Glacial refugia and recolonization pathways in the brown seaweed Fucus serratus

PATERNAL LEAKAGE OF MITOCHONDRIAL DNA IN A FUCUS (PHAEOPHYCEAE) HYBRID ZONE(1)

Eukaryotic mitochondria are mostly uniparentally (maternally) inherited, although mtDNA heteroplasmy has been reported in all major lineages. Heteroplasmy, the presence of more than one mitochondrial genome in an individual, can arise from recombination, point mutations, or by occasional transmission of the paternal mtDNA (=paternal leakage). Here, we report the first evidence of mtDNA paternal leakage in brown algae. In Denmark, where Fucus serratus L. and Fucus evanescens C. Agardh have hybridized for years, we found eight introgressed individuals that possessed the very distinct haplotypes of each parental species. The finding of heteroplasmy in individuals resulting from several generations of backcrosses suggests that paternal leakage occurred in earlier generations and has persisted through several meiotic bottlenecks.

Historical invasions of the intertidal zone of Atlantic North America associated with distinctive patterns of trade and emigration

Early invasions of the North American shore occurred mainly via deposition of ballast rock, which effectively transported pieces of the intertidal zone across the Atlantic. From 1773-1861, >880 European ships entered Pictou Harbor, Nova Scotia, as a result of emigration and trade from Europe. The rockweed Fucus serratus (1868) and the snail Littorina littorea ( approximately 1840) were found in Pictou during this same period. With shipping records (a proxy for propagule pressure) to guide sampling, we used F. serratus as a model to examine the introductions because of its relatively low genetic diversity and dispersal capability. Microsatellite markers and assignment tests revealed 2 introductions of the rockweed into Nova Scotia: 1 from Galway (Ireland) to Pictou and the other from Greenock (Scotland) to western Cape Breton Island. To examine whether a high-diversity, high-dispersing species might have similar pathways of introduction, we analyzed L. littorea, using cytochrome b haplotypes. Eight of the 9 Pictou haplotypes were found in snails collected from Ireland and Scotland. Our results contribute to a broader understanding of marine communities, because these 2 conspicuous species are likely to be the tip of an "invasion iceberg" to the NW Atlantic from Great Britain and Ireland in the 19th Century.

Glacial oceanographic contrasts explain phylogeography of Australian bull kelp

The evolutionary effects of Southern Hemisphere Pleistocene oceanographic conditions - marked by fluctuations in sea levels and water temperatures, and redirected currents - are poorly understood. The southeastern tip of Australia presents an intriguing model system for studying the biological impacts of palaeoceanography. In particular, contrasting oceanographic conditions that existed on eastern vs. western sides of the Bassian Isthmus during Pleistocene glacial periods allow for natural comparisons between putative refugial vs. re-invading populations. Whereas many western Tasmanian marine taxa were likely eliminated by cold subantarctic water during the last glacial period, eastern Tasmanian populations would have persisted in relatively warm temperatures mediated by the ongoing influence of the East Australian Current (EAC). Here we test for the effects of contrasting palaeoceanographic conditions on endemic bull kelp, Durvillaea potatorum, using DNA sequence analysis (COI; rbcL) of more than 100 individuals from 14 localities in southeastern Australia. Phylogenetic reconstructions reveal a deep (maximum divergence 4.7%) genetic split within D. potatorum, corresponding to the 'eastern' and 'western' geographical regions delimited by the Bassian Isthmus, a vicariant barrier during low Pleistocene sea levels. Concordant with the western region's cold glacial conditions, samples from western Tasmania and western Victoria are genetically monomorphic, suggesting postglacial expansion from a mainland refugium. Eastern samples, in contrast, comprise distinct regional haplogroups, suggesting the species persisted in eastern Tasmania throughout recent glacial periods. The deep east-west divergence seems consistent with earlier reports of morphological differences between 'western' and 'eastern' D. potatorum, and it seems likely that these forms represent reproductively isolated species.

Kelp genes reveal effects of subantarctic sea ice during the Last Glacial Maximum

The end of the Last Glacial Maximum (LGM) dramatically reshaped temperate ecosystems, with many species moving poleward as temperatures rose and ice receded. Whereas reinvading terrestrial taxa tracked melting glaciers, marine biota recolonized ocean habitats freed by retreating sea ice. The extent of sea ice in the Southern Hemisphere during the LGM has, however, yet to be fully resolved, with most palaeogeographic studies suggesting only minimal or patchy ice cover in subantarctic waters. Here, through population genetic analyses of the widespread Southern Bull Kelp (Durvillaea antarctica), we present evidence for persistent ice scour affecting subantarctic islands during the LGM. Using mitochondrial and chloroplast genetic markers (COI; rbcL) to genetically characterize some 300 kelp samples from 45 Southern Ocean localities, we reveal a remarkable pattern of recent recolonization in the subantarctic. Specifically, in contrast to the marked phylogeographic structure observed across coastal New Zealand and Chile (10- to 100-km scales), subantarctic samples show striking genetic homogeneity over vast distances (10,000-km scales), with a single widespread haplotype observed for each marker. From these results, we suggest that sea ice expanded further and ice scour during the LGM impacted shallow-water subantarctic marine ecosystems more extensively than previously suggested.

The postglacial recolonization of Northern Europe by Rana arvalis as revealed by microsatellite and mitochondrial DNA analyses

The postglacial history of the moor frog (Rana arvalis) in Northern Europe was investigated with the aid of eight variable microsatellite loci and a 661 bp sequence of the mitochondrial cytochrome b gene. A division between eastern and western mitochondrial lineages was discovered, supporting two recolonization routes to Fennoscandia since the last glacial maximum. This result was corroborated by the microsatellite data, which revealed a contact zone between the two lineages in Northern Sweden. These findings add to the increasing evidence that an intraspecific genetic biodiversity founded on the existence of eastern and western clades is a common element in Fennoscandian fauna and flora.

Phylogeography of the European spiny lobster (Palinurus elephas): Influence of current oceanographical features and historical processes

The European spiny lobster (Palinurus elephas) is a suitable model organism to study the effects of past history and current oceanographic processes on the genetic diversity and population structure of marine species with a long-lived larval phase. A portion of the COI gene was sequenced in 227 individuals from 11 localities, covering most of the present distribution of the species. Divergence was found between Atlantic and Mediterranean regions, which could be explained by restricted gene flow between populations. Moreover, a principal component analysis detected differences within basins. The existence of genetic differentiation between Brittany and Ireland-Scotland populations could be accounted for by the large effect of the Gulf Stream, while mesoscale processes suffered by the incoming Atlantic waters could be responsible of genetic differentiation within the Mediterranean. Furthermore, historical processes could be responsible for a reduction on the overall genetic variability of P. elephas. The haplotypic distribution found in P. elephas, with the presence of one abundant haplotype and a large number of closely related haplotypes, is typical of species experiencing reduction in variability and subsequent expansions. Climatic fluctuations related to glacial cycles could explain the present level of variability and nucleotide diversity found. Interestingly, these glacial events do not seem to have the same impact in other species of the same genus. Our results indicate that recent glacial events could have had a lower impact on Palinurus mauritanicus, a congeneric species that presents an overlapping distribution area but is found in cooler waters than P. elephas.