Genetic variability of the main intermediate host of the Schistosoma mansoni in Brazil,Biomphalaria glabrata (Gastropoda: Planorbidae) assessed by SSR-PCR

The genetic variability of Brazilian Biomphalaria glabrata populations was studied using SSR-PCR. This technique is a variant of the polymerase chain reaction (PCR), which consists of using a single primer directed towards microsatellite regions under high stringency reaction conditions. Twenty snails of each population from eight distant Brazilian localities were analyzed. Morphology and PCR-RFLP were used for previous specific identification of the snails. Bands generated after gel electrophoresis of the SSR-PCR products of each snail were used to study intra- and interpopulation genetic variability. Fifty-five prominent bands were considered in a pairwise band comparison for the determination of genetic variability. Genetic variability was greater between populations than within populations. Snail populations from the field and the laboratory presented almost no genetic differences. No relationship between genetic variability and geographic distance was found. SSR-PCR proved to be a good alternative molecular tool for the population study of B. glabrata.     

Two Triatoma dimidiata clades (Chagas disease vector) associated with different habitats in southern Mexico and Central America

Triatoma dimidiata is the only reported Chagas disease vector in Campeche, Mexico. The purpose of this study was to determine the genetic variability of vectors from Campeche coastal and rain forest areas and establish a phylogenetic relationship with other T. dimidiata populations by analyzing the internal transcribed spacer-2 (ITS-2) region. The sequence length of samples from Campeche ranged from 469 to 478 basepairs. The ITS-2 variability among the populations enabled us to classify them into two clades with an 18-22 nucleotide difference. The genetic distance (0.042) between them confirms this divergence. Phylogenetic analysis of gene genealogies confirmed these two clades. Furthermore, the population genetic analyses showed two groups with little genetic similarity or migration between them. One group was associated with the tropical forest area and the other group was associated with a mainly coastal distribution. This correlation was also observed when T. dimidiata from other regions of Mexico and Central America were analyzed.     

Source and significance of genetic polymorphism of selected parasitic protozoa

The application of biochemical and molecular techniques in parasitological studies has provided increasing evidences of genetic polymorphism among parasite populations. This review presents possible origins of genetic variation within populations of various protozoan species. Since the mode of reproduction has an important influence on genetic polymorphism within parasite populations these considerations refer mainly to some protozoan parasites which have various life cycles, e.g. Giardia, Trypanosoma, Cryptosporidium, Toxoplasma. Also other factors associated with parasites (such as: transmission and passage history in laboratory conditions; occurrence in different hosts or geographic regions; selective pressure of drugs; competitive interactions between populations) that affect parasite genetic diversity are discussed. However, the number of examined isolates of parasites and genetic markers, assortment of methods, probes, primers and reagents used is also of significance. The significance of genetic variability in parasite populations is still the subject of much interest and controversy. A simple interpretation of such variation is impossible because of the complexity of host-parasite interactions. The knowledge of parasite diversity at the nucleic acids level has continually increased, but a corect interpretation of this phenomenon requires at least the same knowledge of genetic variability in host populations. Nevertheless, genetic variability in protozoan parasites has many important implications, e.g. for taxonomy, epidemiology, control and evolution. Genetic differences within parasite populations might also be associated with phenotypic variability, e.g. virulence, antigenicity, infectivity, drug sensitivity, host preference etc.     

Spatial and temporal genetic variation of Echinostoma revolutum (Trematoda: Echinostomatidae) from Thailand and the Lao PDR

A total of 314 individual Echinostoma revolutum were collected at different locations and times from domestic ducks from Khon Kaen Province, Thailand and Vientiane Province, the Lao People's Democratic Republic (PDR). Genetic variation of these parasites was analyzed using multilocus enzyme electrophoresis at three polymorphic loci namely, glucose-6-phosphate dehydrogenase (G6pd), malic enzyme (Me) and peptidase valine-leucine (PepA). High levels of genetic variability were found within and between populations. Significant heterozygote deficiencies compared with the predictions under Hardy–Weinberg equilibrium were detected in populations from Thailand and the Lao PDR for all loci except G6pd-1. Significant genetic differentiation was observed between spatially separated populations from Thailand and the Lao PDR. This as also true for some samples collected at different times in Thailand. The variability found may be consistent with a Wahlund effect, genetic drift and/or other factors such as the population structure of snail hosts. Our data provide further insight into the process of genetic divergence within and among geographically and temporally isolated populations of E. revolutum, and potentially other medically important echinostomes in Southeast Asia.     

Genetic variability of the human filarial parasite, Wuchereria bancrofti in South India

The genetic variability of the lymphatic filarial parasite Wuchereria bancrofti, from three localities (one urban and two rural areas) in southern India, endemic for filariasis was studied using random amplified polymorphic DNA (RAPD) markers. The RAPD profiles were generated for 21 parasite populations (7 populations from each area), using a 10-mer random primer. The analysis of profiles indicated the existence of considerable genetic variability among parasite populations. The Nei's gene diversity between the individual populations in the 2 areas (one urban and another rural) was comparatively greater (0.3372+/-0.1462 & 0.2830+/-0.1764) than that of populations in another village (0.0490+/-0.1373). The greater genetic diversity among the former areas may be due to human migration, endemicity for long time and drug (diethyl-carbamazine citrate) pressure unlike the populations of latter village where the filariasis is relatively a recent introduction and which was never under active chemotherapy. The Nei's genetic distance was estimated and the phylogenetic tree was constructed using 'UPGMA'. These analyses indicated the prevalence of at least two genetically distinct clusters, among the populations studied, their maximum genetic distance being 0.2444. The finding of two genetic 'variants' of W. bancrofti, in the present study, may have important implications in filariasis epidemiology and control/elimination programmes.     

Genetic variability of Triatoma flavida and Triatoma bruneri (Hemiptera: Reduviidae) by RAPD-PCR technique

The Triatominae (Hemiptera:Reduviidae) contains the principal and potential Chagas disease vectors present in Mexico, Central America and South America. Triatoma flavida and T. bruneri are Cuban species. These species are closely related according to morphology and were considered synonyms until 1981, when they were separated on the grounds of external characters of the body and the morphology of male genitalia. The present study seeks to analyze genetic polymorphism of T. flavida and T. bruneri populations using RAPD techniques, and to assess the genetic relationship between these species. Ten random primers were used to evaluate the genetic variability among species using RAPD-PCR. The genetic flow among them was calculated. The dendrogram based on calculated Jaccard distances showed two clearly distinguishable clusters which coincided with the studied species. Within each species, moderate genetic differentiation (Fst 0.05-0.15) and migration rates (N > 1) were found among populations, that reveal gene flow and genetic homogeneity. Between species, the Fst value showed a high genetic differentiation and the migration rate was insufficient to maintain genetic homogeneity, and confirmed the absence of gene flow between them. Our results confirm the genetic variability among T. flavida and T. bruneri species.     

Geographic variations of life history strategies in Drosophila melanogaster II. Analysis of laboratory-adapted populations

Life history traits—hatchability, longevity, and egg production-of five wild-caught populations of Drosophila melanogaster were measured after these populations had been reared in constant laboratory conditions during a 4-year period. The results were analyzed together with those that had been obtained with the same populations just after capture. They are probably the first convincing results that reveal the existence of genetic variability for some life history traits measured in the laboratory. Besides, no significant phenotypic correlations, either positive or negative, between early and late components of fitness were found. Finally, the five populations showed different patterns of genetic correlation between early and late fitness traits. One of the populations showed a negative correlation, another showed a positive correlation, while the remaining three populations showed no correlation at all. This was equally observed at the within- and between-population levels. That result suggests that both the antagonistic pleiotropy hypothesis proposed by Williams and the concordant pleiotropy hypothesis suggested by Lints are not of general validity.     

Genetic variability of Brazilian populations of Lymnaea columella (Gastropoda: Lymnaeidae), an intermediate host of Fasciola hepatica (Trematoda: Digenea)

In Brazil, Lymnaea columella is the most important intermediate host of Fasciola hepatica, the etiological agent of fasciolosis, which is a parasitic disease of veterinarian and human importance. Random amplified polymorphic DNA (RAPD) was used to investigate the genetic variability within and among nine Brazilian populations of L. columella comprising 205 individuals. A number of four primers were used for analysis of molecular variance (AMOVA). Out of 83 RAPD markers, 63 (76%) were polymorphic and revealed 119 unique RAPD profiles. The levels of genetic variability found in the populations were low and most of the genetic variation was interpopulational (81.6%) when compared to intrapopulational variability (18.4%). These results are in accordance with the dynamics and distribution of the populations analyzed.     

Genetic diversity in natural populations of a soil bacterium across a landscape gradient

Genetic diversity in natural populations of the bacterium Pseudomonas cepacia was surveyed in 10 enzymes from 70 clones isolated along a landscape gradient. Estimates of genetic diversity, ranging from 0.54 to 0.70, were higher than any previously reported values of which we are aware and were positively correlated with habitat variability. Patterns of bacterial genetic diversity were correlated with habitat variability. Findings indicate that the source of strains used in genetic engineering will greatly affect the outcome of planned releases in variable environments. Selection of generalist strains may confer a large advantage to engineered populations, while selection of laboratory strains may result in quick elimination of the engineered strains.     

RAPD analysis of the populations of the malaria mosquitoes Anopheles superpictus and A. pulcherrimus in the malaria foci of the central Asia

RAPD-analysis was used to estimate intraspecies-specific variability in malaria mosquitoes, the major vectors of malaria in Central Asia. The high heterogeneity of RAPD-loci was found in 7 populations of An. superpictus and 2 populations of An. pulcherrimus. Genetic distances between the populations were determined on the basis of analysis of the frequencies of 15 RAPD-loci. There was a genetic similarity of An. superpictus populations in the malaria foci of Tadjikistan with those in the neighboring Surkhandariya Region of Uzbekistan.     

Genetic variability maintained in a finite population under mutation and autocorrelated random fluctuation of selection intensity

By using the diffusion equation method, the level of genetic variability maintained under mutation pressure in a finite population is investigated, assuming autocorrelated random fluctuation of selection intensity. An appropriate mathematical model was formulated to treat the change of gene frequencies, incorporating mutation pressure and fluctuating selection. Extensive Monte Carlo simulation experiments were also performed to supplement the theoretical treatments. Excellent agreement between the two results suggests the validity of the present diffusion model for the autocorrelated selection. One of the most important findings from the simulation studies is that mutations and random sampling drift largely determine the level of genetic variability, and that the presence of autocorrelated selection can significantly lower genetic variability only when its strength, as measured by the cumulative variance of selection intensity, is larger than about 10³ times the mutation rate. It is pointed out that the effects of both mutations and random sampling drift have to be incorporated in order to assess the role of various factors for the maintenance of genetic variability in natural populations.     

Occurrence of a sibling species complex within neotropical lymnaeids,snail intermediate hosts of fascioliasis

The delimitation of cryptic species within the genus Lymnaea, which are the main vectors of fascioliasis, remains a topic of controversy. An analysis of genetic variability based on 12 enzyme loci revealed different fixed alleles at 9 loci between two sympatric samples of Lymnaea viatrix at the type locality in Lima, Peru. The absence of heterozygotes within this locality indicates the presence of isolated populations or cryptic species within L. viatrix. Significant genetic differences were also found between these two L. viatrix samples from Lima and other populations of L. viatrix in South America and in addition to species such as L. truncatula, L. cubensis and L. columella. Moreover, the lack of variability within each Lymnaea samples studied indicates the existence of a high selfing rate in each species.     

Genetic structure of Lutzomyia (Nyssomyia) intermedia populations from two ecologic regions in Brazil where transmission of Leishmania (Viannia) braziliensis reflects distinct eco-epidemiologic features

Randomly amplified polymorphic DNA (RAPD) analysis was performed to infer the magnitude and pattern of genetic differentiation among Lutzomyia (Nyssomyia) intermedia populations from two ecologic regions in southeast Brazil where transmission of American cutaneous leishmaniasis (ACL) by L. (Viannia) braziliensis reflects distinct eco-epidemiologic features. Two hundred thirty-five bands from six primers were analyzed for genetic variation within and between sand fly populations. A lower level of intrapopulational variability was found in domestic sand flies (compared with the peridomestic population). Gene flow FST estimates indicated homogeneity in the studied population, with limited population substructuring, but with a tendency of sand fly vectors to form clusters by micro-habitat (domestic and peridomestic). The level of genetic structuring of Lu. intermedia population from each ecologic region is distinct and may reflect association or independence between the domestic and the peridomestic habitats in rural and periurban areas, respectively, both reflecting distinct characteristics of the transmission cycle of ACL.     

Microsatellite loci: determining the genetic variability of Plasmodium vivax

Objective To describe the genetic diversity of Plasmodium vivax isolates from different areas in the Brazilian Amazon using 11 polymorphic microsatellites and to evaluate the correlation between microsatellite variation and repeat array length. Methods Microsatellites with variable repeat units and array lengths were selected using in silico search of the P. vivax genome. We designed primers and amplified the selected loci in DNA obtained from patients with P. vivax acute infections. Results Positive correlation between repeat array length and microsatellite variation was detected independently of the size of repeat unit (di, tri, or tetranucleotide). We used these markers to describe the genetic variability of P. vivax isolates from four geographic regions of the Brazilian Amazon. Substantial variability was observed among P. vivax isolates within populations, concurrent with high levels of multiple‐clone infections and high linkage disequilibrium. Overall, structured populations were observed with moderate to high genetic differentiation. Conclusion The markers studied are useful tools for assessing population structure of P. vivax, as demonstrated for Brazilian populations and for searching for evidence of recent selection events associated with different phenotypes, such as drug resistance.     

Distribution of allelic frequencies in a finite population under stepwise production of neutral alleles.

A formula for the distribution of allelic frequencies in a finite population is derived assuming stepwise production of multiple alleles. Monte Carlo experiments were performed to check the validity of the formula, and excellent agreement was obtained between theoretical distribution and experimental results. The formula should be useful for analyzing genetic variability in natural populations that can be detected by electrophoretic methods.     

Brazilian studies on the genetics of Schistosoma mansoni

The parasite Schistosoma is known to exhibit variations among species, strains and genera, such as, the levels of infectivity, pathogenicity and immunogenicity. These factors may differ among parasite populations according to the local epidemiological conditions. Diversity observed in Schistosoma mansoni from different geographical regions or within individuals of the same region can be determined by differences in the genotype of each parasite strain. However, until recently, finding adequate genetic markers to investigate infectivity or other epidemiological characteristics of a transmission area proved difficult. Several studies have been conducted to evaluate the genetic variability of S. mansoni, using different techniques. Intraspecific variability was observed in morphological characters, isoenzyme studies, mtDNA, ribosomal gene probes, random amplification of polymorphic DNA (RAPD) and microsatellites. The sequencing of the S. mansoni genome was the most important achievement concerning genetic approaches to the study of this parasite and may improve the development of drugs, vaccines and diagnostics of schistosomiasis. The knowledge of the genetic structure of schistosome populations in relation to epidemiological data and host variability is essential for the understanding of the epidemiology of the disease and the design of control strategies.     

The history of effective population size and genetic diversity in the Yellowstone grizzly (Ursus arctos): implications for conservation

Protein, mtDNA, and nuclear microsatellite DNA analyses have demonstrated that the Yellowstone grizzly bear has low levels of genetic variability compared with other Ursus arctos populations. Researchers have attributed this difference to inbreeding during a century of anthropogenic isolation and population size reduction. We test this hypothesis and assess the seriousness of genetic threats by generating microsatellite data for 110 museum specimens collected between 1912 and 1981. A loss of variability is detected, but it is much less severe than hypothesized. Variance in allele frequencies over time is used to estimate an effective population size of approximately 80 across the 20th century and >100 currently. The viability of the population is unlikely to be substantially reduced by genetic factors in the next several generations. However, gene flow from outside populations will be beneficial in avoiding inbreeding and the erosion of genetic diversity in the future.     

Predictors of hypertension. Population studies.

A genetic influence on the variability of blood pressure in populations has been established. This effect has been demonstrated repeatedly by the presence of familial aggregation of blood pressure; however, a unique bimodal distribution in populations has not been established. About one-third to one-half of the blood pressure variance is explained by heredity with the remainder due to environmental or unknown factors. Direct associations of blood pressure with genetic markers in populations have been described infrequently. However, an association between haptoglobin 1 and increased blood pressure has been reported. This relationship is possibly secondary to increased sodium sensitivity in this genetic subgroup. The familial aggregation of urinary kallikrein as well as sodium-lithium countertransport and hypertension represents a successful attempt by investigators to identify correlates of intermediate phenotypes of hypertension. The association between restriction fragment length polymorphisms and elevated blood pressure represents an unmet potential. It should be emphasized that any genetic predictive factor will be affected by environmental factors. This interaction provides the opportunity for possible modification or prevention of hypertension by hygienic means.     

Mate choice theory and the mode of selection in sexual populations

Indirect new data imply that mate and/or gamete choice are major selective forces driving genetic change in sexual populations. The system dictates nonrandom mating, an evolutionary process requiring both revised genetic theory and new data on heritability of characters underlying Darwinian fitness. Successfully reproducing individuals represent rare selections from among vigorous, competing survivors of preadult natural selection. Nonrandom mating has correlated demographic effects: reduced effective population size, inbreeding, low gene flow, and emphasis on deme structure. Characters involved in choice behavior at reproduction appear based on quantitative trait loci. This variability serves selection for fitness within the population, having only an incidental relationship to the origin of genetically based reproductive isolation between populations. The claim that extensive hybridization experiments with Drosophila indicate that selection favors a gradual progression of "isolating mechanisms" is flawed, because intra-group random mating is assumed. Over deep time, local sexual populations are strong, independent genetic systems that use rich fields of variable polygenic components of fitness. The sexual reproduction system thus particularizes, in small subspecific populations, the genetic basis of the grand adaptive sweep of selective evolutionary change, much as Darwin proposed.     

The effect of colonization upon aedes aegypti susceptibility to oral infection with yellow fever virus

Two colonies of Aedes aegypti were established from two independent collections from Vero Beach, Florida. Eleven sequential generations of the first colony were tested for variation in oral susceptibility to infection with yellow fever virus (YFV). Each generation was also assayed for genetic variability at seven enzyme loci using electrophoretic techniques. Significant differences in infection rates were detected between some generations. These differences were significantly correlated with genetic variation at the malate dehydrogenase locus. Seven generations from the second colony were examined simultaneously for variation in susceptibility to YFV. Significant differences were also detected between some of these generations. The results suggest that colonization may have an effect on the genetic and phenotypic variation in a mosquito strain, and that genetically based variation for susceptibility to infection with YFV occurs in populations of Ae. aegypti.