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M Wilkinson

Publications and source records attributed to M Wilkinson.

At least 91 records · Page 5Linked to original sources
PubMed1998

Split support and split conflict randomization tests in phylogenetic inference.

Randomization tests allow the formulation and statistical testing of null hypotheses about the quality of entire data sets or the quality of fit between the data and particular phylogenetic hypotheses. Randomization tests of phylogenetic hypotheses based on the concepts of split support and split conflict are described here, as are tests where splits, rather than the data, are randomly permuted. These tree-independent randomization tests are explored through their application to phylogenetic data for caecilian amphibians. Of these tests, split support randomization tests appear to be the most promising tools for phylogeneticists. These tests seem quite conservative, are applicable to nonpolar data and unordered multistate characters, and do not have the problems of nonindependence that affect split conflict and hierarchy tests. Unlike split conflict tests, their power does not appear to be correlated with split size. However, all tests are sensitive to taxonomic scope. Split support tests may help discern data that are likely to be affected by the problems of long-branches effects. Comparison of test results for mutually incompatible splits may help identify the presence of strong misleading signals in phylogenetic data. Significant split support could be a prerequisite for considering phylogenetic hypotheses to be well supported by the data, and split support randomization tests might be usefully applied prior to or as part of tree construction.

Amphibians↗
PubMed1998

Retroviral diversity and distribution in vertebrates.

We used the PCR to screen for the presence of endogenous retroviruses within the genomes of 18 vertebrate orders across eight classes, concentrating on reptilian, amphibian, and piscine hosts. Thirty novel retroviral sequences were isolated and characterized by sequencing approximately 1 kb of their encoded protease and reverse transcriptase genes. Isolation of novel viruses from so many disparate hosts suggests that retroviruses are likely to be ubiquitous within all but the most basal vertebrate classes and, furthermore, gives a good indication of the overall retroviral diversity within vertebrates. Phylogenetic analysis demonstrated that viruses clustering with (but not necessarily closely related to) the spumaviruses and murine leukemia viruses are widespread and abundant in vertebrate genomes. In contrast, we were unable to identify any viruses from hosts outside of mammals and birds which grouped with the other five currently recognized retroviral genera: the lentiviruses, human T-cell leukemia-related viruses, avian leukemia virus-related retroviruses, type D retroviruses, and mammalian type B retroviruses. There was also some indication that viruses isolated from individual vertebrate classes tended to cluster together in phylogenetic reconstructions. This implies that the horizontal transmission of at least some retroviruses, between some vertebrate classes, occurs relatively infrequently. It is likely that many of the retroviral sequences described here are distinct enough from those of previously characterized viruses to represent novel retroviral genera.

Amino Acid Sequence↗
PubMed1997

Neonatal hypothalamic c-fos expression in an excitotoxicity-induced model of precocious puberty.

We have used immunocytochemical detection of c-fos expression (FOS-like immunoreactivity, FLI) to establish the site of action of monosodium glutamate (MSG) in neonatal rats in a model of lesion-induced precocious puberty. The primary target appears to be the hypothalamic arcuate nucleus (ARC) but other circumventricular organs (CVO) are also affected (e.g. subfornical organ). Single injections of MSG (1-4 mg/g single dose, postnatal day 2 (P2)) which result in precocious puberty induce an area of edema, surrounded by a ring of FLI in the basal hypothalamus. In contrast, a maximal sub-lethal dose of NMDA (N-methyl-D-aspartate; 3 mg/kg) which produces a different pattern of ARC FLI, without edema, has no effect on puberty. Multiple doses of MSG (4 mg/g P2, P4, P6, P8), consistent with severe ARC damage and resultant sterility, markedly attenuates the FLI response by P8, with no visible edematous reaction following the final injection. In efforts to block the effect of MSG, pretreatment with the NMDA receptor-specific antagonist MK-801 (dizocilpine maleate) prevented the appearance of edema, as well as the onset of precocious puberty. However, MK-801 did not completely eliminate the FLI, but transformed the pattern of staining so that the original edematous area now contained many FOS-positive cells. This remaining MSG-induced FLI could not be eliminated by higher doses of MK-801 or by the non-NMDA antagonist DNQX (6,7-dinitroquinoxaline-2,3-dione). The combination of MK-801 and DNQX was also ineffective. MK-801 or DNQX had no effect on FLI when injected alone (i.e., without MSG) or in combination. The receptor which mediates MSG-induced c-fos expression, in the presence of MK-801 or DNQX, needs to be identified. We conclude, in conjunction with our previous work, that MSG induces precocious sexual maturation via an MK-801-sensitive mechanism associated with an edematous response of the basal hypothalamus. Whether the appearance of edema is indicative of an excitotoxic action of MSG, resulting in the removal of neurons inhibitory to sexual maturation, remains to be established.

Animals↗
PubMed1997

FGF-2 antisense RNA encodes a nuclear protein with MutT-like antimutator activity.

Bidirectional transcription of the basic fibroblast growth factor (FGF-2) gene gives rise to multiple polyadenylated sense mRNAs and a unique 1.5 kb antisense transcript (FGF-AS) which is complementary to the 3'-untranslated region of the FGF-2 mRNA. The rat FGF-AS cDNA encodes a novel 35 kDa nuclear protein (GFG) with homology to the MutT family of antimutator NTPases. Antibodies against the deduced amino acid sequence of GFG detected intense immunoreactivity in the nuclei of adult rat hepatocytes. Subcellular fractionation and Western blotting confirmed the presence of a 35 kDa immunoreactive protein in the nuclear fraction and, to a lesser extent, in the mitochondrial fractions of rat liver homogenates. Recombinant GFG suppressed the spontaneous mutation rate of MutT-deficient E. coli in a complementation assay. In-frame deletion of the 53 amino acids encompassing the MutT domain eliminated this activity, confirming the catalytic function of this region in the FGF antisense gene product. These findings demonstrate for the first time that the FGF-AS transcript encodes a functional nuclear protein with MutT-related enzymatic activity.

Amino Acid Sequence↗
PubMed1997

Recovery of hypothalamic NMDA-induced c-fos expression following neonatal glutamate (MSG) lesions.

The neonatal brain is susceptible to neurotoxic insult. In a previous report we showed that a single neonatal injection of MSG, known to cause damage in the arcuate nucleus (ARC), induces a precocious yet otherwise normal puberty in female rats. We have examined this ability of the medial basal hypothalamus (MBH) to recover from an excitotoxic insult using the immediate-early gene c-fos as a developmental marker of ARC response to glutamate receptor stimulation with N-methyl-D-aspartate (NMDA). Groups of neonatal (postnatal day (PD) 2) pups were injected with MSG, then stimulated on subsequent days (PD 3-29) with NMDA, known to induce c-fos expression in ARC. Computer-assisted densitometry was used to quantify Fos-like immunoreactivity (FLI) profiles in ARC. Pups treated neonatally with saline (PD 2) showed a robust, age-specific expression of FLI in the ARC following NMDA treatment. The FLI response was absent in the days immediately following an MSG lesion but subsequently recovered up to 75% of maximum by PD 16. Almost full recovery was seen by PD 29. We also examined the ability of the ARC to recover following chronic MSG treatment (PD 2-8), known to induce extensive hypothalamic damage. These pups displayed an unusual response to subsequent NMDA injection, consisting of 5 min cycles of hyper- and hypoactivity. Stimulation with NMDA revealed only a 50% recovery of FLI even at PD 29. In both treatment groups (acute vs. chronic MSG) the zone of recovery (i.e., reappearance of FLI) was initiated close to the third ventricle and with time radiated towards the periphery of the ARC. Some cells which reacquired FLI in the ARC following lesions presented a highly irregular condensed nuclear morphology. We conclude that the recovery of hypothalamic function (i.e., onset of puberty) after a neonatal MSG lesion is coincident with the reappearance of a normal pattern of c-fos expression in response to NMDA stimulation.

Animals↗
PubMed1997

The effect of lactation on induced Fos-like immunoreactivity in the rat hypothalamic paraventricular nucleus.

Lactating rats display a period of blunted hypothalamo-pituitary-adrenal (HPA) response to a variety of stressors. This hyporesponsiveness is reported to be dependent upon continuous mother-pup interactions. In this study, computer-assisted densitometric methods were used to measure levels of induced Fos-like immunoreactivity (FLI) in the hypothalamic paraventricular nucleus (PVN) of lactating and non-lactating rats. Adrenalectomy (ADX) induces elevated levels of FLI in the PVN of non-lactating rats. We have observed that, between post-partum day (pd) 4 and pd 21, the level of ADX-induced FLI in the PVN of lactating rats follows a U-shaped distribution; that the persistence of this phenomenon is dependent upon continued mother-pup interaction and that sustained mother-pup interaction beyond the end of the normal suckling period (pd 21) does not extend the period of refractoriness. We have further determined that both the non-specific neural activator Metrazole, and the glutamate agonist N-methyl-D,L-aspartate (NMA), induced smaller increases in FLI in the PVN of lactating rats compared to non-lactating cohorts, and that the suppressing effect of lactation on Metrazole-induced FLI does not extend to all brain regions. These results suggest that mechanisms responsible for the onset and maintenance of the so-called lactational stress-hyporesponsive period (LSHRP) include altered function of glutamatergic pathways.

Adrenalectomy↗
PubMed1997

The tumour stroma of oral squamous cell carcinomas show increased vascularity compared with adjacent host tissue.

For tumours to grow they must acquire an adequate blood supply, and the use of drugs to inhibit tumour vascularization is one promising approach to anti-cancer therapy. Clear information is therefore required on the vascular architecture of human tumours and animal tumour models used for testing anti-angiogenic therapies. Many previous studies on animal tumour models have shown that carcinomas are least vascular in their centres and that host tissues become more vascular with proximity to the tumour. However, we have previously found that many human colorectal carcinomas do not show this pattern. The present study on human oral squamous cell carcinomas (SCCs) again reveals significant differences. Paraffin sections from 24 SCCs were immunostained using the QBEnd-10 monoclonal antibody to demonstrate blood vessels, and these were quantified by interactive morphometry using a Kontron Videoplan system. In most carcinomas, viable tumour tissue was no less vascular in the tumour centre than in the tumour periphery. Although tumours are known to release angiogenic factors, viable tumour tissue was less vascular than adjacent host tissues. However, the tumour stroma, by itself, was more vascular than adjacent host tissues. Host tissue adjacent to tumour showed no obvious increase in vascular density with increasing proximity to the tumour edge, which suggests that tumour-released angiogenic factors are only effective over a short distance.

Carcinoma, Squamous Cell↗
PubMed1997

Decline in basic fibroblast growth factor (FGF-2) mRNA expression in female rat hypothalamus at puberty.

There is a growing acceptance of the importance of hypothalamic growth factors in the control of sexual maturation. Basic fibroblast growth factor (bFGF, FGF-2), a potent mitogen and neurotropic factor for brain cells in vitro, including hypothalamic cells, is widely expressed in the post-natal CNS but its physiological functions there are largely unknown. Previously, studies of FGF-2 mRNA regulation in vivo have been hampered by the low levels of FGF-2 mRNA present in post-natal tissues. We have applied a sensitive semi-quantitative procedure based on reverse transcription followed by polymerase chain reaction amplification (RT-PCR) to detect and estimate relative amounts of mRNAs encoding FGF-2 and its receptor in the hypothalamic-hypophyseal axis in individual female rats undergoing sexual maturation. FGF receptor and FGF-2 mRNAs were detectable in all brain regions examined. Injections of the glutamate agonist N-methyl-D-aspartic acid (NMDA) or pregnant mare's serum gonadotropin (PMSG) were used to advance the onset of puberty in immature female rats, and the levels of FGF-2 and FGF receptor mRNA in MBH and cortex were examined. Daily injections of NMDA (20 mg/kg) from day 24-28 resulted in advancement of first ovulation and vaginal opening (VO) in 5 of 9 treated rats. None (0/4) of the saline treated controls achieved first ovulation during the course of the experiment. Expression of FGF-2 mRNA in the medial-basal hypothalamus of the NMDA-treated VO animals, but not nonVO animals, was significantly (P<0.05) reduced by 50% vs saline-treated nonVO controls. There was no effect of NMDA on FGF-2 expression in cerebral cortex of VO Vs nonVO animals. FGF receptor mRNA levels were unaffected by NMDA treatment. To assess the possibility that the decline in hypothalamic FGF-2 mRNA levels was related to puberty and not just to an effect of NMDA, pregnant mare's serum gonadotropin was used to induce first ovulation and vaginal opening. Injection of PMSG to immature female rats on day 26 resulted in precocious first ovulation on day 29. This was accompanied by a significant 40% reduction in the steady-state level of FGF-2 mRNA in the medial basal hypothalamus compared to saline treated controls. As with NMDA treatment, PMSG did not affect FGF-2 mRNA abundance in the cortex, nor the FGF receptor mRNA in MBH or cortex. Immunohistochemical detection of FGF-2 protein in the arcuate nucleus revealed that FGF-2 immunoreactivity was also significantly modified in peri-ovulatory NMDA-treated animals. FGF-2 immunoreactivity in NMDA treated rats was significantly elevated at day 29 (the day of ovulation), but significantly inhibited by day 33. These findings suggest that alterations in the level of FGF-2 mRNA in the hypothalamus may be associated with first ovulation and the onset of sexual maturation in the female rat.

Animals↗
PubMed1997

Different roles of flowering-time genes in the activation of floral initiation genes in Arabidopsis.

We have analyzed double mutants that combine late-flowering mutations at four flowering-time loci (FVE, FPA, FWA, and FT) with mutations at the LEAFY (LFY), APETALA1 (AP1), and TERMINAL FLOWER1 (TFL1) loci involved in the floral initiation process (FLIP). Double mutants between ft-1 or fwa-1 and lfy-6 completely lack flowerlike structures, indicating that both FWA and FT act redundantly with LFY to control AP1. Moreover, the phenotypes of ft-1 ap1-1 and fwa-1 ap1-1 double mutants are reminiscent of the phenotype of ap1-1 cal-1 double mutants, suggesting that FWA and FT could also be involved in the control of other FLIP genes. Such extreme phenotypes were not observed in double mutants between fve-2 or fpa-1 and lfy-6 ap1-1. Each of these showed a phenotype similar to that of ap1-1 or lfy-6 mutants grown under noninductive photoperiods, suggesting a redundant interaction with FLIP genes. Finally, the phenotype of double mutants combining the late-flowering mutations with tfl1-2 were also consistent with the different roles of flowering-time genes.

Arabidopsis↗
PubMed1996

Low-calcium-induced enhancement of chemical synaptic transmission from photoreceptors to horizontal cells in the vertebrate retina.

According to the classical calcium hypothesis of synaptic transmission, the release of neurotransmitter from presynaptic terminals occurs through an exocytotic process triggered by depolarization-induced presynaptic calcium influx. However, evidence has been accumulating in the last two decades indicating that, in many preparations, synaptic transmitter release can persist or even increase when calcium is omitted from the perfusing saline, leading to the notion of a "calcium-independent release" mechanism. Our study shows that the enhancement of synaptic transmission between photoreceptors and horizontal cells of the vertebrate retina induced by low-calcium media is caused by an increase of calcium influx into presynaptic terminals. This paradoxical effect is accounted for by modifications of surface potential on the photoreceptor membrane. Since lowering extracellular calcium concentration may likewise enhance calcium influx into other nerve cells, other experimental observations of "calcium-independent" release may be reaccommodated within the framework of the classical calcium hypothesis without invoking unconventional processes.

Ambystoma↗
PubMed1996

Majority-rule reduced consensus trees and their use in bootstrapping.

Bootstrap analyses are usually summarized with majority-rule component consensus trees. This consensus method is based on replicated components and, like all component consensus methods, it is insensitive to other kinds of agreement between trees. Recently developed reduced consensus methods can be used to summarize much additional agreement on hypothesised phylogenetic relationships among multiple trees. The new methods are "strict" in the sense that they require agreement among all the trees being compared for any relationships to be represented in a consensus tree. Majority-rule reduced consensus methods are described and their use in bootstrap analyses is illustrated with a hypothetical and a real example. The new methods provide summaries of the bootstrap proportions of all n-taxon statements/partitions and facilitate the identification of hypotheses of relationships that are supported by high bootstrap proportions, in spite of a lack of support for particular components or clades. In practice majority-rule reduced consensus profiles may contain many trees. The size of the profile can be reduced by constraints on minimal bootstrap proportions and/or cardinality of the included trees. Majority-rule reduced consensus trees can also be selected a posteriori from the profile. Surrogates to the majority-rule reduced consensus methods using partition tables or tree pruning options provided by widely used phylogenetic inference software are also described. The methods are designed to produce more informative summaries of bootstrap analyses and thereby foster more informed assessment of the strengths and weaknesses of complex phylogenetic hypotheses.

Animals↗
PubMed1996

Interaction of chemical and state effects on ventilation during sleep onset.

Ventilation varies as a function of state, being higher during wakefulness (as indicated by alpha electroencephalogram activity) than during sleep (theta activity). A recent experiment observed a progressive increase in the magnitude of these state-related fluctuations in ventilation over the sleep-onset period (28). The aim of the present experiment was to test the hypothesis that this effect resulted from chemical (feedback-related) amplification of state effects on ventilation. A hyperoxic condition was used to eliminate peripheral chemoreceptor activity. It was hypothesized that hyperoxia would reduce the amplification of changes in ventilation associated with electroencephalogram state transitions. Ventilation was measured over the sleep-onset period under both hyperoxic and normoxic conditions in 10 young healthy male subjects. Sleep onsets were divided into three phases. Phase 1 corresponded to presleep wakefulness; and phases 2 and 3 corresponded to early and late sleep onset, respectively. The magnitudes of state-related changes in ventilation during phases 2 and 3, and under hyperoxic and normoxic conditions were compared using a phase by condition analysis of variance. Results revealed a significant phase by condition interaction, confirming that hyperoxia reduced the amplification of state-related changes in ventilation by selectively decreasing the magnitude of phase 3 state changes in ventilation. However, some degree of amplification was evident during hyperoxia, thus the results demonstrated that peripheral chemoreceptor activity contributed to the amplification of state-related changes in ventilation but that additional factors may also be involved.

Adolescent↗