The presence of a cryptic barrier in the West Pacific Ocean suggests the effect of glacial climate changes on a widespread sea‐dispersed plant,Vigna marina (Fabaceae)

Ocean currents are an important driver of evolution for sea‐dispersed plants, enabling them to maintain reciprocal gene flow via sea‐dispersed diaspores and obtain wide distribution ranges. Although geographic barriers are known to be the primary factors shaping present genetic structure of sea‐dispersed plants, cryptic barriers which form clear genetic structure within oceanic regions are poorly understood. To test the presence of a cryptic barrier, we conducted a phylogeographic study together with past demographic inference for a widespread sea‐dispersed plant, Vigna marina, using 308 individuals collected from the entire Indo‐West Pacific (IWP) region. Chloroplast DNA variation showed strong genetic structure that separated populations into three groups: North Pacific (NP), South Pacific (SP) and Indian Ocean (IN) (F′_CT among groups = 0.954–1.000). According to the Approximate Bayesian computation inference, splitting time between NP and SP was approximately 20,200 years (95%HPD, 4,530–95,400) before present. Moreover, a signal of recent population expansion was detected in the NP group. This study clearly showed the presence of a cryptic barrier in the West Pacific region of the distributional range of V. marina. The locations of the cryptic barrier observed in V. marina corresponded to the genetic breaks found in other plants, suggesting the presence of a common cryptic barrier for sea‐dispersed plants. Demographic inference suggested that genetic structure related to this cryptic barrier has been present since the last glacial maximum and may reflect patterns of past population expansion from refugia.     

MAJOR GENETIC DIFFERENCES BETWEEN CROWN-OF-THORNS STARFISH (ACANTHASTER PLANCI) POPULATIONS IN THE INDIAN AND PACIFIC OCEANS

Spatial variation in allelic frequencies at nine allozyme loci were assayed in 20 populations of the crown-of-thorns starfish, Acanthaster planci, collected throughout the Pacific and Indian Oceans. These data were analyzed together with published data, for the same loci, from an additional 19 populations, giving a total sample size of approximately 1800 individuals. There was a marked discontinuity between the Indian and Pacific Ocean populations, but those off Western Australia and from the Southeast Asian region had a strong Pacific affinity. The genetic groups were congruent with the distributions of two color morph groups: gray-green to red-brown forms in the Pacific and a blue to pale red form in the Indian Ocean. These patterns of genetic structure are similar to those described for the starfish Linckia laevigata, which has similar life-history characteristics. Vicariant events may have influenced some populations within the Pacific, but the allozyme data cannot resolve the effects of these events clearly. Patterns of variation within regions were consistent with isolation by distance, but, at larger scales, were obscured by regional vicariance and some outliers, particularly by apparently high levels of gene flow between Japan and the Great Barrier Reef, Australia. Apparent gene flow between population pairs was not closely related to present-day ocean currents. The results demonstrate a strong influence of allopatric separation on genetic divergence at large geographic scales, but also show evidence of slow rates of change in gene frequencies consistent with the large population sizes of this species. Low levels of divergence between groups demonstrate the genetic structure is recent (Pleistocene) and are likely responses to changes in climate and sea level.     

SPECIATION AND POPULATION GENETIC STRUCTURE IN TROPICAL PACIFIC SEA URCHINS

Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.     

Land masses and oceanic currents drive population structure of Heritiera littoralis,a widespread mangrove in the Indo‐West Pacific

Phylogeographic forces driving evolution of sea‐dispersed plants are often influenced by regional and species characteristics, although not yet deciphered at a large spatial scale for many taxa like the mangrove species Heritiera littoralis. This study aimed to assess geographic distribution of genetic variation of this widespread mangrove in the Indo‐West Pacific region and identify the phylogeographic factors influencing its present‐day distribution. Analysis of five chloroplast DNA fragments’ sequences from 37 populations revealed low genetic diversity at the population level and strong genetic structure of H. littoralis in this region. The estimated divergence times between the major genetic lineages indicated that glacial level changes during the Pleistocene epoch induced strong genetic differentiation across the Indian and Pacific Oceans. In comparison to the strong genetic break imposed by the Sunda Shelf toward splitting the lineages of the Indian and Pacific Oceans, the genetic differentiation between Indo‐Malesia and Australasia was not so prominent. Long‐distance dispersal ability of H. littoralis propagules helped the species to attain transoceanic distribution not only across South East Asia and Australia, but also across the Indian Ocean to East Africa. However, oceanic circulation pattern in the South China Sea was found to act as a barrier creating further intraoceanic genetic differentiation. Overall, phylogeographic analysis in this study revealed that glacial vicariance had profound influence on population differentiation in H. littoralis and caused low genetic diversity except for the refugia populations near the equator which might have persisted through glacial maxima. With increasing loss of suitable habitats due to anthropogenic activities, these findings therefore emphasize the urgent need for conservation actions for all populations throughout the distribution range of H. littoralis.     

Isolation by distance and vicariance drive genetic structure of a coral reef fish in the Pacific Ocean

We studied the genetic diversity of a coral reef fish species to investigate the origin of the differentiation. A total of 727 Acanthurus triostegus collected from 15 locations throughout the Pacific were analyzed for 20 polymorphic loci. The genetic structure showed limited internal disequilibrium within each population; 3.7% of the loci showed significant Hardy-Weinberg disequilibrium, mostly associated with Adh*, and we subsequently removed this locus from further analysis of geographic pattern. The genetic structure of A. triostegus throughout the tropical Pacific Ocean revealed a strong geographic pattern. Overall, there was significant population differentiation (multilocus F(ST) = 0.199), which was geographically structured according to bootstraps of neighbor-joining analysis on Nei's unbiased genetic distances and AMOVA analysis. The genetic structure revealed five geographic groups in the Pacific Ocean: western Pacific (Guam, Philippines, Palau, and Great Barrier Reef); central Pacific (Solomons, New Caledonia, and Fiji); and three groups made up of the eastern populations, namely Hawaiian Archipelago (north), Marquesas (equatorial), and southern French Polynesia (south) that incorporates Clipperton Island located in the northeastern Pacific. In addition, heterozygosity values were found to be geographically structured with higher values grouped within Polynesian and Clipperton populations, which exhibited lower population size. Finally, the genetic differentiation (F(ST)) was significantly correlated with geographic distance when populations from the Hawaiian and Marquesas archipelagos were separated from all the other locations. These results show that patterns of differentiation vary within the same species according to the spatial scale, with one group probably issued from vicariance, whereas the other followed a pattern of isolation by distance. The geographic pattern for A. triostegus emphasizes the diversity of the evolutionary processes that lead to the present genetic structure with some being more influential in certain areas or according to a particular spatial scale.     

EVIDENCE OF A BIOGEOGRAPHIC BREAK BETWEEN POPULATIONS OF A HIGH DISPERSAL STARFISH: CONGRUENT REGIONS WITHIN THE INDO-WEST PACIFIC DEFINED BY COLOR MORPHS,mtDNA, AND ALLOZYME DATA

Both mtDNA variation and allozyme data demonstrate that geographic groupings of different color morphs of the starfish Linckia laevigata are congruent with a genetic discontinuity between the Indian and Pacific Oceans. Populations of L. laevigata sampled from Thailand and South Africa, where an orange color morph predominates, were surveyed using seven polymorphic enzyme loci and restriction fragment analysis of a portion of the mtDNA including the control region. Both allozyme and DNA data demonstrated that these populations were significantly genetically differentiated from each other and to a greater degree from 23 populations throughout the West Pacific Ocean, where a blue color morph is predominant. The genetic structure observed in L. laevigata is consistent with traditional ideas of a biogeographic boundary between the Indian and Pacific Oceans except that populations several hundreds kilometers off the coast of north Western Australia (Indian Ocean) were genetically similar to and had the same color morphs as Pacific populations. It is suggested that gene flow may have continued (possibly at a reduced rate) between these offshore reefs in Western Australia and the West Pacific during Pleistocene falls in sea level, but at the same time gene flow was restricted between these Western Australian populations and those in both Thailand and South Africa, possibly by upwellings. The molecular data in this study suggest that vicariant events have played an important role in shaping the broadscale genetic structure of L. laevigata. Additionally, greater genetic structure was observed among Indian Ocean populations than among Pacific Ocean populations, probably because there are fewer reefs and island archipelagos in the Indian Ocean than in the Pacific, and because present-day surface ocean currents do not facilitate long-distance dispersal.     

Phylogeographic heterogeneity of the brown macroalga Sargassum horneri (Fucaceae) in the northwestern Pacific in relation to late Pleistocene glaciation and tectonic configurations

Pleistocene glacial oscillations and associated tectonic processes are believed to have influenced the historical abundances and distribution of organisms in the Asia Northwest Pacific (ANP). Accumulating evidence indicates that factors shaping tempospatial population dynamics and distribution patterns of marine taxa vary with biogeographical latitude, pelagic behaviour and oceanographic regimes. To detect what kinds of historical and contemporary factors affected genetic connectivity, phylogeographic profiles of littoral macroalga Sargassum horneri in the ANP were analysed based on mitochondrial (Cox3) and chloroplast (rbcL) data sets. Five distinct clades were recovered. A strong signature of biogeographical structure was revealed (Φ(CT) = 0.487, P < 0.0001) derived from remarkable differentiation in clade distribution, as clade I is restricted to Chinese marginal seas (Yellow-Bohai Sea, East China Sea and South China Sea), whereas clades II-V are discontinuously scattered around the main Islands of Japan. Furthermore, two secondary contact regions were identified along the south Japan-Pacific coastline. This significant differentiation between the two basins may reflect historical glacial isolation in the northwestern Pacific, which is congruent with the estimates of clade divergence and demographic expansion during the late Quaternary low sea levels. Analysis of molecular variance and the population-pair statistic F(ST) also revealed significant genetic structural differences between Chinese marginal seas and the Japanese basin. This exceptional phylogeographic architecture in S. horneri, initially shaped by historical geographic isolation during the late Pleistocene ice age and physical biogeographical barriers, can be complicated by oceanographic regimes (ocean surface currents) and relocating behaviour such as oceanic drifting.     

Distinct patterns of hybridization across a suture zone in a coral reef fish (Dascyllus trimaculatus)

Hybrid zones are natural laboratories for investigating the dynamics of gene flow, reproductive isolation, and speciation. A predominant marine hybrid (or suture) zone encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE), where 15 different instances of interbreeding between closely related species from Indian and Pacific Oceans have been documented. Here, we report a case of hybridization between genetically differentiated Pacific and Indian Ocean lineages of the three‐spot dascyllus, Dascyllus trimaculatus (Rüppell, 1829). Field observations indicate there are subtle color differences between Pacific and Indian Ocean lineages. Most importantly, population densities of color morphs and genetic analyses (mitochondrial DNA and SNPs obtained via RADSeq) suggest that the pattern of hybridization within the suture zone is not homogeneous. At CHR, both color morphs were present, mitochondrial haplotypes of both lineages were observed, and SNP analyses revealed both pure and hybrid genotypes. Meanwhile, in CKE, the Indian Ocean color morphs were prevalent, only Indian Ocean mitochondrial haplotypes were observed, and SNP analysis showed hybrid individuals with a large proportion (~80%) of their genotypes assigning to the Indian Ocean lineage. We conclude that CHR populations are currently receiving an influx of individuals from both ocean basins, with a greater influence from the Pacific Ocean. In contrast, geographically isolated CKE populations appear to be self‐recruiting and with more influx of individuals from the Indian Ocean. Our research highlights how patterns of hybridization can be different at scales of hundreds of kilometers, due to geographic isolation and the history of interbreeding between lineages.     

Contrasting demographic history and gene flow patterns of two mangrove species on either side of the Central American Isthmus

Comparative phylogeography offers a unique opportunity to understand the interplay between past environmental events and life‐history traits on diversification of unrelated but co‐distributed species. Here, we examined the effects of the quaternary climate fluctuations and palaeomarine currents and present‐day marine currents on the extant patterns of genetic diversity in the two most conspicuous mangrove species of the Neotropics. The black (Avicennia germinans, Avicenniaceae) and the red (Rhizophora mangle, Rhizophoraceae) mangroves have similar geographic ranges but are very distantly related and show striking differences on their life‐history traits. We sampled 18 Atlantic and 26 Pacific locations for A. germinans (N = 292) and R. mangle (N = 422). We performed coalescence simulations using microsatellite diversity to test for evidence of population change associated with quaternary climate fluctuations. In addition, we examined whether patterns of genetic variation were consistent with the directions of major marine (historical and present day) currents in the region. Our demographic analysis was grounded within a phylogeographic framework provided by the sequence analysis of two chloroplasts and one flanking microsatellite region in a subsample of individuals. The two mangrove species shared similar biogeographic histories including: (1) strong genetic breaks between Atlantic and Pacific ocean basins associated with the final closure of the Central American Isthmus (CAI), (2) evidence for simultaneous population declines between the mid‐Pleistocene and early Holocene, (3) asymmetric historical migration with higher gene flow from the Atlantic to the Pacific oceans following the direction of the palaeomarine current, and (4) contemporary gene flow between West Africa and South America following the major Atlantic Ocean currents. Despite the remarkable differences in life‐history traits of mangrove species, which should have had a strong influence on seed dispersal capability and, thus, population connectivity, we found that vicariant events, climate fluctuations and marine currents have shaped the distribution of genetic diversity in strikingly similar ways.     

Population differentiation in the open sea: insights from the pelagic copepod Pleuromamma xiphias

Although a number of recent studies of marine holoplankton have reported significant genetic structure among populations, little is currently known about the biological and oceanographic processes that influence population connectivity in oceanic plankton. In order to examine how depth preferences influence dispersal in oceanic plankton, I characterized the genetic structure of a copepod with diel vertical migration (DVM) (Pleuromamma xiphias), throughout its global distribution, and compared these results to those expected given the interaction of this species' habitat depth with ocean circulation and bathymetry. Mitochondrial COI sequences from 651 individuals from 28 sites in the Indian, Pacific, and Atlantic Oceans revealed highly significant genetic differentiation both within and among ocean basins. Limited dispersal among distinct pelagic provinces seems to have played a major role in population differentiation in this species, with strong genetic breaks observed across known oceanographic fronts or current systems in all three ocean basins. The Indo-West Pacific (IWP) holds a highly distinct genetic population of this species that was sampled in both the western Pacific and eastern Indian Oceans. This suggests that the IWP does not act as a strong barrier to gene flow between basins, as expected, despite the relatively shallow water depth (<200 m) and vertically extensive (>400 m) diel migration of this species. A pattern of isolation by distance was observed in the Indian Ocean with genetic differentiation among samples down to spatial scales of ～800 km, indicating that realized dispersal in P. xiphias occurs over much smaller spatial scales than in previously reported oceanic holoplankton. Given its highly regionalized population genetic structure, P. xiphias may have some capacity to adapt to local oceanographic conditions, and it should not be assumed that populations of this species in distinct pelagic biomes will respond in the same way to shared physical or climatic forcing.     

INTRASPECIFIC GENETIC DIFFERENTIATION IN CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS) FROM SOUTHERN CALIFORNIA AND THE GULF OF CALIFORNIA

Intraspecific patterns of mitochondrial DNA sequence variation were determined among California sea lions ( Zalophus califomianus californianus ) from three colonies along the Pacific coast of southern and Baja California and one colony in the Gulf of California. We found no variation in 368 base pairs (bp) of cytochrome b sequence among 40 sea lions from these localities, but analysis of 360 base pairs of control region revealed eleven genotypes. The four genotypes found in the Gulf of California population were unique and phylogenetically distinct from those found in sea lions along the Pacific coast. The average sequence divergence between Gulf and Southern California genotypes was 4.3%, suggesting a relatively long period of isolation. However, colonies along the Pacific coast, which are less than 200 km apart, shared mtDNA genotypes, indicating that recent genetic exchange has occurred between them. Therefore, we suggest that regional female philopatry exists in California sea lions. Regional boundaries may be related to oceanic currents or patchiness in the distribution of resources. Further research is needed to better understand the underlying causes of genetic differentiation in the California sea lion.     

Redrawing the map: mtDNA provides new insight into the distribution and diversity of short‐finned pilot whales in the Pacific Ocean

Correlations between morphological and genetic data provide evidence to delineate species or evolutionarily significant units, which then become the units to conserve in management plans. Here, we examine the distribution and genetic differentiation of two morphotypes of short‐finned pilot whale (Globicephala macrorhynchus) in the Pacific Ocean. Mitochondrial control region sequences from 333 samples were combined with 152 previously published sequences to describe genetic variability globally and population structure in the Pacific. Although genetic variability is low, we found strong differentiation at both broad and local levels across the Pacific. Based on genetics, two types are distributed throughout the Pacific, one predominantly in the eastern Pacific and the other in the western and central Pacific. In the eastern Pacific Ocean, no correlation was found between distribution and sea surface temperature. The two types have broad latitudinal ranges, suggesting their distributions are likely driven by more complex factors, such as prey distribution, rather than sea surface temperature.     

The roles of geological history and colonization abilities in genetic differentiation between mammalian populations in the Philippine archipelago

Aim To test hypotheses that: (1) late Pleistocene low sea‐level shorelines (rather than current shorelines) define patterns of genetic variation among mammals on oceanic Philippine islands; (2) species‐specific ecological attributes, especially forest fidelity and vagility, determine the extent to which common genetic patterns are exhibited among a set of species; (3) populations show reduced within‐population variation on small, isolated oceanic islands; (4) populations tend to be most highly differentiated on small, isolated islands; and (5) to assess the extent to which patterns of genetic differentiation among multiple species are determined by interactions of ecological traits and geological/geographic conditions. Location The Philippine Islands, a large group of oceanic islands in Southeast (SE) Asia with unusually high levels of endemism among mammals. Methods Starch‐gel electrophoresis of protein allozymes of six species of small fruit bats (Chiroptera, Pteropodidae) and one rodent (Rodentia, Muridae). Results Genetic distances between populations within all species are not correlated with distances between present‐day shorelines, but are positively correlated with distances between shorelines during the last Pleistocene period of low sea level; relatively little intraspecific variation was found within these ‘Pleistocene islands’. Island area and isolation of oceanic populations have only slight effects on standing genetic variation within populations, but populations on some isolated islands have heightened levels of genetic differentiation, and reduced levels of gene flow, relative to other islands. Species associated with disturbed habitat (all of which fly readily across open habitats) show more genetic variation within populations than species associated with primary rain forest (all of which avoid flying out from beneath forest canopy). Species associated with disturbed habitats, which tend to be widely distributed in SE Asia, also show higher rates of gene flow and less differentiation between populations than species associated with rain forest, which tend to be Philippine endemic species. One rain forest bat has levels of gene flow and heterozygosity similar to the forest‐living rodent in our study. Main conclusions The maximum limits of Philippine islands that were reached during Pleistocene periods of low sea level define areas of relative genetic homogeneity, whereas even narrow sea channels between adjacent but permanently isolated oceanic islands are associated with most genetic variation within the species. Moreover, the distance between ‘Pleistocene islands’ is correlated with the extent of genetic distances within species. The structure of genetic variation is strongly influenced by the ecology of the species, predominantly as a result of their varying levels of vagility and ability to tolerate open (non‐forested) habitat. Readily available information on ecology (habitat association and vagility) and geological circumstances (presence or absence of Pleistocene land‐bridges between islands, and distance between oceanic islands during periods of low sea level) are combined to produce a simple predictive model of likely patterns of genetic differentiation (and hence speciation) among these mammals, and probably among other organisms, in oceanic archipelagos.     

Multilocus phylogeography of the sea snake Hydrophis curtus reveals historical vicariance and cryptic lineage diversity

The Indo‐Australian archipelago (IAA) supports the world's highest diversity of marine fish, invertebrates and reptiles. Many of the marine fish and invertebrates show congruent phylogeographic patterns, supporting a view that the region's complex geo‐climatic history has played an important role in generating its exceptional biodiversity. Here, we examine population genetic structure of the viviparous sea snake, Hydrophis curtus, to assess how past and present barriers to gene flow in the IAA have contributed to genetic and species diversity in a fully marine reptile. Mitochondrial and anonymous nuclear sequences and ten microsatellite loci were used to identify patterns of historical genetic structure and population expansion, reconstruct dated genealogies and assess levels of recent gene flow. These markers revealed strong concordant geographic structure within H. curtus with a prominent genetic break between populations broadly distributed in the Indian Ocean and the West Pacific. These populations were estimated to have diverged in the late Pliocene or early Pleistocene, and microsatellite admixture analyses suggested limited recent gene flow between them despite the current lack of barriers to dispersal, indicating possible cryptic species. Subsequent divergence in the mid–late Pleistocene was detected within the West Pacific clade among the populations in the Phuket‐Thailand region, South‐East Asia and Australia, and two of these populations also showed genetic signals of recent range expansions. Our results show that climatic fluctuations during the Plio‐Pleistocene generated high levels of cryptic genetic diversity in H. curtus, and add to similar findings for diverse other marine groups in the IAA.     

GEOGRAPHIC VARIATION AND SEXUAL DIMORPHISM IN THE SKULL OF DALL'S PORPOISE, PHOCOENOIDES DALLI

Abstract: Two hundred and eighty-nine skulls of Dall's porpoises from the North Pacific Ocean, Sea of Japan, Sea of Okhotsk, and Bering Sea were investigated morphometrically. A clear pattern of geographic variation was found in overall size of the skull. In the North Pacific, overall skull size became gradually smaller eastward from the coast of Japan to the offshore eastern Pacific, and it became larger again off the coast of California. Specimens from the Sea of Japan-Okhotsk and Bering Sea had larger skulls than those from the central North Pacific. The distribution of primary productivity corresponds well with this pattern of geographic variation, suggesting that quantity of food might affect the overall size of Dall's porpoise skulls. By canonical discriminant analyses, about 70% (male) and 90% (female) of specimens of the Sea of Japan-Okhotsk population were distinguished from those of other populations. This agrees with the results of genetic studies.     

Independent origins of cultivated coconut (Cocos nucifera L.) in the old world tropics

As a portable source of food, water, fuel, and construction materials, the coconut (Cocos nucifera L.) played a fundamental role in human migrations and the development of civilization across the humid tropics. Here we investigated the coconut's domestication history and its population genetic structure as it relates to human dispersal patterns. A sample of 1,322 coconut accessions, representing the geographical and phenotypic diversity of the species, was examined using ten microsatellite loci. Bayesian analyses reveal two highly genetically differentiated subpopulations that correspond to the Pacific and Indo-Atlantic oceanic basins. This pattern suggests independent origins of coconut cultivation in these two world regions, with persistent population structure on a global scale despite long-term human cultivation and dispersal. Pacific coconuts show additional genetic substructure corresponding to phenotypic and geographical subgroups; moreover, the traits that are most clearly associated with selection under human cultivation (dwarf habit, self-pollination, and "niu vai" fruit morphology) arose only in the Pacific. Coconuts that show evidence of genetic admixture between the Pacific and Indo-Atlantic groups occur primarily in the southwestern Indian Ocean. This pattern is consistent with human introductions of Pacific coconuts along the ancient Austronesian trade route connecting Madagascar to Southeast Asia. Admixture in coastal east Africa may also reflect later historic Arab trading along the Indian Ocean coastline. We propose two geographical origins of coconut cultivation: island Southeast Asia and southern margins of the Indian subcontinent.     

DISCORDANT DISTRIBUTION OF POPULATIONS AND GENETIC VARIATION IN A SEA STAR WITH HIGH DISPERSAL POTENTIAL

Patiria miniata, a broadcast‐spawning sea star species with high dispersal potential, has a geographic range in the intertidal zone of the northeast Pacific Ocean from Alaska to California that is characterized by a large range gap in Washington and Oregon. We analyzed spatial genetic variation across the P. miniata range using multilocus sequence data (mtDNA, nuclear introns) and multilocus genotype data (microsatellites). We found a strong phylogeographic break at Queen Charlotte Sound in British Columbia that was not in the location predicted by the geographical distribution of the populations. However, this population genetic discontinuity does correspond to previously described phylogeographic breaks in other species. Northern populations from Alaska and Haida Gwaii were strongly differentiated from all southern populations from Vancouver Island and California. Populations from Vancouver Island and California were undifferentiated with evidence of high gene flow or very recent separation across the range disjunction between them. The surprising and discordant spatial distribution of populations and alleles suggests that historical vicariance (possibly caused by glaciations) and contemporary dispersal barriers (possibly caused by oceanographic conditions) both shape population genetic structure in this species.     

Invertébrés Sans Frontières: Large Scales of Connectivity of Selected Freshwater Species among Caribbean Islands

The freshwater fauna (crustaceans, molluscs, fish) of many tropical islands in the Caribbean and Pacific share an amphidromous life‐cycle, meaning their larvae need to develop in saline conditions before returning to freshwater as juveniles. This community dominates the freshwaters of much of the tropics, but is poorly known and at risk from development, in particular dam construction. Amphidromy can theoretically lead to dispersal between different freshwater areas, even to distant oceanic islands, via the sea. The extent and scale of this presumed dispersal, however, is largely unknown in the Caribbean. Recent genetic work in Puerto Rico has shown that many freshwater species have little or no population structure among different river catchments, implying high levels of connectivity within an island, whereas between‐island structure is unknown. We used genetic techniques to infer the geographic scales of population structure of amphidromous invertebrates (a gastropod and a number of crustacean species) between distant parts of the Caribbean, in particular Puerto Rico, Panama and Trinidad. We found virtually no geographic population structure across over 2000 km of open sea for these freshwater species. This implies that they are indeed moving between islands in sea currents as larvae, meaning that continued recruitment requires a continuum of healthy habitat from the freshwater to marine environment. We further discuss the role of amphidromy and suggest its ecological and biogeographic role may be more important than previously presumed.     

Longitudinal differentiation among pelagic populations in a planktic foraminifer

Evolutionary processes in marine plankton have been assumed to be dependent on the oceanic circulation system, which transports plankton between populations in marine surface waters. Gene flow facilitated by oceanic currents along longitudinal gradients may efficiently impede genetic differentiation of pelagic populations in the absence of confounding marine environmental effects. However, how responsible oceanic currents are for the geographic distribution and dispersal of plankton is poorly understood. We examined the phylogeography of the planktic foraminifer Pulleniatina obliquiloculata in the Indo-Pacific Warm Pool (IPWP) by using partial small subunit ribosomal DNA (SSU rDNA) sequences. We found longitudinal clines in the frequencies of three distinct genetic types in the IPWP area. These frequencies were correlated with environmental factors that are characteristic of three water masses in the IPWP. Noteworthy, populations inhabiting longitudinally distant water masses at the Pacific and Indian sides of the IPWP were genetically different, despite transportation of individuals via oceanic currents. These results demonstrate that populations of pelagic plankton have diverged genetically among different water masses within a single climate zone. Changes of the oceanic circulation system could have impacted the geographic patterns of dispersal and divergence of pelagic plankton.     

Color polymorphism and genetic structure in the sea star Pisaster ochraceus

The sea star Pisaster ochraceus is one of the more striking species on the rocky shores of the Northeast Pacific, in part due to the dramatic color polymorphism of the adults. Along the open Pacific coast, Pisaster populations are 6%-28% orange, with a small percentage of brilliant purple stars and a large percentage of reddish-brown to dull purple ones. However, populations in the San Juan Island Archipelago (Washington, USA) and the southern Strait of Georgia (British Columbia, Canada) are almost entirely brilliant purple. The factors that maintain the color polymorphism, and those that contribute to among-site variation in color frequencies, remain unknown. We examined the relationships between color frequencies and several ecological and morphological variables, and conducted a large-scale phylogeographic survey of Pisaster populations. We found very low population genetic structure, suggesting that gene flow is high and geographic variation in color frequencies is not a vestige of Pleistocene glacial refugia. Color frequencies are also unrelated to adult size and to the frequency of injury within a population. However, there are suggestive relationships between color frequency and diet, and with areas of potentially low salinity. We propose that, although the color polymorphism may have an underlying genetic component, the regional-scale variation in color frequency is ecologically controlled.