Discriminating the impact of recent human mediated stock transfer from historical gene flow on genetic structure of European grayling Thymallus thymallus L.

Microsatellite markers were first used to partition individuals of European grayling Thymallus thymallus , from the Danube, Rhine and Main, and Elbe drainage systems into subpopulations and to estimate individual immigrant ancestries over the last few generations. Subsequently, the studied populations were 'purged' from recent immigrants and the proportions of evolutionary lineages within the 'purged' populations were re-estimated by applying mtDNA markers. The results confirmed a high level of admixture of the divergent mtDNA lineages ( i.e. natural secondary contact) in populations sampled at the contact zones of the drainages. In addition, a substantial amount of introgression was observed for several populations that were known to be affected by stocking of European grayling from different origins.     

Microsatellite variation in Bavarian populations of European grayling (Thymallus thymallus): Implications for conservation

European grayling populations in Bavaria have shown steady declines during the last 10–20years. In order to provide guidelines for conservation strategies and future management programs, we investigated the genetic structure of 15 grayling populations originating from three major Central European drainages (the Danube, the Elbe and the Rhine/Main) using 20 microsatellite loci. Genetic divergence between the three drainage systems was substantial as illustrated by highly significant heterogeneity of genotype frequencies, high number of drainage-specific private alleles, high between-drainage F _ST values, high assignment success of individuals to their drainage of origin and the high bootstrap support for the genetic distance based drainage-specific population clusters. In agreement with earlier studies, microsatellites revealed relatively low levels of intrapopulational genetic diversity in comparison to the overall level of variation across populations. Maximum likelihood methods using the coalescent approach revealed that the proportion of common ancestors was generally high in native populations and that the estimates of N _e were correlated with the genetic diversity parameters in all drainages. The number of effective immigrants per generation (N _e m) was less than one for all pairwise comparisons of populations within the drainages, indicating restricted interpopulational gene flow. Based on these findings we recommend a drainage and sub-drainage specific conservation of grayling populations in order to preserve their overall genetic diversity and integrity. For large-scale stocking actions to supplement declining or to restore extinct populations, creation of separate broodstocks for major conservation units (ESUs and MUs) is warranted. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic variation of European grayling (Thymallus thymallus) populations in the Western Balkans

In order to elucidate genetic composition of European grayling (Thymallus thymallus) populations in the Western Balkans, the partial mitochondrial DNA (mtDNA) control region was sequenced and 12 microsatellite loci genotyped in 14 populations originating from tributaries of the Adriatic and Danube drainages. Eleven mtDNA haplotypes were found, one confined to the Adriatic clade, one to the Alpine group and the rest to the ‘Balkan’ grayling phylogenetic clade. Haplotypes from the Balkan clade were confined to the Danube drainage and constituted two groups: northern group with haplotypes found in the Slovenian part of the Danube drainage, and southern group, consisting from Bosnia–Herzegovina and Montenegro. Substantial genetic distance between northern and southern groups of haplotypes (0.75–1.8%) and well supported divisions within the northern group indicate very structured grayling population within the studied Danube basin that most probably did not evolve due to vicariance but rather as a consequence of multiple colonization waves that might have occurred during the Pleistocene. Furthermore, genetic distance of ~4% between Adriatic and Danube populations’ haplotypes, suggest that their separation occurred in mid-Pliocene. These findings imply a complex colonization pattern of the Western Balkans drainages. Microsatellite data also confirm high genetic diversity in Western Balkans populations of grayling (on average 7.5 alleles per microsatellite locus and H _exp 0.58). Limited stocking activities were detected based on microsatellites and mtDNA data. Regarding current knowledge of grayling phylogeography appropriate management strategies were proposed to preserve unique, autochthonous grayling populations in Western Balkan.     

Late glacial history of the cold-adapted freshwater fish Cottus gobio,revealed by microsatellites

The distribution of genetic diversity at 10 highly polymorphic microsatellite loci within the European freshwater fish, Cottus gobio, L. was examined. The sampling range comprised a large geographical scale including lineages known to be highly divergent at both mitochondrial DNA (mtDNA) and allozymes. An analysis of genetic variability within populations showed that expected heterozygosity and allelic richness could be explained largely by current effective population sizes. Evidence was found, however, that historical processes predating the last major glaciation affected allelic richness. In addition to confirming the large-scale patterns from earlier studies, the microsatellite data revealed new insights into recent processes by analysing genetic structure within ancient lineages defined by mtDNA data. Stepwise mutation model (SMM) and nonSMM-based methods demonstrated a clear genetic structuring within the Northwestern European lineage comprising populations from Britain and Belgium, and within the Central European lineage populations from the rivers Danube, Elbe and Main. Supported by an analysis of genetic variability within populations these results showed that the bullhead populations most probably persisted throughout the last major glaciation within the British Isles and within the drainages of the rivers Elbe and Main. Such observations provide the first genetic evidence for a glacial refugium in such close proximity to the European glacial margins.     

Complex patterns of colonization and refugia revealed for European grayling Thymallus thymallus,based on complete sequencing of the mitochondrial DNA control region

The complete mitochondrial DNA (mtDNA) control region (1043 base pairs) and 162-bp of flanking transfer RNA genes were sequenced in 316 European grayling, Thymallus thymallus, from 44 populations throughout the Western European range of the species. A total of 58 haplotypes were revealed with pairwise divergence ranging from 0.001 to 0.038. An inferred intraspecific phylogenetic tree revealed two well-supported clades within the Danube basin, one highly divergent clade in the Adriatic basin, and one large, diverse group representing most other populations. A deeply divergent haplotype fixed in the Loire basin in central France, more groups of haplotypes from distinct Danubian tributaries, and a relatively ancestral haplotype fixed in former tributaries of the Elbe in Denmark all suggest a complex pattern of interglacial and postglacial expansions originating from disjunct refugia throughout central Europe. Despite some evidence of human-mediated stock transfers, parsimony-network-based nested-clade analysis (NCA) supported specific inferences relating to corridors of postglacial expansion such as the lower Rhine (Moselle) and Elbe systems (Danish populations) serving as sources for expansion into the Baltic to the north as well as the upper Rhine and Danube to the south; and specific Rhine populations (Doller, Orbe and Reuss) serving as sources for colonization of the Rhone. The multiple divergent clades representing populations in the upper Danube, as well as the deeply divergent haplotypes found in the Adriatic and Loire basins (> 5% divergence from Asian outgroups) support the theory that European grayling have had a long history in Western Europe, pre-dating Pleistocene glacial cycles. The patterns of mtDNA divergence shown here support a perspective of rich inter- and intrabasin genetic diversity that should be protected from current trends to translocate brood stocks for rearing and release in response to declining populations, especially in southern European basins.     

Genetic and morphological variation in a common European cyprinid, Leuciscus cephalus within and across Central European drainages

Genetic variability and differentiation of the common European cyprinid Leusciscus cephalus was investigated within and across the drainages of the rivers Rhine, Elbe and Danube using starch gel electrophoresis and morphological characters. Eleven of 24 presumptive enzyme loci showed some allozymic variation. Compared to other freshwater fish, genetic variability within all sites was high ( H _e=0·074–0·113). This was explained by the wide geographical and ecological range of the chub. In contrast, genetic and morphological differentiation between sites and drainages was low (F_ST=0·15), which may be due to the high dispersal ability of the chub and translocations of spawn by waterfowl. The phylogeographic structure of Leuciscus cephalus in north-eastern Bavaria showed low congruence with Cottus gobio populations sampled in the same area.     

Differences in levels of heterozygosity in populations of the common gudgeon (Gobio gobio,Cyprinidae) among adjacent drainages in Central Europe: an effect of postglacial range dynamics?

Twenty-nine allozyme loci analyzed in 295 common gudgeons (Gobio gobio) from the Rhine, the upper Danube and the Elbe river systems revealed variability measures of P = 0.590, H(e) = 0.066, and G(ST) = 0.1415. Gene flow was estimated at N(e)m = 1.88 over 223 river km in the Danube basin, and at N(e)m = 1.96 over 300 river km in the Rhine system. Isolation-by-distance was not observed. Danubian gudgeons proved significantly more heterozygous (H(e) = 0.106) than those from the Rhine (H(e) = 0.057) or the Elbe drainages (H(e) = 0.029). Nine polymorphic enzymes contributed to this difference, which probably indicates dispersal bottlenecking of the postglacial immigrants into the Atlantic drainages of the study area. Refugial bottlenecking of gudgeons from Atlantic drainages in a Pleistocene refuge located in oceanic northwest Europe also seems possible. Slightly deeper genetic lineages in the Danube (G(ST) = 0.0859) than in the Rhine (G(ST) = 0.0793) agree with this explanation, as does the greater mean genetic distance among pairs of population samples from the Danube (D = 0.0138) than from the Rhine (D = 0.0054). A genetic distance of D = 0.0085 separated Rhenish and Danubian gudgeons.     

Genetic variability, population size and isolation of distinct populations in the freshwater fish Cottus gobio L.

Analysis of allozyme data of the European freshwater fish Cottus gobio showed marked genetic differentiation across drainage basins in northeastern Bavaria, which points to the existence of at least two cryptic taxa. Genetic variability within populations differed significantly between these two taxa, which could be due to historical (bottlenecks) or ecological reasons (population size). To distinguish between these two hypotheses we sampled 12 distinct populations from Rhine, Elbe and Danube drainages. Using allozyme data we examined the influence of population size and isolation on genetic variability within populations. We used spatial extent of populations (patch size) as a measure for population size. To estimate isolation we calculated a compound measure which took into account patch size and distance to all neighbouring populations. Both patch size and isolation were highly correlated with genetic variability, explaining ≈95% of the variance of genetic variability within populations. Furthermore, analysis of covariance suggests that the difference in genetic variability between taxa may be explained by differences in population size.     

Genetic diversity and differentiation of central European freshwater pearl mussel (Margaritifera margaritifera L.) populations: implications for conservation and management

Despite the fact that mollusc species play an important role in many aquatic ecosystems, little is known about their biodiversity and conservation genetics. Freshwater pearl mussel (Margaritifera margaritifera L.) populations are seriously declining all over Europe and a variety of conservation programs are being established to support the remaining endangered central European populations. In order to provide guidelines for conservation strategies and management programs, we investigated the genetic structure of 24 freshwater pearl mussel populations originating from five major central European drainages including Elbe, Danube, Rhine, Maas and Weser, representing the last and most important populations in this area. We present a nondestructive sampling method of haemolymph for DNA analyses, which is applicable for endangered bivalves. The analyses of nine microsatellite loci with different levels of polymorphism revealed a high degree of fragmented population structure and very different levels of genetic diversity within populations. These patterns can be explained by historical and demographic effects and have been enforced by anthropogenic activities. Even within drainages, distinct conservation units were detected, as revealed from high F(ST) values, private alleles and genetic distance measures. Populations sampled close to contact zones between main drainage systems showed lowest levels of correct assignment to present-day drainage systems. Populations with high priority for conservation should not only be selected by means of census population size and geographical distance to other populations. Instead, detailed genetic analyses are mandatory for revealing differentiation and diversity parameters, which should be combined with ecological criteria for sustainable conservation and recovery programs.     

Mitochondrial and nuclear DNA phylogeography of European grayling (Thymallus thymallus): evidence for secondary contact zones in central Europe

Mitochondrial and microsatellite DNA markers were applied to infer the phylogeography, intraspecific diversity and dynamics of the distributional history of European grayling (Thymallus thymallus) with focus on its central and northern European distribution range. Phylogenetic and nested clade analyses revealed at least four major mtDNA lineages, which evolved in geographical isolation during the Pleistocene. These lineages should be recognized as the basic evolutionary significant units (ESUs) for grayling in central and northern Europe. In addition, and in contrast to previous work on grayling, the results of Bayesian analysis of individual admixture coefficients, two-dimensional scaling analysis and spatial analysis of molecular variance provided evidence for a high level of admixture among major lineages in contact zones between drainages (e.g. the low mountain range of Germany), most likely resulting from glacial perturbations and ancient river connections between drainages during the Pleistocene glaciations. Even within river systems, a high level of differentiation among populations was revealed as indicated by the microsatellite data. Grayling sampled from 29 sites displayed high levels of differentiation (overall F(ST) = 0.367), a high number of private alleles and high bootstrap support for the genetic distance-based population clusters across 12 loci. We specifically discuss our results in context of phylogeograpic studies on other European freshwater fish species with habitat preferences similar to those of grayling. Our study shows that both large-scale phylogeographical and detailed genetic analyses on a fine scale are mandatory for developing appropriate conservation guidelines of endangered species.     

Phylogeography of the bullhead Cottus gobio (Pisces: Teleostei: Cottidae) suggests a pre-pleistocene origin of the major central European populations

The bullhead Cottus gobio is a small, bottom-dwelling fish consisting of populations that have not been subject to transplantations or artificial stocking. It is therefore an ideal model species for studying the colonization history of central European freshwater systems, in particular with respect to the possible influences of the Pleistocene glaciation cycles. We sampled Cottus populations across most of its distribution range, with a special emphasis on southern Germany where the major European drainage systems are in closest contact. Mitochondrial D-loop sequencing of more than 400 specimens and phylogenetic network analysis allowed us to draw a detailed picture of the colonization of Europe by C. gobio. Moreover, the molecular distances between the haplotypes enabled us to infer an approximate time frame for the origin of the various populations. The founder population of C. gobio stems apparently from the Paratethys and invaded Europe in the Pliocene. From there, the first colonization into central Europe occurred via the ancient lower Danube, with a separate colonization of the eastern European territories. During the late Pliocene, one of the central European populations must have reached the North Sea in a second step after which it then started to colonize the Atlantic drainages via coastal lines. Accordingly, we found very distinct populations in the upper and lower Rhine, which can be explained by the fact that the lower Rhine was disconnected from the upper Rhine until approximately 1 million years ago (Ma). More closely related, but still distinct, populations were found in the Elbe, the Main and the upper Danube, all presumably of Pleistocene origin. Intriguingly, they have largely maintained their population identity, despite the strong disturbance caused by the glaciation cycles in these areas. On the other hand, a mixing of populations during postglacial recolonization could be detected in the lower Rhine and its tributaries. However, the general pattern that emerges from our analysis suggests that the glaciation cycles did not have a major impact on the general population structure of C. gobio in central Europe.     

Parasite infection reflects host genetic diversity among non-native populations of pumpkinseed sunfish in Europe

Species introductions often coincide with loss of genetic diversity and natural enemies. Anthropogenic translocation of the North-American pumpkinseed Lepomis gibbosus (L., 1758) (Centrarchidae) and its further spread have resulted in recent species establishment in most European countries. This study determines genetic differentiation of non-native European pumpkinseed populations and identifies how their genetic structure relates to the distribution and abundance of parasite species. Microsatellite analysis indicated presence of three genetic lineages, which were well supported by discriminant analysis based on parasite abundance data. The first lineage clustered pumpkinseed populations from northern and southern France and showed high allelic richness, heterozygosity and parasite richness. The second included populations along the “Southern invasion corridor” connecting the rivers Rhine, Main and Danube. The fish exhibited low to high genetic and parasite diversity and generally high parasite abundance. The third lineage clustered populations with low genetic and parasite diversity, located in Portuguese reservoirs and water bodies along the upper Elbe. Parasite species richness was significantly associated with host microsatellite heterozygosity and allelic richness, a trend partially affected by richness of North-American parasites. Furthermore, our results indicate that parasite community composition may serve as a useful biological tool to discriminate non-native fish populations and their inter-relationships.

     

Phylogeography of European grayling, <Emphasis Type="Italic">Thymallus thymallus</Emphasis> (Actinopterygii,Salmonidae), within the Northern Adriatic basin: evidence for native and exotic mitochondrial DNA lineages

Phylogeographic information on European grayling, Thymallus thymallus, is still fragmentary for the Northern Adriatic basin. In this article, we provide complete mitochondrial DNA control region sequence data of 456 grayling specimens from 21 sampling sites across distinct river drainages. Thirty-seven haplotypes were resolved and clustered into Danubian, Atlantic and Adriatic lineages. The latter clade, composed of 16 new haplotypes, was identified in 12 out of 17 Adriatic sampling sites and reached frequencies of 0.97 within single water courses of the Adige and the Po drainages. However, native Adriatic haplotypes were accompanied by Danubian and/or Atlantic variants in all cases. A positive correlation between hatchery haplotype frequency and annual stocking input pointed to a direct effect of stocking intensity on the genetic architecture of wild populations, although natural trans-basin colonisation may have additionally complicated the situation. However, both the extent and patterns of introgression between native and foreign strains, as well as microgeographic population structure within the Adriatic lineage will be clarified by future molecular surveys, based on nuclear genetic markers. Until then, conservation management must include an immediate cessation of stocking of commercial grayling stocks, as well as the prohibition of grayling translocation, even at the intra-drainage level.     

Historic speciation and recent colonization of Eurasian monkey gobies (Neogobius fluviatilis and N. pallasi) revealed by DNA sequences,microsatellites, and morphology

Aim Hidden diversity within an invasive ‘species’ can mask both invasion pathways and confound management goals. We assessed taxonomic status and population structure of the monkey goby Neogobius fluviatilis across Eurasia, comparing genetic variation across its native and invasive ranges. Location Native populations were analysed within the Black and Caspian Sea basins, including major river drainages (Dnieper, Dniester, Danube, Don and Volga rivers), along with introduced locations within the upper Danube and Vistula river systems. Methods DNA sequences and 10 nuclear microsatellite loci were analysed to test genetic diversity and divergence patterns of native and introduced populations; phylogenetic analysis of mtDNA cytochrome b and nuclear RAG‐1 sequences assessed taxonomic status of Black and Caspian Sea lineages. Multivariate analysis of morphology was used to corroborate phylogenetic patterns. Population genetic structure within each basin was evaluated with mtDNA and microsatellite data using F_ST analogues and Bayesian assignment tests. Results Phylogenetic analysis of mitochondrial and nuclear sequences discerned a pronounced genetic break between monkey gobies in the Black and Caspian Seas, indicating a long‐term species‐level separation dating to c. 3 million years. This pronounced separation further was confirmed from morphological and population genetic divergence. Bayesian inference showed congruent patterns of population structure within the Black Sea basin. Introduced populations in the Danube and Vistula River basins traced to north‐west Black Sea origins, a genetic expansion pattern matching that of other introduced Ponto‐Caspian gobiids. Main conclusions Both genetic and morphological data strongly supported two species of monkey gobies that were formerly identified as subspecies: N. fluviatilis in the Black Sea basin, Don and Volga Rivers, and the Kumo‐Manych Depression, and Neogobius pallasi in the Caspian Sea and Volga River delta. Genetic origins of introduced N. fluviatilis populations indicated a common invasion pathway shared with other introduced Ponto‐Caspian fishes and invertebrates.     

Coarse-grained population structure in Central European sculpin (Cottus gobio L.): secondary contact or ongoing genetic drift?

Population genetic affinities of 261 European sculpins Cottus gobio L. across the Rhenanian-Danubian and the Rhenanian-Rhónian watersheds were assessed by horizontal agarose-gel electrophoresis of up to 20 allozyme systems (encoded by 29 genetic loci). Polymorphism P_mean= 0.0689 (range: 0.000-0.1379), and heterozygosity H_e(mean)= 0.0167 (range: 0.000-0.0507) indicated low genetic variability within local stocks from single streams. Significant genetic distances D_mean= 0.1917 ± 0.0336 (D_max= 0.2407), based on differential fixation at 3–6 loci (Acp-1**, Pgdh**, Fh**, Est-1**, Gpi-2**, and Pgm-1**) distinguished populations from the Neckar catchment basin from those of the Hochhrein-Oberrhein and Danube basins. Differential fixation of alleles and pronounced genetic distances also separated sculpins of the Rh6nian tributary Doubs from Neckarian populations (D_men= 0.2131 ± 0.0033; Ah**, Acp-1**, Fh**), of the Doubs from the Danube (D_mean= 0.2177 ± 0.0028; Gpi-2**, Pgm-1**, Pgdh**, Ah**), and of the Doubs from the Hochrhein-Oberrhein (D_mean= 0.1780; Pgm-1**, Pgdh**, Ah**). Genetic distances between streams within these drainages proved low (Neckar: D_men= 0.0047 ± 0.0014, Danube: D_mean= 0.026 ± 0.0179, Rhine: D_mean= 0.0308). Screening of another 16 small-size samples consisting of 55 sculpins for nine diagnostic loci (Aat-2**, Acp-1**, Acp-2**, Ah**, Est-1**, Fh**, Gpi-2**, Pgdh** and Pgm-1**) confirmed the genetic homogeneity of sculpins within the Danubian and Neckarian drainage systems, but Neckarian sculpins were similar to those from the river Main. Populations from Hochrhein-Oberrhein resembled the Danubian stock but contained a decreasing frequency of ‘Neckarian’ markers when approaching the Danubian region. The genetic divergence between Neckarian, Danubian and Rhdnian sculpins suggests the existence of hitherto neglected taxa of anteglacial divergence.     

Distribution and invasion genetics of the quagga mussel (<Emphasis Type="Italic">Dreissena rostriformis bugensis</Emphasis>) in German rivers

The quagga mussel Dreissena rostriformis bugensis, native to the Dnieper and the northern Black Sea, has become a major invasive species in both the Volga River and the North American Great Lakes since the early 1990s. Findings in the Netherlands (2006) and Germany (2007) mark the start of its establishment in Western Europe. We investigated the current distribution, time of first arrival and population structure of D. rostriformis bugensis from the rivers Rhine, Main and in the Main-Danube canal in Germany. Two putative sources of the German populations were analysed by genetically comparing these populations to older invasive populations from North America and the southeast Danube. Dreissena rostriformis bugensis was abundant in the Main and in three Rhine harbours, but rare in the actual Rhine river and absent south of the Main-Danube canal. Mussels found in the Rhine harbours were significantly smaller than in the Main. Population genetic analyses found no sign of founder effects and minimal differentiation between German, North American and southeast Danube populations. The genetic data suggest that these invasive populations derive from a common and rapidly expanding source. Based on the non-continuous distribution and shell size differences of Rhine harbour and Main populations, our results indicate that expansion in Germany involved at least two independent settling events, one of which happened before 2005, and most likely was caused by jump dispersal.     

Genetic differentiation of European grayling (Thymallus thymallus) populations in Serbia,based on mitochondrial and nuclear DNA analyses

Background

The structure and diversity of grayling (Thymallus thymallus) populations have been well studied in most of its native habitat; however the southernmost populations of the Balkan Peninsula remain largely unexplored. The purpose of this study was to assess the genetic diversity of Serbian grayling populations, detect the impact of stocking and provide guidelines for conservation and management.

Methods

Eighty grayling individuals were collected from four rivers (Ibar, Lim, Drina and Rzav). The mitochondrial DNA control region (CR; 595 bp of the 3''end and 74 bp of flanking tRNA) and the ATP6 gene (630 bp fragment) were sequenced for 20 individuals (five from each locality). In addition, all individuals were genotyped with 12 microsatellite loci. The diversity and structure of the populations as well as the recent and ancient population declines were studied using specialized software.

Results

We detected three new haplotypes in the mtDNA CR and four haplotypes in the ATP6 gene of which three had not been described before. Previously, one CR haplotype and two ATP6 gene haplotypes had been identified as allochthonous, originating from Slovenia. Reconstruction of phylogenetic relations placed the remaining two CR haplotypes from the River Danube drainage of Serbia into a new clade, which is related to the previously described sister Slovenian clade. These two clades form a new Balkan clade. Microsatellite marker analysis showed that all four populations are genetically distinct from each other without any sign of intra-population structure, although stocking of the most diverse population (Drina River) was confirmed by mtDNA analysis. Recent and historical population declines of Serbian grayling do not differ from those of other European populations.

Conclusions

Our study shows that (1) the Ibar, Lim and Drina Rivers grayling populations are genetically distinct from populations outside of Serbia and thus should be managed as native populations in spite of some introgression in the Drina River population and (2) the Rzav River population is not appropriate for further stocking activities since it originates from stocked Slovenian grayling. However, the Rzav River population does not represent an immediate danger to other populations because it is physically isolated from these.     

Detection of a G/T base substitution in two bleak (Alburnus alburnus) populations by RAPD

A G/T substitution has been found in a 462 bp fragment produced by random PCR amplification (RAPD) of genomic DNA in bleak ( Alburnus alburnus ). Two bleak populations from the rivers Main and Danube were screened for the two variants by allele-specific PCR. The allele frequencies were 0.62 and 0.31 for variant T, and 0.38 and 0.69 for variant G in the river populations of Main and Danube, respectively. Allele frequencies differed significantly (P < 0.01).     

Recolonizing lost habitat—how European beavers (<Emphasis Type="Italic">Castor fiber</Emphasis>) return to south-western Germany

For the last decades, the European beaver (Castor fiber) has been recolonizing its original habitats. Reintroductions of beavers from different relict populations into southern Germany have resulted in several admixed populations, which are spreading out along various river systems. The eastern part of the German state of Baden-Württemberg is a melting pot of colonization waves originating from various introduced populations. The aim of this study was to exemplify origins and dispersal behaviour of beavers in this region using genetic fingerprint methods. Sequence analysis of hypervariable region 1 (HV1) of the mitochondrial control region and fragment length analyses at 11 microsatellite loci resulted in genetic profiles for 84 samples. The study region is being populated from three different local origins of beavers: the north of the Main-Tauber district, the Neckar River near the city of Mosbach and the Danube tributaries in the east. Main-Tauber samples were most diverse, including microsatellite alleles and HV1 haplotypes specific to C. f. albicus (from the German Elbe relict population). In view of the geographical proximity of this region to a release site of C. f. albicus in the Spessart area, this finding strongly suggests gene flow between beaver populations of different provenience. Two remaining local origins at the Neckar and Danube tributaries are closely related to each other, thus possibly descending from the same original (admixed) population. This study is intended to serve as a starting point for follow-up fine-scale research into dispersal behaviour of European beavers currently recolonizing their original habitats.     

Hierarchical organization of genetic variation in the Costa Rican livebearing fish Brachyrhaphis rhabdophora (Poeciliidae)

I examined the geographic distribution of genetic variation in the livebearing freshwater fish Brachyrhaphis rhabdophora in northwestern Costa Rica as revealed by allozymes and mitochondrial DNA sequences. Allelic variability at 11 enzyme-coding loci surveyed across 12 localities revealed marked genetic differentiation among populations within drainages (0_P= 0.36) and among drainages within regions (0_D=0.17), but not between northern and southern geographic regions (0_R=– 0.02). Allozyme variation was hierarchically organized such that populations found within stream drainages were more similar to each other than to populations found in adjacent drainages, a result confirmed by cluster analysis. In contrast to the allozyme data, there was extremely little DNA sequence variation among populations in the mitochondrial control region (3 variable nucleotide positions out of 444 bp examined). The difference in genetic divergence between allozyme and mtDNA markers was unexpected and is discussed in terms of biogeographical colonization events and a molecular selective swéep on the mitochondrial genome, both processes that could explain the lack of mitochondrial variability in this highly subdivided species.