Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. II. Daphnia cucullata feeding on mixed diets*

1. The effects of feeding on suboptimal foods were investigated in Daphnia cucullata a zooplankton common in many types of lakes. Eleven clones of D. cucullata were collected from four lakes of varying trophic levels and fed a high (1 mg C l^–1) concentration of one of two diets: (i) a 1 : 9 mixture of the cryptophyte Cryptomonas pyrenoidifera with the green alga Scenedesmus obliquus ; and (ii) a 1 : 9 mixture of the cryptophyte C. pyrenoidifera with the filamentous cyanobacterium Oscillatoria limnetica.
2. As expected, the diet dominated by Oscillatoria was found to be significantly poorer compared with the diet dominated by Scenedesmus. All D. cucullata clones were, however, able to achieve positive population growth rates on both diets, supporting the view that daphnids can grow and reproduce on a diet dominated by filamentous cyanobacteria.
3. Clones originating from different types of lakes differed in their responses to the two diets. On the Scenedesmus -dominated diet, clones originating from moderately eutrophic lakes had lower population growth rates than clones from hypertrophic lakes. In contrast, on the Oscillatoria -dominated diet, the clones from moderately eutrophic lakes had higher population growth rates than clones from hypertrophic lakes. Contrary to expectation, clones originating from lakes dominated by filamentous cyanobacteria had more difficulty utilizing filamentous cyanobacteria as food than clones from less eutrophic lakes which contain less filamentous cyanobacteria.
4. As the reactions of clones originating from same type of lake resembled each other, it is hypothesized that the life histories of D. cucullata populations are locally adapted to environmental factors other than abundance of filamentous cyanobacteria, which are correlated with trophic levels.     

Population genetic structure of Carchesium polypinum (Ciliophora: Peritrichia) in four Chinese lakes inferred from ISSR fingerprinting: high diversity but low differentiation

Although the peritrichous ciliate Carchesium polypinum is common in freshwater, its population genetic structure is largely unknown. We used inter-simple sequence repeat (ISSR) fingerprinting to analyze the genetic structure of 48 different isolates of the species from four lakes in Wuhan, central China. Using eight polymorphic primers, 81 discernible DNA fragments were detected, among which 76 (93.83%) were polymorphic, indicating high genetic diversity at the isolate level. Further, Nei's gene diversity (h) and Shannon's Information index (I) between the different isolates both revealed a remarkable genetic diversity, higher than previously indicated by their morphology. At the same time, substantial gene flow was found. So the main factors responsible for the high level of diversity within populations are probably due to conjugation (sexual reproduction) and wide distribution of swarmers. Analysis of molecular variance (AMOVA) showed that there was low genetic differentiation among the four populations probably due to common ancestry and flooding events. The cluster analysis and principal component analysis (PCA) suggested that genotypes isolated from the same lake displayed a higher genetic similarity than those from different lakes. Both analyses separated C. polypinum isolates into subgroups according to the geographical locations. However, there is only a weak positive correlation between the genetic distance and geographical distance, suggesting a minor effect of geographical distance on the distribution of genetic diversity between populations of C. polypinum at the local level. In conclusion, our studies clearly demonstrated that a single morphospecies may harbor high levels of genetic diversity, and that the degree of resolution offered by morphology as a marker for measuring distribution patterns of genetically distinct entities is too low.     

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

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Loss of genetic integrity correlates with stocking intensity in brook charr (Salvelinus fontinalis)

Supportive breeding and stocking performed with non‐native or domesticated fish to support sport fishery industry is a common practice throughout the world. Such practices are likely to modify the genetic integrity of natural populations depending on the extent of genetic differences between domesticated and wild fish and on the intensity of stocking. The purpose of this study is to assess the effects of variable stocking intensities on patterns of genetic diversity and population differentiation among nearly 2000 brook charr (Salvelinus fontinalis) from 24 lakes located in two wildlife reserves in Québec, Canada. Our results indicated that the level of genetic diversity was increased in more intensively stocked lakes, mainly due to the introduction of new alleles of domestic origin. As a consequence, the population genetic structure was strongly homogenized by intense stocking. Heavily stocked lakes presented higher admixture levels and lower levels of among lakes genetic differentiation than moderately and un‐stocked lakes. Moreover, the number of stocking events explained the observed pattern of population genetic structure as much as hydrographical connections among lakes in each reserve. We discuss the implications for the conservation of exploited fish populations and the management of stocking practices.     

Application of sequence-specific labeled 16S rRNA gene oligonucleotide probes for genetic profiling of cyanobacterial abundance and diversity by array hybridization

DNA sequence information for the small-subunit rRNA gene (16S rDNA) obtained from cyanobacterial cultures was used to investigate the presence of cyanobacteria and their abundance in natural habitats. Eight planktonic communities developing in lakes characterized by relatively low algal biomass (mesotrophic) and in lakes with correspondingly high biomass (eutrophic) were selected for the study. The organismal compositions of the water samples were analyzed genetically, using multiplex sequence-specific labeling of oligonucleotide probes targeted to 16S rDNA and subsequent hybridization of the labeled probes to their respective complements spotted onto a solid support (DNA array). Ten probes were established to determine the relative abundances of the discernible cyanobacteria encountered in the selected lakes. The probes were generally specific for their targets, as determined through analyses of clone cultures. Reproducible abundance profiles were established for the lakes investigated in the subsequent analyses of natural cyanobacterial communities. The results from the genetic analyses were then compared with information obtained from standard hydrobiological and hydrochemical analyses. Qualitatively, there were relatively good correlations among the groups of organisms (Nostoc, Microcystis, and Planktothrix species) found in the different lakes. The levels of correlation were lower for the quantitative data. This may, however, be due to differences in sample processing technique. The conclusions from these comparisons are that the genetic abundance profiles may provide a foundation for separating and quantifying genetically distinct groups of cyanobacteria in their natural habitats.     

Comparing Alkaline Phosphatase PhoX-Encoding Genes in Two Contrasting Habitats of the Large Eutrophic Lake Taihu,China

River mouth areas of eutrophic lakes have frequent cyanobacterial blooms and high organophosphate mineralization rates. To determine the genetic background of the high organophosphate mineralization rate in river mouth areas of eutrophic lakes, we compared the diversity and abundance of bacterial alkaline phosphatase PhoX-encoding genes in the river mouth areas and the lake center within the large eutrophic Lake Taihu, China. Results showed that the seasonal average values of phosphorus content, bacterial alkaline phosphatase activity, diversity, and abundance of bacterial phoX in the river mouths were significantly higher than those in the lake center, and 47% of all identified genotypes were present only in the river mouths. The genotypes most similar to alphaproteobacterial phoX dominated in both regions, with 72% and 64% of the total genotypes in the lake center and river mouths, respectively. More than 40% of the total phoX genotypes were only found in the river mouths, however, indicating the special microbial nature in this hypereutrophic habitat. The differences in contents of dissolved organic carbon and phosphorus were related to the divergences of bacterial phoX genotypes in these two contrasting habitats. Because some dominant genotypes most similar to phoX genes of alphaproteobacterial genus were observed only in the river mouth areas, such as Microvirga sp. WSM3557, and because the hydraulic retention time of Lake Taihu is insufficient to shape the special bacterial groups in the river mouths, allochthonous inputs of nutrients and bacterial species could make contributions to explain the different organophosphate mineralization rates in these two contrasting habitats of the large eutrophic lake.     

Genetic variation and spatial structure in sugar maple (Acer saccharum Marsh.) and implications for predicted global-scale environmental change

Current ecosystem model predictions concerning the effects of global temperature increase on forest responses do not account for factors influencing long‐term evolutionary dynamics of natural populations. Population structure and genetic variability may represent important factors in a species' ability to adapt to global‐scale environmental change without experiencing major alterations in current range limits. Genetic variation and structure in sugar maple (Acer saccharum Marsh.) were examined across three regions, between two stands within regions, and among four to five open‐pollinated families within stands (total N = 547 genotypes) using 58 randomly amplified polymorphic DNA (RAPD) markers. Differences within open‐pollinated families account for the largest portion of the total variation (29%), while differences among regions represent less than 2% of the total variation. Genetic diversity, as indicated by estimates of percent polymorphic loci, expected heterozygosity, fixation coefficients, and genetic distance, is greatest in the southern region, which consists of populations with the maximum potential risk due to climate change effects. The high level of genetic similarity (greater than 90%) among some genotypes suggests that gene flow is occurring among regions, stands, and families. High levels of genetic variation among families indicate that vegetational models designed to predict species' response to global‐scale environmental change may need to consider the degree and hierarchical structure of genetic variation when making large‐scale inferences.     

Targeted deep sequencing reveals high diversity and variable dominance of bloom-forming cyanobacteria in eutrophic lakes

Cyanobacterial blooms in eutrophic lakes are severe environmental problems worldwide. To characterize the spatiotemporal heterogeneity of cyanobacterial blooms, a high-throughput method is necessary for the specific detection of cyanobacteria. In this study, the cyanobacterial composition of three eutrophic waters in China (Taihu Lake, Dongqian Lake, and Dongzhen Reservoir) was determined by pyrosequencing the cpcBA intergenic spacer (cpcBA-IGS) of cyanobacteria. A total of 2585 OTUs were obtained from the normalized cpcBA-IGS sequence dataset at a distance of 0.05. The 238 most abundant OTUs contained 92% of the total sequences and were classified into six cyanobacterial groups. The water samples of Taihu Lake were dominated by Microcystis, mixed Nostocales species, Synechococcus, and unclassified cyanobacteria. Besides, all the samples from Taihu Lake were clustered together in the dendrogram based on shared abundant OTUs. The cyanobacterial diversity in Dongqian Lake was dramatically decreased after sediment dredging and Synechococcus became exclusively dominant in this lake. The genus Synechococcus was also dominant in the surface water of Dongzhen Reservoir, while phylogenetically diverse cyanobacteria coexisted at a depth of 10 m in this reservoir. In summary, targeted deep sequencing based on cpcBA-IGS revealed a large diversity of bloom-forming cyanobacteria in eutrophic lakes and spatiotemporal changes in the composition of cyanobacterial communities. The genus Microcystis was the most abundant bloom-forming cyanobacteria in eutrophic lakes, while Synechococcus could be exclusively dominant under appropriate environmental conditions.     

Patterns in freshwater diatom taxonomic distinctness along an eutrophication gradient

1. A variety of species richness measures have been used to assess the effects of environmental degradation on biodiversity. Such measures can be highly influenced by sample size, sampling effort, habitat type or complexity, however, and typically do not show monotonic responses to human impact. In addition to being independent of the degree of sampling effort involved in data acquisition, effective measures of biodiversity should reflect the degree of taxonomical relatedness among species within ecological assemblages and provide a basis for understanding observed diversity for a particular habitat type. Taxonomic diversity or distinctness indices emphasize the average taxonomic relatedness (i.e. degree of taxonomical closeness) between species in a community. 2. Eutrophication of freshwater ecosystems, mainly due to the increased availability of nutrients, notably phosphorus, has become a major environmental problem. Two measures of taxonomic distinctness (Average Taxonomic Distinctness and Variation in Taxonomic Distinctness) were applied to surface sediment diatoms from 45 lakes across the island of Ireland to examine whether taxonomic distinctness and nutrient enrichment were significantly related at a regional scale. The lakes span a range of concentrations of epilimnic total phosphorus (TP) and were grouped into six different types, based on depth and alkalinity levels, and three different categories according to trophic state (ultra‐oligotrophic and oligotrophic; mesotrophic; and eutrophic and hyper‐eutrophic). 3. The taxonomic distinctness measures revealed significant differences among lakes in the three different classes of trophic state, with nutrient‐rich lakes generally more taxonomically diverse than nutrient‐poor lakes. This implies that enrichment of oligotrophic lakes does not necessarily lead to a reduction in taxonomic diversity, at least as expressed by the indices used here. Furthermore, taxonomic distinctness was highly variable across the six different lake types regardless of nutrient level. 4. Results indicate that habitat availability and physical structure within the study lakes also exert a strong influence on the pattern of taxonomic diversity. Overall the results highlight problems with the use of taxonomic diversity measures for detecting impacts of freshwater eutrophication based on diatom assemblages.     

Functional groups of phytoplankton shaping diversity of shallow lake ecosystems

Phytoplankton of eutrophic shallow lakes are frequently dominated by one species or species of the same functional group, resulting in species-pure algal assemblages. Knowledge of the structure of these assemblages is essential to understand their functioning; therefore, species and functional diversity were investigated in five sub-types of eutrophic shallow lake. Among the sub-types, astatic saline lakes and hypertrophic ponds had type-specific assemblages dominated by S_N and W0, W1 codons. The diversity of the phytoplankton in the sub-types was quite similar, except for the astatic saline lakes, which were characterised by lower values of both functional and species diversity. We found that both functional and species diversity were low when bloom-forming cyanobacteria (H1, S_N functional groups) became dominant. Dominance of other groups (J, Y, L_O and W1) did not coincide with decrease in species diversity. Analysis of the biovolume versus diversity relationships revealed that decrease in diversity might be expected at biovolume >20 mm³ l⁻¹ for shallow lakes.     

Application of Sequence-Specific Labeled 16S rRNA Gene Oligonucleotide Probes for Genetic Profiling of Cyanobacterial Abundance and Diversity by Array Hybridization

DNA sequence information for the small-subunit rRNA gene (16S rDNA) obtained from cyanobacterial cultures was used to investigate the presence of cyanobacteria and their abundance in natural habitats. Eight planktonic communities developing in lakes characterized by relatively low algal biomass (mesotrophic) and in lakes with correspondingly high biomass (eutrophic) were selected for the study. The organismal compositions of the water samples were analyzed genetically, using multiplex sequence-specific labeling of oligonucleotide probes targeted to 16S rDNA and subsequent hybridization of the labeled probes to their respective complements spotted onto a solid support (DNA array). Ten probes were established to determine the relative abundances of the discernible cyanobacteria encountered in the selected lakes. The probes were generally specific for their targets, as determined through analyses of clone cultures. Reproducible abundance profiles were established for the lakes investigated in the subsequent analyses of natural cyanobacterial communities. The results from the genetic analyses were then compared with information obtained from standard hydrobiological and hydrochemical analyses. Qualitatively, there were relatively good correlations among the groups of organisms (Nostoc, Microcystis, and Planktothrix species) found in the different lakes. The levels of correlation were lower for the quantitative data. This may, however, be due to differences in sample processing technique. The conclusions from these comparisons are that the genetic abundance profiles may provide a foundation for separating and quantifying genetically distinct groups of cyanobacteria in their natural habitats.     

Specific microsatellite loci for brook charr reveal strong population subdivision on a microgeographic scale

We have isolated specific microsatellite loci from a partial genomic library of brook charr Salvelinus fontinalis. Their usefulness was investigated by measuring intra- and inter-population genetic diversity at four loci among 20 individuals from each of five lakes located 3 to 22 km apart in La Mauricie national park (Canada). These markers were moderately to highly polymorphic. A total of five, six, 16 and 18 alleles per locus were detected, and their overall expected heterozygosity was 0.53, 0.58, 0.86 and 0.87. Strong inter-population diversity was detected. Highly significant differences in allelic frequencies were found in all but two pairwise χ² permutation tests between populations at all loci. Numerous population unique alleles were observed in all five lakes. Consequently, a highly significant component of total genetic diversity was due to interpopulation variance, as exemplified by G _ST values of 0.33, 0.42, 0.47 and 0.84 for each individual locus. Altogether, the results indicated that these loci should be valuable in addressing fine scale population genetics questions in brook charr. To our knowledge, they also represent the first available microsatellites developed in the genus Salvelinus.     

Genetic variation of the bloom-forming Cyanobacterium Microcystis aeruginosa within and among lakes: implications for harmful algal blooms

To measure genetic variation within and among populations of the bloom-forming cyanobacterium Microcystis aeruginosa, we surveyed a suite of lakes in the southern peninsula of Michigan that vary in productivity (total phosphorus concentrations of approximately 10 to 100 microg liter(-1)). Survival of M. aeruginosa isolates from lakes was relatively low (i.e., mean of 7% and maximum of 30%) and positively related to lake total phosphorus concentration (P = 0.014, r2 = 0.407, n = 14). In another study (D. F. Raikow, O. Sarnelle, A. E. Wilson, and S. K. Hamilton, Limnol. Oceanogr. 49:482-487, 2004), survival rates of M. aeruginosa isolates collected from an oligotrophic lake (total phosphorus of approximately 10 mug liter(-1) and dissolved inorganic nitrogen:total phosphorus ratio of 12.75) differed among five different medium types (G test, P of <0.001), with higher survival (P = 0.003) in low-nutrient media (28 to 37% survival) than in high-nutrient media. Even with the relatively low isolate survivorship that could select against detecting the full range of genetic variation, populations of M. aeruginosa were genetically diverse within and among lakes (by analysis of molecular variance, Phi(sc) = 0.412 [Phi(sc) is an F-statistic derivative which evaluates the correlation of haplotypic diversity within populations relative to the haplotypic diversity among all sampled populations], P = 0.001), with most clones being distantly related to clones collected from lakes directly attached to Lake Michigan (a Laurentian Great Lake) and culture collection strains collected from Canada, Scotland, and South Africa. Ninety-one percent of the 53 genetically unique M. aeruginosa clones contained the microcystin toxin gene (mcyA). Genotypes with the toxin gene were found in all lakes, while four lakes harbored both genotypes possessing and genotypes lacking the toxin gene.     

REDUCED GENETIC DIVERSITY AND INCREASED POPULATION DIFFERENTIATION IN PERIPHERAL AND OVERHARVESTED POPULATIONS OF GIGARTINA SKOTTSBERGII (RHODOPHYTA,GIGARTINALES) IN SOUTHERN CHILE1

This study assesses two hypotheses on the genetic diversity of populations of Gigartina skottsbergii Setchell et Gardner (Rhodophyta, Gigartinales) at the border of the species distribution: 1) peripheral populations display a reduced genetic diversity compared with central populations, and 2) genetic differentiation is higher among peripheral than among central populations. Two peripheral and four central populations were sampled along the Chilean coast and 113 haploid individuals were analyzed using 17 random amplification of polymorphic DNA loci. The genetic diversity was estimated by allele diversity (H_e), allele richness (Â), and the mean pair‐wise differences among multilocus genotypes. All three estimates consistently and significantly indicated a lower genetic diversity within the peripheral than within the central populations. Genetic differentiation between the two peripheral populations was stronger (F_ST=0.35) than between central populations at similar spatial scales (F_ST ranging from 0 to 0.25). In addition, it appeared from the distribution of pair‐wise differences that peripheral populations are in demographic expansion after a recent bottleneck. The results are discussed in the specific context of potential overharvesting of these wild populations.     

Genetic diversity among Synechococcus spp. (cyanobacteria) isolated from the pelagial of Lake Constance

Abstract: Seven phycoerythrin (PE)-rich and six phycocyanin (PC)-rich unicellular cyanobacteria of the Synechococcus type were isolated from the pelagial of Lake Constance. The genetic diversity among the isolates was evaluated using restriction fragment length polymorphism (RFLP) within the psbA gene family. Nine out of 13 isolates exhibit different DNA structures in the probed areas and, furthermore, they differ from morphologically similar strains collected from other lakes. The data set does not support a principal distinction between PC-rich and PE-rich strains but it reveals less heterogeneity in the coding region of the psbA genes among PE-rich isolates than among PC-rich isolates. The isolation of distinct strains in different seasons suggests species diversity and seasonal occurrence of Synechococcus spp. in Lake Constance.     

Consumption of cyanobacteria by roach (Rutilus rutilus): useful or harmful to the fish?

1. The ability of roach to use cyanobacterial food is generally believed to be one reason for the dominance of roach over perch in eutrophic European lakes. The aim of this study was to test whether cyanobacteria really are a suitable food for juvenile roach. Special attention was paid to differences between the two cyanobacteria species Aphanizomenon and Microcystis which are common in eutrophic lakes and are ingested by roach there.
2. We performed growth and behaviour experiments with juvenile roach fed with zooplankton and the different cyanobacteria. Growth rate with Aphanizomenon was lower than with Daphnia but significantly higher than without food, whereas growth rate with Microcystis was as low as without food.
3. In cultivation experiments of roach faeces, Microcystis was found not to have been digested and grew exponentially after passing through the gut whereas Aphanizomenon stayed at low biomass. Differences in growth were not related to the toxin content of cyanobacteria. Investigations of roach motility showed no differences whether fed with Aphanizomenon or Microcystis .
4. In contrast to Microcystis , Aphanizomenon can be regarded as a suitable food source for juvenile roach probably because of its better digestability. We conclude that the ability to feed on cyanobacteria is not a general competitive advantage for roach, but the outcome depends on the species composition of the cyanobacteria.     

Species composition and cyanotoxin production in periphyton mats from three lakes of varying trophic status

In lakes, benthic micro-algae and cyanobacteria (periphyton) can contribute significantly to total primary productivity and provide important food sources for benthic invertebrates. Despite recognition of their importance, few studies have explored the diversity of the algal and cyanobacterial composition of periphyton mats in temperate lakes. In this study, we sampled periphyton from three New Zealand lakes: Tikitapu (oligotrophic), ōkāreka (mesotrophic) and Rotoiti (eutrophic). Statistical analysis of morphological data showed a clear delineation in community structure among lakes and highlighted the importance of cyanobacteria. Automated rRNA intergenic spacer analysis (ARISA) and 16S rRNA gene clone libraries were used to investigate cyanobacterial diversity. Despite the close geographic proximity of the lakes, cyanobacterial species differed markedly. The 16S rRNA gene sequence analysis identified eight cyanobacterial OTUs. A comparison with other known cyanobacterial sequences in GenBank showed relatively low similarities (91-97%). Cyanotoxin analysis identified nodularin in all mats from Lake Tikitapu. ndaF gene sequences from these samples had very low (≤ 89%) homology to sequences in other known nodularin producers. To our knowledge, this is the first detection of nodularin in a freshwater environment in the absence of Nodularia. Six cyanobacteria species were isolated from Lake Tikitapu mats. None were found to produce nodularin. Five of the species shared low (< 97%) 16S rRNA gene sequence similarities with other cultured cyanobacteria.     

Genetic diversity and recombination within populations of Fusarium pseudograminearum from western Canada.

Genetic diversity within populations of Fusarium pseudograminearum isolated from wheat grains from the Canadian provinces of Alberta and Saskatchewan was investigated. Three restriction enzymes (EcoRI, HaeIII, and PstI) were used to carry out restriction analysis of the nuclear ribosomal DNA (nrDNA) intergenic spacer region (IGS region) and eight primers were used to generate inter-simple sequence-repeat (ISSR) molecular markers. Our study indicated substantially high genetic diversity within these two populations, but low genetic differentiation and frequent gene flow among populations. The IGS data showed no genetic distinction between the two Alberta populations and only minor genetic differentiation between the Saskatchewan and Alberta populations. Analysis of molecular variance indicated that most genetic variability resulted from differences among isolates within populations. Multilocus linkage disequilibrium analysis suggested a panmictic population genetic structure and the occurrence of significant recombination in F. pseudograminearum. Regular gene flow and random mating between isolates from different populations could result in novel genotypes with both improved pathological and biological traits.     

Higher diversity of Rhizobium leguminosarum biovar viciae populations in arable soils than in grass soils

The bacterial genetic diversity after long-term arable cultivation was compared with that under permanent grassland using replicated paired contrasts. Pea-nodulating Rhizobium leguminosarum populations were sampled from pairs of arable and grass sites at four locations in Yorkshire, United Kingdom. Isolates were characterized using both chromosomal (16S-23S ribosomal DNA internal transcribed spacer PCR-restriction fragment length polymorphism) and plasmid (group-specific repC PCR amplification) markers. The diversities of chromosomal types, repC profiles, and combined genotypes were calculated using richness in types (adjusted to equal sample sizes by rarefaction), Shannon-Wiener index, and Simpson's index. The relative differences in diversity within each pair of sites were similar for all three diversity measures. Chromosomal types, repC profiles, and combined genotypes were each more diverse in arable soils than in grass soils at two of the four locations. The other comparisons showed no significant differences. We conclude that rhizobial diversity can be affected by differences between these two management regimens. Multiple regression analyses indicated that lower diversity was associated with high potential nitrogen and phosphate levels or with acidity.     

Do assemblages of Coregonus (Teleostei: Salmoniformes) in the Central Alpine region of Europe represent species flocks?

To examine models of evolution for Coregonus from the Central Alpine region of Europe, 20 populations from nine lakes were assessed for variation at six microsatellite DNA loci. Patterns of variation were tested against three evolutionary models: phenotypic plasticity, multiple invasions of lakes by divergent forms, and within-lake radiation of species flocks. All sympatric and all but one allopatric pairs of populations were significantly divergent in allele frequencies. Pairwise F -statistics indicated reduced gene flow among phenotypically divergent sympatric populations. These results reject the hypothesis that within-lake morphological and ecological diversity reflects phenotypic plasticity within a single gene pool. Genetic similarity was higher among forms within lakes than between populations of the same form in different lakes. Among-lake divergence was primarily a product of allele size differences. Mantel tests contrasting patterns of genetic divergence against patterns predicted from the multiple invasions and species flocks models indicated that the latter is the best explanation of the observed genetic variation. Thus, reproductively isolated species diverged within lakes, with similar patterns repeatedly emerging among lakes. While this study argues for a particular mode of evolution in Central Alpine Coregonus , the taxonomy of these forms remains unresolved.