Low Arctic Site Research Articles

Apomictic parthenogenesis and genotypic diversity in Cypridopsis vidua (Ostracoda: Cyprididae)

Based on sex ratios, the crustacean subclass Ostracoda appears to have made frequent transitions from sexual to asexual reproduction. However few genetic data are available verifying their mode of reproduction. The current report provides such evidence for apomictic pathenogenesis in the cosmopolitan cypridid ostracode Cypridopsis vidua. Sex ratio analysis indicated that natural populations are exclusively female. Allozyme phenotypes in 18 of 20 natural populations showed strong departures from Hardy–Weinberg expectations at single loci, and three of four populations had non-random associations of phenotypes at MPI and PGM. Allozyme phenotypes at these loci suggested that polyploids were present in half these populations and were more common in the low arctic site than in the temperate sites. Most natural populations of C. vidua displayed high levels of genotypic diversity, with a total of 77 unique clones detected and up to 24 clones per pond. Breeding studies on 64 isofemale lines established that parthenogenesis is apomictic and that mutation rates are less than 10−3.

Asymbiotic dinitrogen fixation by tundra in the Imnavait Creek drainage, Alaska, USA

N2 fixation by free-living microorganisms was investigated at an intensively studied low Arctic site near Toolik Lake in the northern foothills of the Brooks Range, Alaska, during July 1987. Four characteristic vegetation associations along an elevational gradient were assayed using minimally disruptive in situ acetylene reduction assay methods. The acetylene reduction rates did not differ significantly among vegetation associations. The mean rate for the site was 9.60 μmol m−2 h−1 or 90 μg N m−2 day−1, which is within the range of values given for other Arctic and alpine tundra studies. The complex microtopography and resulting patchy distribution of free-living and phycobiont diazotrophs is the most likely cause of the high spatial variability in acetylene reduction activity. Rates were most variable among samples from the lowest position, a riparian site. The potential contribution of heterotrophic diazotrophs was examined through a laboratory enrichment study. Soils from the two lower slope positions showed dramatic responses to added C, suggesting that heterotrophs may contribute fixed N2 to this system.

Microgeographic genetic heterogeneity of melanic Daphnia pulex at a low-Arctic site

Clonal diversity of obligately parthenogenetic Daphnia pulex was examined in 147 ponds at 13 sites near Churchill, Manitoba. Cellulose acetate electrophoresis of four polymorphic loci allowed the detection of 16, multi-locus genotypes (clones). The three most common clones accounted for 86 per cent of the 3291 animals surveyed. An average of 1·5 clones coexisted in single ponds, while an average of 3·8 clones was present at each site. Fifteen of the sixteen clones show unbalanced electromorph phenotypes at one or more loci suggesting that they are polyploids. There was significant spatial autocorrelation of the distribution of certain clones within a given site on a scale of tens of metres. Clinal patterns in clone distributions were often found. No significant between-site spatial autocorrelation was found on a scale of 0–20 kilometres, although between-site heterogeneity of clonal frequencies was frequently seen. Microgeographic heterogeneity of D. pulex clones at Churchill is influenced by salinity gradients on a scale of tens of meters. Mechanisms that might influence the clonal structure of this apomictic complex are discussed.

Prey preference and utilization by Mesostoma lingua (Turbellaria, Rhabdocoela) at a low arctic site

The rhabdocoel Mesostoma lingua is a common inhabitant of rock bluff ponds in the area of Churchill, Manitoba. Unlike most flatworms it has the ability to prey actively on zooplankton. Among the zooplankton, Mesostoma prefers Daphnia pulex, Simocephalus vetulus, and Diaptomus tyrrelli. Larger calanoid copepods are difficult to capture and take much longer to consume. Growth in Mesostoma is maintained when individuals consume 0.5 Daphnia day−1 and is maximal at 4 Daphnia day−1. In comparison, Mesostoma maintained growth only when offered the copepod Diaptomus victoriaensis at the rate of 8–10 individuals-day−1. Mesostoma may be an important agent in structuring pond zooplankton communities in the arctic, producing a shift in dominance away from cladocerans such as Daphnia toward larger calanoid copepods.

Ecology of the dominant copepod species at a Low Arctic site

Four species of calanoid copepod regularly co-occur in tundra ponds near Churchill, Manitoba. Despite the physical uniformity of the habitats they occupy, twofold variation in body size and fivefold variation in brood size existed among conspecific populations of each species. Size shifts were concordant in the sexes, but the extent of sexual dimorphism was size dependent in each species. Size differences among populations persisted through two summers. The three herbivorous species showed parallel variation in body sizes and fecundities, while the predatory species did not. The size covariation in the herbivorous taxa may reflect significant overlap in resource use despite their size divergence.

THE BOTANY OF SOMERSET ISLAND, DISTRICT OF FRANKLIN

The geology, physiography, general appearance, climate and weather, and history of exploration of Somerset I. are discussed briefly. The principal habitats and the areas visited are described. Data are presented on prevailing winter wind directions on Somerset I. and Boothia Pen., drawn from observations on residual snowbanks and plant growth. A few phenological data are presented, which supply further evidence that spring development at low arctic sites lags behind that at both subarctic and high arctic sites. Annotated lists are given of the fungi (28 species) and vascular plants (90 species) collected. Among the fungi Puccinia helicalis sp. nov. on Pedicularis capitata, and Doassansia nearctica sp. nov. on Ranunculus hyperboreus are described; and Exobasidium warmingii, intermediate between Exobasidium and Kordyana, is made the type of Arcticomyces gen. nov. Thirty-two species of vascular plants are added to the recorded flora of the island.