Restricted dispersal in a sea of gene flow.

How far do marine larvae disperse in the ocean? Decades of population genetic studies have revealed generally low levels of genetic structure at large spatial scales (hundreds of kilometres). Yet this result, typically based on discrete sampling designs, does not necessarily imply extensive dispersal. Here, we adopt a continuous sampling strategy along 950 km of coast in the northwestern Mediterranean Sea to address this question in four species. In line with expectations, we observe weak genetic structure at a large spatial scale. Nevertheless, our continuous sampling strategy uncovers a pattern of isolation by distance at small spatial scales (few tens of kilometres) in two species. Individual-based simulations indicate that this signal is an expected signature of restricted dispersal. At the other extreme of the connectivity spectrum, two pairs of individuals that are closely related genetically were found more than 290 km apart, indicating long-distance dispersal. Such a combination of restricted dispersal with rare long-distance dispersal events is supported by a high-resolution biophysical model of larval dispersal in the study area, and we posit that it may be common in marine species. Our results bridge population genetic studies with direct dispersal studies and have implications for the design of marine reserve networks.

Persistence of butterfly populations in fragmented habitats along urban density gradients: motility helps.

In a simulation study of genotypes conducted over 100 generations for more than 1600 butterfly's individuals, we evaluate how the increase of anthropogenic fragmentation and reduction of habitat size along urbanisation gradients (from 7 to 59% of impervious land cover) influences genetic diversity and population persistence in butterfly species. We show that in areas characterised by a high urbanisation rate (>56% impervious land cover), a large decrease of both genetic diversity (loss of 60-80% of initial observed heterozygosity) and population size (loss of 70-90% of individuals) is observed over time. This is confirmed by empirical data available for the mobile butterfly species Pieris rapae in a subpart of the study area. Comparing simulated data for P. rapae with its normal dispersal ability and with a reduced dispersal ability, we also show that a higher dispersal ability can be an advantage to survive in an urban or highly fragmented environment. The results obtained here suggest that it is of high importance to account for population persistence, and confirm that it is crucial to maintain habitat size and connectivity in the context of land-use planning.

Genomic resources and their influence on the detection of the signal of positive selection in genome scans.

Genome scans represent powerful approaches to investigate the action of natural selection on the genetic variation of natural populations and to better understand local adaptation. This is very useful, for example, in the field of conservation biology and evolutionary biology. Thanks to Next Generation Sequencing, genomic resources are growing exponentially, improving genome scan analyses in non-model species. Thousands of SNPs called using Reduced Representation Sequencing are increasingly used in genome scans. Besides, genome sequences are also becoming increasingly available, allowing better processing of short-read data, offering physical localization of variants, and improving haplotype reconstruction and data imputation. Ultimately, genome sequences are also becoming the raw material for selection inferences. Here, we discuss how the increasing availability of such genomic resources, notably genome sequences, influences the detection of signals of selection. Mainly, increasing data density and having the information of physical linkage data expand genome scans by (i) improving the overall quality of the data, (ii) helping the reconstruction of demographic history for the population studied to decrease false-positive rates and (iii) improving the statistical power of methods to detect the signal of selection. Of particular importance, the availability of a high-quality reference genome can improve the detection of the signal of selection by (i) allowing matching the potential candidate loci to linked coding regions under selection, (ii) rapidly moving the investigation to the gene and function and (iii) ensuring that the highly variable regions of the genomes that include functional genes are also investigated. For all those reasons, using reference genomes in genome scan analyses is highly recommended.

Statistical analysis of amplified fragment length polymorphism data: a toolbox for molecular ecologists and evolutionists.

Recently, the amplified fragment length polymorphism (AFLP) technique has gained a lot of popularity, and is now frequently applied to a wide variety of organisms. Technical specificities of the AFLP procedure have been well documented over the years, but there is on the contrary little or scattered information about the statistical analysis of AFLPs. In this review, we describe the various methods available to handle AFLP data, focusing on four research topics at the population or individual level of analysis: (i) assessment of genetic diversity; (ii) identification of population structure; (iii) identification of hybrid individuals; and (iv) detection of markers associated with phenotypes. Two kinds of analysis methods can be distinguished, depending on whether they are based on the direct study of band presences or absences in AFLP profiles ('band-based' methods), or on allelic frequencies estimated at each locus from these profiles ('allele frequency-based' methods). We investigate the characteristics and limitations of these statistical tools; finally, we appeal for a wider adoption of methodologies borrowed from other research fields, like for example those especially designed to deal with binary data.

A new individual-based spatial approach for identifying genetic discontinuities in natural populations.

The population concept is central in evolutionary and conservation biology, but identifying the boundaries of natural populations is often challenging. Here, we present a new approach for assessing spatial genetic structure without the a priori assumptions on the locations of populations made by adopting an individual-centred approach. Our method is based on assignment tests applied in a moving window over an extensively sampled study area. For each individual, a spatially explicit probability surface is constructed, showing the estimated probability of finding its multilocus genotype across the landscape, and identifying putative migrants. Population boundaries are localized by estimating the mean slope of these probability surfaces over all individuals to identify areas with genetic discontinuities. The significance of the genetic discontinuities is assessed by permutation tests. This new approach has the potential to reveal cryptic population structure and to improve our ability to understand gene flow dynamics across landscapes. We illustrate our approach by simulations and by analysing two empirical datasets: microsatellite data of Ursus arctos in Scandinavia, and amplified fragment length polymorphism (AFLP) data of Rhododendron ferrugineum in the Alps.

The genetical bandwidth mapping: a spatial and graphical representation of population genetic structure based on the Wombling method.

Characterizing the spatial variation of allele frequencies in a population has a wide range of applications in population genetics. This article introduces a new nonparametric method, which provides a two-dimensional representation of a structural parameter called the genetical bandwidth, which describes genetic structure around arbitrary spatial locations in a study area. This parameter corresponds to the shortest distance to areas of significant allele variation, and its computation is based on the Womble's systemic function. A simulation study and application to data sets taken from the literature give evidence that the method is particularly demonstrative when the fine-scale structure is stronger than the large-scale structure, and that it is generally able to locate genetic boundaries or clines precisely.

Assumed and inferred spatial structure of populations: the Scandinavian brown bears revisited.

We reanalysed the spatial structure of the Scandinavian brown bear (Ursus arctos) population based on multilocus genotypes. We used data from a former study that had presumed a priori a specific population subdivision based on four subpopulations. Using two independent methods (neighbour-joining trees and Bayesian assignment tests), we analysed the data without any prior presumption about the spatial structure. A subdivision of the population into three subpopulations emerged from our study. The genetic pattern of these subpopulations matched the three geographical clusters of individuals present in the population. We recommend considering the Scandinavian brown bear population as consisting of three (instead of four) subpopulations. Our results underline the importance of determining genetic structure from the data, without presupposing a structure, even when there seems to be good reason to do so.

Genetic diversity and differentiation in Eryngium alpinum L. (Apiaceae): comparison of AFLP and microsatellite markers.

Genetic diversity and structure of 12 populations of Eryngium alpinum L. were investigated using 63 dominant amplified fragment length polymorphism (AFLP) and seven codominant microsatellite (48 alleles) markers. Within-population diversity estimates obtained with both markers were not correlated, but the microsatellite-based fixation index Fis was correlated with both AFLP diversity indices (number of polymorphic bands and Nei's expected heterozygosity). Only AFLP diversity indices increased with the size of populations, although they did not significantly differ among them (Kruskall-Wallis test). The discrepancy between AFLPs and microsatellites may be explained by a better coverage of the genome with numerous AFLPs, the higher mutation rates of microsatellites or the absence of significant difference among within-population diversity estimates. Genetic differentiation was higher with AFLPs (theta=0.40) than with microsatellites (theta=0.23), probably due to the higher polymorphism of microsatellites. Thus, we considered global qualitative patterns rather than absolute estimates to compare the performance of both types of markers. On a large geographic scale, the Mantel test and multivariate analysis showed that genetic patterns were more congruent with the spatial arrangement of populations when inferred from microsatellites than from AFLPs, suggesting higher homoplasy of AFLP markers. On a small spatial scale, AFLPs managed to discriminate individuals from neighboring populations whereas microsatellites did not (multivariate analysis), and the percentage of individuals correctly assigned to their population of origin was higher with AFLPs than with microsatellites. However, dominant AFLPs cannot be used to study heterozygosity-related topics. Thus, distinct molecular markers should be used depending on the biological question and the geographical scale investigated.

A seroepidemiological study of toxocariasis and risk factors for infection in children in Sri Lanka.

A seroepidemiology study using TES-ELISA was carried out in 1,020 children aged 1-12 years in the Hindagala Community Health Project, Sri Lanka. Toxocariasis seroprevalence was 43% with 16.6% showing high antibody levels. Unconditional logistic regression analysis showed 7-9 year olds to be at the highest risk (OR 3.0820; CI = 1.95-4.87). Dog ownership, especially puppies (OR 29.28; CI = 7.40-116.0), and geophagia-pica (OR 6.3732; CI = 3.87-10.50), were significant risk factors. Family clustering of toxocariasis was significant (chi2 = 88.000; p = 0.0001). Abdominal pain (45%), cough (30%), limb pain (23%) and skin rashes (20%) were significantly associated with seropositivity indicating that toxocariasis causes covert morbidity. These findings are, overall, applicable to other areas in Sri Lanka. However, in the dry zone, survival of infective eggs in the soil could be affected by the climate while more importantly, in agricultural areas with a high buffalo population, Toxocara vitulorum could account for human toxocariasis. Using a species specific double sandwich ELISA based on 57 kDa protein of T. canis ES antigen, it is demonstrated that 91% of the seropositives were due to T. canis. Thus along with rabies and dirofilariasis, toxocariasis is an important zoonotic health hazard from dogs in Sri Lanka and prevention is indicated.