Probability of Fixation and Mean Fixation Time of an Overdominant Mutation

The probability of fixation of an overdominant mutation in a finite population depends on the equilibrium gene frequency in an infinite population (m) and the product (A) of population size and selection intensity. If m < 0.5 (disadvantageous overdominant genes), the probability is generally much lower than that of neutral genes; but if m is close to 0.5 and A is relatively small, it becomes higher. If m > 0.5 (advantageous overdominant genes), the probability is largely determined by the fitness of heterozygotes rather than that of mutant homozygotes. Thus, overdominance enhances the probability of fixation of advantageous mutations. The average number of generations until fixation of an overdominant mutation also depends on m and A. This average time is long when m is close to 0.5 but short when m is close to 0 or 1. This dependence on m and A is similar to that of Robertson's retardation factor.     

Rapid Two-Temperature Formalin Fixation

Formalin fixation is a mainstay of modern histopathologic analysis, yet the practice is poorly standardized and a significant potential source of preanalytical errors. Concerns of workflow and turnaround time drive interest in developing shorter fixation protocols, but rapid protocols can lead to poor histomorphology or inadequate downstream assay results. Additionally, assays such as immunohistochemistry for phosphorylated epitopes have historically been challenging in the context of formalin-fixed tissue, indicating that there may be room for improvement in this process that is fundamental to the practice of anatomic pathology. With these issues in mind, we studied basic formalin biochemistry to develop a novel formalin fixation protocol that involves a pre-incubation in subambient temperature formalin prior to a brief exposure to heated formalin. This new protocol is more rapid than standard protocols yet preserves histomorphology and yields tissue that is compatible with an expanded set of downstream clinical and research assays, including immunohistochemistry for phosphorylated epitopes.     

Phospholipid Fixation by Dichromate-Sublimate

Phospholipids may be preserved for study in paraffin sections by fixing the tissue 1 to 3 days at room temperature in 2.5% potassium dichromate dissolved in 5% mercuric chloride. After this dichromate-sublimate fixative and paraffin embedding, the phospholipids are readily stained with Sudan black.     

Nitrogen Fixation in Seawater

S ummary . The acetylene reduction technique was used for a 3-year period to monitor potential nitrogen fixation by aerobic heterotrophic bacteria in the sea 2 miles offshore in Cardigan Bay. Samples from depths down to 15 m were membrane-filtered and the residues incubated aerobically or anaerobically in acetylene-containing gas mixtures in sealed Millipore Field Monitors. Pure cultures of aerobic heterotrophs isolated from spread-plates or monitor membranes supplied with 'nitrogen-free'media, were tested for ability to reduce acetylene. Glucose was the best of 14 substrates tested to support both growth and acetylene reduction by marine bacteria. The results suggested the presence of a few aerobic or facultatively anaerobic nitrogen fixers among much more numerous and efficient 'fixed-nitrogen-scavengers'. The acetylene-reducing capacity of pure cultures was inexplicably variable, even under closely-standardized conditions: this is discussed. The more consistent acetylene reducers included various-sized rods (Gram positive and negative), coccobacilli and yeasts, which latter may have had non-culturable bacteria associated with them. No recognizable Azotobacter sp. was isolated, despite the strongly-selective conditions imposed. Similar results were obtained for seawater samples from the Irish Sea and an Iceland fjord.     

Quantitative Micro-Complement Fixation Test

A quantitative micro-complement fixation test capable of detecting nanogram quantities of antigen is described. The test is simple to perform and is highly reproducible. Typical results from three antigen-antibody systems are given.     

Nitrogen Fixation in Wild Legumes

Isolations of the rhizobia from nodules of field plants of Ulexeuropaeus and Medicago lupulina have been examined for effectivenessin fixation of nitrogen by inoculating them into host plantsgrowing in a medium free of combined nitrogen. The results indicatethat vigorous fixation may be presumed to occur in associationwith field plants of the Ulex species, but that fixation inMedicago is probably decreased by the presence of ineffectiverhizobia in some nodules. The general importance of fixationin wild legumes is discussed, and it is pointed out that dueattention must be paid to non-legume genera with root nodules.These genera are locally abundant, and in former periods werestill more prominent in some regions of the world.     

Golgi Impregnation After Formalin Fixation

Formalin fixed (10% aqueous) brain from cat, rabbit and man cut to blocks 3-4 mm. thick was placed in a mixture of potassium bichromate, 5 g.; chloral hydrate, 3 g. and water 90 ml. for 24 hours. The specimens were rinsed through 3 changes of water, and transferred through 3 changes of 1% silver nitrate, 1-3 minutes each, then placed for 24 hours in 1.5% silver nitrate. Frozen sections, 40-50 μ were dehydrated and mounted with a cover glass, using Permount. No deterioration of the stain was seen after 5 months. Some brains had been in formalin for 9 months; others only 7 days.     

Genetic Engineering and Nitrogen Fixation

Molecular cloning is based on isolation of a DNA sequence of interest to obtain multiple copies of it in vitro. Application of this technique has become an increasingly important tool in clinical microbiology due to its simplicity, cost effectiveness, rapidity, and reliability. This review entails the recent advances in molecular cloning and its application in the clinical microbiology in the context of polymicrobial infections, recombinant antigens, recombinant vaccines, diagnostic probes, antimicrobial peptides, and recombinant cytokines. Culture-based methods in polymicrobial infection have many limitation, which has been overcome by cloning techniques and provide gold standard technique. Recombinant antigens produced by cloning technique are now being used for screening of HIV, HCV, HBV, CMV, Treponema pallidum, and other clinical infectious agents. Recombinant vaccines for hepatitis B, cholera, influenza A, and other diseases also use recombinant antigens which have replaced the use of live vaccines and thus reduce the risk for adverse effects. Gene probes developed by gene cloning have many applications including in early diagnosis of hereditary diseases, forensic investigations, and routine diagnosis. Industrial application of this technology produces new antibiotics in the form of antimicrobial peptides and recombinant cytokines that can be used as therapeutic agents.     

Micro-Complement Fixation in Klebsiella Classification

The alkaline phosphatases of 29 strains of bacteria assigned by various authors to the genera Aerobacter, Klebsiella and Enterobacter were compared by the micro-complement fixation technique. On the basis of phosphatase resemblance, we recommend that all strains hitherto assigned to Aerobacter aerogenes and Enterobacter aerogenes be assigned to the genus Klebsiella.     

Microwave Fixation of the Lung

A technique for microwave fixation of inflated rat lung is described. Conventional intratracheal fixation with instillation of fixative into the airways at a constant pressure results in pressure artifacts as well as flushing and disruption of cells and exudates. Microwave fixation fixes these elements in situ without disruption and thus is valuable when evaluating the distribution of inflammatory infiltrates. Exudative pneumonitis was produced in the rat using intratracheal instillations of either endotoxin or silica and comparisons were made between histologic sections fixed using either standard formalin fixation or microwave fixation.