Patterns of morphological changes and hybridization between sympatric whitefish morphs (Coregonus spp.) in a Swiss lake: a role for eutrophication?

Whitefish, genus Coregonus, show exceptional levels of phenotypic diversity with sympatric morphs occurring in numerous postglacial lakes in the northern hemisphere. Here, we studied the effects of human-induced eutrophication on sympatric whitefish morphs in the Swiss lake, Lake Thun. In particular, we addressed the questions whether eutrophication (i) induced hybridization between two ecologically divergent summer-spawning morphs through a loss of environmental heterogeneity, and (ii) induced rapid adaptive morphological changes through changes in the food web structure. Genetic analysis based on 11 microsatellite loci of 282 spawners revealed that the pelagic and the benthic morph represent highly distinct gene pools occurring at different relative proportions on all seven known spawning sites. Gill raker counts, a highly heritable trait, showed nearly discrete distributions for the two morphs. Multilocus genotypes characteristic of the pelagic morph had more gill rakers than genotypes characteristic of benthic morph. Using Bayesian methods, we found indications of recent but limited introgressive hybridization. Comparisons with historical gill raker data yielded median evolutionary rates of 0.24 haldanes and median selection intensities of 0.27 for this trait in both morphs for 1948-2004 suggesting rapid evolution through directional selection at this trait. However, phenotypic plasticity as an alternative explanation for this phenotypic change cannot be discarded. We hypothesize that both the temporal shifts in mean gill raker counts and the recent hybridization reflect responses to changes in the trophic state of the lake induced by pollution in the 1960s, which created novel selection pressures with respect to feeding niches and spawning site preferences.

A pyrosequencing assay for the rapid discrimination of mitochondrial lineages in the Salmo trutta species complex

The European trout (Salmo trutta species complex) is genetically very diverse consisting of five distinct mitochondrial lineages that probably originated in the Pleistocene. Here, we describe a novel pyrosequencing protocol to generate two short sequence reads from the mitochondrial control region, which allow the unambiguous identification of all five lineages. The approach was found to be easily transferable between laboratories and should be a valuable tool for the assessment of genetic diversity in trout. Pyrosequencing-based assays for molecular species identification are expected to be generally useful whenever multiple positions in a short DNA sequence need to be assessed.

Hybridization between distant lineages increases adaptive variation during a biological invasion: stickleback in Switzerland

The three-spined stickleback is a widespread Holarctic species complex that radiated from the sea into freshwaters after the retreat of the Pleistocene ice sheets. In Switzerland, sticklebacks were absent with the exception of the far northwest, but different introduced populations have expanded to occupy a wide range of habitats since the late 19th century. A well-studied adaptive phenotypic trait in sticklebacks is the number of lateral plates. With few exceptions, freshwater and marine populations in Europe are fixed for either the low plated phenotype or the fully plated phenotype, respectively. Switzerland, in contrast, harbours in close proximity the full range of phenotypic variation known from across the continent. We addressed the phylogeographic origins of Swiss sticklebacks using mitochondrial partial cytochrome b and control region sequences. We found only five different haplotypes but these originated from three distinct European regions, fixed for different plate phenotypes. These lineages occur largely in isolation at opposite ends of Switzerland, but co-occur in a large central part. Across the country, we found a strong correlation between a microsatellite linked to the high plate ectodysplasin allele and the mitochondrial haplotype from a region where the fully plated phenotype is fixed. Phylogenomic and population genomic analysis of 481 polymorphic amplified fragment length polymorphism loci indicate genetic admixture in the central part of the country. The same part of the country also carries elevated within-population phenotypic variation. We conclude that during the recent invasive range expansion of sticklebacks in Switzerland, adaptive and neutral between-population genetic variation was converted into within-population variation, raising the possibility that hybridization between colonizing lineages contributed to the ecological success of sticklebacks in Switzerland.

Sympatric colour polymorphisms associated with nonrandom gene flow in cichlid fish of Lake Victoria

Colour polymorphisms have fascinated evolutionary ecologists for a long time. Yet, knowledge on the mechanisms that allow their persistence is restricted to a handful of well-studied cases. We studied two species of Lake Victoria cichlid fish, Neochromis omnicaeruleus and Neochromis greenwoodi, exhibiting very similar sex-linked colour polymorphisms. The ecology and behaviour of one of these species is well studied, with colour-based mating and aggression preferences. Here, we ask whether the selection potentially resulting from female and male mating preferences and aggression biases reduces gene flow between the colour morphs and permits differentiation in traits other than colour. Over the past 14 years, the frequencies of colour morphs have somewhat oscillated, but there is no evidence for directional change, suggesting the colour polymorphism is persistent on an ecological timescale. We find limited evidence of ecomorphological differentiation between sympatric ancestral (plain) and derived (blotched) colour morphs. We also find significantly nonrandom genotypic assignment and an excess of linkage disequilibrium in the plain morph, which together with previous information on mating preferences suggests nonrandom mating between colour morphs. This, together with negative frequency-dependent sexual selection, found in previous studies, may facilitate maintenance of these polymorphisms in sympatry