indicating that induction of positive selection does not convey immediate resistance to negative selection. In contrast, newly generated CD4+ CD8minus sign CD69+ cells are not only resistant to deletion by SEB, but respond to SEB-mediated T-cell receptor-ligation by activation, indicating that the acquisition of functional competence occurs at the newly generated CD4+ CD8minus sign CD69+ stage. Finally, by using direct retroviral infection of primary CD4+ CD8+ thymocytes, we also show that Notch-1 activation in CD4+ CD8+ thymocytes does not correlate with, nor convey resistance to superantigen-mediated negative selection. Thus, our data suggest that although Notch-1 has been implicated in resistance to thymocyte apoptosis, the acquisition of resistance to negative selection occurs independently of Notch-1 signalling.

Animals↗
PubMed2000

Timing of developmentally programmed excision and circularization of Paramecium internal eliminated sequences.

Paramecium internal eliminated sequences (IESs) are short AT-rich DNA elements that are precisely eliminated from the germ line genome during development of the somatic macronucleus. They are flanked by one 5'-TA-3' dinucleotide on each side, a single copy of which remains at the donor site after excision. The timing of their excision was examined in synchronized conjugating cells by quantitative PCR. Significant amplification of the germ line genome was observed prior to IES excision, which starts 12 to 14 h after initiation of conjugation and extends over a 2- to 4-h period. Following excision, two IESs were shown to form extrachromosomal circles that can be readily detected on Southern blots of genomic DNA from cells undergoing macronuclear development. On these circular molecules, covalently joined IES ends are separated by one copy of the flanking 5'-TA-3' repeat. The similar structures of the junctions formed on the excised and donor molecules point to a central role for this dinucleotide in IES excision.

Animals↗