Genotypic Overlap Research Articles

Importance of seedling recruitment for regeneration and maintaining genetic diversity of Cyperus papyrus during drawdown in Lake Naivasha, Kenya

Drawdown-flooding cycles occur commonly in many water bodies and influence plant succession in many ways including the expansion of macrophytes through seedling recruitment. We investigated a regeneration event of Cyperus papyrus in Lake Naivasha and documented seed production, seedling recruitment, zonation progression, dispersal and establishment in relation to evidence from genetic relatedness between seedlings and mature stands using microsatellite loci. Seed estimate counts in five papyrus umbels reached high values between 98,000 and 337,000. The drawdown drying phase led to desiccation accompanied by cracking of mudflat soils, and the oxidation and demise of littoral aquatic plants. Reflooding led to distinct zones of young papyrus and hygrophilous ephemerals. In the final flooding phase when the lake reached normal water levels, hygrophilous ephemerals died off while papyrus survived. Young floating papyrus mats dispersed through wind and wave action joined existing mature stands or spread into formerly unoccupied shoreline areas.Microsatellite analysis of seedlings in the drawdown zone and a neighboring stand of mature and juvenile shoots reveals high overall gene diversity (Ho=0.476, He=0.576, Ae=2.8) reflecting an underlying sexual reproduction. A large overlap of genotypes was found in seedling and parent stands indicating a single gene pool. A total of 40 alleles were observed across 3 life stages (clonal juvenile, mature and seedlings), however, more private alleles and higher allelic diversity were detected in seedlings than in the parent individuals, showing their contribution to an increase in the local gene pool. Fine-scaled spatial genetic structuring was detected at about 100m distance in parent stands for both juvenile and mature life stages indicating a potential influence caused by local seed rain.

Recovery of novel alkaline phosphatase-encoding genes (phoX) from eutrophic Lake Taihu

To expand current knowledge on the molecular aspects of alkaline phosphatase PhoX in shallow eutrophic freshwaters, we investigated the genetic diversity and abundance of the PhoX-encoding gene (phoX) in 4 ecological regions in Lake Taihu, China, following a gradient in total phosphorus concentrations ranging from hypereutrophic to mesotrophic. Bacterial phoX was heterogeneously distributed with the highest diversity in the eutrophic regions and the highest abundance in the mesotrophic Xukou Bay. The concentrations of total phosphorus and enzymatically hydrolyzable phosphorus determined the distribution of bacterial phoX in Lake Taihu. Most (70.8%) of the phoX-translated proteins had <90% similarity to the PhoX proteins in the GenBank database, suggesting the presence of novel phoX genotypes in Lake Taihu. The low overlap in phoX genotypes (15.8%) between Lake Taihu and some marine ecosystems, and the dominance of the translated proteins most similar to the Alphaproteobacteria-affiliated PhoX, demonstrate the uniqueness of PhoX in eutrophic freshwaters.

Phytophthora infestans Field Isolates from Gansu Province, China are Genetically Highly Diverse and Show a High Frequency of Self Fertility

The genetic diversity of 85 isolates of Phytophthora infestans collected in 2007 from Gansu province in China was determined and compared with 21 isolates collected before 2004. Among them, 70 belonged to the A1 mating type and 15 were self-fertile (SF). The mitochondrial DNA haplotypes revealed both Ia (25%) and IIa (75%) haplotypes. Metalaxyl resistance occurred with high frequency (54%) in Gansu. Simple sequence repeat (SSR) genotyping revealed 26 genotypes (13 from the Tianshui region) among the 85 isolates, and 18 genotypes among the 21 isolates collected before 2004, without overlap in genotypes detected in the two groups. Cluster analysis showed clear subdivisions within the different mating type isolates. Among Gansu's isolates, Nei's and Shannon's diversity indices were highest in isolates collected in Tianshui where both A1 and SF isolates were found. Analysis of molecular variance of isolates from Gansu indicated that 51% and 49% of the variance was explained by within-area and among-area variance, respectively. The results suggest that the occurrence of SF isolates increases the risk of sexual reproduction, the formation of oospore as initial inocula in the field, and affects the genotypic diversity in the population.

Genotypes and antibiotic resistance of Campylobacter coli in fattening pigs

Campylobacter coli is a food-borne zoonotic pathogen causing human gastroenteritis worldwide. The organism is a commensal in the intestine of many food production animals including fattening pigs. The role of the pig as a potential reservoir for C. coli affecting human either directly or via poultry has hardly been investigated and genetic characterization of porcine strains is needed to address this question. For this aim multilocus sequence typing (MLST) and flaB typing was applied to 256 C. coli isolates from faeces of fattening pig collected during 2009 at different slaughterhouses in Switzerland. In addition genotypic resistances towards macrolides and quinolones based on point mutations in the 23S rRNA and gyrA genes, respectively, were determined. Of the 67 sequence types (STs) obtained by MLST, 37 were found for the first time. flaB typing revealed 46 different types with 14 of them being novel and was useful to further differentiate strains with an identical ST. Quinolone resistance was detected in 33.6% and macrolide resistance was found in 10.6% of isolates. Comparison with 99 C. coli pig isolates from 2001 revealed a significant decrease in antibiotic resistance towards both groups of antibiotics and there was high overlap between genotypes of 2001 and 2009. Little overlap of porcine genotypes was found with 97 C. coli isolates from poultry collected 2008, however, macrolide resistance was significantly higher in pig isolates. In conclusion, C. coli from Swiss pig are heterogeneous containing many novel STs, findings that could reflect the partitioned Swiss pig production with almost no international breed exchange. The antibiotic resistance echoes the use of corresponding drugs in the Swiss livestock production and indicates the efficacy of restrictive application of antibiotics in order to reduce resistances.

Clonal Differences between Non-Typhoidal Salmonella (NTS) Recovered from Children and Animals Living in Close Contact in The Gambia

BackgroundNon-Typhoidal Salmonella (NTS) is an important cause of invasive bacterial disease and associated with mortality in Africa. However, little is known about the environmental reservoirs and predominant modes of transmission. Our study aimed to study the role of domestic animals in the transmission of NTS to humans in rural area of The Gambia.MethodologyHuman NTS isolates were obtained through an active population-based case-control surveillance study designated to determine the aetiology and epidemiology of enteric infections covering 27,567 Gambian children less than five years of age in the surveillance area. Fourteen children infected with NTS were traced back to their family compounds and anal swabs collected from 210 domestic animals present in their households. Identified NTSs were serotyped and genotyped by multi-locus sequencing typing.Principal FindingsNTS was identified from 21/210 animal sources in the households of the 14 infected children. Chickens carried NTS more frequently than sheep and goats; 66.6%, 28.6% and 4.8% respectively. The most common NTS serovars were S. Colindale in humans (21.42%) and S. Poona in animals (14.28%). MLST on the 35 NTS revealed four new alleles and 24 sequence types (ST) of which 18 (75%) STs were novel. There was no overlap in serovars or genotypes of NTS recovered from humans or animal sources in the same household.ConclusionOur results do not support the hypothesis that humans and animals in close contact in the same household carry genotypically similar Salmonella serovars. These findings form an important baseline for future studies of transmission of NTS in humans and animals in Africa.

New insights into the mycorrhizal Rhizoscyphus ericae aggregate: spatial structure and co‐colonization of ectomycorrhizal and ericoid roots

• Fungi in the Rhizoscyphus ericae aggregate have been recovered from the roots of co-occurring ericaceous shrubs and ectomycorrhizal trees. However, to date, there is no evidence that the same individual genotypes colonize both hosts, and no information on the extent of the mycelial networks that might form. • Using spatially explicit core sampling, we isolated fungi from neighbouring Pinus sylvestris (ectomycorrhizal) and Vaccinium vitis-idaea (ericoid mycorrhizal) roots and applied intersimple sequence repeat (ISSR) typing to assess the occurrence and extent of shared genets. • Most isolates were identified as Meliniomyces variabilis, and isolates with identical ISSR profiles were obtained from neighbouring ericoid and ectomycorrhizal roots on a number of occasions. However, genet sizes were small (< 13 cm), and several genets were found in a single soil core. Genetic relatedness was independent of spatial separation at the scales investigated (< 43 m) and M. variabilis populations from sites 20 km apart were genetically indistinguishable. • We conclude that individual genets of M. variabilis can simultaneously colonize Scots pine and Vaccinium roots, but there is no evidence for the formation of large mycelial networks. Our data also suggest significant genotypic overlap between widely separated populations of this ubiquitous root-associated fungus.

W1224 Genotypic Overlap Between Crohn's Disease (CD) and Ulcerative Colitis (UC) in a Large Cohort of Patients With Inflammatory Bowel Disease (IBD)

W1224 Genotypic Overlap Between Crohn's Disease (CD) and Ulcerative Colitis (UC) in a Large Cohort of Patients With Inflammatory Bowel Disease (IBD)

Speciation in sexually deceptive orchids: pollinator-driven selection maintains discrete odour phenotypes in hybridizing species

Ophrys orchids mimic the female sex pheromones of their pollinator species to attract males for pollination. Reproductive isolation in Ophrys is based on the selective attraction of only a single pollinator species. A change of floral odour can result in the attraction of a new pollinator species that acts as an isolation barrier towards other sympatrically occurring Ophrys species. Ophrys lupercalis, Ophrys bilunulata, and Ophrys fabrella grow sympatrically and bloom consecutively on Majorca and are pollinated by three species of Andrena. We investigated variation of phenotypic and genotypic flower traits, aiming to study the role of the floral odour for reproductive isolation and speciation. Using chemical and electrophysiology (gas chromatography coupled with an electroantennographic detector) methods, we show that the three Ophrys species use the same odour compounds for pollinator attraction, but in different proportions. A comparison of the floral odour bouquets in a multivariate analysis revealed a clear grouping of plants from the same species, although with an overlap between species. A comparison of the same plants using molecular markers gave a contrasting result. Although O. lupercalis and O. fabrella were genetically well separated, plants of O. bilunulata did not form a distinct group but were similar to either O. lupercalis or O. fabrella. Our data indicate gene flow and hybridization to occur between O. bilunulata and O. lupercalis as well as between O. bilunulata and O. fabrella. All plants of O. bilunulata, despite having different genotypes, showed a very similar floral odour. This reflects a strong selective pressure by the pollinating males. The overlap of genotypes of O. bilunulata and O. fabrella supports our hypothesis that O. fabrella diverged from O. bilunulata by scent variation and the attraction of a new pollinator species, Andrena fabrella.

Association of the dystrobrevin binding protein 1 gene (DTNBP1) in a bipolar case–control study (BACCS)

Recent studies suggest a degree of overlap in genetic susceptibility across the traditional categories of schizophrenia and bipolar disorder. There is some evidence for an association of the dystrobrevin binding protein 1 gene (DTNBP1) with schizophrenia, and, thus, this gene has also become a focus of further investigation in bipolar disorder (BD). The aim of our study is to explore the association of DTNBP1 with BD and with a sub phenotype, presence/absence of psychotic symptoms, in a sample of 515 patients with BD (ICD10/DSMIV) and 1,316 ethnically matched control subjects recruited from the UK. Seven DTNBP1 SNPs: rs2743852 (SNP C), rs760761 (P1320), rs1011313 (P1325), rs3213207 (P1635), rs2619539 (P1655), rs16876571 and rs17470454 were investigated using the SNPlex genotyping system and 1 SNP (rs2619522) genotypes were imputed. Association analyses were conducted in a sample of 452 cases and 956 controls. We found significant differences in genotypic and allelic frequencies of rs3213207 and rs760761 of DTNBP1 between bipolar patients and controls. We also showed a global haplotypic association and an association of a particular haplotype with BD. Our results are consistent with previous studies in term of a general association between DTNBP1 and bipolar disorder and provide additional evidence that a portion of the genotypic overlap between schizophrenia and bipolar affective disorder is attributable to this gene.

Burkholderia pseudomallei Is Genetically Diverse in Agricultural Land in Northeast Thailand

BackgroundThe soil-dwelling Gram-negative bacterium Burkholderia pseudomallei is the cause of melioidosis. Extreme structuring of genotype and genotypic frequency has been demonstrated for B. pseudomallei in uncultivated land, but its distribution and genetic diversity in agricultural land where most human infections are probably acquired is not well defined.MethodsFixed-interval soil sampling was performed in a rice paddy in northeast Thailand in which 100 grams of soil was sampled at a depth of 30 cm from 10×10 sampling points each measuring 2.5 m by 2.5 m. Soil was cultured for the presence of B. pseudomallei and genotyping of colonies present on primary culture plates was performed using a combination of pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST).Principal Findings B. pseudomallei was cultured from 28/100 samples. Genotyping of 630 primary colonies drawn from 11 sampling points demonstrated 10 PFGE banding pattern types, which on MLST were resolved into 7 sequence types (ST). Overlap of genotypes was observed more often between sampling points that were closely positioned. Two sampling points contained mixed B. pseudomallei genotypes, each with a numerically dominant genotype and one or more additional genotypes present as minority populations.ConclusionsGenetic diversity and structuring of B. pseudomallei exists despite the effects of flooding and the physical and chemical processes associated with farming. These findings form an important baseline for future studies of environmental B. pseudomallei.

A case report of a family with overlapping features of autosomal dominant febrile seizures and GEFS+

Familial febrile seizures occur in both generalized epilepsy with febrile seizures plus (GEFS+) and autosomal dominant febrile seizures (ADFS). The literature largely separates families with GEFS+ from those with ADFS. However, there is clinical overlap, and families with ADFS also include individuals with afebrile seizures. The phenotypic spectrum of GEFS+ is broader now than when first described, resulting in unclear boundaries between these two familial syndromes. The purpose of this report is to highlight the phenotypic similarities of GEFS+ and ADFS. A multigenerational family with febrile and afebrile seizures is described and the clinical features are compared to those of previously reported GEFS+ and ADFS families. This family meets the requirements for both ADFS and the broader definition of GEFS+. Linkage analysis has shown no clear linkage to known febrile seizure or GEFS+ loci. Despite locus heterogeneity, identified mutations in reported GEFS+ have so far all been in sodium channel or gamma-aminobutyric acid (GABA)-receptor genes, with other modifier genes postulated to affect individual phenotypes. The two mutations identified in families with ADFS are in genes implicated in GEFS+, SCN1A, and GABRG2. We conclude that it is inappropriate to separate GEFS+ and ADFS at present given the clinical and genotypic overlap.

Association of DISC1 and TSNAX genes and affective disorders in the depression case–control (DeCC) and bipolar affective case–control (BACCS) studies

The gene known as Disrupted-in-Schizophrenia-1, DISC1, was originally discovered in a large family, in which it also co-segregated with bipolar affective disorder (BD) and with major depressive disorder (MDD). The TSNAX (Translin-associated factor X) gene, located immediately upstream of DISC1, has also been suggested as a candidate gene in relation to psychiatric illness, as one transcript resulting from intergenic splicing encodes a novel TSNAX-DISC1 fusion protein. We explored the TSNAX-DISC1 gene region for an association with BD and MDD in a sample of 1984 patients (1469 MDD, 515 BD) and 1376 ethnically matched controls. Eight single nucleotide polymorphisms (SNPs) within the TSNAX-DISC1 region (rs766288, rs3738401, rs2492367, rs6675281, rs12133766, rs1000731, rs7546310 and rs821597) were investigated using the SNPlex Genotyping System. We found a significant allelic and genotypic association of the TSNAX-DISC1 gene region with BD, whereas a haplotypic association was found for both BD and MDD. Therefore, our results suggest an association between the TSNAX-DISC1 region and both forms of affective disorders, and support the hypothesis that a portion of the genotypic overlap between schizophrenia and affective disorders is attributable to this gene.

Analysis of an acute Chagas disease outbreak in the Brazilian Amazon: human cases, triatomines, reservoir mammals and parasites

An outbreak of Chagas disease occurred in Mazagão, Amapá, Brazilian Amazon in 1996. Seventeen of 26 inhabitants presented symptoms compatible with acute Chagas disease and were submitted to parasitological and serological tests. All 17 were positive in at least one parasitological test and 11 were also IgM or IgG anti-Trypanosoma cruzi positive. The nine asymptomatic patients were negative for parasites and one was positive for IgG anti-T. cruzi. Sixty-eight triatomines were captured (66 Rhodnius pictipes; two Panstrongylus geniculatus); 45 were infected with T. cruzi (43 R. pictipes; two P. geniculatus). Thirteen trypanosomatid strains were isolated: eight from humans and five from R. pictipes. Four were genotyped as T. cruzi I (two from humans; two from R. pictipes), seven as T. cruzi Z3 (six from humans; one from R. pictipes) and two as T. cruzi Z3 and T. rangeli (from R. pictipes). Treatment started for all patients leading to a decrease in parasitaemia in 16 during the follow-up period (6 months, 1, 5 and 7 years). All were serologically negative 7 years post-treatment. There was an overlap of genotypes in the same ecotope, raising the possibility of transmission through the oral route and the need for early therapeutic intervention for better patient management in the Brazilian Amazon.

Molecular Characterization of Giardia Isolates from Calves and Humans in a Region in Which Dairy Farming Has Recently Intensified

Giardiasis is a notifiable disease of high prevalence in New Zealand, but there is limited knowledge about the sources of Giardia duodenalis genotypes that can potentially cause human infections. Dairy calves are one environmental source of Giardia isolates, but it is unknown whether they harbor genotypes that are potentially capable of causing infections in humans. To address these questions, 40 Giardia isolates from calves and 30 from humans, living in the same region and collected over a similar period, were genotyped using the beta-giardin gene. The G. duodenalis genetic assemblages A and B were identified from both calves and humans, and genotype comparisons revealed a substantial overlap of identical genotypes from the two hosts for both assemblages. Significantly, no assemblage E (the genotype commonly found in cattle elsewhere in the world) has been detected in New Zealand livestock to date. Given recent and rapid land use conversions to dairy farming in many South Island regions of New Zealand, an increasingly large concentration of domestic cattle harboring genotypes potentially capable of causing infections in humans is particularly concerning.

Cognitive dysfunction in adults with Van der Woude syndrome

PurposeVan der Woude syndrome is an autosomal dominant disorder manifested in clefts of the lip and/or palate and lip pits. There is phenotypic and genotypic overlap between Van der Woude syndrome and isolated cleft of the lip and/or palate. Subjects with isolated cleft of the lip and/or palate have been shown to have cognitive dysfunction. Given the similarities between Van der Woude syndrome and isolated cleft of the lip and/or palate, the current study was designed to evaluate the pattern of cognitive function in adults with Van der Woude syndrome. MethodsFourteen adults with Van der Woude syndrome were compared with age- and gender-matched controls. A battery of cognitive tests was administered to determine general IQ as well as more specific cognitive performance measures. ResultsAll subjects with Van der Woude syndrome showed deficits in performance on an executive function task. In addition, males with Van der Woude syndrome had significantly lower scores on intelligence measures and on a verbal fluency task compared with controls. ConclusionThe pattern of cognitive function in Van der Woude syndrome is very similar to that seen in isolated cleft of the lip and/or palate. In addition, the findings have a significant gender effect in which males are more severely affected than females. This pattern is common to other conditions with a neurodevelopmental etiology, supporting the notion that the cognitive deficits of Van der Woude syndrome and isolated cleft of the lip and/or palate are due to abnormal brain development.

Choroid plexus carcinomas and rhabdoid tumors: phenotypic and genotypic overlap.

Five of six poorly differentiated choroid plexus carcinomas identified at our institution contained cells displaying a rhabdoid phenotype. Immunoperoxidase stains showed focal positivity for cytokeratin, epithelial membrane antigen, glial fibrillary acidic protein, S100, and vimentin. The MIB-1 proliferative index ranged from 7.0% to 27.1%. All six tumors were p53 positive. Only the one child with Li-Fraumeni syndrome had a p53 germline mutation. Electron microscopy verified choroid plexus differentiation and the co-existence of rhabdoid cells. Of the five studied, four had deletions of chromosome 22 [three with monosomy 22 and one with del(22)(q12)]. Thus, there was a phenotypic and genotypic overlap between choroid plexus carcinomas and rhabdoid tumors.

Systematic mutation analysis of the catechol O-methyltransferase gene as a candidate gene for schizophrenia.

Catechol O-methyltransferase (COMT) is involved in the degradation of catecholamine neurotransmitters. Recent linkage studies of schizophrenia and molecular studies of velocardiofacial syndrome suggest that the COMT gene might be a candidate gene for schizophrenia. The authors systematically searched for mutations and microdeletion of the COMT gene in 177 Chinese schizophrenic patients from Taiwan; 99 comparison subjects were also studied. Five molecular variants were identified: c.186C > T at exon 3, c.408C > G at exon 4, c.472G > A at exon 4, c.597G > A at exon 5, and c.821-827insC at the 3' untranslated region. However, no differences in the genotype and haplotype frequencies of these molecular variants between the schizophrenic and comparison subjects were detected. Furthermore, no microdeletion was identified among the patients. These data suggest that the COMT gene does not play a major role in the pathogenesis of schizophrenia, and the genotypic overlap between schizophrenia and velocardiofacial syndrome was rare in this cohort.

Seasonal variation as a determinant of population structure in rotifers reproducing by cyclical parthenogenesis

Monogonont rotifers live in habitats that display extensively variation in both biotic and abiotic components. Much of this variation is seasonal and therefore predictable for a given pond or lake. In 1972, King proposed one physiological and two genetic models presenting alternative modes of adaptation to this temporal variation. Our purpose in the present paper is to review and evaluate how our knowledge of the seasonal structure of rotifer populations has changed in the past 25 years. Seasonal changes in clone frequencies have been reported from three studies of natural populations using electrophoretic analysis of isozymes. In one of these studies there was evidence for substantial temporal overlap of multilocus genotypes suggesting that these clones were broad-niched generalists. By contrast, both the genetic and ecological analyses in the other two studies support a non-overlap model in which clonal groups are composed of narrow-niched specialists that undergo seasonal succession. In both of these studies the clonal groups appear to have achieved the status of sibling species, a phenomenon that we conclude is probably common in monogonont rotifers. Strong competition promotes reproductive isolation between successive groups of seasonal specialists. The existence of this competition has been inferred from natural populations and demonstrated by studies in the laboratory. Also required, and also supported by field observations, is a temporal separation of periods of mictic (sexual) reproduction. A final requirement of the nonoverlap model is seasonal variation in the timing of resting egg hatching. That is, clones established from hatching of resting eggs must enter a physiologically appropriate habitat if they are to increase in number and achieve a competitive advantage. Unfortunately, we still have little information on this topic. Finally, we present the results of a study analyzing the effects of variation in the mictic ratio (i.e., the relative frequency of mictic females) on the adaptive structure of rotifer populations. Mixis may shift the balance between costs and benefits of specialization thereby producing seasonally specialized populations that overlap in space but not time. Life history patterns may therefore provide fundamental insights on the adaptation of rotifers to the extensive temporal variation in their environments.

Phenotypic and genotypic overlap between atelosteogenesis type 2 and diastrophic dysplasia

Mutations in the diastrophic dysplasia sulfate transporter gene DTDST have been associated with a family of chondrodysplasias that comprises, in order of increasing severity, diastrophic dysplasia (DTD), atelosteogenesis type 2 (AO2), and achondrogenesis type 1B (ACG1B). To learn more about the molecular basis of DTDST chondrodysplasias and about genotype-phenotype correlations, we studied fibroblast cultures of three new patients: one with AO-2, one with DTD, and one with an intermediate phenotype (AO2/DTD). Reduced incorporation of inorganic sulfate into macromolecules was found in all three. Each of the three patients was found to be heterozygous for a c862t transition predicting a R279W substitution in the third extracellular loop of DTDST. In two patients (DTD and AO2/DTD), no other structural mutation was found, but polymerase chain reaction amplification and single-strand conformation polymorphism analysis of fibroblast cDNA showed reduced mRNA levels of the wild-type DTDST allele: these two patients may be compound heterozygotes for the "Finnish" mutation (as yet uncharacterized at the DNA level), which causes reduced expression of DTDST. The third patient (with AO2) had the R279W mutation compounded with a novel mutation, the deletion of cytosine 418 (delta c418), predicting a frameshift with premature termination. Also the delta c418 allele was underrepresented in the cDNA, in accordance with previous observations that premature stop codons reduce mRNA levels. The presence of the DTDST R279W mutation in a total of 11 patients with AO2 or DTD emphasizes the overlap between these conditions. This mutation has not been found so far in 8 analyzed ACG1B patients, suggesting that it allows some residual activity of the sulfate transporter.

Isozyme evidence for host races of the fungus Atkinsonella hypoxylon (Clavicipitaceae) infecting the Danthonia (Poaceae) complex in the southern Appalachians

On granite outcrops around Highlands, North Carolina four sympatric Danthonia grass species are infected by the fungus Atkinsonella hypoxylon (Clavicipitaceae). Danthonia epilis and D. sericea (rare hosts) are infected only in this small region, although they range more widely, while D. compressa and D. spicata (widespread hosts) are infected throughout their ranges in eastern North America. To test the hypothesis that infection of the rare hosts has recently spread from the widespread hosts, the genetic structure of fungal populations on different Danthonia hosts was investigated by isozyme electrophoresis. Two hundred isolates were collected from 46 sites and analyzed for ten polymorphic isozyme loci to examine levels of variation, genetic differentiation, and gene flow among fungal populations infecting different hosts. Genetic identities (I) indicated that isolates from within each of the widespread and rare host species pairs were homogeneous (I > 0.99), but the pairs were highly differentiated (0.66 < I < 0.69). With one exception there was no overlap in fungal genotypes isolated from the widespread hosts (12 genotypes) vs. the rare hosts (seven genotypes). Two plants of D. compressa growing sympatrically with infected D. epilis at one site were infected with a genotype characteristic of the latter species. Estimates of gene flow suggest a high degree of host-mediated reproductive isolation in A. hypoxylon, indicating that the fungus forms distinct, long-standing host races or sibling species in the southern Appalachians.