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Heterozygosity-fitness correlations (HFCs) have been reported in populations of many species. We provide evidence for a positive correlation between genetic variability and growth rate at 12 allozyme loci in a catadromous marine fish species, the European eel (Anguilla anguilla L.). More heterozygous individuals show a significantly higher length and weight increase and an above average condition index in comparison with more homozygous individuals. To a lesser extent, six microsatellite loci show a similar pattern, with positive but not significant correlations between heterozygosity and growth rate. The HFCs observed could be explained by an effect of either direct allozyme over-dominance or associative overdominance. Selection affecting some of the allozyme loci would explain the greater strength of the HFCs found at allozymes in comparison with microsatellites and the lack of correlation between MLH at allozymes and MLH at microsatellites. Associative overdominance (where allozyme loci are merely acting as neutral markers of closely linked fitness loci) might provide an explanation for the HFCs if we consider that allozyme loci have a higher chance than microsatellites to be in linkage disequilibrium with fitness loci.

The anadromous-freshwater three-spined stickleback (Gasterosteus aculeatus) system allows for inferring the role of adaptation in speciation with a high level of accuracy because the freshwater ecotype has evolved multiple times from a uniform anadromous ancestor. A cause for concern is that independent evolution among drainages is not guaranteed in areas with a poorly resolved glacial history. This is the case for the west European great rivers, whose downstream valleys flanked the southern limit of the late Pleistocene ice sheet. We tested for independent and postglacial colonization of these valleys hypothesizing that the relationships among anadromous and freshwater sticklebacks correspond to a raceme structure. We compared the reduction in plate number accompanying this colonization to the genetic differentiation using 13 allozyme and five microsatellite loci in 350 individuals. Overall microsatellite differentiation (FST = 0.147) was twice as large as allozyme differentiation (FST = 0.066). Although habitat-specific gene flow may mask the ancestral relationships among both ecotypes, levels of microsatellite differentiation supported the hypothesis of raceme-like divergence, reflecting independent colonizations rather than the presence of two distinct evolutionary clades. Under an infinite alleles model and in the absence of gene flow, the observed freshwater divergence might be reached after 440 (microsatellites) to 4500 (allozymes) generations. Hence, the anadromous-freshwater stickleback system most likely diverged postglacially. We conclude that the reduction in plate number in two freshwater basins probably occurred independently, and that its considerable variation among populations is not in agreement with the time since divergence.

Marine fish seem to experience evolutionary processes that are expected to produce genetically homogeneous populations. We have assessed genetic diversity and differentiation in 15 samples of the sand goby Pomatoschistus minutus (Pallas, 1770) (Gobiidae, Teleostei) from four major habitats within the Southern Bight of the North Sea, using seven microsatellite and 13 allozyme loci. Despite its high dispersal potential, microsatellite loci revealed a moderate level of differentiation (overall FST=0.026; overall RST=0.058). Both hierarchical analysis of molecular variance and multivariate analysis revealed significant differentiation (P

The genetic variability and structure of the European eel (Anguilla anguilla L.) in populations throughout Europe was reassessed using 15 allozymic loci, seven of which were polymorphic. Seven sites were sampled on a latitudinal gradient across the natural continental range, extending from southern France to southern Norway. Heterozygosity (He = 0.05) and level of polymorphism (P = 0.43) were comparable to other marine fish. Populations were poorly differentiated (GST = 0.014, FST = 0.002), which is not surprising considering the high dispersal capability of the European eel. However, a significant geographical cline was detected at two alleles (IDh-1*100 and GPI-1*110), andgenetie distances (DCE) were concordant with geographical coastal distances. Mantel tests, pairwise FST's and multidimensional scaling analyses identify three distinct groups: Northern Europe, Western Europe and the Mediterranean Sea. We propose that the clinal genetic structure in the European eel may be due to (1) isolation by distance (as recently detected with microsatellites), (2) temporal reproductive separation, (3) post-larval selective forces, (4) contact between formerly separated groups or (5) some combination thereof.