ISOLATION BY DISTANCE

Spatial analysis of nuclear and mitochondrial RFLP genotypes in populations of the chestnut blight fungus, Cryphonectria parasitica

Spatial structure of both nuclear and mitochondrial RFLPs were studied in several populations of the chestnut blight fungus, Cryphonectria parasitica, using a variety of spatial autocorrelation tests designed to detect nonrandom patterns. Fungal individuals were sampled from cankers on infected chestnut trees, and the location of each tree was mapped. Single-locus nuclear RFLPs, nuclear fingerprints, and mitochondrial DNA haplotypes were determined for each individual. Individuals with the same DNA fingerprint genotypes occurred closer together than would be expected at random in four of the five plots, while mitochondrial DNA haplotypes were aggregated in all five plots. Genetic distances between individuals, expressed as one minus the proportion of shared restriction fragment size classes for fingerprints and mitochondrial haplotypes, were significantly correlated with Euclidean distances between individuals in four of the five populations, but these correlations were very weak (r < 0.18). The same DNA fingerprint and single-copy nuclear RFLP alleles occurred on the same trees or immediately neighbouring trees more often than would be expected at random. Most of the aggregation for all three genetic markers occurred among individuals within the same cluster of chestnut stems or on neighbouring trees. Lack of spatial autocorrelation in one population was probably due to sampling on a larger scale that was too coarse to detect any patterns. Significant aggregation of genotypes in C. parasitica is most likely caused by some degree of restricted dispersal within populations. The implications of restricted dispersal are discussed in relation to the breeding system and isolation by distance in populations of C. parasitica.

Spatial distributions of genotypes under isolation by distance

The spatial distributions of single-locus diploid genotypes, produced within populations of sexually reproducing individuals under isolation by distance, are measured and characterized in detail by quantifying the join-counts for simulated model populations. The models more realistically reflect spatially explicit distributions of genotypes in populations, because unlike the classical theory, they include the stochasticity inherent in the process of matings between genotypes. This stochasticity causes the formation of large areas or patches containing mostly one homozygous genotype, which is not predicted by the classical theory. A number of previously uncharacterized features of the spatial structures produced under isolation by distance are revealed. Spatial autocorrelation measures based on counts of pairs of homozygotes are highly consistent quantifications of the concentrations of homozygotes in patchy genotypic distributions for a given level of dispersal. Most strikingly, the degree of intermixing of homozygotes with heterozygotes over small spatial scales is much higher than previously thought, unless dispersal is extremely limited (e.g., Wright's neighborhood size less than approximately 5.0). Among implications for studies of spatial structure of real populations, one is that the results provide a basis for using join-counts as estimators of gene dispersal based solely on genetic data.

Wheat storage proteins: glutenin diversity in wild emmer, Triticum dicoccoides, in Israel and Turkey. 2. DNA diversity detected by PCR

Germplasm of Triticum dicoccoides collected from different environments in Israel was evaluated by using the PCR as a molecular marker. Two pairs of primers were used in the PCR in amplifying the DNA tracts coding the high-and low-molecular-weight glutenin subunits. Analyses reveal great variability within and between populations indicating the high values of this germplasm for future breeding programs to improve the protein quality in wheat.

Estimation, variance and optimal sampling of gene diversity II. Diploid locus

Nei's analysis of diversity at a diploid locus is extended to a population subdivided into a large number of subpopulations. The diversities and the heterozygotes frequency are defined with respect to the total population and unbiasedly estimated in a two-stage random cluster sampling. The fixation indices F IS, F IT andF ST are derived, then inter- and intra-population variances of the estimated parameters are studied. We show that there is a unique sample size per population which yields the best accuracy in estimatingF ST and F IS, respectively, at a given locus. These results are illustrated with an analysis of DNA diversity in a forest tree and compared to those obtained under the Hardy-Weinberg assumption.

The effect of relatedness on likelihood ratios and the use of conservative estimates

DNA profiling can be used to identify criminals through their DNA matching that left at the scene of a crime. The strength of the evidence supplied by a match in DNA profiles is given by the likelihood ratio. This, in turn, depends upon the probability that a match would be produced if the suspect is innocent. This probability could be strongly affected by the possibility of relatedness between the suspect and the true source of the scene-of-crime DNA profile. Methods are shown that allow for the possibility of such relatedness, arising either through population substructure or through a family relationship. Uncertainties about the likelihood ratio have been taken as grounds for the use of very conservative estimates of this quantity. The use of such conservative estimates can be shown to be neither necessary nor harmless.

Population subdivision and gene flow in Danish house mice

Genetic subdivision in local populations of the European house mice, Mus musculus domesticus and M. m. musculus, was analysed to study patterns of gene flow. The data consisted of frequencies of microsatellite alleles in 16 samples (250 individuals) from a total of 11 sites in Jutland, which included successive samples from three sites. Sequences of the control region of mitochondrial DNA in three successive samples from one site were also analysed. Microsatellite genotype frequencies within samples were close to Hardy-Weinberg expectations. Levels of microsatellite differentiation among samples (theta = 0.05-0.21) corresponded to limited gene flow at migration-drift equilibrium (Nm = 1-5). Weak isolation by distance for microsatellites in M.m. musculus suggested that gene flow tends to occur among neighbouring sites. Estimates of effective population size over a few generations were much lower than those corresponding to the long periods needed for arrival at mutation-drift equilibrium. This suggested that subpopulations had been influenced by gene flow since formation, or had originated recently from genetically diverse founders.

Population structure of morphological traits in Clarkia dudleyana. I. Comparison of FST between allozymes and morphological traits

Studies of genetic variation at allozyme loci, assumed to be selectively neutral, have provided valuable insights into the genetic structure of numerous populations. The degree to which population structure of allozyme variation reflects that of quantitative traits, however, is not well resolved. Here, we compare estimates of population differentiation (FST) of 11 populations for allozymes with those for nine discrete and nine continuous morphological traits. Overall, the allozymes have the lowest FST estimates, indicating relatively little population differentiation. Excepting two traits, petal width and long internode length, the continuous morphological traits have estimates similar to those from allozymes. The discrete morphological traits tend to have the highest estimates. On a single trait basis, estimates of FST for four discrete and two continuous traits are higher than those for allozymes. A more detailed (narrow-sense quantitative) genetic study of two populations suggests that these estimates of FST may underestimate the true value because of dominance. Clustering analyses show that the pattern of differentiation for the discrete morphological traits strongly reflects the geographical distribution of the populations, whereas the patterns for the continuous traits and allozymes do not. These results suggest that selection has been occurring on the discrete morphological traits, selecting toward a common optimum within each geographic group, and optima differing among geographic groups.

Spatial structure of two-locus genotypes under isolation by distance

Extensive Monte Carlo simulations are conducted of spatial distributions of two-locus genotypes in large, continuous populations under isolation by distance models. The results show that substantial patches of double homozygotes are present in the spatial structures, even when loci are unlinked. The stochastic spread of identical two-locus genotypes largely outpowers the tendency for recombination to decouple patterns for separate loci. A spatial patch is a large area containing mostly one double homozygous genotype in a highly contiguous constellation. This patch structure is reflected in high positive spatial autocorrelations and large excesses of pairs, or joins, of identical double homozygotes at short-to-intermediate distances of spatial separation. Although spatial patches of double homozygotes are the dominant spatial feature, and the major contributors to overall high levels of autocorrelations among two-locus genotypes, other substantial features include areas of concentrations of identical genotypes heterozygous at only one locus. One implication of the patch structure is the presence of high levels of linkage disequilibrium, caused by isolation by distance even for unlinked loci, at some spatial scales; yet the disequilibrium in the large total populations is near 0. Thus linkage disequilibrium produced by isolation by distance is highly dependent on spatial scale. Another implication is that high degrees of spatial structuring and autocorrelations are produced for genetic variation controlling quantitative traits, at least when the number of loci is relatively small, under a wide range of situations, even if the trait is selectively neutral. The significance of the results to field studies is also examined.

Multiple-population versus hierarchical conifer breeding programs: a comparison of genetic diversity levels

Advanced-generation domestication programs for forest-tree species has raised some concerns about the maintenance of genetic diversity in forest-tree breeding programs. Genetic diversity in natural stands was compared with two genetic conservation options for a third-generation elite Pinus taeda breeding population. The breeding population was subdivided either on the basis of geographic origin and selection goals (multiple-population or MPBS option) or stratified according to genetic value (hierarchical or HOPE option). Most allelic diversity in the natural stands of loblolly pine is present in the domesticated breeding populations. This was true at the aggregate level for both multiple-population (MPBS) and the hierarchical (HOPE) populations. Individual subpopulations within each option had less genetic diversity but it did not decline as generations of improvement increased. Genetic differentiation within the subdivided breeding populations ranged from 1 to 5%, genetic variability is within each subpopulation rather than among subpopulations for both MPBS (>95%) and the HOPE approaches (>98%). Nei's Gst estimates for amongpopulation differentiation were biased upwards relative to estimates of θ from Weir and Cockerham (1984).

Population structure, stepwise mutations, heterozygote deficiency and their implications in DNA forensics

In a substructured population the overall heterozygote deficiency can be predicted from the number of subpopulations (s), their time of divergence (t), and the nature of the mutations. At present the true mutational mechanisms at the hypervariable DNA loci are not known. However, the two existing mutation models (the infinite allele model (IAM) and the stepwise mutation model (SMM)) provide some guides to predictions from which the possible effect of population substructuring may be evaluated, assuming that the subpopulations do not exchange any genes among them during evolution. The theory predicts that the loci with larger mutation rate, and consequently showing greater heterozygosity within subpopulations, should exhibit a smaller proportional heterozygote deficiency (GST) and, hence, the effects of population substructuring should be minimal at the hypervariable DNA loci (an order of magnitude smaller than that at the blood group and protein loci). Applications of this theory to data on six Variable Number of Tandem Repeat (VNTR) loci and five short tandem repeat (STR) loci in the major cosmopolitan populations of the USA show that while the VNTR loci often exhibit a large significant heterozygote deficiency, the STR loci do not show a similar tendency. This discordant finding may be ascribed to the limitations, coalescence and nondetectability of alleles associated with the restriction fragment length polymorphism (RFLP) analysis through which the VNTR loci are scored. Such limitations do not apply to the polymerase chain reaction (PCR) method, through which the STR loci are scored. The implications of these results are discussed in the context of the forensic use of DNA typing data.

Estimation, variance and optimal sampling of gene diversity : I. Haploid locus

An extension of Nei's analysis of diversity in a subdivided population is proposed for a haploid locus. The differentiation G STbecomes a natural extension of Wright's F STand generalizes Weir and Cockerham's parameter of co-ancestry by relaxing the assumption of identical correlation for all the alleles. Inter- and intrapopulation variances of the estimated diversities and differentiation are derived. Finally, the optimal sampling strategy for measuring G STwhen a fixed number of individuals can be analysed is considered. It is shown that, at a given locus, there is a unique sample size per population which yields the smallest variance of G ST,regardless of the number of populations studied. These theoretical developments are illustrated with an analysis of chloroplast DNA diversity in a forest tree. The results emphasize the necessity of sampling many populations, rather than many individuals per population, for an accurate measurement of the subdivision of gene diversity at a single locus.

Mitochondrial DNA and Ascaris microepidemiology: the composition of parasite populations from individual hosts, families and villages

Patterns of genetic subdivision in parasite populations can provide important insights into transmission processes and complement information obtained using traditional epidemiological techniques. We describe mitochondrial sequence variation in 265 Ascaris collected from 62 individual hosts (humans and pigs) from 35 households in 3 Guatemalan locations. Restriction mapping of individual worms revealed 42 distinct mitochondrial genotypes. We ask whether the mitochondrial genotypes found in worms from individual hosts, from families of hosts and from villages represent random samples from the total Ascaris population. Patterns of genetic subdivision were quantified using F-statistics, while deviations from the null hypothesis of randomness were evaluated by a simple resampling procedure. The analysis revealed significant deviations from panmixia. Parasite populations were strongly structured at the level of the individual host in both humans and pigs: parasites bearing the same mitochondrial genotype were found more frequently than would be expected by chance within hosts. Significant heterogeneity was also observed among populations from different villages, but not from different families within a village. The clustering of related parasites within hosts suggests a similar clustering of related infective stages in the environment and may explain why sex ratios in Ascaris are female-biased. We discuss aspects of Ascaris biology which may lead to the observed patterns.

Genetic population structure and gene flow in the Atlantic cod Gadus morhua: a comparison of allozyme and nuclear RFLP loci

High levels of gene flow have been implicated in producing uniform patterns of allozyme variation among populations of many marine fish species. We have examined whether gene flow is responsible for the limited population structure in the Atlantic cod, Gadus morhua L., by comparing the previously published patterns of variation at 10 allozyme loci to 17 nuclear restriction fragment length polymorphism (RFLP) loci scored by 11 anonymous cDNA clones. Unlike the allozyme loci, highly significant differences were observed among all populations at the DNA markers in a pattern consistent with an isolation-by-distance model of population structure. The magnitude of allele frequency variation at the nuclear RFLP loci significantly exceeded that observed at the protein loci (chi 2 = 24.6, d.f. = 5, P < 0.001). Estimates of gene flow from the private alleles method were similar for the allozymes and nuclear RFLPs. From the infinite island model, however, estimates of gene flow from the DNA markers were fivefold lower than indicated by the proteins. The discrepancy between gene flow estimates, combined with the observation of a large excess of rare RFLP alleles, suggests that the Atlantic cod has undergone a recent expansion in population size and that populations are significantly displaced from equilibrium. Because gene flow is a process that affects all loci equally, the heterogeneity observed among populations at the DNA level eliminates gene flow as the explanation for the homogeneous allozyme patterns. Our results suggest that a recent origin of cod populations has acted to constrain the extent of population differentiation observed at weakly polymorphic loci and implicate a role for selection in affecting the distribution of protein variation among natural populations in this species.

Hierarchical structure of mitochondrial DNA gene flow among humpback whales Megaptera novaeangliae, world-wide

The genetic structure of humpback whale populations and subpopulation divisions is described by restriction fragment length analysis of the mitochondrial (mt) DNA from samples of 230 whales collected by biopsy darting in 11 seasonal habitats representing six subpopulations, or 'stocks', world-wide. The hierarchical structure of mtDNA haplotype diversity among population subdivisions is described using the analysis of molecular variance (AMOVA) procedure, the analysis of gene identity, and the genealogical relationship of haplotypes as constructed by parsimony analysis and distance clustering. These analyses revealed: (i) significant partitioning of world-wide genetic variation among oceanic populations, among subpopulations or 'stocks' within oceanic populations and among seasonal habitats within stocks; (ii) fixed categorical segregation of haplotypes on the south-eastern Alaska and central California feeding grounds of the North Pacific; (iii) support for the division of the North Pacific population into a central stock which feeds in Alaska and winters in Hawaii, and an eastern or 'American' stock which feeds along the coast of California and winters near Mexico; (iv) evidence of genetic heterogeneity within the Gulf of Maine feeding grounds and among the sampled feeding and breeding grounds of the western North Atlantic; and (v) support for the historical division between the Group IV (Western Australia) and Group V (eastern Australia, New Zealand and Tonga) stocks in the Southern Oceans. Overall, our results demonstrate a striking degree of genetic structure both within and between oceanic populations of humpback whales, despite the nearly unlimited migratory potential of this species. We suggest that the humpback whale is a suitable demographic and genetic model for the management of less tractable species of baleen whales and for the general study of gene flow among long-lived, mobile vertebrates in the marine ecosystem.

The effect of relatives on the likelihood ratio associated with DNA profile evidence in criminal cases

In DNA profiling sometimes a match is declared between the DNA profile from a suspect and that from a scene-of-crime DNA sample. DNA evidence has frequently been presented in the form of a likelihood ratio, the ratio of the probabilities of the data set under the two hypotheses of a single and two sources for the matching DNA profiles. The calculation of the probability of a match is usually performed using a product rule with information from an appropriate database. This approach has been criticized for failing to allow for genetic relatedness, such that the suspect could be a close relative of the source of the scene-of-crime DNA profile. This paper suggests ways of incorporating the possibility of relatives into the likelihood ratio, and shows that unless there is strong evidence implicating a full sibling of the accused, allowing for possible relatedness has very little impact.

HLA-DPB1 DNA polymorphism in the Swiss population: linkage disequilibrium with other HLA loci and population genetic affinities

Allelic diversity at the HLA-DPB1 locus was determined by PCR-oligotyping in a sample of 125 healthy Swiss individuals. A total of 17 alleles were detected among which four main alleles (DPB1*0401, *0201, *0301, *0402) reached a cumulative frequency of 74.8%. HLA-A and -B (by serology) and HLA-DRB1 (by oligotyping) allelic polymorphisms were analysed also. HLA-B and HLA-DRB1 loci were highly polymorphic with 25 and 28 alleles respectively and similar heterozygosity levels of 0.93 and 0.92. These two loci were found to be more polymorphic than expected under neutrality, while lower heterozygosity levels were found for HLA-A (0.87) and DPB1 (0.81) loci. This paper presents also a global comparison of DPB1 allelic frequencies among 15 populations from four continents. As opposed to the DRB1 locus, overall DPB1 is shown to have a lower level of polymorphism and may be considered as neutral in all tested populations. DPB1 genetic diversity is correlated significantly with geography also, as found previously for DRB1. Two- and four-locus haplotype frequencies were determined and the significance of their linkage disequilibrium tested by an original non-parametric method. A significant positive linkage disequilibrium was found for 11 A-B, 16 B-DRB1, 7 DRB1-DPB1 and 3 A-B-DRB1-DPB1 haplotypes. The overall linkage disequilibrium between DRB1 and DPB1 was much lower than expected from the physical distance and lower than for A-B and B-DRB1 pairs. The implications of these results for bone marrow transplantation and for the evolution of HLA loci are discussed.

Two new members in the Contracaecum osculatum complex (Nematoda, Ascaridoidea) from the Antarctic

The genetic structure of adults and larvae of Contracaecum osculatum (sensu lato) from the Antarctic is analyzed on the basis of 24 enzyme loci. Significant deviations of genotype frequencies from the Hardy-Weinberg equilibrium were found, even in samples recovered from the same host. These data indicate that two distinct, reproductively isolated species coexist in C. osculatum (sensu lato) samples from the Antarctic. They were provisionally designated C. osculatum D and E, as they do not correspond to any of the three species previously detected in this complex from the Atlantic Arctic Boreal region (C. osculatum A, B and C). An allozyme diagnostic key for the identification of the five members of the C. osculatum complex, at the larval and adult stage and in both sexes, is given. Species D and E were found to be genetically quite variable: average P99 = 84.3, A = 3.3 and He = 0.23. Both showed high values of intraspecific gene flow: Nm = 4.6 and 6.1 respectively; similar values were found for the Arctic-Boreal C. osculatum A, B and C. The most related members of the complex are the Antarctic species E and the Arctic-Boreal species A (DNei = 0.21), while the most differentiated ones are the Arctic-Boreal species B and C (DNei = 0.76). The evolutionary divergence of C. osculatum C started more than 3 million years ago, in a Pliocene refugium (Baltic Sea). As to the other C. osculatum species, their evolutionary divergence took place during Pleistocene, when this complex achieved a bipolar distribution. This process involved two distinct colonizations of the marine Antarctic region by ancestors of the northern hemisphere, about 1.5 and 1 million years ago, giving origin to C. osculatum D and E respectively.

Population structure of a predatory beetle: the importance of gene flow for intertrophic level interactions

Migration and gene flow of natural enemies play an important role in the stability of predator-prey interactions and community organization in both natural and managed systems. Yet, relative to that of their herbivorous insect prey, the genetic structure of natural enemy populations has been little studied. We present evidence that populations of the predatory coccinellid beetle Coleomegilla maculata (Coleoptera: Coccinellidae), are not genetically subdivided and that levels of gene flow among these populations are extremely high. Furthermore, in the same geographical area, gene flow of C. maculata was significantly (one order of magnitude) greater than that of an abundant prey species, the Colorado potato beetle Leptinotarsa decemlineata (Coleoptera: Chrysomelidae). The high mobility of this natural enemy relative to the insect herbivores on which it feeds may contribute to its effectiveness as a biological control agent in agricultural systems.

The genetic structure of metapopulations and conservation biology

A range of models describing metapopulations is surveyed and their implications for conservation biology are described. An overview of the use of both population genetic elements and demographic theory in metapopulation models is given. It would appear that most of the current models suffer from either the use of over-simplified demography or the avoidance of selectively important genetic factors. The scale for which predictions are made by the various models is often obscure. A conceptual framework for describing metapopulations by utilising the concept of fitness of local populations is provided and some examples are given. The expectation that any general theory, such as that of metapopulations, can make useful predictions for particular problems of conservation is examined and compared with the prevailing 'state of the art' recommendations.

Application of a random walk model to geographic distributions of animal mitochondrial DNA variation

In rapidly evolving molecules, such as animal mitochondrial DNA, mutations that delineate specific lineages may not be dispersed at sufficient rates to attain an equilibrium between genetic drift and gene flow. Here we predict conditions that lead to nonequilibrium geographic distributions of mtDNA lineages, test the robustness of these predictions and examine mtDNA data sets for consistency with our model. Under a simple isolation by distance model, the variance of an mtDNA lineage's geographic distribution is expected be proportional to its age. Simulation results indicated that this relationship is fairly robust. Analysis of mtDNA data from natural populations revealed three qualitative distributional patterns: (1) significant departure of lineage structure from equilibrium geographic distributions, a pattern exhibited in three rodent species with limited dispersal; (2) nonsignificant departure from equilibrium expectations, exhibited by two avian and two marine fish species with potentials for relatively long-distance dispersal; and (3) a progression from nonequilibrium distributions for younger lineages to equilibrium distributions for older lineages, a condition displayed by one surveyed avian species. These results demonstrate the advantages of considering mutation and genealogy in the interpretation of mtDNA geographic variation.

Effective population size, genetic diversity, and coalescence time in subdivided populations

A formula for the effective population size for the finite island model of subdivided populations is derived. The formula indicates that the effective size can be substantially greater than the actual number of individuals in the entire population when the migration rate among subpopulations is small. It is shown that the mean nucleotide diversity, coalescence time, and heterozygosity for genes sampled from the entire population can be predicted fairly well from the theory for randomly mating populations if the effective population size for the finite island model is used.

Heterogeneous selection in subdivided populations

The dynamics of allele frequencies changing under migration and heterogeneous selection in a subdivided population are investigated. Using perturbation techniques, a stationary state is obtained when migration and selection are both small. Heterogeneous selection leads to a positive correlation between values of F-statistics and heterozygosities when these are compared among sets of subdivided populations. This contrast with a negative value of the correlation obtained under Wright's classical model of homogeneous selection, and with the absence of correlation in the completely neutral situation.

Spatial and space-time correlations in systems of subpopulations with genetic drift and migration

The geographic distribution of genetic variation is an important theoretical and experimental component of population genetics. Previous characterizations of genetic structure of populations have used measures of spatial variance and spatial correlations. Yet a full understanding of the causes and consequences of spatial structure requires complete characterization of the underlying space-time system. This paper examines important interactions between processes and spatial structure in systems of subpopulations with migration and drift, by analyzing correlations of gene frequencies over space and time. We develop methods for studying important features of the complete set of space-time correlations of gene frequencies for the first time in population genetics. These methods also provide a new alternative for studying the purely spatial correlations and the variance, for models with general spatial dimensionalities and migration patterns. These results are obtained by employing theorems, previously unused in population genetics, for space-time autoregressive (STAR) stochastic spatial time series. We include results on systems with subpopulation interactions that have time delay lags (temporal orders) greater than one. We use the space-time correlation structure to develop novel estimators for migration rates that are based on space-time data (samples collected over space and time) rather than on purely spatial data, for real systems. We examine the space-time and spatial correlations for some specific stepping stone migration models. One focus is on the effects of anisotropic migration rates. Partial space-time correlation coefficients can be used for identifying migration patterns. Using STAR models, the spatial, space-time, and partial space-time correlations together provide a framework with an unprecedented level of detail for characterizing, predicting and contrasting space-time theoretical distributions of gene frequencies, and for identifying features such as the pattern of migration and estimating migration rates in experimental studies of genetic variation over space and time.

Hierarchical gene diversity and genetic structure of tribal populations of Andhra Pradesh, India

Gene diversity and genetic structure of tribal populations of Andhra Pradesh, India, have been analyzed under a hierarchical model consisting of five regions of the state, tribes within the regions, and local subpopulations within the tribes. Average gene diversity has been estimated from gene frequency data for 15 polymorphic loci by using nested gene diversity analysis of GST. The intralocation coefficient of gene diversity was estimated at 96% of the total, whereas the intertribal, within--and between--regional gene diversities were found to be only 1.90, 0.95, and 1.43%, respectively. The estimate of gene diversity was higher for loci with higher degrees of polymorphism such as ABO, MN, ESD, and PTC and lower for loci with low-level polymorphism and extreme gene frequencies such as Hb, Tf, PHI, 6PGD, and Hp. The nature of selective preference or neutrality at the loci seems to be important in this respect. Tribes of the plains exhibit the least gene diversity, apparently because of higher gene flow among them. The contribution of loci with intermediate gene frequencies in intertribal and regional gene diversity was found to be higher than for loci with extreme allelic frequencies. These results suggest that the most significant component of variation is between individuals within locations and that variation between local subpopulations is negligible in the genetic structure of a population. Forces like selection, gene flow and drift also influence the diversity depending upon the nature of the locus.

Three sibling species within Contracaecum osculatum (Nematoda, Ascaridida, Ascaridoidea) from the Atlantic Arctic-Boreal region: reproductive isolation and host preferences

Genetic variation within and between population samples from 22 locations of the Atlantic Arctic-Boreal region, including 1657 specimens morphologically assigned to Contracaecum osculatum, was electrophoretically analysed at 17 loci. Highly significant deviations from the Hardy-Weinberg equilibrium were found at various loci in several samples, owing to the existence of three distinct gene pools within C. osculatum (sensu lato) from the study area. These gene pools correspond to three biological species (provisionally designated A, B and C), characterized by distinct genotypes at several diagnostic loci. Reproductive isolation between C.osculatum A, B and C is confirmed by the lack of F1, recombinant, or backcross genotypes in sympatric areas, despite the occurrence of multiple infections. Mean heterozygosity per locus is on average 0.11 in species A, 0.10 in B and 0.07 in C. High levels of gene flow were found within each of the three species, the values of Nm (number of migrant individuals) ranging from 3.41 (C. osculatum C) to 5.77 (C. osculatum A). Average Nei's genetic distance is 0.46 between A and B, 0.50 between A and C and 0.77 between B and C. From these values, times of evolutionary divergence from 2 to 4 million years can be estimated. Genetic relationships among populations and species of the C. osculatum complex are illustrated by principal component analysis. The role of both geographical isolation and host preferences in the speciation of C. osculatum (sensu lato) is discussed. A morphological distinction of the three species has not yet been possible (sibling species). However, there is evidence that the name C. osculatum (sensu stricto) should be used for species C, which shows a geographical distribution and definitive host corresponding to the neotype of C. osculatum (sensu stricto). Finally, a comparison is made between the members of the C. osculatum complex from the Atlantic Arctic-Boreal region and those of the Pseudoterranova decipiens complex from the same area, as to: (i) times of evolutionary divergence, (ii) geographical distribution, and (iii) host preferences.

Coevolution of genes and languages revisited

In an earlier paper it was shown that linguistic families of languages spoken by a set of 38 populations associate rather strongly with an evolutionary tree of the same populations derived from genetic data. While the correlation was clearly high, there was no evaluation of statistical significance; no such test was available at the time. This gap has now been filled by adapting to this aim a procedure based on the consistency index, and the level of significance is found to be much stronger than 10(-3). Possible reasons for coevolution of strictly genetic characters and the strictly cultural linguistic system are discussed briefly. Results of this global analysis are compared with those obtained in independent local analysis.

Genetic diversity at electrophoretic loci in the house fly, Musca domestica L

Vertical polyacrylamide gel electrophoresis was used to separate enzyme proteins at 73 putative loci in natural house fly populations sampled in central Iowa. Thirty-nine of the loci were polymorphic (53%). The mean effective number of alleles per polymorphic locus was 1.93 and 1.47 alleles among 68 scored loci. Observed and expected heterozygosities at 34 house fly loci were 0.1628 and 0.1834, respectively. No statistically significant differentiation was detected among nine central Iowa fly populations in 1989 or among nine Iowa and three Minnesota populations in 1990.