On the importance of being structured: instantaneous coalescence rates and human evolution--lessons for ancestral population size inference?

Most species are structured and influenced by processes that either increased or reduced gene flow between populations. However, most population genetic inference methods assume panmixia and reconstruct a history characterized by population size changes. This is potentially problematic as population structure can generate spurious signals of population size change through time. Moreover, when the model assumed for demographic inference is misspecified, genomic data will likely increase the precision of misleading if not meaningless parameters. For instance, if data were generated under an n-island model (characterized by the number of islands and migrants exchanged) inference based on a model of population size change would produce precise estimates of a bottleneck that would be meaningless. In addition, archaeological or climatic events around the bottleneck's timing might provide a reasonable but potentially misleading scenario. In a context of model uncertainty (panmixia versus structure) genomic data may thus not necessarily lead to improved statistical inference. We consider two haploid genomes and develop a theory that explains why any demographic model with structure will necessarily be interpreted as a series of changes in population size by inference methods ignoring structure. We formalize a parameter, the inverse instantaneous coalescence rate, and show that it is equivalent to a population size only in panmictic models, and is mostly misleading for structured models. We argue that this issue affects all population genetics methods ignoring population structure which may thus infer population size changes that never took place. We apply our approach to human genomic data.

The demographic history of populations experiencing asymmetric gene flow: combining simulated and empirical data.

Population structure can significantly affect genetic-based demographic inferences, generating spurious bottleneck-like signals. Previous studies have typically assumed island or stepping-stone models, which are characterized by symmetric gene flow. However, many organisms are characterized by asymmetric gene flow. Here, we combined simulated and empirical data to test whether asymmetric gene flow affects the inference of past demographic changes. Through the analysis of simulated genetic data with three methods (i.e. bottleneck, M-ratio and msvar), we demonstrated that asymmetric gene flow biases past demographic changes. Most biases were towards spurious signals of expansion, albeit their strength depended on values of effective population size and migration rate. It is noteworthy that the spurious signals of demographic changes also depended on the statistical approach underlying each of the three methods. For one of the three methods, biases induced by asymmetric gene flow were confirmed in an empirical multispecific data set involving four freshwater fish species (Squalius cephalus, Leuciscus burdigalensis, Gobio gobio and Phoxinus phoxinus). However, for the two other methods, strong signals of bottlenecks were detected for all species and across two rivers. This suggests that, although potentially biased by asymmetric gene flow, some of these methods were able to bypass this bias when a bottleneck actually occurred. Our results show that population structure and dispersal patterns have to be considered for proper inference of demographic changes from genetic data.

Ancient DNA from an Early Neolithic Iberian population supports a pioneer colonization by first farmers.

The Neolithic transition has been widely debated particularly regarding the extent to which this revolution implied a demographic expansion from the Near East. We attempted to shed some light on this process in northeastern Iberia by combining ancient DNA (aDNA) data from Early Neolithic settlers and published DNA data from Middle Neolithic and modern samples from the same region. We successfully extracted and amplified mitochondrial DNA from 13 human specimens, found at three archaeological sites dated back to the Cardial culture in the Early Neolithic (Can Sadurní and Chaves) and to the Late Early Neolithic (Sant Pau del Camp). We found that haplogroups with a low frequency in modern populations-N* and X1-are found at higher frequencies in our Early Neolithic population (∼31%). Genetic differentiation between Early and Middle Neolithic populations was significant (F(ST) ∼0.13, P<10(-5)), suggesting that genetic drift played an important role at this time. To improve our understanding of the Neolithic demographic processes, we used a Bayesian coalescence-based simulation approach to identify the most likely of three demographic scenarios that might explain the genetic data. The three scenarios were chosen to reflect archaeological knowledge and previous genetic studies using similar inferential approaches. We found that models that ignore population structure, as previously used in aDNA studies, are unlikely to explain the data. Our results are compatible with a pioneer colonization of northeastern Iberia at the Early Neolithic characterized by the arrival of small genetically distinctive groups, showing cultural and genetic connections with the Near East.

Population divergence with or without admixture: selecting models using an ABC approach.

Genetic data have been widely used to reconstruct the demographic history of populations, including the estimation of migration rates, divergence times and relative admixture contribution from different populations. Recently, increasing interest has been given to the ability of genetic data to distinguish alternative models. One of the issues that has plagued this kind of inference is that ancestral shared polymorphism is often difficult to separate from admixture or gene flow. Here, we applied an approximate Bayesian computation (ABC) approach to select the model that best fits microsatellite data among alternative splitting and admixture models. We performed a simulation study and showed that with reasonably large data sets (20 loci) it is possible to identify with a high level of accuracy the model that generated the data. This suggests that it is possible to distinguish genetic patterns due to past admixture events from those due to shared polymorphism (population split without admixture). We then apply this approach to microsatellite data from an endangered and endemic Iberian freshwater fish species, in which a clustering analysis suggested that one of the populations could be admixed. In contrast, our results suggest that the observed genetic patterns are better explained by a population split model without admixture.

The population genetic effects of ancestry and admixture in a subdivided cattle breed.

The genetic structure of the Dexter, a minority cattle breed with complex demographic history, was investigated using microsatellite markers and a range of statistical approaches designed to detect both admixture and genetic drift. Modern representatives of two putative ancestral populations, the Devon and Kerry, together with the different populations of the Dexter, which have experienced different demographic histories, were analysed. Breed units showed comparatively high levels of genetic variability (H(E) = 0.63-0.68); however, distinct genetic subgroups were detected within the Dexter, which could be attributed to known demographic events. Much lower diversity was identified in three small, isolated Dexter populations (H(E) = 0.52-0.55) and higher differentiation (F(ST) > 0.13) was found. For one of these populations, where strong selection has taken place, we also found evidence of a demographic bottleneck. Three methods for quantifying breed admixture were applied and substantial method-based variation in estimates for the genetic contribution of the two proposed ancestral populations for each subdivision of the Dexter was found. Results were consistent only in the case of a group consisting of selected Traditional Dexter animals, where the ancestor of the modern Kerry breed was also determined as the greater parental contributor to the Dexter. The inconsistency of estimation of admixture proportions between the methods highlights the potentially confounding role of genetic drift in shaping small population structure, and the consequences of accurately describing population histories from contemporary genetic data.

Origin and number of founders in an introduced insular primate: estimation from nuclear genetic data.

Cynomolgus macaques (Macaca fascicularis) were introduced on the island of Mauritius between 400 and 500 years ago and underwent a strong population expansion after a probable initial founding event. However, in practice, little is known of the geographical origin of the individuals that colonized the island, on how many individuals were introduced, and of whether the following demographic expansion erased any signal of this putative bottleneck. In this study, we asked whether the current nuclear genome of the Mauritius population retained a signature that would allow us to answer these questions. Altogether, 21 polymorphic autosomal and sex-linked microsatellites were surveyed from 81 unrelated Mauritius individuals and 173 individuals from putative geographical sources in Southeast Asia: Java, the Philippines islands and the Indochinese peninsula. We found that (i) the Mauritius population was closer to different populations depending on the markers we used, which suggests a possible mixed origin with Java playing most probably a major role; and (ii) the level of diversity was lower than the other populations but there was no clear and consistent bottleneck signal using either summary statistics or full-likelihood methods. However, summary statistics strongly suggest that Mauritius is not at mutation-drift equilibrium and favours an expansion rather than a bottleneck. This suggests that on a short time scale, population decline followed by growth can be difficult to deduce from genetic data based on mutation-drift theory. We then used a simple Bayesian rejection algorithm to estimate the number of founders under different demographic models (exponential, logistic and logistic with lag) and pure genetic drift. This new method uses current population size estimates and expected heterozygosity of Mauritius and source population(s). Our results indicate that a simple exponential growth is unlikely and that, under the logistic models, the population may have expanded from an initial effective number of individuals of 10-15. The data are also consistent with a logistic growth with different lag values, indicating that we cannot exclude past population fluctuation.

Estimation of admixture proportions: a likelihood-based approach using Markov chain Monte Carlo.

When populations are separated for long periods and then brought into contact for a brief episode in part of their range, this can result in genetic admixture. To analyze this type of event we considered a simple model under which two parental populations (P1 and P2) mix and create a hybrid population (H). After that event, the three populations evolve under pure drift without exchange during T generations. We developed a new method, which allows the simultaneous estimation of the time since the admixture event (scaled by the population size t(i) = T/N(i), where N(i) is the effective population size of population i) and the contribution of one of two parental populations (which we call p1). This method takes into account drift since the admixture event, variation caused by sampling, and uncertainty in the estimation of the ancestral allele frequencies. The method is tested on simulated data sets and then applied to a human data set. We find that (i) for single-locus data, point estimates are poor indicators of the real admixture proportions even when there are many alleles; (ii) biallelic loci provide little information about the admixture proportion and the time since admixture, even for very small amounts of drift, but can be powerful when many loci are used; (iii) the precision of the parameters' estimates increases with sample size n = 50 vs. n = 200 but this effect is larger for the t(i)'s than for p1; and (iv) the increase in precision provided by multiple loci is quite large, even when there is substantial drift (we found, for instance, that it is preferable to use five loci than one locus, even when drift is 100 times larger for the five loci). Our analysis of a previously studied human data set illustrates that the joint estimation of drift and p1 can provide additional insights into the data.

Effects of chemical contaminants on genetic diversity in natural populations: implications for biomonitoring and ecotoxicology.

The conservation of genetic diversity has emerged as one of the central issues in conservation biology. Although researchers in the areas of evolutionary biology, population management, and conservation biology routinely investigate genetic variability in natural populations, only a handful of studies have addressed the effects of chemical contamination on population genetics. Chemical contamination can cause population reduction by the effects of somatic and heritable mutations, as well as non-genetic modes of toxicity. Stochastic processes in small populations, increased mutation load, and the phenomenon of mutational meltdown are compounding factors that cause reduced fitness and accelerate the process of population extirpation. Although the original damage caused by chemical contaminants is at the molecular level, there are emergent effects at the level of populations, such as the loss of genetic diversity, that are not predictable based solely on knowledge of the mechanism of toxicity of the chemical contaminants. Therefore, the study of evolutionary toxicology, which encompasses the population-genetic effects of environmental contaminants, should be an important focus of ecotoxicology. This paper reviews the issues surrounding the genetic effects of pollution, summarizes the technical approaches that can be used to address these issues, and provides examples of studies that have addressed some of them.

2BAD: an application to estimate the parental contributions during two independent admixture events.

Several approaches have been developed to calculate the relative contributions of parental populations in single admixture event scenarios, including Bayesian methods. In many breeds and populations, it may be more realistic to consider multiple admixture events. However, no approach has been developed to date to estimate admixture in such cases. This report describes a program application, 2BAD (for 2-event Bayesian ADmixture), which allows the consideration of up to two independent admixture events involving two or three parental populations and a single admixed population, depending on the number of populations sampled. For each of these models, it is possible to estimate several parameters (admixture, effective sizes, etc.) using an approximate Bayesian computation approach. In addition, the program allows comparing pairs of admixture models, determining which is the most likely given data. The application was tested through simulations and was found to provide good estimates for the contribution of the populations at the two admixture events. We were also able to determine whether an admixture model was more likely than a simple split model.

SPAms: A user-friendly software to simulate population genetics data under complex demographic models.

Patterns of genetic diversity can be used to reconstruct populations demographic history. Thus, it is crucial to understand the statistical properties of data sets generated under different scenarios. SPAms is a graphical application that allows the simulation of data under a set of demographic models of increasing complexity: (i) population size change (ii) admixture and (iii) population structure (n-island or stepping stone model). SPAms generates data under the infinite site and the stepwise mutation models and computes a set of commonly used statistics. It should thus be a useful tool for both research and teaching purposes.

Philopatry and reproductive success in Bornean orang-utans (Pongo pygmaeus).

Behavioural observations suggest that orang-utans are semi-solitary animals with females being philopatric and males roaming more widely in search of receptive partners, leading to the prediction that females are more closely related than males at any given site. In contrast, our study presents evidence for male and female philopatry in the orang-utan. We examined patterns of relatedness and parentage in a wild orang-utan population in Borneo using noninvasively collected DNA samples from animals observed to defecate, and microsatellite markers to assess dispersal and mating strategies. Surprisingly, resident females were equally as related to other resident females (mean r(xy) = 0.303) as resident males were to other resident males (mean r(xy) = 0.305). Moreover, resident females were more related to each other and to the resident males than they were to nonresident females, and resident males were more related to each other (and resident females) than they were to nonresident males. We assigned genetic mothers to 12 individuals in the population, while sires could be identified for eight. Both flanged males and unflanged males achieved paternity, similar to findings reported for Sumatran orang-utans.

Patterns of genetic diversity and migration in increasingly fragmented and declining orang-utan (Pongo pygmaeus) populations from Sabah, Malaysia.

We investigated the genetic structure within and among Bornean orang-utans (Pongo pygmaeus) in forest fragments of the Lower Kinabatangan flood plain in Sabah, Malaysia. DNA was extracted from hair and faecal samples for 200 wild individuals collected during boat surveys on the Kinabatangan River. Fourteen microsatellite loci were used to characterize patterns of genetic diversity. We found that genetic diversity was high in the set of samples (mean H(E) = 0.74) and that genetic differentiation was significant between the samples (average F(ST) = 0.04, P < 0.001) with F(ST) values ranging from low (0.01) to moderately large (0.12) values. Pairwise F(ST) values were significantly higher across the Kinabatangan River than between samples from the same river side, thereby confirming the role of the river as a natural barrier to gene flow. The correlation between genetic and geographical distance was tested by means of a series of Mantel tests based on different measures of geographical distance. We used a Bayesian method to estimate immigration rates. The results indicate that migration is unlikely across the river but cannot be completely ruled out because of the limited F(ST) values. Assignment tests confirm the overall picture that gene flow is limited across the river. We found that migration between samples from the same side of the river had a high probability indicating that orang-utans used to move relatively freely between neighbouring areas. This strongly suggests that there is a need to maintain migration between isolated forest fragments. This could be done by restoring forest corridors alongside the river banks and between patches.

[Strong differences of mitochondrial DNA between Mediterranean sea and Eastern Atlantic populations of Sardinella aurita]. / Fortes différences des ADN mitochondriaux de populations de Sardinella aurita de la mer Méditerranée et de l'Atlantique Est.

Digestion by restriction enzymes has been carried out on polymerase chain reaction products of the mitochondrial DNA control region of round sardine (Sardinella aurita) samples coming from the Eastern Atlantic and the Mediterranean Sea. The results show i) that, though the habitat is continuous, there is no gene flow between the two basins where two genetically differentiated groups can be recognized, ii) that each basin is genetically homogeneous and, iii) that the haplotypic diversity in the Mediterranean is between two and three times smaller than that observed in the Atlantic. These results can hardly be explained by a recent colonization from the Eastern Atlantic. This suggests that, for S. aurita and some other species, the Mediterranean Sea is genetically little influenced by the Eastern Atlantic.

Population genetic structure of and inbreeding in an insular cattle breed, the Jersey, and its implications for genetic resource management.

The Jersey is a ubiquitous and successful breed of cattle that originates from the UK Channel Island of Jersey. While the breed has been exported extensively, no imports have taken place to the island since 1789, leading to a concern regarding possible losses of genetic diversity and increased inbreeding. We have conducted the first large-scale genetic analysis of the Jersey cattle using only samples from the island. A total of 223 cattle from all parishes except one were genotyped for 12 microsatellite loci. The average number of alleles per locus and expected heterozygosity were found to be comparatively high (n(A)=4, H(e)=0.64) with respect to that observed in a number of continental breeds. Only breeds that have been upgraded and are therefore the result of admixture are clearly more variable than the Jersey. We also found a significant but limited amount of genetic differentiation between parishes (F(st)=0.013), or even between farms (F(st)=0.035) despite an apparent lack of movement. This is confirmed by the application of two recent statistical methods. A Bayesian partition analysis shows that the most probable value of K, the number of possible hidden partitions, is 1 (P approximately 0.98). K=2 has a much lower probability (P approximately 0.02) while other values are essentially zero. Similarly, we were able to show that there was no support for departure from panmixia other than due to population structure, and thus that there is sufficient background gene flow across the island to overcome local drift. Overall, it appears that the current level of genetic diversity and its distribution within the island means it is unnecessary to import unrelated genetic material to the island for management purposes.

Clinal variation in the nuclear DNA of Europeans.

Allele frequencies are clinally distributed for many protein polymorphisms in Europe, suggesting that the current populations are derived from an ancestral group that expanded from the Near East. It is not yet fully established whether that expansion took place during the Neolithic or earlier or whether the detectable protein variation faithfully reflects the underlying molecular variation. In this study we address the latter question by describing geographic patterns of genetic diversity at seven highly polymorphic DNA markers. Two of these markers are minisatellites, four are microsatellites, and the seventh is a locus of the HLA system. By analyzing a database of 304 samples, with more than 130,000 chromosomes, we found evidence for a major clinal component of genetic variation. At most loci spatially close populations resemble each other genetically, and the degree of genetic similarity, as measured by spatial autocorrelation statistics, decreases at increasing distances. The observed patterns of molecular variation do not seem to differ qualitatively from those identified for protein polymorphisms. This suggest that low levels of population structuring, described in some mitochondrial DNA studies, may reflect different evolutionary histories for nuclear and maternally inherited markers or, alternatively, that spatial patterns of mitochondrial DNA variation may need more sensitive statistical methods to be recognized.

Clines of nuclear DNA markers suggest a largely neolithic ancestry of the European gene pool.

Comparisons between archaeological findings and allele frequencies at protein loci suggest that most genes of current Europeans descend from populations that have been expanding in Europe in the last 10, 000 years, in the Neolithic period. Recent mitochondrial data have been interpreted as indicating a much older, Paleolithic ancestry. In a spatial autocorrelation study at seven hypervariable loci in Europe (four microsatellites, two larger, tandem-repeat loci, and a sequence polymorphism) broad clinal patterns of DNA variation were recognized. The observed clines closely match those described at the protein level, in agreement with a possible Near Eastern origin for the ancestral population. Separation times between populations were estimated on the basis of a stepwise mutation model. Even assuming low mutation rates and long generation times, we found no evidence for population splits older than 10,000 years, with the predictable exception of Saami (Lapps). The simplest interpretation of these results is that the current nuclear gene pool largely reflects the westward and northward expansion of a Neolithic group. This conclusion is now supported by purely genetic evidence on the levels and patterns of microsatellite diversity, rather than by correlations of biological and nonbiological data. We argue that many mitochondrial lineages whose origin has been traced back to the Paleolithic period probably reached Europe at a later time.

Microsatellite analysis of genetic variation among and within Alpine marmot populations in the French Alps.

The genetic structure of the Alpine marmot, Marmota marmota, was studied by an analysis of five polymorphic microsatellite loci. Eight locations were sampled in the French Alps, one from Les Ecrins valley (n = 160), another from La Sassière valley (n = 289) and the six others from the Maurienne valley (n = 139). Information on social group structure was available for both Les Ecrins and La Sassière but not for the other samples. The high levels of genetic diversity observed are at odds with the results obtained using microsatellites, minisatellites and allozymes on Alpine marmots from Germany, Austria and Switzerland. Strong deficits in heterozygotes were found in Les Ecrins and La Sassière. They are caused by a Wahlund effect due to the family structure (i.e. differentiation between the family groups). The family groups exhibit excess of heterozygotes rather than deficits. This may be caused by outbreeding and this is compatible with recent results from the genetics of related social species when information on the social structure is taken into account. The observed outbreeding could be the result of females mating with transient males or males coming from neighbouring colonies. Both indicate that the species may not be as monogamous as is usually believed. The results are also compatible with a male-biased dispersal but do not allow us to exclude some female migration. We also found a significant correlation between geographical and genetic distance indicating that isolation by distance could be an issue in marmots. This study is the first that analysed populations of marmots taking into account the social structure within populations and assessing inbreeding at different levels (region, valley, population, and family groups). Our study clearly demonstrated that the sampling strategy and behavioural information can have dramatic effects on both the results and interpretation of the genetic data.