Understanding the factors that influence population differentiation in temperate taxa can be difficult because the signatures of both historic and contemporary demographics are often reflected in population genetic patterns. Fortunately, analyses based on coalescent theory can help untangle the relative influence of these historic and contemporary factors. Common murres (Uria aalge) are vagile seabirds that breed in the boreal and low arctic waters of the Northern Hemisphere. Previous analyses revealed that Atlantic and Pacific populations are genetically distinct; however, less is known about population genetic structure within ocean basins. We employed the mitochondrial control region, four microsatellite loci and four intron loci to investigate population genetic structure throughout the range of common murres. As in previous studies, we found that Atlantic and Pacific populations diverged during the Pleistocene and do not currently exchange migrants. Therefore, Atlantic and Pacific murre populations can be used as natural replicates to test mechanisms of population differentiation. While we found little population genetic structure within the Pacific, we detected significant east-west structuring among Atlantic colonies. The degree that population genetic structure reflected contemporary population demographics also differed between ocean basins. Specifically, while the low levels of population differentiation in the Pacific are at least partially due to high levels of contemporary gene flow, the east-west structuring of populations within the Atlantic appears to be the result of historic fragmentation of populations rather than restricted contemporary gene flow. The contrasting results in the Atlantic and Pacific Oceans highlight the necessity of carefully considering multilocus nonequilibrium population genetic approaches when reconstructing the demographic history of temperate Northern Hemisphere taxa.

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Abstmct: Forty-three species of seedless vascular plants were assessed for modifications to root cortical cell walls. All species except Lycopodium had an endodermis with distinct Casparian bands. Experiments with the apoplastic tracer berberine hemisulfate showed that walls of all root cortical cells in the two Lycopodium species tested were permeable to this tracer. Although most species examined lacked a hypodermis several Equisetum species had a hypodermis with modified walls. Three Selaginella species had distinct Casparian bands in this cortical cell layer. This layer, therefore, is an exodermis in Selaginello and its presence limited the inward diffusion of the apoplastic tracer berberine hemisulfate.

Quaternary glaciations affected the distribution of many species. Here, we investigate whether the Arctic represented a glacial refugium during the Last Glacial Maximum or an area of secondary contact following the ice retreat, by analyzing the genetic population structure of the thick-billed murre (Uria lomvia), a seabird that breeds throughout the North Atlantic, North Pacific and Arctic Oceans. The thick-billed murre is a species of socio-economic importance and faces numerous threats including hunting, oil pollution, gill netting, and climate change. We compared variation in the mitochondrial DNA (mtDNA) control region (n = 424), supplemented by 4 microsatellite loci (n = 445), among thick-billed murres sampled throughout their range. MtDNA data indicated that colonies comprise 4 genetically differentiated groups (Φst = 0.11-0.81): 1) Atlantic Ocean plus New Siberian Islands region, 2) Cape Parry, 3) Chukchi Sea, and 4) Pacific Ocean. Microsatellite variation differed between Atlantic and Pacific populations. Otherwise, little substructure was found within either ocean. Atlantic and Pacific populations appear to have been genetically isolated since the last interglacial period and should be considered separate evolutionary significant units for management. The Chukchi Sea and Cape Parry appear to represent areas of secondary contact, rather than arctic refugial populations.

Ancient Murrelets (Synthliboramphus antiquus) are subarctic seabirds that breed on islands from British Columbia through Alaska to China. In this study, we used sequence variation in the mitochondrial control region and cytochrome b gene to estimate the extent of genetic differentiation and gene flow among populations both within British Columbia and across the North Pacific. Results suggest that genetic differentiation is low and female-mediated gene flow is high among colonies within British Columbia, in agreement with banding studies. Surprisingly, genetic differentiation appears to be low and gene flow high between British Columbia and Asia. The effective female population size appears to be stable, but the species may have undergone a range expansion. These results suggest that Ancient Murrelets from throughout the North Pacific may represent a single management unit for conservation. El ADN Mitocondrial Sugiere Alto Flujo Génico en Synthliboramphus antiquus Resumen. Synthliboramphus antiquus es una especie de ave marina subártica que se reproduce en islas desde British Columbia a través de Alaska hasta China. En este estudio estimamos el grado de diferenciación genética y de flujo génico entre poblaciones localizadas dentro de British Columbia y a través del Pacífico Norte. Nos basamos en la variación en secuencias de la región control y el gen citocromo b del ADN mitocondrial. Los resultados sugieren baja diferenciación genética y alto flujo génico mediado por las hembras entre las colonias de British Columbia, lo que es consistente con estudios de aves anilladas. Sorprendentemente, la diferenciación genética parece ser baja y el flujo génico alto entre British Columbia y Asia. El tamaño efectivo de la población de hembras parece estar estable, pero la especie podría haber expandido su rango de distribución. Estos resultados sugieren que los S. antiquus de todo el Pacífico Norte pueden representar una sola unidad de manejo en términos de conservación.