Microsatellite variation in natural Drosophila melanogaster populations from New South Wales (Australia) and Tasmania

Microsatellite variation was studied at 48 microsatellite loci in 10 Drosophila melanogaster populations to investigate the population structure on the Australian east coast. Low, but statistically significant population differentiation was observed among most populations. The populations on the Australian mainland did not show evidence for isolation by distance. We conclude that the population structure of D. melanogaster on the Australian mainland is probably the result of a shared history (recent colonization). The observed differences between local D. melanogaster populations probably reflect variation in effective population sizes rather than patterns of gene flow. Two populations from Tasmania were more differentiated from the Australian mainland than a population from Israel, raising the question whether they are derived from the Australian mainland or colonized from a different source population.     

Shifting dispersal modes at an expanding species’ range margin

While it is generally recognized that noncontiguous (long‐distance) dispersal of small numbers of individuals is important for range expansion over large geographic areas, it is often assumed that colonization on more local scales proceeds by population expansion and diffusion dispersal (larger numbers of individuals colonizing adjacent sites). There are few empirical studies of dispersal modes at the front of expanding ranges, and very little information is available on dispersal dynamics at smaller geographic scales where we expect contiguous (diffusion) dispersal to be prevalent. We used highly polymorphic genetic markers to characterize dispersal modes at a local geographic scale for populations at the edge of the range of a newly invasive grass species (Brachypodium sylvaticum) that is undergoing rapid range expansion in the Pacific Northwest of North America. Comparisons of Bayesian clustering of populations, patterns of genetic diversity, and gametic disequilibrium indicate that new populations are colonized ahead of the invasion front by noncontiguous dispersal from source populations, with admixture occurring as populations age. This pattern of noncontiguous colonization was maintained even at a local scale. Absence of evidence for dispersal among adjacent pioneer sites at the edge of the expanding range of this species suggests that pioneer populations undergo an establishment phase during which they do not contribute emigrants for colonization of neighbouring sites. Our data indicate that dispersal modes change as the invasion matures: initial colonization processes appear to be dominated by noncontiguous dispersal from only a few sources, while contiguous dispersal may play a greater role once populations become established.     

Genetic structure of an expanding Armillaria root rot fungus (Armillaria ostoyae) population in a managed pine forest in southwestern France

The Landes de Gascogne forest (southwestern France) is the largest maritime pine ( Pinus pinaster ) plantation in Europe. Armillaria root disease ( Armillaria ostoyae ) has been reported since the early 1920s in the coastal area (western sector), but its incidence over the last 20 years has increased in the eastern sector. We investigated the genetic structure of the A. ostoyae population in this forest, focusing particularly on geographical differentiation potentially indicative of disease expansion in this area. In total, 531 isolates obtained from mycelial fans on symptomatic trees or undecayed stumps in 31 different disease foci were genotyped at five microsatellite loci. In 20 of these disease foci, a single genotype dominated, reflecting a predominantly clonal local spread of A. ostoyae . By contrast, at the regional scale, A. ostoyae probably spreads mostly via basidiospores (sexual spores), as no genotype common to several disease foci was identified. The absence of a clear pattern of isolation by distance may indicate either substantial gene flow or stochastic colonisation independent of spatial distance. The gradient of genetic diversity from the coast inwards and the greater genetic divergence of the eastern disease foci are consistent with the expansion of the A. ostoyae population from the coast eastwards.     

Nested clade analyses of phylogeographic data: testing hypotheses about gene flow and population history

Since the 1920s, population geneticists have had measures that describe how genetic variation is distributed spatially within a species' geographical range. Modern genetic survey techniques frequently yield information on the evolutionary relationships among the alleles or haplotypes as well as information on allele frequencies and their spatial distributions. This evolutionary information is often expressed in the form of an estimated haplotype or allele tree. Traditional statistics of population structure, such as F statistics, do not make use of evolutionary genealogical information, so it is necessary to develop new statistical estimators and tests that explicitly incorporate information from the haplotype tree. One such technique is to use the haplotype tree to define a nested series of branches (clades), thereby allowing an evolutionary nested analysis of the spatial distribution of genetic variation. Such a nested analysis can be performed regarding the geographical sampling locations either as categorical or continuous variables (i.e. some measure of spatial distance). It is shown that such nested phylogeographical analyses have more power to detect geographical associations than traditional, nonhistorical analyses and, as a consequence, allow a broader range of gene-flow parameters to be estimated in a precise fashion. More importantly, such nested analyses can discriminate between phylogeographical associations due to recurrent but restricted gene flow vs. historical events operating at the population level (e.g. past fragmentation, colonization, or range expansion events). Restricted gene flow and historical events can be intertwined, and the cladistic analyses can reconstruct their temporal juxtapositions, thereby yielding great insight into both the evolutionary history and population structure of the species. Examples are given that illustrate these properties, concentrating on the detection of range expansion events.     

Population structure and gene flow of the Atlantic walrus (Odobenus rosmarus rosmarus) in the eastern Atlantic Arctic based on mitochondrial DNA and microsatellite variation

The population structure of the Atlantic walrus, Odobenus rosmarus rosmarus , was studied using 11 polymorphic microsatellites and restriction fragment length polymorphism detected in the NADH-dehydrogenase ND1, ND2 and ND3/4 segments in mtDNA. A total of 105 walrus samples were analysed from northwest (NW) Greenland, east (E) Greenland, Svalbard and Franz Joseph Land. Two of the 10 haplotypes detected in the four samples were diagnostic for the NW Greenland sample, which implied that the group of walruses in this area is evolutionary distinct from walruses in the other three areas. One individual sampled in E Greenland exhibited a Pacific haplotype, which proved a connection between the Pacific walrus and walruses in eastern Greenland. The Franz Joseph Land, Svalbard and E Greenland samples shared the most common haplotype, indicating very little differentiation at the mtDNA level. Gene flow ( Nm ) estimates among the four areas indicated a very restricted exchange of female genes between NW Greenland and the more eastern Atlantic Arctic samples, and a closer relationship between the three samples composing the eastern Atlantic Arctic. The genetic variation at 11 polymorphic microsatellite loci grouped individuals into three populations, NW Greenland, E Greenland and a common Franz Joseph Land–Svalbard population, which were connected by moderate gene flow.     

Isolation and characterization of microsatellites in Sparganium emersum and cross‐species amplification in the related species S. erectum

We developed seven novel polymorphic microsatellite loci for the aquatic macrophyte Sparganium emersum (Sparganiaceae). These were characterized on 62 individuals collected from nine different populations. In this set of individuals, seven to 20 alleles per locus were detected and observed heterozygosity ranged between 0.16 and 0.95. Cross‐species amplification was tested in the related species Sparganium erectum, and was successful for five of the seven microsatellite loci.     

The population genetic structure of a large temperate pollinator species, Bombus pascuorum (Scopoli) (Hymenoptera: Apidae)

The genetic population structure of the bumble bee Bombus pascuorum was studied using six microsatellite loci and a partial sequence of the mitochondrial gene cytochrome b . Eighteen populations from central and northern Europe were included in the analysis. Observed levels of genetic variability and heterozygosity were high. Estimates of population differentiation based on F - and Φ-statistics revealed significant genetic differentiation among B. pascuorum populations and suggest that two partially isolated gene pools, separated by the Alps, do exist. The distribution of mtDNA haplotypes supports this view and presents direct evidence for gene flow across the Alps. Estimates of the number of migrants exchanged among populations north of the Alps suggest that historical events may have left a strong imprint on population structure.     

To hybridize or not to hybridize: what separates two genetic lineages of the Chalk‐hill Blue Polyommatus coridon (Lycaenidae,Lepidoptera) along their secondary contact zone throughout eastern Central Europe?

As a consequence of postglacial range expansion, hybrid zones evolved where different genetic lineages met. In this study, we analysed the Chalk‐hill Blue Polyommatus coridon all along the contact zone of two expansive lineages. This zone stretches from the sandy areas of north‐eastern Germany, along the mountain ranges of the German–Czech border and throughout the eastern Alps. We studied allozymes (19 loci) of 38 populations (1542 individuals) and compared these data sets against 15 populations of the western and 15 populations of the eastern lineages and found different degrees of hybridization. Thus, the calcareous regions of Thuringia and Sachsen‐Anhalt were mostly colonized by the western lineage. The middle mountain ranges between Bavaria and Bohemia represented a strong barrier blocking further expansion and thus completely impeding hybridization in this region. More intense hybridization was detected in the populations of the eastern Alps, especially in the north‐eastern part, where the Danube most probably acted as an expansion corridor for both lineages followed by intensive hybridization. In the south‐eastern Alps, hybrid populations were mostly detected in the easternmost parts and along the larger river valley of Drava and Mur; pure western populations dominated in the other areas of this region. These results show that the degree of hybridization along a contact zone is correlated with the ecological demands of a species and the regional physical geographic circumstances. This finding was proved for the Chalk‐hill Blue in our study but is also the most likely scenario in most animal and plant species.     

Genetic differentiation of the tephritid fly Urophora cardui in Europe as evidence for its biogeographical history

Gaps in the large-scale distribution of the tephritid fly Urophora cardui in Europe have been explained as the results of an ongoing re-immigration from Pleistocene refugia due to a very low dispersal capacity. Following evidence of a much greater dispersal capacity of U. cardui than previously assumed, the pattern of genetic differentiation of 41 populations from 16 European regions was studied using allozyme electrophoresis. In these analyses 18 enzyme systems were scored consistently providing 27 alleles. Allozyme variation indicated high gene flow and low levels of genetic differentiation within and between sampling regions as well as in recently colonized areas. No geographical pattern of heterozygosity or allozyme differentiation could be found matching the previously suggested recent immigration pattern. An observed south-north gradient in allozyme frequencies was interpreted as a geographical cline due to environmental factors. The results corroborate evidence from more recent studies that U. cardui is a highly mobile species which is likely to have repeatedly colonized some suboptimal European regions since the Pleistocene after retreats during 'little ice ages'. Patterns resulting from postglacial immigration processes are likely to have been long wiped out through high exchange rates.     

Rolling stones and stable homes: social structure,habitat diversity and population genetics of the Hawaiian spinner dolphin (Stenella longirostris)

Spinner dolphins (Stenella longirostris) exhibit different social behaviours at two regions in the Hawaiian Archipelago: off the high volcanic islands in the SE archipelago they form dynamic groups with ever‐changing membership, but in the low carbonate atolls in the NW archipelago they form long‐term stable groups. To determine whether these environmental and social differences influence population genetic structure, we surveyed spinner dolphins throughout the Hawaiian Archipelago with mtDNA control region sequences and 10 microsatellite loci (n = 505). F‐statistics, Bayesian cluster analyses, and assignment tests revealed population genetic separations between most islands, with less genetic structuring among the NW atolls than among the SE high islands. The populations with the most stable social structure (Midway and Kure Atolls) have the highest gene flow between populations (mtDNA Φ_ST < 0.001, P = 0.357; microsatellite F_ST = ?0.001; P = 0.597), and a population with dynamic groups and fluid social structure (the Kona Coast of the island of Hawai’i) has the lowest gene flow (mtDNA 0.042 < Φ_ST < 0.236, P < 0.05; microsatellite 0.016 < F_ST < 0.040, P < 0.001). We suggest that gene flow, dispersal, and social structure are influenced by the availability of habitat and resources at each island. Genetic comparisons to a South Pacific location (n = 16) indicate that Hawaiian populations are genetically depauperate and isolated from other Pacific locations (mtDNA 0.216 < F_ST < 0.643, P < 0.001; microsatellite 0.058 < F_ST < 0.090, P < 0.001); this isolation may also influence social and genetic structure within Hawai’i. Our results illustrate that genetic and social structure are flexible traits that can vary between even closely‐related populations.