Ecology, phylogeography and genetic diversity of an Arctic Daphnia species complex
This project forms an extension of a research programme called “Ecological genetics of the Daphnia pulex complex – An Arctic, circumpolar survey” (Hobæk 1995; Hobæk and Weider 1999). Our programme is based on collaborative efforts by researchers in Canada, Germany/USA and Norway. The general objective is to map clonal diversity, genetic variation and phylogeographic patterns within the complex throughout the Arctic, and to investigate the role of various ecological factors in structuring clonal composition.
Project description
Previous work has revealed conspicuous differences between Arctic regions with respect to clonal composition, richness and diversity (Colbourne et al. 1998; Hobæk and Weider 1999; Hobæk et al. 1993; Weider and Hobæk 1994; 1997; Weider et al. 1996; 1999a; 1999b). One major factor determining clonal variation seems to be glaciation history, regions impacted by Pleistocene glaciations being significantly poorer in clonal diversity. The Beringian refuge stands out as a major centre of high diversity levels. Another possible refuge is the NE corner of the Canadian Archipelago, including portions of Ellesmere and Baffin Islands and Peary Land (Greenland). The present project aims to analyse patterns of biodiversity and phylogeography in the Canadian Arctic in more detail by means of a geographically more comprehensive sampling. We also hope to test several hypotheses based on our previous work.
Based on the sequencing of a mitochondrial (mt) genome fragment, the complex can be broken in to two major clades, each with two or more subclades (Colbourne et al. 1998). One major clade (called the pulicaria group) is found from Western Russia, Fennoscandia and Svalbard westwards across Iceland, Greenland and the Canadian High Arctic, with its highest diversity in Canada. The other major clade (called the tenebrosa group) occurs from Svalbard eastwards, dominating Eurasia and extends deep in to the American Arctic, but is absent in Greenland and Iceland. Within both major clades there is further geographic structuring of subclades (Weider et al. 1999a).
Using the Tundra Northwest 1999 expedition as a logistic vehicle, we sampled as many populations as possible of the Daphnia pulex-complex. The main habitat was tundra ponds, but we included several lakes and rock pools where members of this complex can also be found.
Assignment to major group sand subclades (Weider et al. 1999a) will be conducted by analysis of restriction fragment length polymorphism (RFLP) of a 2 kb mtDNA fragment amplified via the polymerase chain reaction (PCR) . Clonal diversity will be estimated by analysing variation at 6 to 8 enzyme loci (Weider et al. 1999b). Further nuclear markers (microsatellites) are also available for increased resolution.
This study will also contribute to two subthemes (Comparative Phylogeography and Genetic Diversity) within theme B, Biodiversity of the Arctic Tundra.
Field-work du ring Tundra Northwest 1999
Anders Hobæk participated on Leg l (18 June–1 August), while Lawrence J. Weider took over on Leg 2 (2 August–4 September). Based in ”Camp Blue”, we surveyed as many freshwater localities as possible on foot. Zooplankton samples were gathered using a dip net while wading in to the ponds. One sample was preserved in the field, while another sample consisted of animals hand-picked from the net catches and brought back alive to the ship. In addition, we also brought water samples for later analyses, and recorded the position as well as descriptive parameters for each pond.
In the lab on board the Louis S. St-Laurent, animals were sorted into cryotubes and frozen at -80°C. In addition to Daphnia, we also picked several other Crustaceans for later genetic analyses.
After the expedition terminated, the frozen samples were divided into two batches. One batch was taken in liquid nitrogen to Oklahoma, while the other was taken to Sweden and later to Norway on dry ice. Thus, each lab now holds a backup of samples for each other.
Analysis of the material collected
Morphological variation between populations was conspicuous, within as well as between different sites. Both hyaline (unpigmented) and melanic animals were encountered at many sites. So far, no males have been observed in the samples.
The genetic analysis of the material has just started and no results are yet available. Initially, each population will be screened for allozyme variation at loci known to be polymorphic in other Arctic areas (Weider et al. 1999b). Several other loci may be screened later, depending on the initial results. DNA extraction is also underway. After completion of the initial allozyme analyses, a survey of mtDNA RFLP genotypes will be carried out in accordance with Weider et al. (1999a).
The planned data analyses include phylogeographic patterns relative to putative glacial refuges and dispersal corridors, and clonal diversity along gradients such as north-south and island-mainland. Furthermore, we plan to contribute to joint comparative analyses of diversity patterns within the various organisms sampled during the expedition (mosses, vascular plants, terrestrial insects, lemmings).
The number of molecular markers available increases steadily, and the possibilities for refined analyses keep growing. Therefore, some of the material collected will be stored. Also, culture isolates established from resting eggs will provide a continuous source of material for future analyses.
Dates
18 June–4 September 1999
Participants
Principal investigator
Anders Hobæk
Norwegian Institute for Water Research
Bergen, Norway
Principal investigator
Lawrence J. Weider
Biological Station, University of Oklahoma
Kingston, USA
References
Colbourne, J.K., Crease, T.J., Weider, L.J., Hebert, P.D.N., Dufresne, F. and Hobæk. A, (1998). Phylogenetics and evolution of a circumarctic species complex (Cladocera: Daphnia pulex) during the Pleistocene. Biol. J. Linnean Soc. 65, 347-365.
Hobæk, A. (1995). Ecological genetics of the Daphnia pulex complex – An Arctic, circumpolar survey. Swedish-Russian Tundra Ecology-Expedition 1994. A Cruise Report Grönlund, E. and Melander, O. (ed.),. Swedish Polar Research Secretariat, Stockholm, 216-224.
Hobæk, A, Weider, L.J. and Wolf, H.G. (1993). Ecological genetics of Norwegian Daphnia III. Clonal richness in Arctic, apomictic complex. Heredity 71, 323-330.
Hobæk , A and Weider L.J. (1999). A circumpolar study of Arctic biodiversity: Phylogeographic patterns in the Daphnia pulex complex. Ambio 28, 245-250.
Weider, L.J. and Hobæk., A (1994). Molecular biogeography of clonal lineages in a High-Arctic apomictic Daphnia. Molecular Ecology 8, 1-13.
Weider, L.J. and Hobæk, A (1997). Postglacial dispersal, glacial refugia, and clonal structure in Russian/Siberian populations of the arctic Daphnia pulex complex. Heredity 78, 363-372.
Weider, L.J., Hobæk, A, Crease, T.J. and Stibor, H. (1996). Molecular characterization of clonal population structure and biogeography of Arctic apomictic Daphnia from Iceland and Greenland. Molecular Ecology 5, 107-118.
Weider, L.J. Hobæk, A., Dufresne, F., Colbourne, J. K, Crease T.J. and Hebert, P.D.N. (1999a). Holarctic phylogeography of an asexual species complex. I: mtDNA variation in Arctic Daphnia. Evolution 53, 777-792.
Weider, L.J., Hobæk, A , Hebert, P.D.N. and Crease. T.J. (1999b). Holarctic phylogeography of an asexual species complex. II: Allozymic variation and clonal structure in Arctic Daphnia. Molecular Ecology 8, 1-13.