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31.
Robert J. Baker Scott K. Davis Robert D. Bradley Meredith J. Hamilton Ronald A. Van Den Bussche 《Evolution; international journal of organic evolution》1989,43(1):63-75
We studied 75 individuals of the Plains pocket gopher, Geomys bursarius, from eastern New Mexico, where the subspecies major and knoxjonesi hybridize. Each individual was examined for chromosome number, ribosomal DNA, mitochondrial DNA, and three protein systems for which reference parental populations were fixed for alternative alleles. Twenty individuals were indistinguishable from parental major, 14 individuals were indistinguishable from parental knoxjonesi, and 41 individuals had genotypes composed of combinations of character states that distinguish the two parental types. The parental types appear to represent discrete genetic entities that have restricted introgression across a narrow hybrid zone (width approximately 3 km, using the 20/80 criterion). Parental types overlap in geographic distribution near the center of the zone, and changes in mitochondrial DNA and the five nuclear markers are concordant across the zone. It is probable that there is premating isolation between knoxjonesi males and major females. The frequencies of individuals with certain genotypic combinations within our sample imply differential reproductive success of certain genotypes. We propose that F1's and highly heterozygous males are sterile and that hybrid females are less fertile than parental females. These postmating factors, along with premating isolation for one of the reciprocal crosses, probably account for the restriction of gene flow across the contact zone. The structure of the zone can be explained by the “dynamic equilibrium” model. 相似文献
32.
Mutational analysis of the Escherichia coli spoT gene identifies distinct but overlapping regions involved in ppGpp synthesis and degradation 总被引:4,自引:1,他引:3
The spoT gene of Escherichia coli encodes a guanosine 3′,5′-bis(diphosphate) 3′-pyrophosphohydrolase (ppGppase) as well as an apparent guanosine 3′,5′-bis(diphosphate) synthetase (designated PSII). To determine the regions of the SpoT protein that are required for these two competing activities, we analysed plasmid-borne deletion mutations for their ability to complement chromosomal mutations defective in each activity. We found that a region containing the first 203 amino acids of the 702-amino-acid SpoT protein was sufficient for ppGppase activity while an overlapping region containing residues 67–374 was sufficient for PSII activity. These data indicate that the catalytic sites involved in the two activities are separate but closely linked in the primary sequence of the SpoT protein. A ppGppase-defective Δ1–58 deletion mutant strain failed to synthesize ppGpp in response to nutrient limitation, also supporting the notion that PSII activity from wild-type SpoT does not increase in response to nutrient limitation. Using a strain lacking PSII activity but retaining ppGppase activity, we determined the contribution of the RelA protein (ppGpp synthetase I, PSI) to ppGpp synthesis following glucose starvation. We found that the RelA protein activity accounts for the initial burst of ppGpp synthesis at the onset of glucose starvation but that this source of synthesis is absent when amino acids are present during glucose starvation. 相似文献
33.
34.
The cyanophage AN-15 was found to have a requirement for either 1 mM calcium or 1 mM magnesium ions to maintain viral stability,
whereas 1 mM calcium ions alone were essential for the infection process to proceed in Anabaena sp. strain PCC 7120. Following
prolonged incubation, phage-resistant cells were detected at a high frequency (approximately 10-5) in lysates, as either renewed growth in liquid cultures, or as colonies in confluently lysed lawns. Southern hybridisation
failed to detect AN-15 DNA in any of the resistant strains, implying that resistance is unlikely to be due to the presence
of temperate phages. A high rate of spontaneous mutation is therefore likely to be the cause of resistance. Two classes of
resistant cells were identified; those in which AN-15 failed to attach to host cells, and those in which attachment occurred,
but subsequent replication was defective. However, it was possible to overcome phage resistance by the isolation of spontaneous
mutants of AN-15, capable of infecting phage-resistant cells. These observations imply that if cyanophages are to be assessed
as a means of controlling cyanobacterial blooms in freshwater bodies, the ionic (notably calcium) concentration of the water
must be considered, together with the possible need to employ alternative cyanophage strains if resistance to the original
one arises.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
35.
Analysis of DNA sequences of 132 introns and 140 exons from 42 pairs of orthologous genes of mouse and rat was used to compare
patterns of evolutionary change between introns and exons. The mean of the absolute difference in length (measured in base
pairs) between the two species was nearly five times as high in the case of introns as in the case of exons. The average rate
of nucleotide substitution in introns was very similar to the rate of synonymous substitution in exons, and both were about
three times the rate of substitution at nonsynonymous sites in exons. G+C content of introns and exons of the same gene were
correlated; but mean G+C content at the third positions of exons was significantly higher than that of introns or positions
1–2 of exons from the same gene. G+C content was conserved over evolutionary time, as indicated by strong correlations between
mouse and rat; but the change in G+C content was greatest at position 3 of exons, intermediate in introns, and lowest at positions
1–2 in introns.
Received: 23 December 1996 / Accepted: 1 April 1997 相似文献
36.
Sera from 146 cancer patients at risk for disseminated candidiasis were studied prospectively with immunodiffusion (ID), counterelectrophoresis
(CEP), and latex agglutination (LA) tests to determine their diagnostic value in the detection of antibodies to theCandida species. Serial serum samples, cultures, and clinical data were obtained after a malignancy was diagnosed. Patients were
classified into three groups (I, II, and III) on the basis of cultural, histological, and clinical evidence for superficial
(Group I) versus disseminated (Group III)Candida infection. Thirty-two of 78 patients (41%) in Group I had positive ID, CEP, and LA titers. In Group II, those patients lacking
histological confirmation of disseminated infection, 16 of 18 (89%) had positive titers. Thirty-six of 50 (72%) in Group III
were positive by all three tests. Heavy colonization of the gastrointestinal tract, without evidence of tissue invasion, produced
positive test results. Negative serologic tests were encountered in immunosuppressed patients with rapidly progressive candidiasis.C. krusei infections produced specific antibody titers detected by the homologous antigen but not byC. albicans antigen. Stable or decreasing LA titers were correlated with clinical improvement in patients receiving effective antifungal
therapy. 相似文献
37.
Alwyn H. Gentry 《Plant Systematics and Evolution》1981,137(1-2):95-105
Field studies in Amazonian Peru show that thePassiflora vitifolia complex, a microcosm of many larger taxa, is concentrated in Amazonia with 4 ecologically separated species. One species is restricted to seasonally inundated forests, a second to non-inundated white sand soils, a third to non-inundated lateritic soils, and the fourth mostly to non-inundated alluvial soils. A new combination is formally proposed to recognize the white sand taxon, previously regarded as a variety, as a distinct species:P. involucrata. The importance of -diversity in maintaining high Amazonian species diversity is emphasized. 相似文献
38.
Meredith F. Small 《International journal of primatology》1981,2(1):91-95
Using weight and skinfold thickness to calculate relative body fat, the fat content of 21 captive female rhesus macaques was
estimated. Although age and. pregnancy had no effect on fat, rank was significant, especially during winter, with high-rank
females having the highest fat scores. Rank therefore has a significant effect on the health of captive animals and may also
affect the health of individuals in feral groups. 相似文献
39.
R.K. Campbell T.M. Perring G.L. Barnes R.D. Eikenbary C.R. Gentry 《Journal of invertebrate pathology》1978,31(3):289-295
Natural isolates of two entomogenous fungi, Beauveria bassiana and Metarrhizium anisopliae, were cultured in liquid culture media containing 24 amino acids and KNO3 to determine their effect on growth and sporulation. In addition, the growth and medium pH changes for each isolate grown on an asparagine-containing medium were compared. Tryptophan and alanine were most effective for growth and sporulation of B. bassiana, although glutamine and KNO3 also produced large numbers of regularly shaped spores. Tryptophan, glutamic acid, and histidine were all well utilized for both growth and sporulation of M. anisopliae. Nitrogen sources containing sulfur were poorly utilized for sporulation by M. anisopliae. In general, B. bassiana produces greater mycelial mass and much larger numbers of spores than M. anisopliae. Both fungi attained nearly the same growth maximum on asparagine medium though B. bassiana exhibited an initially more rapid growth rate. In both fungi this rapid growth phase was accompanied by a decline in medium pH followed by a rise in pH during the decline phase of growth. 相似文献
40.
K Hoffken I D Meredith R A Robins R W Baldwin C J Davies R W Blamey 《BMJ (Clinical research ed.)》1977,2(6081):218-220
In a retrospective study in women with breast cancer circulating immune complex levels were measured by radioimmunoprecipitation with 125I-Clq. Before operation all the patients showed plasma immune complex levels significantly higher than those in controls. Twelve months after mastectomy patients identified clinicopathologically as having a good prognosis had almost normal levels of immune complexes. By contrast, patients with detectable dissemination on diagnosis or those who died within 22 months after mastectomy had significantly raised plasma levels. The tumour-specific nature of the immune complexes detected remains to be shown and suggestions about the applicability of this test not only for prognosis but also for monitoring the course of malignant diseases need to be confirmed by further investigations. 相似文献