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Biological systems in the McMurdo Dry Valleys of Antarctica are severely restricted by extreme environmental conditions including cold temperatures and low moisture levels, resulting in a cold desert ecosystem that is much simpler than temperate regions (Campbell and Claridge 1981). Continental Antarctica has a low diversity of soil nematodes: 11 species in six genera (Maslen 1979). Nematodes are a dominant soil fauna in the McMurdo Dry Valleys of continental Antarctica, and in the drier soils, they represent the top of the food chain (Freckman and Virginia 1991, in press). Only three genera have been found in McMurdo Dry Valley soils: the microbial feeders Scottnema lindsayae Timm 1971 and Plectus antarcticus de Man 1904 (Overhoff, Freckman, and Virginia 1993; Yeates et al. 1993) and the omnivore/predator Eudorylaimus antarcticus (Steiner 1916), Yeates 1970 (Yeates et al. 1993). Scottnema dominates McMurdo Dry Valley soil communities (Freckman and Virginia 1991) and is the only endemic genus in continental Antarctica (Maslen 1979). As such, Scottnema is an important component of the McMurdo Dry Valley soil ecosystem.
We examined the genetic diversity of Scottnema lindsayae to determine if variation across valleys was low, suggesting dispersal, or if valleys were associated with unique populations that evolved with little interaction between valleys. Biota were extracted from samples collected from 1-square-meter grids at sites in three McMurdo Dry Valleys during the 1993-1994 austral summer (table 1). Approximately 1,500 grams of soil per sample at 0-10 centimeters depth was collected with presterilized plastic sampling scoops (Nasco Sampling Equipment) and placed in sterile Whirl-pak® bags (Freckman and Virginia 1993). Soil samples were refrigerated at 1°C (Overhoff, Freckman, and Virginia 1993), and nematodes were extracted by sugar centrifugation within 48 hours of collection (Freckman and Virginia 1993).
The genetic variation of Scottnema was examined using molecular techniques because morphological variation indicated more than one interbreeding group may exist. Scottnema were extracted from samples collected at Lake Bonney, Lake Hoare, and Lake Fryxell in Taylor Valley, Victoria Lower Glacier in Victoria Valley, and Lake Brownworth in Wright Valley (table 1). Two segments of ribosomal DNA (rDNA) that encode the D2 and D3 expansion segments of the nuclear large rRNA subunit were sequenced. Genomic DNA was extracted from individual nematodes using 15-microliter lysis buffer (60 micrograms per milliliter proteinase potassium in 10-millimolar Tris pH 8.8, 50 millimolar potassium chloride, 2.5 millimolar magnesium chloride, 0.45 percent Tween 20, and 0.05 percent gelatin) (Williams et al. 1992).
Nematodes (living and dead) were found in 61 percent of all soils sampled. Three nematode species were found: Scottnema lindsayae, Eudorylaimus antarcticus, and Plectus antarcticus (table 1). Viable nematodes were found in 58 percent of all samples. Scottnema was the most abundant nematode in each valley. Eudorylaimus was found in both Taylor and Wright Valleys whereas Plectus was found at only one location (north shore of Lake Hoare in Taylor Valley). Only one sample of the 99 total samples contained three living species, and 18 had two species. In all cases of two or three species complexes, S. lindsayae was present. Scottnema was the dominant nematode in all the samples indicating low species diversity and a one-link trophic chain.
Three different genotypes were identified for the D2 region based on 75 sequences (table 2). Sixty-six nematodes possessed a common genotype that has a cytosine (C) at position 91. Nematodes with this common genotype were found at all the locations sampled. Only three nematodes had a rare genotype with an adenine (A) at position 91. Nematodes with this rare genotype originated from soil samples collected at Victoria Lower Glacier in Victoria Valley. In addition, six individuals containing both an A and a C at position 91 in approximately equal ratios were identified from the following locations: Lake Bonney in Taylor Valley (one nematode) and Victoria Lower Glacier in Victoria Valley (five nematodes).
Three genotypes were also found for the D3 region based on 76 sequences (table 2). A common genotype that contains a C at position 103 was found in 69 nematodes and across all the sites sampled. A rare genotype that contains a thymine (T) at position 103 was found in two nematodes, one from Lake Fryxell in Taylor Valley and one from the north shore of Lake Hoare in Taylor Valley. Five individuals containing both a C and a T at position 103 in approximately equal ratios were identified from the following locations: the south shore of Lake Hoare in Taylor Valley (three nematodes) and Lake Brownworth in Wright Valley (two nematodes).
A common, rare, and combined genotype were identified for both D2 and D3 (table 2). The occurrence of individuals with both the rare and common sequence in approximately equal ratios suggests that S. lindsayae may be evolving into two species. The pattern of nuclear variation is most consistent with a single species of nematode defined morphologically as S. lindsayae.
S. lindsayae occurs more frequently in this harsh environment than the other two species (E. antarcticus and P. antarcticus). In this extreme environment, differences in the soil microclimate habitat and limitations to dispersal and establishment may be contributing to the evolution of two distinct species of Scottnema as evidenced by the nuclear variation in the D2 and D3 regions.
We thank M. Ho and L.E. Powers for their assistance in sampling and laboratory analyses. We also gratefully acknowledge the logistic support of the National Science Foundation (NSF) McMurdo Station laboratory staff and the U.S. VXE-6 and Royal New Zealand Air Force helicopter crews. This research was supported by NSF grant OPP 92-11773, the McMurdo Dry Valley Long-Term Ecological Research (LTER), and is a contribution to NSF grant OPP 91-20123 to D.W. Freckman and R.A. Virginia.
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