Spatial structure of lemming populations (Dicrostonyx groenlandicus) fluctuating in density.

The pattern and scale of the genetic structure of populations provides valuable information for the understanding of the spatial ecology of populations, including the spatial aspects of density fluctuations. In the present paper, the genetic structure of periodically fluctuating lemmings (Dicrostonyx groenlandicus) in the Canadian Arctic was analysed using mitochondrial DNA (mtDNA) control region sequences and four nuclear microsatellite loci. Low genetic variability was found in mtDNA, while microsatellite loci were highly variable in all localities, including localities on isolated small islands. For both genetic markers the genetic differentiation was clear among geographical regions but weaker among localities within regions. Such a pattern implies gene flow within regions. Based on theoretical calculations and population census data from a snap-trapping survey, we argue that the observed genetic variability on small islands and the low level of differentiation among these islands cannot be explained without invoking long distance dispersal of lemmings over the sea ice. Such dispersal is unlikely to occur only during population density peaks.

Reliable noninvasive genotyping based on excremental PCR of nuclear DNA purified with a magnetic bead protocol.

A new protocol for extraction of DNA from faeces is presented. The protocol involves gentle washing of the surface of the faeces followed by a very simple DNA extraction utilizing the wash supernatant as the source of DNA. Unlike most other protocols, it does not involve the use of proteinase K and/or organic extraction, but is instead based on adsorption of the DNA to magnetic beads. The protocol was tested by microsatellite genotyping across six loci for sheep and reindeer faeces. Comparison with DNA extracted from blood demonstrated that the protocol was very reliable, even when used on material stored for a long time. The protocol was compared with another simple, solid-phase DNA-binding protocol, with the result that the bead-based protocol gave a slightly better amplification success and a lower frequency of allelic drop-outs. Furthermore, our experiments showed that the surface wash prior to DNA extraction is a crucial step, not only for our protocol, but for other solid-phase protocols as well.

Genetic evidence for different migration routes of freshwater fish into Norway revealed by analysis of current perch (Perca fluviatilis) populations in Scandinavia.

To elucidate the colonization of freshwater fish into Norway following the last deglaciation of Europe 10,000 years ago, we have performed a survey using mitochondrial DNA variation (20 populations) and multilocus DNA fingerprinting (14 populations) of the widely distributed perch (Perca fluviatilis) from the Scandinavian peninsula and the Baltic Sea. Sequence analysis of a 378 bp segment of the perch mitochondrial control region (D-loop) revealed 12 different haplotypes. A nested clade analysis was performed with the aim of separating population structure and population history. This analysis revealed strong geographical structuring of the Scandinavian perch populations. In addition, the level of genetic diversity was shown to differ considerably among the various populations as measured by the bandsharing values (S-values) obtained from multilocus DNA fingerprinting, with intrapopulation S-values ranging from 0.19 in Sweden to 0.84 in the central part of Norway. Analysis of the intrapopulation S-values, with S-value as a function of lake surface area and region, showed that these differences were significant. The mitochondrial and DNA fingerprinting data both suggest that the perch colonized Norway via two routes: one from the south following the retreating glacier, and the other through Swedish river systems from the Baltic Sea area. Perch utilizing the southern route colonized the area surrounding Oslofjord and the lakes which shortly after deglaciation were close to the sea. Fish migrating from the Baltic Sea seem to have reached no further than the east side of Oslofjord, where they presumably mixed with perch which had entered via the southern route. It seems likely that the migration events leading to the current distribution of perch also apply to other species of freshwater fish showing a similar distribution pattern.

Lack of concordance between mtDNA gene flow and population density fluctuations in the bank vole.

The genetic structure of bank voles Clethrionomys glareolus was determined from analyses of mitochondrial DNA (mtDNA) sequences, and compared with previous data on geographical synchrony in population density fluctuations. From 31 sample sites evenly spaced out along a 256-km transect in SE Norway a total of 39 distinct mtDNA haplotypes were found. The geographical distribution of the haplotypes was significantly non-random, and a cladistic analysis of the evolutionary relationship among haplotypes shows that descendant types were typically limited to a single site, whereas the ancestral types were more widely distributed geographically. This geographical distribution pattern of mtDNA haplotypes strongly indicates that the range and amount of female dispersal is severely restricted and insufficient to account for the previously observed synchrony in population density fluctuations. We conclude that geographical synchrony in this species must be caused by factors that are external to the local population, such as e.g. mobile predators.