The Antarctic silverfish Pleuragramma antarcticum (Nototheniidae) is the most abundant pelagic fish inhabiting Antarctic waters. In this study we investigated, through partial sequencing of the D-loop mitochondrial region, samples collected at four different locations in the Southern Ocean, three in the Atlantic and one in the Pacific sector. Sampling was replicated in two different years at two locations. Sequence analysis showed a remarkably high polymorphism, with 110 haplotypes over the 256 investigated specimens, and about 80% of haplotypes occurring only once. Neutrality tests indicated that all samples were not at mutation-drift equilibrium, and suggested a past population expansion. This result was supported by the presence of a star-like topology in the D-loop gene tree, and by results of mismatch distribution. The start of the expansion was dated, using a specifically calibrated clock, between 111 and 126 thousand years ago. This value corresponds to the start of the cooling period that led to the last glaciation peak, and is in close agreement with a recently suggested range expansion for pelagic Antarctic ecosystems. Analysis of molecular variation indicated a small, though highly significant, value of differentiation between samples. This result, together with the lack of association between clades and geographical locations, indicates a weak population structure for the species.

BackgroundSouthern Ocean fauna represent a significant amount of global biodiversity, whose origin may be linked to glacial cycles determining local extinction/eradication with ice advance, survival of refugee populations and post-glacial re-colonization. This pattern implies high potential for differentiation in benthic shelf species with limited dispersal, yet consequences for pelagic organisms are less clear. The present study investigates levels of genetic variation and population structure of the Antarctic krill Euphausia superba using mitochondrial DNA and EST-linked microsatellite markers for an unprecedentedly comprehensive sampling of its populations over a circum-Antarctic range.ResultsMtDNA (ND1) sequences and EST-linked microsatellite markers indicated no clear sign of genetic structure among populations over large geographic scales, despite considerable power to detect differences inferred from forward-time simulations. Based on ND1, few instances of genetic heterogeneity, not significant after correction for multiple tests, were detected between geographic or temporal samples. Neutrality tests and mismatch distribution based on mtDNA sequences revealed strong evidence of past population expansion. Significant positive values of the parameter g (a measure of population growth) were obtained from microsatellite markers using a coalescent-based genealogical method and suggested a recent start (60 000 - 40 000 years ago) for the expansion.ConclusionsThe results provide evidence of lack of genetic heterogeneity of Antarctic krill at large geographic scales and unequivocal support for recent population expansion. Lack of genetic structuring likely reflects the tight link between krill and circum-Antarctic ocean currents and is consistent with the hypothesis that differentiation processes in Antarctic species are largely influenced by dispersal potential, whereas small-scale spatial and temporal differentiation might be due to local conditions leading to genetic patchiness. The signal of recent population growth suggests differential impact of glacial cycles on pelagic Antarctic species, which experienced population expansion during glaciations with increased available habitat, versus sedentary benthic shelf species.EST-linked microsatellites provide new perspectives to complement the results based on mtDNA and suggest that data-mining of EST libraries will be a useful approach to facilitate use of microsatellites for additional species.

Meganyctiphanes norvegica (M. Sars 1856), the northern krill, is the largest and most abundant euphausiid species in the northern hemisphere, where it represents a key component of many pelagic communities. Although planktonic, krill could be considered a nektonic organism, as it is capable of active movements. This behaviour may be adaptive, because it allows these organisms to maintain their geographic position, leading to stable population structure, despite being continuously exposed to the heterogeneous oceanic conditions. By means of single-strand conformation polymorphism and DNA sequencing, we analyzed allelic variation of the Subunit 1 of NADH dehydrogenase mtDNA locus in 23 populations of the northern krill Meganyctiphanes norvegica, from 15 locations spanning the distribution range of the species. Analysis of the data indicated that the genetic structure, as revealed by analysis of population samples collected at the same site in consecutive years, was stable during that sampling period. Our results revealed the existence of 4 genetically and geographically distinct gene pools of M. norvegica, 2 occurring in the NE Atlantic ('northern' NE Atlantic and 'southern' NE Atlantic), 1 in the NW Atlantic, and 1 in the Mediterranean (Ligurian) Sea.

We characterized eight polymorphic microsatellites in the Scotia icefish Chaenocephalus aceratus (Perciformes, Notothenioidei, Channichthyidae) that is endemic of Southern Ocean waters surrounding the tip of the Antarctic Peninsula. Microsatellites were isolated from a partial genomic library enriched for an AC motif. The number of alleles ranged from two to 19 with a mean observed hererozygosity of 0.71. Loci were in Hardy–Weinberg equilibrium and no evidence for linkage disequilibrium was found. These molecular markers will be useful to investigate Scotia icefish genetic structure, possibly providing insights on its effective population size and demographic history.