Evolution at HLA: possible explanations for the deficiency of homozygotes in two populations

There is substantial evidence that some form of balancing selection is important for loci in the HLA region. Two classic studies found a large deficiency of homozygotes for these loci. Four possible proposed selective explanations were proposed: (1) heterozygous advantage; (2) linked lethal; (3) maternal-fetal interaction, and (4) resistance to infectious disease. These hypotheses as well as another explanation, different male and female gametic frequencies, have been examined here to try and evaluate their potential impact on genotypic frequencies. For any of these selective mechanisms to alone account for the observed homozygous deficiency, selection and/or others factors would have to be extremely strong. The linked lethal selection model and the hypothesis based on different male and female gametic frequencies appear to be unlikely explanations for these observations. Other factors that may influence genotypic frequencies are also discussed.     

Active Maintenance of T Cell Memory in Acute and Chronic Viral Infection Depends on Continuous Expression of FOXO1

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POPULATION GENETICS OF INTRAGAMETOPHYTIC SELFING

Intragametophytic selfing is a mode of reproduction occurring in homosporous ferns where two gametes from the same haploid gametophyte form a completely homozygous sporophyte. The inbreeding equilibrium is derived for a population with partial intragametophytic selfing, selfing, and outcrossing. Procedures for directly estimating the extent of intragametophytic selfing and selfing using parent-offspring data are given. The conditions for a stable polymorphism from a heterozygous-advantage fitness model are more restrictive for partial intragametophytic selfing than for selfing. The rate of decay of gametic disequilibrium is slower for partial intragametophytic selfing than for selfing. Based on these findings, one would predict that plants with intragametophytic selfing would have less polymorphism for loci with a heterozygous advantage and more gametic disequilibrium between neutral loci than is expected for populations with an equivalent amount of selfing. Data from several studies are consistent with these predictions.     

Development and Evaluation of a Simple Latex Agglutination Test for Diagnosis of Tuberculosis

A simple latex agglutination test (SLAT) based on modifications of existing serodiagnostic techniques, in which commercially available reagents are used, was developed for detection of antibodies against Mycobacterium tuberculosis. Tests performed on 553 serum samples from 316 individuals, including 117 bacteriologically confirmed active tuberculosis patients, showed 80% positive titers. Sera from 12 patients with arrested tuberculosis showed 91% positive titers. Nonspecific reactions were noted in 5% of 160 serums from selected normal individuals and patients with diseases other than tuberculosis. The antibodies detected by the SLAT method were found to be relatively stable when exposed to low temperatures, whereas high temperatures reduced the antibody titer considerably. Disodium ethylenediaminetetraacetic acid inactivation of serum complement was found to be satisfactory. No variation of tuberculosis antibody titer was noted in tests on multiple specimens from patients whose conditions were stabilized. However, considerable fluctuation was encountered in antibody titers obtained on recently detected individuals. Data obtained in this study indicate that the modified procedures of the SLAT method could replace the tuberculin skin test for simple screening of tuberculosis in adults.     

Distribution of lectin binding sites in Xenopus laevis egg jelly.

Eggs from the anuran Xenopus laevis are surrounded by a thick jelly coat that is required during fertilization. The jelly coat contains three morphologically distinct layers, designated J1, J2, and J3. We examined the lectin binding properties of the individual jelly coat layers as a step in identifying jelly glycoproteins that may be essential in fertilization. The reactivity of 31 lectins with isolated jelly coat layers was examined with enzyme-linked lectin-assays (ELLAs). Using ELLA we found that most of the lectins tested showed some reactivity to all three jelly layers; however, two lectins showed jelly layer selectivity. The lectin Maackia amurensis (MAA) reacted only with J1 and J2, while the lectin Trichosanthes kirilowii (TKA) reacted only with J2 and J3. Some lectins were localized in the jelly coat using confocal microscopy, which revealed substantial heterogeneity in lectin binding site distribution among and within jelly coat layers. Wheat germ agglutinin (WGA) bound only to the outermost region of J3 and produced a thin, but very intense, band of fluorescence at the J1/J2 interface while the remainder of J2 stained lightly. The lectin MAA produced an intense fluorescence-staining pattern only at the J1/J2 interface. Several lectins were also tested for the ability to inhibit fertilization. WGA, MAA, and concanavalin A significantly inhibited fertilization and WGA was found to block fertilization by preventing sperm from penetrating the jelly. Using Western blotting, we identified high-molecular-weight components in J1 and J2 that may be important in fertilization.     

Distribution of Lectin Binding Sites in Xenopus laevis Egg Jelly

Eggs from the anuran Xenopus laevis are surrounded by a thick jelly coat that is required during fertilization. The jelly coat contains three morphologically distinct layers, designated J1, J2, and J3. We examined the lectin binding properties of the individual jelly coat layers as a step in identifying jelly glycoproteins that may be essential in fertilization. The reactivity of 31 lectins with isolated jelly coat layers was examined with enzyme-linked lectin-assays (ELLAs). Using ELLA we found that most of the lectins tested showed some reactivity to all three jelly layers; however, two lectins showed jelly layer selectivity. The lectin Maackia amurensis (MAA) reacted only with J1 and J2, while the lectin Trichosanthes kirilowii (TKA) reacted only with J2 and J3. Some lectins were localized in the jelly coat using confocal microscopy, which revealed substantial heterogeneity in lectin binding site distribution among and within jelly coat layers. Wheat germ agglutinin (WGA) bound only to the outermost region of J3 and produced a thin, but very intense, band of fluorescence at the J1/J2 interface while the remainder of J2 stained lightly. The lectin MAA produced an intense fluorescence-staining pattern only at the J1/J2 interface. Several lectins were also tested for the ability to inhibit fertilization. WGA, MAA, and concanavalin A significantly inhibited fertilization and WGA was found to block fertilization by preventing sperm from penetrating the jelly. Using Western blotting, we identified high-molecular-weight components in J1 and J2 that may be important in fertilization.