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The mitochondrial genomics of parasitic nematodes of socio-economic importance: recent progress, and implications for population genetics and systematics.

Mitochondria are subcellular organelles in which oxidative phosphorylation and other important biochemical functions take place within the cell. Within these organelles is a genome, called the mitochondrial (mt) genome, which is distinct from, but cooperates closely with the nuclear genome of the cell. Investigating mt genomes has significant implications for various fundamental research areas, including mt biochemistry and physiology, and, importantly, such genomes provide a rich source of markers for population genetic and systematic studies. While approximately 250 complete mt genome sequences have been determined for a range of metazoan organisms from various phyla, few of these represent parasitic helminths. Until 1998, only two mt genome sequences had been determined for parasitic nematodes, in spite of their socio-economic importance and the need for investigations into their population genetics, taxonomy and evolution. However, since that time, there has been some progress. The main focus of the present chapter is to review the state of knowledge of the mt genomics for parasitic nematodes, to describe recent technological improvements to mt genome sequencing, to summarize applications of mt gene markers for studying the systematics and population genetics of parasitic nematodes, and to emphasize prospects and opportunities for future research in these areas.

Animals↗

Population genetics of 14 ethnic groups using phenotypic data from VNTR loci.

Population genetic studies were performed using five VNTR loci (D2S44, D4S163, D14S13, D17S79, D18S27). The populations examined were Caucasian (Australia, Brazil and U.S.A.), Australian aborigine, Chinese, Amerindian (Cheyenne, Maya, Navajo, Pima, Tobas/Wicnis), North American Black, North American Hispanic (California, Miami, New York, Texas). The overall size range of the alleles for these loci, in PstI-digested DNA, was the same in all populations. The major difference among populations was the relative frequency of particular groups of alleles. These differences were small among similar ethnic groups, while sometimes varying several fold among some of the more distinct populations. However, groups of alleles that were rare in the major ethnic groups (Caucasian, Black, Chinese) were also rare in the other populations. The frequency databases generated by typing individuals for 4 loci were used to compare the random DNA profile frequencies among populations. The results show that the estimated frequency of any 4 locus profile is very low in all populations examined (e.g., median value < 10(-8)). Analysis of relative genetic similarity among populations was used to create the most likely clustering of these ethnic groups. Results show an uncanny similarity between the clusters generated and genetic distance measurements obtained with traditional calculations of conventional genetic markers.

Alleles↗

Population genetic structure of the African malaria mosquito Anopheles funestus in Kenya.

Anopheles funestus Giles is a major malaria vector in Africa, but little is known about the genetic structure of natural populations. In this study, microsatellite markers were used to investigate the genetic structure of A. funestus populations from Kenya. Two populations from western Kenya 80 km apart and two from coastal Kenya 50 km apart were collected and examined for allelic variation at five trinucleotide microsatellite loci. We found A. funestus Giles was the predominant species (> 98%) in the A. funestus group in these populations. The western Kenya populations exhibited higher genetic diversity than the coastal populations. No significant genetic structure for populations within the coastal or western Kenya regions was detected. However, population genetic differentiation between the two regions was high (F(ST) = 0.208, R(ST) = 0.158), approximately two-fold higher than A. gambiae populations from the same area. The results suggest that the minimum area associated with a deme of A. funestus in western or coastal Kenya is larger than 50 km in diameter. The Great Rift Valley in east Africa, high-elevation mountains in western Kenya, and the vast arid area east to the Great Rift Valley may all play a role in restricting A. funestus gene flow between coastal and western Kenya.

Alleles↗

Dispersal and distribution of the tick Ixodes uriae within and among seabird host populations: the need for a population genetic approach.

The aim of this study was to characterize the spatial distribution of the tick Ixodes uriae within and among populations of its seabird hosts and to consider the potential insight that could be gained by a population genetic approach to the issue of dispersal of this tick. Analyses of data collected around the Avalon Peninsula, Newfoundland, indicated that both the prevalence and mean abundance of ticks varied significantly among sample locations. Whereas ticks were found on all 4 host species examined (Rissa tridactyla, Uria aalge, Alca torda, Fratercula arctica), infestation prevalence and mean abundance differed among the species. On R. tridactyla, ticks were significantly aggregated at the among-nest scale and nestling infestation was spatially autocorrelated. Conversely, ticks were not aggregated among chicks within nests. These results enabled us to make a priori predictions regarding tick dispersal and host specificity and suggest there may be spatial structure of Ixodes uriae populations at both macro- and microgeographic scales. Investigating the population genetic structure of ticks within and among populations of hosts with different breeding biologies should provide direct insight into the metapopulation dynamics of such a spatially structured system.

Animals↗

Pathogen population genetics, evolutionary potential, and durable resistance.

We hypothesize that the evolutionary potential of a pathogen population is reflected in its population genetic structure. Pathogen populations with a high evolutionary potential are more likely to overcome genetic resistance than pathogen populations with a low evolutionary potential. We propose a flexible framework to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. According to this framework, pathogens that pose the greatest risk of breaking down resistance genes have a mixed reproduction system, a high potential for genotype flow, large effective population sizes, and high mutation rates. The lowest risk pathogens are those with strict asexual reproduction, low potential for gene flow, small effective population sizes, and low mutation rates. We present examples of high-risk and low-risk pathogens. We propose general guidelines for a rational approach to breed durable resistance according to the evolutionary potential of the pathogen.

Bacteria↗

The population genetics of host specificity: genetic differentiation in dove lice (Insecta: Phthiraptera).

Some species of parasites occur on a wide range of hosts while others are restricted to one or a few host species. The host specificity of a parasite species is determined, in part, by its ability to disperse between host species. Dispersal limitations can be studied by exploring the genetic structure of parasite populations both within a single species of host and across multiple host species. In this study we examined the genetic structure in the mitochondrial cytochrome oxidase I (COI) gene of two genera of lice (Insecta: Phthiraptera) occurring on multiple sympatric species of doves in southern North and Central America. One genus, Columbicola, is generally less host-specific than the other, Physconelloides. For both genera we identified substantial genetic differentiation between populations of conspecific lice on different host species, generally 10-20% sequence divergence. This level of divergence is in the range of that often observed between species of these two genera. We used nested clade analysis to explore fine scale genetic structure within species of these feather lice. We found that species of Physconelloides exhibited more genetic structure, both among hosts and among geographical localities, than did species of Columbicola. In many cases, single haplotypes within species of Columbicola are distributed on multiple host species. Thus, the population genetic structure of species of Physconelloides reveals evidence of geographical differentiation on top of high host species specificity. Underlying differences in dispersal biology probably explain the differences in population genetic structure that we observed between Columbicola and Physconelloides.

Animals↗

Approximate Bayesian computation in population genetics.

We propose a new method for approximate Bayesian statistical inference on the basis of summary statistics. The method is suited to complex problems that arise in population genetics, extending ideas developed in this setting by earlier authors. Properties of the posterior distribution of a parameter, such as its mean or density curve, are approximated without explicit likelihood calculations. This is achieved by fitting a local-linear regression of simulated parameter values on simulated summary statistics, and then substituting the observed summary statistics into the regression equation. The method combines many of the advantages of Bayesian statistical inference with the computational efficiency of methods based on summary statistics. A key advantage of the method is that the nuisance parameters are automatically integrated out in the simulation step, so that the large numbers of nuisance parameters that arise in population genetics problems can be handled without difficulty. Simulation results indicate computational and statistical efficiency that compares favorably with those of alternative methods previously proposed in the literature. We also compare the relative efficiency of inferences obtained using methods based on summary statistics with those obtained directly from the data using MCMC.

Bayes Theorem↗

The cohesive population genetics of molecular drive.

The long-term population genetics of multigene families is influenced by several biased and unbiased mechanisms of nonreciprocal exchanges (gene conversion, unequal exchanges, transposition) between member genes, often distributed on several chromosomes. These mechanisms cause fluctuations in the copy number of variant genes in an individual and lead to a gradual replacement of an original family of n genes (A) in N number of individuals by a variant gene (a). The process for spreading a variant gene through a family and through a population is called molecular drive. Consideration of the known slow rates of nonreciprocal exchanges predicts that the population variance in the copy number of gene a per individual is small at any given generation during molecular drive. Genotypes at a given generation are expected only to range over a small section of all possible genotypes from one extreme (n number of A) to the other (n number of a). A theory is developed for estimating the size of the population variance by using the concept of identity coefficients. In particular, the variance in the course of spreading of a single mutant gene of a multigene family was investigated in detail, and the theory of identity coefficients at the state of steady decay of genetic variability proved to be useful. Monte Carlo simulations and numerical analysis based on realistic rates of exchange in families of known size reveal the correctness of the theoretical prediction and also assess the effect of bias in turnover. The population dynamics of molecular drive in gradually increasing the mean copy number of a variant gene without the generation of a large variance (population cohesion) is of significance regarding potential interactions between natural selection and molecular drive.

Biological Evolution↗

[Population genetics study of the differential fertility in urban populations].

In an urban population with widespread birth control practice the distribution of the number of pregnancies, births and abortions was studied in a cohort of women of completed fertility. The mean number of pregnancies per woman was 4.03 +/- 0.08 (sigma = = 2.98); the mean number of births - 1.12 +/- 0.02 (sigma = 0.77). 7.4% of women which had completed their reproductive performance had no pregnancies and 19.5% - no births. The Crow's Index of the Opportunity for Selection and its components connected with differential fertility and differential mortality were estimated. In the population under study two components of selection - selection at the prenatal stages and selection associated with infertility - are shown to be still significant. Such type of selection is exemplified by investigation of couples suffering from repeated spontaneous abortions.

Abortion, Habitual↗

Bio++: a set of C++ libraries for sequence analysis, phylogenetics, molecular evolution and population genetics.

BACKGROUND: A large number of bioinformatics applications in the fields of bio-sequence analysis, molecular evolution and population genetics typically share input/output methods, data storage requirements and data analysis algorithms. Such common features may be conveniently bundled into re-usable libraries, which enable the rapid development of new methods and robust applications. RESULTS: We present Bio++, a set of Object Oriented libraries written in C++. Available components include classes for data storage and handling (nucleotide/amino-acid/codon sequences, trees, distance matrices, population genetics datasets), various input/output formats, basic sequence manipulation (concatenation, transcription, translation, etc.), phylogenetic analysis (maximum parsimony, markov models, distance methods, likelihood computation and maximization), population genetics/genomics (diversity statistics, neutrality tests, various multi-locus analyses) and various algorithms for numerical calculus. CONCLUSION: Implementation of methods aims at being both efficient and user-friendly. A special concern was given to the library design to enable easy extension and new methods development. We defined a general hierarchy of classes that allow the developer to implement its own algorithms while remaining compatible with the rest of the libraries. Bio++ source code is distributed free of charge under the CeCILL general public licence from its website http://kimura.univ-montp2.fr/BioPP.

Algorithms↗

Applicability of bovine microsatellite markers for population genetic studies on African buffalo (Syncerus caffer).

The applicability of bovine autosomal microsatellite markers for population genetic studies on African buffalo was investigated. A total of 168 microsatellite markers were tested for PCR amplification on a test panel of seven African buffalo. Amplification was observed for 139 markers (83%), and 101 markers were studied further with 91 (90%) being polymorphic. The mean number of alleles per marker was 5.0 (SE = 0.2) and the mean heterozygosity per marker was 0.61 (SE = 0.03). Considering the overall high level of polymorphism, it was concluded that most bovine microsatellite markers are applicable in African buffalo.

Alleles↗

Mitochondrial control-region sequences in two shorebird species, the turnstone and the dunlin, and their utility in population genetic studies.

We determined the mitochondrial control-region sequences of five turnstones (Arenaria interpres) and three dunlins (Calidris alpina). Comparisons revealed that the central part (part II) is conserved relative to much more variable parts at the beginning (part I) and the end (part III). This pattern of sequence conservation is also found in the control regions of other vertebrates. The average sequence divergence between turnstone and dunlin was 21.8% for part I, 7.5% for part II, and 29.5% for part III. Within-species sequence divergence over the entire control region was much lower, at 0.9% for turnstones and 2.0% for dunlins. In both shorebird species, part III contains a repetitive sequence composed only of A and C nucleotides, which has not been found in the control regions of other birds. A survey of the part I sequences of 25 turnstones and 25 dunlins sampled around the world revealed that these species have very different population genetic structures. Dunlins are not only much more differentiated in their sequences but also have a strongly subdivided population genetic structure. Pleistocene vicariant events combined with strong natal philopatry and high mutation rates of the sequences are likely responsible for this population genetic subdivision. Conversely, part I sequences of turnstones are weakly differentiated and are geographically unstructured. We argue that this is not the result of global gene flow but that, instead turnstones have recently expanded from a refugial population that was bottlenecked.

Animals↗

Variable segment number in centipedes: population genetics meets evolutionary developmental biology.

The case studies of population genetics focus on intraspecific variation, but most cases--at least where the variation is polymorphic--deal with characters that are not directly linked to organismic structure or ontogeny. Conversely, the case studies of evolutionary developmental biology focus directly on structure/ontogeny, but usually involve only interspecific comparisons. To integrate these complementary approaches, it is desirable to have a model system that permits study of intraspecific variation in development, using a character whose genetic basis either is already known or can be elucidated. Segment number in geophilomorph centipedes is proposed as a possible model system of this kind. Segment number is variable in natural populations of geophilomorphs, while in the other centipede orders it is fixed, either completely (scutigeromorphs, lithobiomorphs), or at least within species (scolopendromorphs). Statistical analysis of data on the extent of variation in different geophilomorph species suggests that segment number may be of selective importance, rather than the variation being merely an inevitable consequence of the difficulty of achieving a high degree of repeatability when there is a large number of segments.

Animals↗

The population genetics of a biological control introduction: mitochondrial DNA and microsatellie variation in native and introduced populations of Aphidus ervi, a parisitoid wasp.

Introductions of biological control agents may cause bottlenecks in population size despite efforts to avoid them. We examined the population genetics of Aphidius ervi (Hymenoptera: Braconidae), a parasitoid that was introduced to North America from Western Europe in 1959 to control pea aphids. To explore the phylogeographical relationships of A. ervi we sequenced 1249 bp of mitochondrial DNA (mtDNA) from 27 individuals from the native range and 51 individuals from the introduced range. Most individuals from Western Europe, the Middle East and North America shared one of two common haplotypes, consistent with the known history of the introduction. However, some A. ervi from the Pacific Northwest have a haplotype that is most similar to haplotypes found in Japan, raising the possibility of a second accidental introduction. To examine population structure and assess whether a bottleneck occurred upon introduction to North America, we assayed variation at 5 microsatellite loci in 62 individuals from 2 native populations and 230 individuals from 6 introduced populations. Introduced samples had fewer rare alleles than native samples (F1,34 = 13.5, P = 0.0008), but heterozygosity did not differ significantly. These results suggest that a mild bottleneck occurred in spite of the introduction of over 1000 individuals. Using a hierarchical Bayesian approach, the founding population size was estimated to be 245 individuals. amova showed significant genetic differentiation between the European and North American samples, and a Bayesian assignment approach clustered individuals into four groups, with most European individuals in one group and most North American individuals in the other three. These results highlight that genetic changes are associated with founder events in rapidly growing natural populations, even when the founding population size is relatively large.

Analysis of Variance↗

Population genetics of short tandem repeat (STR) loci.

To investigate the population genetics of short tandem repeat (STR) polymorphisms in human populations, we have studied the allele frequency distributions of four STR loci (HUMTH01, HUMVWA31, HUMF13A1 and HUMFES) in 16 different population surveys which can be categorised within three broadly defined ethnic groups: Caucasian, Asian (Indian subcontinent), and African (Afro-Caribbean and US black). We have observed that allele frequency distributions of populations within ethnic groups are similar; consequently, genetic distances are an order of magnitude lower than between ethnic groups. Inbreeding coefficients (F-statistics) and calculations of the number of mean heterozygous loci per individual, along with estimates of variance, did not suggest that the populations were substructured. This included a study of an immigrant Asian population known to comprise at least three different sub-groups. Finally, an indication of the discriminating power is given by calculation of likelihood ratios (LR) of each individual tested across all four loci. Approximately 70% of Caucasians give an LR of greater than 10,000; the test is even more discriminating in Afro-Caribbeans--approximately 90% of tests are greater than 10,000.

Alleles↗

Fine-scale population genetic structure of a wildlife disease vector: the southern house mosquito on the island of Hawaii.

The southern house mosquito, Culex quinquefasciatus, is a widespread tropical and subtropical disease vector. In the Hawaiian Islands, where it was introduced accidentally almost two centuries ago, it is considered the primary vector of avian malaria and pox. Avian malaria in particular has contributed to the extinction and endangerment of Hawaii's native avifauna, and has altered the altitudinal distribution of native bird populations. We examined the population genetic structure of Cx. quinquefasciatus on the island of Hawaii at a smaller spatial scale than has previously been attempted, with particular emphasis on the effects of elevation on population genetic structure. We found significant genetic differentiation among populations and patterns of isolation by distance within the island. Elevation per se did not have a limiting effect on gene flow; however, there was significantly lower genetic diversity among populations at mid elevations compared to those at low elevations. A recent sample taken from just above the predicted upper altitudinal distribution of Cx. quinquefasciatus on the island of Hawaii was confirmed as being a temporary summer population and appeared to consist of individuals from more than one source population. Our results indicate effects of elevation gradients on genetic structure that are consistent with known effects of elevation on population dynamics of this disease vector.

Animals↗

Population genetic structure of the lemon shark (Negaprion brevirostris) in the western Atlantic: DNA microsatellite variation.

DNA microsatellite markers were used to characterize the population genetic structure of the lemon shark, Negaprion brevirostris, in the western Atlantic. This study demonstrates for the first time the usefulness of microsatellites to study population genetic structure and mating systems in the Chondricthyes. Lemon sharks (mostly juveniles) were sampled non-destructively from four locations, Gullivan Bay and Marquesas Key in Florida, Bimini, Bahamas, and Atol das Rocas, Brazil. At least 545 individuals were genotyped at each of four dinucleotide loci. The number of alleles per locus ranged from 19 to 43, and expected heterozygosities ranged from 0.69 to 0.90. Relatively little genetic structure was found in the western Atlantic, with small but significant values for estimators of F(ST) and R(ST) among populations, theta (0.016) and rho (0.026), respectively. No sharp discontinuities were found between the Caribbean sites and Brazil, and most alleles were found at all four sites, indicating that gene flow occurs throughout the western Atlantic with no evidence for distinct stocks.

Animals↗

Population genetics of STR loci in Caucasians.

STR loci are becoming increasingly important in forensic casework. In order to be used fairly and efficiently, the population genetics of these loci must be investigated and the implications for forensic inference assessed. A key population genetics parameter is the "coancestry coefficient", or FST, which is the correlation between two genes sampled from distinct individuals within a subpopulation. We present analyses of STR data, at geographic scales which range from national to regional, from the UK and other European sources. We implement a likelihood-based method of estimating FST, which has important advantages over alternative methods: it allows a range of plausible values to be assessed, rather than presenting a single point estimate, and it allows a subpopulation to be compared with a larger population from which a database has been drawn, which is the relevant comparison in forensic work. Our results suggest that values of FST appropriate to forensic applications in Europe are too large to be ignored. With appropriate allowance, however, it is possible to make use of STR evidence in a way which is efficient yet avoids overstatement of evidential strength.

Cross-Cultural Comparison↗