Pohjois-Euroopan murtovesisimpukoiden alkuperä ja historia

This study addressed the large-scale molecular zoogeography in two brackish water bivalve molluscs, Macoma balthica and Cerastoderma glaucum, and genetic signatures of the postglacial colonization of Northern Europe by them. The traditional view poses that M. balthica in the Baltic, White and Barents seas (i.e. marginal seas) represent direct postglacial descendants of the adjacent Northeast Atlantic populations, but this has recently been challenged by observations of close genetic affinities between these marginal populations and those of the Northeast Pacific. The primary aim of the thesis was to verify, quantify and characterize the Pacific genetic contribution across North European populations of M. balthica and to resolve the phylogeographic histories of the two bivalve taxa in range-wide studies using information from mitochondrial DNA (mtDNA) and nuclear allozyme polymorphisms. The presence of recent Pacific genetic influence in M. balthica of the Baltic, White and Barents seas, along with an Atlantic element, was confirmed by mtDNA sequence data. On a broader temporal and geographical scale, altogether four independent trans-Arctic invasions of Macoma from the Pacific since the Miocene seem to have been involved in generating the current North Atlantic lineage diversity. The latest trans-Arctic invasion that affected the current Baltic, White and Barents Sea populations probably took place in the early post-glacial. The nuclear genetic compositions of these marginal sea populations are intermediate between those of pure Pacific and Atlantic subspecies. In the marginal sea populations of mixed ancestry (Barents, White and Northern Baltic seas), the Pacific and Atlantic components are now randomly associated in the genomes of individual clams, which indicates both pervasive historical interbreeding between the previously long-isolated lineages (subspecies), and current isolation of these populations from the adjacent pure Atlantic populations. These mixed populations can be characterized as self-supporting hybrid swarms, and they arguably represent the most extensive marine animal hybrid swarms so far documented. Each of the three swarms still has a distinct genetic composition, and the relative Pacific contributions vary from 30 to 90 % in local populations. This diversity highlights the potential of introgressive hybridization to rapidly give rise to new evolutionarily and ecologically significant units in the marine realm. In the south of the Danish straits and in the Southern Baltic Sea, a broad genetic transition zone links the pure North Sea subspecies M. balthica rubra to the inner Baltic hybrid swarm, which has about 60 % of Pacific contribution in its genome. This transition zone has no regular smooth clinal structure, but its populations show strong genotypic disequilibria typical of a hybrid zone maintained by the interplay of selection and gene flow by dispersing pelagic larvae. The structure of the genetic transition is partly in line with features of Baltic water circulation and salinity stratification, with greater penetration of Atlantic genes on the Baltic south coast and in deeper water populations. In all, the scenarios of historical isolation and secondary contact that arise from the phylogeographic studies of both Macoma and Cerastoderma shed light to the more general but enigmatic patterns seen in marine phylogeography, where deep genetic breaks are often seen in species with high dispersal potential.Pohjolaa reunustavien merialueiden eliöyhteisöt ovat muotoutuneet vasta jääkauden jälkeen, viimeisten 10 000 vuoden aikana. Tätä ennen aluetta peitti paksu mannerjää, jonka väistyttyä vapautuneet reunameret, etenkin Itämeri, ovat olleet valtamerilajeille haasteellisia elinympäristöjä. Niissä menestyvät vain harvat, vähäsuolaisuutta sietävät lajit. Näitä ovat liejusimpukka (Macoma balthica) ja idänsydänsimpukka (Cerastoderma glaucum). Ennen oletettiin, että reunamerten simpukat polveutuvat suoraan läheisen Koillis-Atlantin populaatioista. Uudempi tutkimus on kuitenkin viitannut täysin erilaiseen historiaan, kun eräiden Itämeren simpukoiden lähimmät sukulaiset ovatkin löytyneet Tyynestämerestä. Väitöstutkimuksessa jäljitettiin simpukoiden historiaa Pohjois-Euroopan reunamerissä ja laajemmin pohjoisissa merissä erilaisten geenituntomerkkien sisältämän informaation perusteella. Mitokondrio-DNA:n polveutumistutkimus osoitti, että liejusimpukat käsittävät useita eri kehityslinjoja, jotka historian saatossa ovat eri aikoina itsenäisesti saapuneet Tyynestämerestä Jäämeren kautta Atlantin altaaseen. Euroopassa näitä linjoja on kaksi: jo ennen jääkausiaikaa saapunut Pohjanmeren linja (alalaji) ja toinen, ilmeisesti vasta pian jääkauden jälkeen Tyynestämerestä saapunut, Itämeressä ja Vienanmeressä nyt vallitseva linja. Idänsydänsimpukan osalta, joka puolestaan on pelkästään eurooppalainen laji, tutkimus kumosi oletuksen perusjaosta Välimeren ja Atlantin populaatioiden välillä; vain Mustanmeren ja itäisimmän Välimeren sydänsimpukat eroavat toisesta, yleiseurooppalaisesta kehityslinjasta, jonka levinneisyys ulottuu Kreikasta Itämereen. Erityisesti liejusimpukka-analyysin tulokset muuttavat perusteellisesti perinteisiä käsityksiä Itämeren ja Pohjois-Euroopan muiden reunamerten eliöstön historiasta ja omaperäisyydestä. Ensiksi, hiljattain mutta kuitenkin ennen ihmisen väliintuloa Tyynestämerestä saapunut simpukkakanta edustaa täällä ennen tuntematonta eläinmaantieteellistä elementtiä. Tämä korostaa reunamerten asemaa eri alkuperiä olevien eliöstöjen omaleimaisina kokooma-alueina. Toiseksi, tulokset haastavat perinteisen käsityksen siitä, että esim. Itämeren jääkauden jälkeinen historia olisi ollut liian lyhyt omaperäisten eliölajien tai muotojen synnylle. Tuman geenien analyysi nimittäin vahvisti, että Itämeren, Vienanmeren ja Barentsinmeren nykyiset liejusimpukkakannat ovat syntyneet jääkauden jälkeen Tyynestämerestä saapuneen ja Euroopassa pidempään eläneen alalajin genomien sekoittuessa perinpohjaisen risteytymisprosessin tuloksena. Pohjoisen Itämeren simpukoiden perimästä noin 60 % on nyt Tyynenmeren alkuperää, Vienan- ja Barentsinmeressä osuus vaihtelee 30-90 % välillä. Kullakin merialueella on näin verrattain nopeasti kehittynyt ainutkertainen simpukkakanta, jossa yhdistyy kauan eri valtamerissä eristyksissä eläneiden alalajien ominaisuuksia. Reunamerten liejusimpukat edustavat toistaiseksi laajimpia tunnettuja risteytyvien kantojen sulaumia ('hybrid swarm') meriympäristöissä. Sulaumapopulaation muodostumisen voi yhtäältä katsoa merkitsevän jo alkaneen lajiutumisprosessin keskeytymistä; toisaalta sitä voidaan pitää äkillisenä lajiutumistapahtumana, jossa vanhan monimuotoisuuden synteesinä syntyy nopeasti uudenlainen eliömuoto. Sulaumapopulaatioiden geneettinen monimuotoisuus on suurempi kuin kantapopulaatioiden, mikä on saattanut edistää niiden menestymistä reunamerten äärevissä olosuhteissa

Data from: Comparison of population-genetic structuring in congeneric kelp- versus rock-associated snails: a test of a dispersal-by-rafting hypothesis

Phylogeographic studies indicate that many marine invertebrates lacking autonomous dispersal ability are able to achieve trans-oceanic colonization by rafting on buoyant macroalgae. However, less is known about the impact of rafting on on-going population-genetic connectivity of intertidal species associated with buoyant macroalgae. We hypothesize that such species will have higher levels of population-genetic connectivity than those exploiting nonbuoyant substrates such as rock. We tested this hypothesis by comparing nuclear multilocus population-genetic structuring in two sister topshell species, which both have a planktonic larval phase but are fairly well segregated by their habitat preference of low-tidal bull-kelp holdfasts versus mid-to-low tidal bare rock. We analyzed population samples from four sympatric sites spanning 372 km of the east coast of southern New Zealand. The sampled region encompasses a 180 km wide habitat discontinuity and is influenced by a stable, northward coastal current. The level of connectivity was high in both species, and neither of them showed significant correlation between genetic and geographic distances. However, a significant negative partial correlation between genetic distance and habitat discontinuity was found in the rock-associated species, and estimates of migrant movement between sites were somewhat different between the two species, with the kelp-associated species more often yielding higher estimates across the habitat discontinuity, whereas the rock-associated species more often exhibited higher estimates between sites interspersed by rock habitats. We conclude that for species with substantial means of autonomous dispersal, the most conspicuous consequence of kelp dwelling may be enhanced long-distance dispersal across habitat discontinuities rather than a general increase of gene flow

Circumpolar dispersal by rafting in two subantarctic kelp-dwelling crustaceans

Long-distance oceanic rafting is frequently invoked as an explanation for broad geographic distributions of sedentary marine taxa, but evidence for this ecological process remains elusive. We explored empirically the potential of rafting as a dispersal mechanism by comparing circumpolar mtDNA variability in 3 codistributed subantarctic taxa: 2 direct-developing epifaunal crustacean species (Limnoria stephenseni, Parawaldeckia kidderi) and their macroalgal host (Durvillaea antarctica). A previous study of D. antarctica suggests that its subantarctic populations were established only postglacially, and we predict that the epifaunal invertebrates associated with it experienced a parallel island-colonization episode, facilitated by kelp-rafting. We generated and analysed mtDNA sequence data (cytochrome oxidase subunit I [COI] gene, >900 base pairs) from 89 L. stephenseni and 62 P. kidderi specimens and reanalysed previously published comparable data for D. antarctica. Both epifaunal species exhibited wide-scale circumpolar distributions of a single haplotype. Little sequence diversity was found within island samples, with the exception of P. kidderi in the Falkland Islands. The phylogeographic diversity and structuring of the invertebrates was very similar to that of their kelp host and consistent with a scenario of subantarctic recolonization and population expansion. The dependence of these otherwise non-dispersive crustaceans on macroalgal holdfasts for food and habitat, as well as the great abundance of D. antarctica adrift in the Antarctic Circumpolar Current (ACC), support rafting as their most plausible recolonization mechanism. We suggest that macroalgal rafting may explain similarities in the species composition of intertidal marine communities across the subantarctic

Data from: Evolutionary consequences of microhabitat: population-genetic structuring in kelp- versus rock-associated chitons

Rafting has long been invoked as a key marine dispersal mechanism, but biologists have thus far produced little genetic evidence to support this hypothesis. We hypothesise that coastal species associated with buoyant seaweeds should experience enhanced population connectivity due to rafting. In particular, invertebrates strongly associated with the buoyant bull-kelp Durvillaea antarctica might be expected to have lower levels of population genetic differentiation than taxa mainly exploiting non-buoyant substrates. We undertook a comparative genetic study of two co-distributed, congeneric chiton species, assessing population connectivity at scales of 61-516 km, using ≥186 polymorphic AFLP loci per species. Consistent with predictions, population genetic differentiation was weaker in the kelp-associated Sypharochiton sinclairi than in the rock-associated S. pelliserpentis. Additionally, while we found a significant positive correlation between genetic and oceanographic distances in both chiton species, the correlation was stronger in S. pelliserpentis (R2 = 0.28) than in S. sinclairi (R2 = 0.18). These data support the hypothesis that epifaunal taxa can experience enhanced population-genetic connectivity as a result of their rafting-ability

Data from: Passive rafting is a powerful driver of transoceanic gene flow

Dispersal by passive oceanic rafting is considered important for the assembly of biotic communities on islands. However, not much is known about levels of population genetic connectivity maintained by rafting over transoceanic distances. We assess the evolutionary impact of kelp-rafting by estimating population genetic differentiation in three kelp-associated invertebrate species across a system of islands isolated by oceanic gaps for over 5 million years, using mtDNA and AFLP markers. The species occur throughout New Zealand's subantarctic islands, but lack pelagic stages and any opportunity for anthropogenic transportation, and hence must rely on passive rafting for long-distance dispersal. They all have been directly observed to survive transoceanic kelp-rafting journeys in this region. Our analyses indicate that regular gene flow occurs among populations of all three species between all of the islands, especially those on either side of the subtropical front oceanographic boundary. Notwithstanding its perceived sporadic nature, long-distance kelp-rafting appears to enable significant gene flow among island populations separated by hundreds of kilometres of open ocean

Poleward bound: biological impacts of Southern Hemisphere glaciation

Postglacial recolonisation patterns are well documented for the Northern Hemisphere biota, but comparable processes in the Southern Hemisphere have only recently been examined. In the largely terrestrial Northern Hemisphere, recession of ice after the Last Glacial Maximum (LGM) allowed various taxa, including slow-moving terrestrial species, to migrate poleward. By contrast, the Southern Hemisphere polar region is completely ringed by ocean, and recolonisation of Antarctica and the sub-Antarctic islands has thus presented considerable challenges. Although a few highly dispersive marine species have been able to recolonise postglacially, most surviving high-latitude taxa appear to have persisted throughout glacial maxima in local refugia. These contrasting patterns highlight the importance of habitat continuity in facilitating biological range shifts in response to climate change