No abstract

SUMMARY 1. Biological invasions often involve close taxonomic relatives either as native/invader pairs or as invader/invader pairs. Precise identification and differentiation of species is therefore of paramount importance to reconstruct the invasion history. Genetic studies are indispensable in the case of morphologically conservative taxonomic groups. 2. We analysed the Pontocaspian freshwater amphipods Dikerogammarus that have successfully invaded the benthos of large Central European rivers. Taxonomic uncertainties were clarified by phylogenetic analyses of mitochondrial 16S and COI genes. The three‐way partitioning of allozyme genotypes in a syntopic population further corroborated the taxonomic status of the three species Dikerogammarus haemobaphes, D. villosus and D. bispinosus. Dikerogammarus bispinosus had been prior misidentified as a subspecies of D. villosus. The conspicuous colour types of D. villosus, however, appeared to be conspecific. 3. The genetic identification of the previously more abundant D. haemobaphes individuals in old samples supported the ‘successive invasion wave’ hypothesis with D. haemobaphes as the first invader displaced by the second invader D. villosus. Dikerogammarus bispinosus could be a potential future invader. 4. Haplotype differentiation was apparent between two invasion lines of D. haemobaphes, but the occurrence of a single widespread haplotype indicates genetic impoverishment during rapid colonisation.

Since the mid-1980s the zebra mussel, Dreissena polymorpha, Pallas 1771, has become the protagonist of a spectacular freshwater invasion in North America due to its large economic and biological impact. Several genetic studies on American populations have failed to detect any large-scale geographical patterns. In western Europe, where D. polymorpha has been a classical invader from the Pontocaspian since the early 19th century, the situation is strikingly different. Here, we show with genetic markers that two major western European invasion lineages with lowered genetic variability within and among populations can be discriminated. These two invasion lineages correspond with two separate navigable waterways to western Europe. We found a rapid and asymmetrical genetic interchange of the two invasion lines after the construction of the Main-Danube canal in 1992, which interconnected the two waterways across the main watershed.

The population structure of Gammarus fossarum Koch was analysed at six enzyme loci in 38 populations from the Danube and Rhine drainage system in the southern part of Germany. The species-restricted distribution to the upper reaches of rivers was reflected in the large genetic differentiation of all populations. The genetic pattern can be explained as a dynamic balance between genetic drift and restricted gene flow. As migration mainly occurs along the rivers, a treelike pattern showing the genetic relationship of the populations corresponded well with the geographical position of the populations in the river system. The overall gene flow was estimated as low, although small rivers have a higher migration rate than large rivers. The watershed between the Rhine and Danube drainage basins affects gene flow at about the same level as the major migration barriers within the Danube drainage basin.Each sampled population seemed to be panmictic with respect to the variation at single loci. In two-locus analyses, linkage disequilibria were found in several populations at combinations of three of the loci. A probable cause is that the linkage disequilibria are produced through random genetic drift (possibly through bottlenecks) and that the three loci are so closely linked that the recombination rate is too low to establish linkage equilibrium.