Glacial refugia, haplotype distributions, and clonal richness of the Daphnia pulex complex in arctic Canada.

As part of a large international Arctic biodiversity expedition (Tundra Northwest '99), we examined the distribution of members of the arctic Daphnia pulex complex (Cladocera, Anomopoda) from 121 tundra ponds, spread across 16 sites spanning a large portion of arctic Canada (i.e. from 62 degrees 22' N to 79 degrees 01' N; 66 degrees 45' W to 139 degrees 37' W). Using allozyme electrophoresis and mitochondrial (mt)DNA analyses, we examined the population genetic (clonal) structure of these populations. The following taxa were detected in this complex: Daphnia pulicaria, D. middendorffiana and D. tenebrosa. Clear geographical differences in mean clonal richness and diversity were observed, with most western sites exhibiting higher clonal richness and diversity, than sites in the eastern Canadian Arctic. For both the pulicaria group (i.e. D. pulicaria and D. middendorffiana) and D. tenebrosa, the highest mean regional clonal richness was detected from the southern section of Banks Island, an unglaciated site situated on the edge or directly in the eastern fringe of the Beringian glacial refuge. A significant negative correlation was found between geographical distance from the Beringian edge, and overall regional clonal richness (i.e. sites closer to the edge harboured greater clonal richness). These results clearly indicate that more recently deglaciated regions (i.e. eastern Canadian Arctic) harbour lower levels of clonal richness than western regions nearer Beringia. We discuss the role that glacial refugia have played in influencing both biotic and genetic diversity in arctic taxa.

Natural selection for grazer resistance to toxic cyanobacteria: evolution of phenotypic plasticity?

We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor-quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcvstis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj). We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log-transformed) changes in gj. Log(gj) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in gj and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in gj among genotypes), a significant genotype x food-type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log-transformed data, a significant sediment-genotype-age x food-type interaction. The latter shows that phenotypic plasticity evolved over the period studied. Two constraints act on response to selection in the D. galeata-Lake Constance system. First, gj on a diet containing poor food is highly correlated with gj on a diet of good food, thus evolving resistance also meant evolving an increase in gj on both diets. Second, because genotypes with a high gj also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential gj after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized gj and led to animals of small adult body size even if their genotypes had the potential for high gj and large size. With realized gj reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.

Molecular characterization of clonal population structure and biogeography of arctic apomictic Daphnia from Greenland and Iceland.

As part of a continuing international project to characterize the population genetic (clonal) structure of arctic members of the Daphnia pulex complex, 34 populations from western Iceland (N = 1373), and 76 populations from western Greenland (N = 2917), were surveyed for allozymic variation at six polymorphic enzyme loci. Mean clonal richness (+/- SE) was 1.91 +/- 0.19 and 1.50 +/- 0.12 for Iceland and Greenland populations, respectively. Mean clonal diversity (+/-1 SE) was 1.20 +/- 0.07 and 1.13 +/- 0.04 for Iceland and Greenland populations, respectively. Four widely distributed clones constituted 92.2% of the total animals surveyed from Iceland, while three locally abundant clones constituted 80.1% of the total animals collected primarily from Disko Island, western Greenland. Selected populations were screened for mitochondrial (mt)DNA variation using PCR-based RFLP analysis of a 2100 bp fragment containing part of the ND4 and ND5 genes. One mtDNA haplotype was very widespread in both western Greenland and western Iceland, although, a number of mutational derivatives were also detected. These data indicate the potential for long-distance dispersal of mtDNA lineages, of the order of hundreds or thousands of kilometers across the arctic. Phylogenetic analysis of the sequence of a 254 base pair (bp) fragment of the control region of the mtDNA molecule revealed two major clades one of which consisted solely of non-melanic lineages, and the other of which consisted almost exclusively of melanic lineages (i.e. one non-melanic lineage also clustered in this clade). Sequence divergence between the two clades averaged 7.3%. Both mitochondrial analyses did not reveal any distinct intraregional clustering of lineages. We discuss our results in reference to previous molecular work done on this arctic Daphnia complex, and we attempt to infer phylogeographic patterning based on geological/glaciological historical events in this region of the arctic.

Molecular biogeography of clonal lineages in a high-arctic apomictic Daphnia complex.

An electrophoretic survey of 81 populations of arctic Daphnia pulex from around the Svalbard archipelago revealed the presence of 49 unique allozyme clones (N = 3357). Two closely related clones accounted for 66% of the total sample, and were widespread across the archipelago. Restriction fragment length polymorphisms (RFLPs) of a 2.1-kb fragment of mtDNA (NADH-4 and NADH-5 subunits), amplified using the polymerase chain reaction (PCR), revealed the presence of eight mtDNA haplotypes. One haplotype was particularly widespread, and the two most abundant allozyme clones shared this haplotype. Nonrandom distribution patterns of clones were observed, and are most likely the result of historical events (i.e. founder effects) related to the past glacial history of the archipelago. The data are discussed with reference to past glaciation events, and attempts are made to discern the colonization history of this apomictic complex.

Life-history variation in a hybrid species complex ofDaphnia.

Life-history variation was examined among members of theDaphnia longispina group, which consists ofD. galeata, D. hyalina, D. cucullata, and hybrids. Factorial experiments were conducted at two temperatures (14° and 20° C) and two food concentrations (0.2 and 1.0 mg Cl-1). Differences in life-history features (size at maturity, age at first reproduction, size of first clutch, offspring size in first clutch) under the different environmental conditions were assessed among eightDaphnia clones, which represented members of this species complex. Significant differences between parentals and hybrids for most life-history features were observed under various treatments; generally, hybrid clones showed intermediate life-history traits when compared with parentals. When comparisons were made among clones within a given species (i.e.D. galeata, D. galeata xcucullata, D. cucullata), clonal differences were also noted for certain life-history traits. The data are discussed with reference to the formation and maintenance of hybrid species complexes in nature.

Population genetics of Polyphemus pediculus (Cladocera: Polyphemidae).

Population genetic studies of cyclically parthenogenetic organisms have focussed on members of the freshwater cladoceran crustaceans, and mainly on those populations which inhabit intermittent or permanent ponds. Little is known of the genetic structure of large-lake populations. The genetic structure of populations of the predatory freshwater littoral-zone cladoceran, Polyphemus pediculus from 20 palearctic lakes in Northern Germany was examined by electrophoresis to determine its breeding system and level of genetic polymorphism, and to compare its genetic structure with those of other Cladocera. Polyphemus was polymorphic for three of eight enzyme loci and most (20/23) genotypic frequencies were in Hardy-Weinberg equilibrium. These populations are thus panmictic, and most likely reproduce by cyclical parthenogenesis. There were heterozygote deficiencies at each of the polymorphic loci in the three deviant populations. Genetic differentiation between populations was evident at each locus, with significant spatial autocorrelation found at the Pgm locus. Population heterozygosity averaged 9.2 per cent with a mean of 1.5 alleles/locus. Mean number of composite genotypes per population was 6.0+/-1.0 (S.E.) with a range of 1-17 clones per population. P. pediculus thus has a population genetic structure similar to that of large-lake Daphnia populations.

Life history variation among low-arctic clones of obligately parthenogenetic Daphnia pulex: a diploid-polyploid complex.

Laboratory life table experiments were conducted using nine clones of obligately parthenogenetic Daphnia pulex that were collected from a site in the Canadian low-arctic. Two of the nine clones were diploids, while the other seven clones were polyploids. Significant clonal differences in age at first reproduction, size at first reproduction, number of offspring in each of the first three broods, offsrring sizes for the first two broods, and intrinsic rates of natural increase were detected. Differences in life histories were evident between polyploids and diploids. Generally, polyploid clones reached maturity at later ages, matured at larger sizes, produced smaller broods, and larger offspring than the diploid clones. The data are discussed in reference to potential biotic (i.e. invertebrate predation) and abiotic factors (i.e. physicochemical gradients) that may influence life history variation in this clonal assemblage.