Within-population Component Research Articles

Local Differentiation Among Populations of Arabis stelleri var. japonica in a Sand Dune Habitat

Local variation in ecological and genetic traits was analysed in the perennial herb Arabis stelleri var. japonica in a sand dune area in western Japan. Plants of this species are established in three contiguous habitats (subpopulations): sand dune, forest and an ecotonal zone between both habitats. Morphological, ecological and genetic traits of A. stelleri var. japonica plants were analysed to discover the extent of microdifferentiation among these subpopulations. All plants in nine quadrats were marked and followed during 2 years. The ecotonal zone presented the highest number of mature individuals per quadrat. Plants may produce the same number of shoots per plant but rosette size was larger in plants from sand dune and forest subpopulations than the ecotone. Plants in the sand dune showed the highest number of large capsules per plant. Mortality of seedlings and mature plants was slightly higher in the ecotone than in the other two subpopulations. Sand dune plants had the highest variation among quadrats in almost all traits measured. In the electrophoretic analyses, 21 loci were detected on twelve enzyme systems. Fifteen loci were monomorphic and six loci had 1·2 alleles per loci. Genetic distances and genetic identity Nei's values were very similar among subpopulations (means I = 0·994, D = 0·006). The total genetic diversity (HT) was 0·340 and the within-population component (Hs) was 0·333 indicating low genetic differentiation among subpopulations. Gene flow varied between genes but most of the values were high (mean Nm = 188·3). Subpopulations of A. stelleri showed differentiation in morphological and ecological traits and low genetic differentiation in the loci studied. This may be related with the plastic response of plants to different microenvironments and a significant gene flow between habitats.

Geographical patterns of isozyme variation in Mediterranean populations of perennial ryegrass

Twenty-four natural populations of perennial ryegrass, sampled from the Mediterranean area (Italy, Corsica, Sardinia, South of France), have been studied for allelic variation at 13 enzyme loci. The total genetic diversity results mainly from the within-population component whereas a part of the between-population component can be explained by the geographical origins of the populations. Relationships between accessions have been studied using cluster and multivariate analyses such as principal component and discriminant analyses. Finally, the frequency and distribution of the most discriminating allele have been represented on geographic maps. The results indicate that Corsican populations are similar to Italian accessions whereas both French and Sardinian populations differ from all the others. The origins and relationships between each geographical group of accessions are discussed and, taking historical factors into account, we can suggest that Corsican populations have been introduced from Italy to the eastern side of the island.

Genetic variation in Bromus tectorum (Poaceae): comparison between native and introduced populations

The amount and distribution of genetic variation in 51 native (Eurasian and northern African) populations of Bromus tectorum were assessed at 25 loci using starch gel electrophoresis and were compared with our previous results for introduced (North American) populations of this predominantly cleistogamous grass. More alleles and variable loci were detected across populations in the native range than in North American populations. Within populations, however, the level of polymorphism is higher in the introduced range than in the native range. Deviation from Hardy-Weinberg expectation among native populations is almost as severe as for introduced populations. Eurasian populations exhibit greater genetic differentiation than those in North America; populations from Southwest Asia are the most genetically differentiated. Comparison of single-locus genotypes suggests possible source populations in both Europe and Southwest Asia for the populations now so prominent in western North America. Genetic differences between introduced and native populations of B. tectorum stem from two opposing factors: the reduction in genetic variability across populations produced by founder effects combined with an increase in the within-population component of genetic variation from multiple introductions.

Detection of genetic variation between and within populations of Gliricidia sepium and G. maculata using RAPD markers.

Gliricidia sepium and G. maculata are multi-purpose leguminous trees native to Central America and Mexico. Research programmes have been initiated to define the native distribution of Gliricidia and sample the spectrum of genetic variation. To date, there has been little systematic assessment of genetic variability in multi-purpose tree species. Accurate estimates of diversity between- and within-populations are considered a prerequisite for the optimization of sampling and breeding strategies. We have used a PCR-based polymorphic assay procedure (RAPDs) to monitor genetic variability in Gliricidia. Extensive genetic variability was detected between species and the variability was partitioned into between- and within-population components. On average, most (60 per cent) of the variation occurs between G. sepium populations but oligonucleotide primers differed in their capacity to detect variability between and within populations. Population-specific genetic markers were identified. RAPDs provide a cost-effective method for the precise and routine evaluation of variability and may be used to identify areas of maximum diversity. The approaches outlined have general applicability to a range of organisms and are discussed in relation to the exploitation of multi-purpose tree species of the tropics.

Hierarchical structure of pathogenic variation among Rhynchosporium secalis populations in Idaho and Oregon

Ninety-four isolates of Rhynchosporium secalis, the causal organism of the barley scald disease, from four populations (infected fields) in Idaho and eight in Oregon, were tested for pathogenicity to 14 barley differential cultivars; 60 distinct pathotypes, 33 in Idaho and 28 in Oregon, were found among the 94 isolates. Pathogenicity to most of the differentials was found in each local population sampled. One differential cultivar, CI 5831, was resistant to all isolates tested. Isolates from the same cultivar in different locations usually had different pathogenicities, and most pathotypes were genetically heterogeneous mixtures of many different genotypes based on previous electrophoretic data. The sampled cultivars generally had few or no resistance genes, suggesting that the R. secalis populations in Idaho and Oregon are characterized by large amounts of seemingly unnecessary pathogenicity. When the total pathogenic diversity was partitioned into hierarchical components using the Shannon information statistic, 58% was accounted for by the within-population component, 33% among populations within regions, and only 9% by the between-region component. Because most of the pathogenic variation is within local populations, these populations should adapt rapidly to introduced resistance genes; therefore traditional methods of breeding for resistance to R. secalis using single major resistance genes are not likely to be effective in the Pacific Northwest. Key words: pathogenicity index.

Chloroplast DNA diversity within and among populations of Trifolium pratense

In contrast to animal mitochondrial DNA, intraspecific variation in chloroplast DNA is thought to be very rare. This presumption has prevented the application to plant population biology of the diversity of molecular genetic techniques now well established for animal mitochondrial DNA. In Trifolium pratense, however, extensive intrapopulational variation does exist. In two paper I report a characterization of unprecedented restriction fragment profile variation within single populations. Populations typically contain a common genotype and many rare ones; often the rare genotypes differ from population to population. While both nucleon and nucleotide diversity, as well as estimates of Wright's F ST, indicate a large within-population component and relatively little diversity among populations, the distribution of plastid genotype frequencies in each population is not homogeneous. Estimates of migration rate based on chloroplast DNA genotypes suggest a moderate number of migrants per generation. The unusually high level of genetic variation in Trifolium chloroplast DNA provides the first opportunity to use the plastid genome of plants to study population differentiation. Furthermore, it suggests that the plastid genome may not be as invariant as previously believed, but may instead exhibit high levels of genetic diversity at the population level.

Temporal variation in the ribosomal DNA nontranscribed spacer of Aedes albopictus (Diptera: Culicidae)

Length variation in ribosomal DNA (rDNA) nontranscribed spacer within and among populations of the mosquito Aedes albopictus was studied over a 3-year period in eight different populations from Texas. Statistically significant variation was detected among populations at each sampling date. Significant, but nondirectional, changes were observed within populations over time. An average of 78% of the total variance in the frequencies of spacer variants was attributable to the within-population component, suggesting local differentiation. The results corroborate those from a previous study on spacer length diversity in populations of A. albopictus from around the world and are similar to those from a study on temporal variation in allozyme frequencies in some of the same populations and sampling dates. We propose that a high rate of production of length variants and localized breeding structure could explain the observed high levels of within- and among-population variation in A. albopictus.

Genetics of Brazil nut (Bertholletia excelsa Humb. & Bonpl.: Lecythidaceae)

We provide an estimate of genetic variation within and between two populations of Bertholletia excelsa (Brazil nut), a large canopy tree found in the rain forests of South America. Average heterozygosity is 0.190, and 54.3% of the sampled loci are polymorphic. The population structure deviates significantly from Hardy-Weinberg expectations for Fest2 and Pgm2 (F =0.405 and 0.443, respectively) in one population, and highly significantly (F=-0.341) for Gdh in the other population. Although allele frequencies of the two populations differ significantly for Aat2, Est5, Mdh1, and Mdh2B, Nei's coefficient of gene differentiation (Gst) indicates that the between-population component (Dst) of genic diversity represents only 3.75% of the size of the within-population component (Hs). The implications of these findings in terms of conservation genetics are that much of the genetic diversity of this species may be preserved within one or a few populations. However, such populations must be very large because it appears that the large amount of genetic variation in Brazil nut populations is maintained by extensive gene flow and bonds of mating over a large area. The genetic architecture of Bertholletia excelsa is similar to that expected for an extensively diploidized paleopolyploid species.

Geographic components of linkage disequilibrium in natural populations of Escherichia coli.

Geographic variation in the genetic structure of natural enteric populations of Escherichia coli was assessed at both the single-locus and dilocus levels from allozyme genotypes at 12 enzyme loci in 178 cell lines isolated from human hosts in Sweden, Iowa, and Tonga. Although there was significant heterogeneity in allele frequencies at six of the 12 loci, geographic variation accounted for only 2.0% of the total genetic diversity (HT = 0.518). Ohta's D-statistics were used to partition the total variance of dilocus linkage disequilibrium into within-population and between-population components. The observed total variance in disequilibrium (0.0339), averaged over 66 locus-pairs, was significantly greater than would be expected (0.0103) if alleles were randomly associated in an unstructured total population; and both within-locality and between-locality components made substantial contributions to the total variance. Half the locus-pairs exhibited the specific dual relationship among components expected when random factors are generating disequilibrium, but 20% of the locus-pairs showed the opposite relationship, reflecting systematic allele associations. The magnitude of dilocus disequilibrium apparently is unrelated to the chromosomal distance between loci. This and other evidence indicates that substitutive recombination rates in natural populations are sufficiently low to permit indirect periodic selection to play a prominent role in generating multilocus genetic structure.

Local variation in seeds of ponderosa pine

Variation in seed morphology among populations of ponderosa pine growing in south central Oregon was examined. A total of nine populations was sampled from a range of elevations within each of two distinct soil types: basalt residuals derived from Pliocene basalt flows and wind-deposited pumice originating from the eruption of Mt. Mazama 7600 years ago. Overall, an unusually diverse array of seed morphology was encountered. Slight but significant differences in seed shape and color were present between populations from the two soil types. Except for seed weight, no significant differences were found between populations within soils. Variation in seed weight was negatively correlated with seed source moisture as measured by elevation. For all traits studied, variance components for populations and soils were relatively minor, the within-population component accounting for a major portion of the total variation. Hypotheses are offered to explain the high levels of within-population diversity in seed traits as an adaptation to a spatially heterogeneous environment, as well as a defense against the foraging behavior of seed predators. Also, the possible adaptive significance of the observed variation in seed traits is discussed in relation to seedbed properties of the respective soils.