Anders Hobæk

Sexual reproduction in Daphnia magna requires three stimuli

The factors inducing sexual eggs, as well as the offspring sex ratio preceding sexual egg formation, were investigated in a cyclic parthenogen, Daphnia magna (Crustacea, Cladocera). Laboratory experiments were conducted on individual animals living in low-through chambers, making it possible to separate the effects of two density-dependent factors: food limitation and a chemically mediated cue. These factors were studied under a short-day photoperiod and in permanent light. The simultaneous actions of an inductive photoperiod, food limitation and chemically mediated crowding, were all needed to induce sexual egg formation. When only two stimuli were present, the offspring sex ratio was 0.50 or lower, and no sexual eggs were produced

HOLARCTIC PHYLOGEOGRAPHY OF AN ASEXUAL SPECIES COMPLEX I. MITOCHONDRIAL DNA VARIATION IN ARCTIC DAPHNIA

Pleistocene glacial cycles undoubtedly altered the evolutionary trajectories of many taxa, yet few studies have examined the impact of such events on genetic differentiation and phylogeography at large geographic scales. Here we present the results of a circumarctic survey of mitochondrial DNA diversity in members of the Daphnia pulex complex. The analysis involved the survey of restriction site polymorphisms in a 2100‐bp fragment of the NADH‐4 (ND4) and NADH‐5 (ND5) genes for 276 populations representing the two major groups (tenebrosa and pulicaria) in this complex across their Holarctic range. A comparison of the distribution patterns for seven clades in this complex revealed very clear phylogeographic structuring. Most notably, pulicaria group lineages were restricted primarily to the Nearctic, with some colonization of formerly glaciated portions of northern Europe. This group was not detected from vast expanses of northern Eurasia, including the Beringian glacial refuge. In contrast, tenebrosa group haplotypes showed considerable intercontinental divergence between Eurasian and North American lineages, but were absent from Greenland and Iceland, as well as the Canadian arctic archipelago. Dispersal in Eurasia was primarily in a westerly direction from Beringia, whereas dispersal in the Nearctic followed proglacial drainage patterns. Long‐distance dispersal of certain lineages was observed in both groups, and variation in haplotype richness and nucleotide diversity allowed us to make inferences about the positioning of putative glacial refugia. Overall, the phylogeographic pattern of diversification in this arctic complex is characterized by the apparently unique postglacial histories for each clade, indicating that even closely allied taxa can respond independently to the allopatric effects of glacial cycles. This is in sharp contrast to other phylogeographic studies of species assemblages from more southern (unglaciated) latitudes, which are often characterized by concordant patterns.

A taxonomic reappraisal of the European Daphnia longispina complex (Crustacea, Cladocera, Anomopoda)

The Daphnia longispina complex contains some of the most common water flea species in the northern hemisphere, and has been a model organism for many ecological and evolutionary studies. Nevertheless, the systematics and nomenclature of this group, in particular its Palaearctic members, have been in flux for the past 150 years; this hinders the correct interpretation of scientific results and promotes the erroneous use of species names. We revise the systematics of this species complex based on mitochondrial sequence variation (12S rDNA and COI) of representative populations across Europe, with a special focus on samples from type localities of the respective taxa. Combining genetic evidence and morphological assignments of analysed individuals, we propose a comprehensive revision of the European members of the D. longispina complex. We show that D. hyalina and D. rosea morphotypes have evolved several times independently, and we find no evidence to maintain these morphotypes as distinct biological species. Alpine individuals described as D. zschokkei are conspecific with the above‐mentioned lineage. We suggest that this morphologically and ecologically plastic but genetically uniform hyalina–rosea–zschokkei clade should be identified as D. longispina (O. F. Müller, 1776). The valid name of Fennoscandian individuals labelled D. longispina sensu stricto in the recent literature is D. lacustris G. O. Sars, 1862. Additionally, we discovered another divergent lineage of this group, likely an undescribed species, in southern Norway. Our results present a solution for several prevailing taxonomic problems in the genus Daphnia, and have broad implications for interpretation of biogeographical patterns, and ecological and evolutionary studies.

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°22′ N to 79°01′ N; 66°45′ W to 139°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.

Factors influencing species richness in lacustrine zooplankton

Frequent dispersal events are expected to elevate local species richness in island-like habitats such as lakes. However, the importance of dispersal can be hard to evaluate if other factors cause large background variability in species composition and richness. In this paper, we review empirical studies on ecological factors known or expected to influence species richness in zooplankton communities of inland lakes. We then present summaries of two recent case studies. Our objectives are twofold: we first look for effects of biotic interactions on species richness and species composition, and then evaluate whether the expected effects of dispersal are likely to be detected on a background of large variability caused by other ecological factors and interactions. Species richness within lakes appears to be primarily controlled by factors related to lake size, lake productivity, water quality, and fish predation levels. One case study indicated a slight, but significant, positive effect of lake density and lake area in the surrounding landscape on species richness, suggesting that frequent dispersal events may enhance species richness. This local variation in species richness is superimposed on regional variation in species pools.

Holoarctic phylogeography of an asexual species complex. - II. Allozymic variation and clonal structure in arctic Daphnia.

Previous mitochondrial (mt)DNA sequencing and restriction fragment length polymorphism (RFLP) studies have shown that the Holarctic Daphnia pulex complex is divisible into two major groups (pulicaria and tenebrosa) that exhibit distinct phylogeographic patterns. Here we examine allozymic variation at six polymorphic enzyme loci to ascertain clonal structure and clonal distribution patterns within each group. Specimens were collected from a total of 850 populations encompassing the Arctic. A significant negative relationship (Mantel test) between similarity of regional clonal arrays and geographical distance was observed. A small fraction of clones in each group was widespread (in the order of 1000s of kilometres). However, most clones were restricted to single regions, and were often found only in a single population. These data indicate that the population genetic structure is highly fragmented in this complex, but the potential for long‐distance passive dispersal exists. Further, ‘hot spots’ of high clonal richness and diversity were found in each group, which is concordant with earlier work. In addition, ≈ 20% of pulicaria group clones possess nuclear genes from tenebrosa, while approximately 10% of tenebrosa group clones harbour pulicaria nuclear genes. These data indicate nuclear introgression between the two groups, which was found to be prominent in a broad zone of secondary contact encompassing parts of northwestern Russia, northern Fennoscandia, Svalbard, and extending into the high eastern Canadian Arctic.

Postglacial dispersal, glacial refugia, and clonal structure in Russian/Siberian populations of the arctic Daphnia pulex complex

Results from an extensive allozymic survey of the genetic (clonal) structure of Russian/Siberian populations in the arctic Daphnia pulex complex yielded high clonal diversity/richness estimates, at both the intrapopulational and intraregional levels. Highest levels of clonal diversity were detected in the heart of the Beringian glacial refuge, with lower levels detected in glaciated regions (western Russia). mtDNA haplotype diversity (based on RFLPs) showed similar spatial trends, with the highest levels of haplotype diversity again being found in Beringia. Several haplotypes were extremely widespread (in the order of thousands of kilometres across the breadth of Eurasia), and evidence for presumed mutationally derived haplotypes was plentiful. Spatial autocorrelation analysis revealed a significant clinal pattern in the degree of genetic similarity among mtDNA haplotypes. In addition, similarity of clonal arrays (based on allozymes) showed a significant inverse association with geographical distance; i.e. similarity of clonal arrays between populations increased with decreasing geographical distance between populations. Sporadic sexual reproduction (based on Hardy–Weinberg expectations) was detected in Beringia, and has undoubtedly played a role in fuelling the production of novel genotypes. Our results suggest that vast expanses of northern Eurasia have been colonized by a small number of geographically widespread haplotypes, with more regionally restricted haplotypes constituting the remaining lineages. We discuss our results within the general framework of biogeographical colonization events of the Arctic by members of the D. pulex complex, and place these data into the broader picture of the importance of Beringia as a glacial refuge for many taxa during the Pleistocene.

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.

Restoring Daphnia lacustris G.O. Sars, 1862 (Crustacea, Anomopoda): a cryptic species in the Daphnia longispina group

While molecular markers have revealed several distinct species within the Daphnia longispina group, there is a need to reconcile these species with traditional nomenclature. Here we show that one such species, called D. longispina in recent literature based on molecular markers, can reliably be associated with the described taxon Daphnia lacustris G.O. Sars, 1862. Both mitochondrial and nuclear molecular markers readily distinguish this species from others in the D. longispina group. D. lacustris is absent in the region from which D. longispina was first described (Denmark), and the designation D. longispina must be reserved for another widespread species represented by Danish lineages. While the diagnosis of D. lacustris (and other species of the D. longispina group) by molecular markers is unequivocal, distinguishing it morphologically from other species is still problematic. The presently known distribution range of D. lacustris includes most of Norway, northern Finland and a single lake in the Polish Tatra Mountains. Its typical habitat is oligotrophic lakes without intense fish predation.

Daphnia galeata · D. longispina hybrids in western Norway

We describe the occurrence of D. galeata×longispina hybrids in two lakes of western Norway. Parental species and interspecific hybrids were characterised by both nuclear and mitochondrial molecular markers. In one of the populations, hybrids were shown to dominate the population over several years. A few individuals in both populations were probably not F1 hybrids, but possibly backcrosses or F2 hybrids. Most (possibly all) F1 hybrids were of D. galeata maternal origin. In addition, interspecific hybrids could be identified based on morphological characters, which were intermediate between the parental species. Interspecific hybridisation between these two species is remarkable, since they are distantly related.