Genetic structuring and gene flow in Cerastoderma glaucum (Bivalvia: Cardiidae): evidence from allozyme variation at different geographic scales

Genetic variation at 16 enzyme loci was analysed in 20 Mediterranean and 1 Baltic population of Cerastoderma glaucum. Spatial genetic variation at different geographic scales was investigated. In general, this species was fairly genetically structured (over all loci Š=0.088), according to the fragmentary nature of its habitat. Spatial structuring of genetic diversity was shown to follow different models, depending on the geographic scale considered: a stepping-stone model provided a good fit at a wide scale, with gene flow inversely related to geographic distance, whereas at small scale, genetic relationships among samples could not be interpreted as simply the effect of physical distance among populations. Results are discussed taking into account the intrinsic and extrinsic factors which are most likely to affect the patterns of genetic structuring in coastal marine animals. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0753-x     

Genetic structure of round scad mackerel <Emphasis Type="Italic">Decapterus macrosoma</Emphasis> (Carangidae) in the Indo-Malay archipelago

Round scad mackerel sampled in 1995-1998 were analysed for genetic variation using mitochondrial-DNA and nuclear-DNA markers. Sequence variation for a fragment of the cytochrome b gene (mitochondrial), amplified by polymerase chain reaction, was screened across individuals using single strand conformation polymorphism (SSCP). Sequence analysis of all SSCP haplotypes indicated two mitochondrial clades separated by, on average, 2.3% nucleotide divergence. The geographic distribution of haplotypes was homogeneous (Weir and Cockerham's [^(q)] \hat \theta =-0.002). Also, no geographic heterogeneity was detected for length polymorphism of Intron 1 of the gene encoding aldolase B ([^(q)] \hat \theta =0.005). Although homogeneity in allele frequencies throughout the Indo-Malay archipelago conformed to the expectations for a widely distributed pelagic fish in a highly connected habitat, this was at variance with the sharp geographic structure previously uncovered in Indian scad mackerel, Decapterus russelli, a fish with life-history characteristics similar to D. macrosoma. A remarkable similarity, however, was the occurrence of two similarly distinct clades within each species, suggesting a common history of geographic isolation. Low sea levels in the Pleistocene might have caused the separation and vicariance of populations within both D. macrosoma and D. russelli. Subsequent genetic exchange between populations would then have erased allele-frequency differences at the cytochrome b and aldolase B loci in D. macrosoma while some barrier to gene flow was maintained in D. russelli.     

Morphological, reproductive, and genetic variability among three populations of Crucibulum quiriquinae (Gastropoda: Calyptraeidae) in northern Chile

Crucibulum quiriquinae (Lesson, 1830) is the only species of Crucibulum currently recognized in northern Chile. Recent analysis of three Crucibulum populations obtained in northern Chile demonstrates the existence of morphological, genetic, and reproductive differences among populations. Two populations present in Bahía Tongoy (30°15'S), one inhabiting the shells of the snail Turritella cingulata and the other shells of the pectinid Argopecten purpuratus, showed morphological differences. However, both had planktonic larval development and show low genetic divergence (D=0.002). A third population from Bahía La Herradura (29°58'S), which also inhabits the shells of T. cingulata, did not show morphological differences compared with its counterpart from Bahía Tongoy. However the Bahía La Herradura population had intracapsular development and metamorphosis, and a larger genetic distance (D=0.06) from both Tongoy populations. The results of the reproductive and genetic analyses strongly suggested that the two Tongoy populations, although showing morphological differences, are biologically the same species, Crucibulum quiriquinae, whereas the La Herradura population is a new species.     

Population structure of red drum (Sciaenops ocellatus) in the northern Gulf of Mexico, as inferred from variation in nuclear-encoded microsatellites

Allelic variation at eight nuclear-encoded microsatellites was assayed among 967 red drum (Sciaenops ocellatus) sampled from four consecutive cohorts at seven geographic localities (=28 samples total) in the northern Gulf of Mexico (Gulf). Number of alleles per microsatellite ranged from 6 to 21; average direct-count heterozygosity values per sample (-SE) ranged from 0.560ǂ.018 to 0.903ǂ.009. Tests of Hardy-Weinberg equilibrium revealed significant departures from expected genotype proportions at one microsatellite, which was omitted from further analysis. Tests of genotypic equilibrium indicated that genotypes between pairs of microsatellites were randomly associated. Homogeneity tests of allele distributions across cohorts within localities were non-significant following correction for multiple tests executed simultaneously, and results from molecular analysis of variance indicated that the genetic variance component attributable to variation among cohorts did not differ significantly from zero. Homogeneity tests of allele distributions among localities (cohorts pooled) revealed significant differences both before and after correction for multiple tests. Neighbor-joining clustering of a pairwise matrix of Š values (an unbiased estimator of F_ST), spatial autocorrelations, and regression analysis revealed a pattern of isolation by distance, where genetic divergence among geographic samples increases with geographic distance between sample localities. The pattern and degree of temporal and spatial divergence in the nuclear-encoded microsatellites paralleled almost exactly those of mitochondrial (mt) DNA, as determined in a prior study. Stability of both microsatellite and mtDNA allele distributions within localities indicates that the small but significant genetic divergence among geographic samples represents true signal and that overlapping populations of red drum in the northern Gulf may be influenced by independent population dynamics. The degree of genetic divergence in microsatellites and mtDNA is virtually identical, indicating that genetic effective size of microsatellites and mtDNA in red drum are the same. This, in turn, suggests either that gene flow in red drum in the northern Gulf could be biased sexually or that red drum populations may not be in equilibrium between genetic drift and migration. If a sexual bias exists, the observation that divergence in mtDNA is considerably less than 4 times that of microsatellites could suggest female-mediated dispersal and/or male philopatry. The observed isolation-by-distance effect indicates a practical limit to dispersal. Approximate estimates of geographic neighborhood size suggest the limit is in the range 700-900 km. Although the genetic studies of red drum indicate significant genetic divergence across the northern Gulf, the genetic differences do not delimit specific populations or stocks with fixed geographic boundaries.     

Co-existing populations of Pacific ocean perch, Sebastes alutus, in Queen Charlotte Sound, British Columbia

Variation at five microsatellite loci (Sal1, Sal2, Sal3, Sal4 and Sal5) was examined in approximately 1300 Pacific ocean perch (Sebastes alutus) sampled from 14 coastal sites in British Columbia, Canada. Mean observed heterozygosities by locus ranged from 71% to 88%, and by sample ranged from 75% to 84%. Theta values ranged from 0 to 0.04 over the five loci, and averaged 0.015. Among Pacific ocean perch samples, Š ranged from 0.001 to 0.056. Canonical discriminate analysis of multilocus genotypes and neighbour-joining analysis of pairwise genetic distances between samples both indicated the presence of three populations, one off the west coast of Vancouver Island (the Vancouver Island population) and two co-existing populations in Queen Charlotte Sound, Dixon Entrance and along the west coast of the Queen Charlotte Islands (the eastern and western QCI populations). Pacific ocean perch of the eastern and western QCI populations were caught in close proximity to each other, but individual samples showed little evidence of admixture. Fall and spring samples collected within geographic areas were genetically similar, indicating seasonally stable population structure. Restricted gene flow between the Vancouver Island and the two more northerly populations may result from limited adult dispersal and larval retention within the California Current and Alaska Gyre, respectively, but the presence of two populations within Queen Charlotte Sound cannot be explained entirely by larval retention hypotheses. The presence of two Pacific ocean perch populations in central British Columbia has implications for fisheries management.     

Phylogeography of two squid (Loligo pealei and L. plei) in the Gulf of Mexico and northwestern Atlantic Ocean

The loliginid squids Loligo pealei LeSueur and L. plei Blaineville (both recently proposed for reclassification as Doryteuthis) are commercially important, similar in appearance, and sympatric throughout much of the northwestern Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. To investigate possible cryptic speciation and population structure, we examined samples (collected from 1995 to 1997) of both species for restriction fragment length polymorphisms (RFLPs) in PCR products of the mitochondrial gene cytochrome c oxidase (subunit I). RFLP haplotypes were further characterized by direct sequencing. In North American waters, cryptic speciation was rejected by the far greater nucleotide sequence divergence between species (~14%) versus within species (<1%). Each species displayed about a dozen RFLP haplotypes, but only three of their respective haplotypes were found among 90% of L. pealei specimens (n=356) and 97% of L. plei specimens (n=431). For L. pealei, a genetic break existed between the northern Gulf of Mexico and the Atlantic Ocean; among sample units within each population, gene flow was consistent with panmixia. The phylogeography of L. pealei is likely a consequence of the eastward currents of the Florida Straits, the elevated temperatures of those surface waters, and the restriction of this species to the continental shelf. For L. plei, a genetic break existed between longitudes 88°W and 89°W, with the northwestern Gulf of Mexico and the northeastern Gulf-Atlantic Ocean comprising separate populations; among sample units within each population, gene flow fit an isolation-by-distance model. If the genetic break found for L. plei represents resident populations separated by nearshore physical parameters (e.g. effects of the Mississippi River and the sediment boundary at longitude 88°W), the lack of structure within the Gulf for L. pealei might be due to its distribution farther from shore. However, the two populations of L. plei probably represent annual recolonization from the southwestern Gulf of Mexico and from the eastern Caribbean Sea, whereas the populations of L. pealei probably are permanent residents within their respective regions.     

Sympatry of distinct mitochondrial DNA lineages in a copepod inhabiting estuarine creeks in the southeastern USA

The population genetic structure of the meiobenthic harpacticoid copepod Microarthridion littorale (Poppe) was examined with a geographic survey of a 348 bp fragment of the mitochondrial cytochrome b gene. Copepods were collected from ten locations on the coast from North Carolina to Georgia, USA, from January 1997 to November 1998. Sequence divergence among 198 individuals was as much as 4.3%, and three divergent mitochondrial clades were uncovered that differed by six to nine nucleotide changes. A rapid assay was developed to distinguish among mitochondrial clades, and an additional 333 specimens were surveyed. The three lineages co-occurred in seven of ten sampling locations. Data analyses were carried out separately for individuals assayed by DNA sequencing as well as for a combined data set that included individuals typed by restriction endonuclease digestion. An analysis of molecular variance indicated that a significant proportion of the total genetic variance could be partitioned among populations, although no significant correlation between geographical and genetic distance was detected.     

Genetic, morphological, and chemical divergence in the sponge genus Latrunculia (Porifera: Demospongiae) from New Zealand

We undertook a comprehensive study of Latrunculia in New Zealand to determine the relationship between taxonomic, environmental, and chemical variation within the genus. Sponges were collected from five locations around New Zealand: Three Kings Islands, Tutukaka, Wellington, Kaikoura, and Doubtful Sound. Allozyme electrophoresis at nine polymorphic loci indicated that sponges from each geographic location were genetically distinct, and that they displayed genetic differences of the magnitude usually associated with reproductively isolated species (Nei's D between locations =0.375-2.476). Additionally, the comparisons revealed that the green and brown colour morphs of Latrunculia that are sympatric at Three Kings Islands and Kaikoura are distinct from each other, and that there are two genetic groups within the green sponges in Doubtful Sound. On the basis of genetic data we conclude that there are at least eight species of Latrunculia in New Zealand waters, not one to four as had been previously thought. Morphological comparisons of the eight genetic species based on skeletal characters (i.e. skeletal organisation of the choanosome, spicule composition, size, and geometry) indicated that the eight Latrunculia species fell into only two morphological groups that could be easily diagnosed on the basis of discorhabd type. Within these two primary morphological groups, skeletal characteristics among the eight species largely overlap and are not diagnostic. These findings emphasise the limitations of traditional taxonomic methods based solely on skeletal characters for distinguishing species of Latrunculia. However, multivariate analysis (MANOVA and CDA) based on six measured skeletal variables did reveal significant morphological variation among the species (Pillai's Trace=3.28, F=6.90, P<0.0001), supporting the division of the genus into eight species. Comparisons of chemical extracts from Latrunculia also showed that the amounts of five different bioactive compounds (discorhabdins A, B, C, D, and J) varied predictably among the eight species. This finding suggests that discorhabdin variation within Latrunculia, previously thought to be associated with intra-specific environmental variability, is more likely to reflect differences among species rather than phenotypic plasticity. Our results also highlight the importance of thorough taxonomic studies associated with marine natural products research to understand fully the variation in bioactive properties among individuals. The potential processes underlying the unusually high speciation rates in New Zealand Latrunculia that are indicated in our study are discussed.     

Genetic structure of camouflage grouper,<Emphasis Type="Italic"> Epinephelus polyphekadion</Emphasis> (Pisces: Serranidae), in the western central Pacific

The objective of the present study was to investigate the population genetic structure of the commercially important camouflage grouper, Epinephelus polyphekadion (Bleeker, 1849), in the western and central Pacific Ocean to improve existing management. Camouflage grouper are widely distributed in the Indo-Pacific and form brief, seasonal, spawning aggregations that are often heavily fished. The present study examined populations sampled in 1997-1998 at five sites in the western central Pacific spanning a geographic distance of ~5,000 km: New Caledonia, Great Barrier Reef, Palau, Marshall Islands, and Pohnpei (Micronesia). Primer pairs were developed to examine genetic variation at three polymorphic microsatellite loci. Cluster analysis, using genetic distance, revealed three regional groupings: (1) Palau, (2) Pohnpei and the Marshall Islands, and (3) the Great Barrier Reef and New Caledonia. Highly significant allele frequency differences were observed among sites. At Pohnpei, significant allele frequency differences in successive years were also apparent, possibly related to genetic variation among cohorts or between local spawning groups. The inter-annual differences at Pohnpei suggest that there may be further genetic structuring over relatively modest distances, a finding relative to determining management units for this commercially valuable species and suggests that future studies need to incorporate possible small-scale temporal or spatial components into study design.     

Some quantitative aspects of seagrass ecology in a coastal lagoon of Mauritius

Seagrass distribution was recorded by snorkel dives on a grid of stations in the waterfront of Club Méditerranée at Mon Choisy-Trou Aux Biches lagoon (NW Mauritius) and subsequently mapped using SURFER 6 computer software. Above-ground (AG) and below-ground (BG) standing biomass in terms of dry weight (DW) and ash-free dry weight (AFDW) as well as shoot density and shoot length were monitored monthly from June1997 to May1998 in a mixed stand of Halodule uninervis and Syringodium isoetifolium (dominant) at a shallow, nearshore station in the lagoon. Measurements of physical and chemical parameters [water temperature, current speed, salinity, pH, dissolved oxygen (DO), nitrate and phosphate concentrations] were made simultaneously, as well as at a reference station (ORE) outside the coral reef. The bottom sediment was analysed for grain size and type composition. Variation patterns were examined and statistical correlations drawn to relate plant performance to the environmental variables measured. The SURFER 6 programme generated a satisfactory contour map of seagrass distribution in the lagoon with a cover range of 0-60%. The densest patches occurred adjacent to the shoreline experiencing weaker water currents (3-13 cm s^-1) rather than near the reef (5-35 cm s^-1), where seagrasses were absent. Sand (0.063-2 mm grain size) constituted 97.2% and 77.6% of the nearshore and near-reef sediment, respectively. The dominant grain types were derived from corals (about 80%) and mollusc shells (about 14%). The recorded range of total standing biomass for H. uninervis was 243.1-468.2 g DW m^-2 (326.9ᇛ.7 g) or 71.7-141.2 g AFDW m^-2 (96.8ᆨ.1 g) and for S. isoetifolium it was 271.7-758 g DW m^-2 (460.4끯.1 g) or 119-220.5 g AFDW m^-2 (155.1ᆮ.5 g), with a maximum biomass increase during September-December in both species. AG:BG biomass ratios were generally <1 and approximated 1 during the warmest months of December-February only. Mean shoot density (1,077-4,364 shoots m^-2 in the overall range of 998-4,428 shoots m^-2) and mean shoot length (10.9-20.8 cm in the overall range of 7-31 cm) in S. isoetifolium were higher than in H. uninervis (1,732-4,137 shoots m^-2 in the overall range of 1,522-4,327 shoots m^-2 and 7.9-13.7 cm in the overall range of 6-20 cm, respectively). Temperature showed strong positive correlations with total AFDW biomass of both species (r=0.755, P<0.01 for H. uninervis; r=0.679, P<0.02 for S. isoetifolium) and with DO (r=0.925, P<0.01). High DO levels (10.7-11.2 mg l^-1) coincided with optimum standing biomass at 27.2°C. Correlations were also strong with shoot density (r=0.881, P<0.01 for H. uninervis; r=0.952, P<0.01 for S. isoetifolium) and shoot length (r=0.752, P<0.01 for H. uninervis; r=0.797, P<0.01 for S. isoetifolium). Under optimal environmental conditions, nutrient inputs from surface run-off or underground freshwater seepage in the lagoon due to heavy rainfall may boost up seagrass biomass, as suggested by positive significant correlations between phosphate levels and AG AFDW biomass (r=0.63, P<0.05 for H. uninervis; r=0.65, P<0.05 for S. isoetifolium) and shoot density (r=0.6, P<0.05 for H. uninervis; r=0.687, P<0.02 for S. isoetifolium). The results generated in this study suggest local seagrass standing biomass is comparable to that reported in monospecific stands from elsewhere. Anthropogenic activities increasingly draw down the resilience of the seagrass beds around Mauritius, and preventative measures are indispensable to achieve coastal ecological stability.     

Mitochondrial DNA variation in Eritrean hamadryas baboons (Papio hamadryas hamadryas): life history influences population genetic structure

The hamadryas baboon, Papio hamadryas hamadryas, represents a rare exception from the pattern of female philopatry and male-biased dispersal predominant in mammals including primates. To elucidate the possible consequences of the dispersal pattern on the population genetic structure of hamadryas baboons, we sequenced the maternally transmitted mitochondrial hypervariable region I of 74 individuals from ten sampling locations in different ecogeographic zones of Eritrea. To this end, individual fecal samples were collected at sleeping cliffs. Upon comparing the individual sequences by means of phylogenetic tree reconstructions and AMOVA, we could not detect a population genetic structure corresponding to a geographic pattern. Tree reconstructions revealed the existence of two profoundly different lineages both present at most of the sampling locations. These findings and Mantel correlations of genetic distances and the frequency of shared haplotypes to geographic distances point to the presence of female dispersal. Female-mediated gene flow is detectable over geographic distances exceeding those between neighboring subpopulations. Our study therefore corroborates local behavioral observations on a broad geographic scale. After inclusion of geographically closely situated olive baboons, P. h. anubis, in the analyses, all anubis sequences fell within one hamadryas clade. Possible scenarios leading to this situation including long-term hybridization processes are discussed.     

Population genetics of the fissiparous holothurians Stichopus chloronotus and Holothuria atra (Aspidochirotida): a comparison between the Torres Strait and La Réunion

Collections of about 50 individuals from each of five populations of the fissiparous holothurian species Stichopus chloronotus and four populations of Holothuria atra were made in 1999. These populations were located in the Torres Strait (western Pacific) and La Réunion (western Indian Ocean). Allozyme electrophoretic surveys of five (S. chloronotus) and six (H. atra) loci were conducted to compare patterns of asexual reproduction and to investigate connectivity between regions separated by large geographic distances. Deviations from genotype frequencies expected under Hardy-Weinberg equilibrium, mostly heterozygote excesses, were observed in all populations of both species. The maximum contribution of sexual reproduction (calculated as the maximum number of sexually produced individuals: sample size=N*/N_i) was similar for all S. chloronotus (58-64%) and H. atra (76-92%) populations, and on the same level as previously reported for midshelf reefs of the Great Barrier Reef. The higher values in the latter species indicated greater contributions of asexual reproduction to S. chloronotus populations. Variability was strongly reduced in S. chloronotus populations at La Réunion, with only one locus being variable in that population. When the dataset was reduced to one representative per multi-locus genotype per population to reduce the effect of asexual reproduction on calculations on gene flow, F_ST values were not significantly different from zero, suggesting high gene flow between these regions. However UPGMA cluster analyses using Rogers' genetic distance, roughly clustered populations by region. In the case of H. atra, pooled populations within each region were significantly different from those of the other region. Thus, although some restrictions in gene flow and greater genetic distances between the regions may exist, those differences are distinctly less than those reported in previous studies on echinoderms over similar geographic scales. Despite the importance of asexual reproduction for the maintenance of local population size, this study also confirmed that the potential for widespread dispersal mediated by sexually produced larvae is large.     

Cryptic speciation in a high gene flow scenario in the oviparous marine sponge Chondrosia reniformis

Sponge systematics has been traditionally based on the study of the skeleton (spicules and spongin fibres). However, sponges of the genus Chondrosia are devoid of those skeletal features, making it difficult to distinguish between different species in the genus. Chondrosia reniformis Nardo, 1847, the type species of the genus, was described from the Mediterranean Sea. The lack of distinguishing morphological features may have been responsible for the widespread assignment of specimens of the genus to this species; as a result C. reniformis is considered to be a cosmopolitan species. In this work, populations of C. reniformis from the western Mediterranean (France) and the West Atlantic (Bermuda and Brazil) were analysed using allozyme electrophoresis for 13 enzyme loci. Levels of mean heterozygosity were high (Bermuda and Brazil H=0.27 and W Mediterranean H=0.12), as is often observed in sponge species. Gene identities observed between West Atlantic and Mediterranean populations were low (I=0.40-0.52, typical values for congeneric species), including the presence of four diagnostic loci. This level of divergence clearly shows that they are not conspecific. Hence, a worldwide or cosmopolitan distribution of C. reniformis would seem improbable. However, the West Atlantic samples (Bermuda and Brazil) were genetically similar (gene identity, I=0.88-0.95) over a distance of 8,000 km. This is the first report of genetic homogeneity in a sponge species over such a large geographical distance.     

Panmixia in Pocillopora verrucosa from South Africa

The genetic structure of six local collections of Pocillopora verrrucosa from six coral reefs in KwaZulu-Natal, South Africa, was examined using allozyme electrophoresis. The six separate reefs lie within two different reef complexes. Twenty-two enzymes were screened on five buffer systems, but only five polymorphic loci (Gpi-1, Gdh-1, Lgg-2, Lpp-1, Est-1) could be consistently resolved. No significant differences in allelic frequencies were detected among the six sites. All local collections were genotypically diverse, with evidence of only very limited clonal replication at each site. Indeed, the ratio of observed to expected genotypic diversity (mean Go:Ge=0.64ǂ.05 SD), the ratio of observed number of genotypes to the number of individuals (mean Ng:N=0.65ǂ.04 SE), and deviations from the Hardy-Weinberg equilibrium indicate that sexual reproduction plays a major role in the maintenance of the populations. No genetic differentiation was found either within (FSR=0.026ǂ.003 SE) or between (FRT=0.000ǂ.001 SE) reef complexes. The homogeneity of the gene frequencies across the six reefs strongly supports the assumption that the KwaZulu-Natal reef complexes are highly connected by gene flow (Nem=44). The reefs in the southern and central reef complexes along the northern Maputaland coastline can therefore be considered part of a single population.     

Population structure of yellowfin tuna (Thunnus albacares) in the western Pacific Ocean, inferred from microsatellite loci

Five polymorphic microsatellite loci were examined in 1391 yellowfin tuna (Thunnus albacares) from eight regions of the western (Coral Sea, eastern Australia, Fiji, Indonesia, Philippines and Solomon Islands) and eastern (California and Mexico ) Pacific Ocean. Across all samples, numbers of alleles per locus ranged from 7 to 30 (mean: 17.0), and observed heterozygosities per locus ranged from 0.223 to 0.955 (mean: 0.593). Temporal collections were available for three areas: no significant temporal heterogeneity was observed for the Coral Sea (1991/1992 and 1995/1996 collections) or eastern Australia (1994/1995, 1995/1996, 1996/1997 and 1997/1998), but there was slight but significant heterogeneity at one locus (cmrTa-161) between the two Philippines collections (1994/1995 and 1996/1997). Genotypes generally showed a good fit to Hardy-Weinberg expectations within populations; only cmrTa-208 in the pooled Coral Sea population gave a significant deviation after Bonferroni correction for 40 tests, with a small but significant excess of homozygotes. Four loci showed no evidence of population differentiation following contingency Chi-squared and F_ST analyses. The fifth locus, cmrTa-161, showed small but significant differentiation (F_ST=0.002, P<0.001). This heterogeneity was largely a result of the Philippines 1994/1995 and Fiji collections; there was no correlation with geographic distance. The average F_ST across all five loci was very low (F_ST=0.002), but it was significant (P<0.001). It is unclear whether this low but significant differentiation reflects noise in the dataset, perhaps arising from experimental error, or real population differentiation. The finding of very limited population heterogeneity accords with most of the earlier allozyme and mitochondrial DNA studies of yellowfin tuna in the Pacific Ocean.     

DNA sequence evidence for speciation,paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods

Because the classification of extant and fossil articulate brachiopods is based largely upon shell characters observable in fossils, it identifies morphotaxa whose biological status can, in practice, best be inferred from estimates of genetic divergence. Allozyme polymorphism and restriction fragment length polymorphism of mitochondrial DNA (mtDNA RFLP) have been used to show that nuclear and mitochondrial genetic divergence between samples of the cancellothyridid brachiopods Terebratulina septentrionalis from Canada and T. retusa from Europe is compatible with biological speciation, but the genetic distances obtained were biased by methodological limitations. Here, we report estimates of divergence in 12S rDNA mitochondrial sequences within and between samples of these brachiopods. The sequence-based genetic distance between these samples (5.98ǂ.07% SE) is at least 10 times greater than within them and, since they also differ in a complex life-history trait, their species status is considered to be securely established. Divergence levels between 12S rDNA genes of three other cancellothyridids, T. unguicula from Alaska, T. crossei from near Japan, and Cancellothyris hedleyi from near Australia are higher than between the two North Atlantic species, and the mean nucleotide distance between all these cancellothyrids is similar to the mean distance between species of Littorina (Mollusca: Gastropoda). Sequences of both 12S and 16S genes from cancellothyridids and other short-looped brachiopod species show neither saturation nor lineage-specific rate differences and, when analysed with different outgroups, either separately or together, yield one unexpected, but well-supported, tree with Alaskan T. unguicula basal and C. hedleyi nested within Terebratulina, i.e. these genera are paraphyletic. A geologically dated divergence between Antarctic and New Zealand species of the short-looped brachiopod Liothyrella is used to calibrate the rate of 12S divergence at ca. 0.1% per million years (MY), and this rate is used to infer that T. septentrionalis and T. retusa have been diverging for ca. 60 MY and that they and T. unguicula have been diverging from their last common ancestor for ca. 100 MY. This indicates a Mesozoic origin for the present-day distribution of cancellothyridids and the basal position of T. unguicula suggests a possible North Pacific centre of origin, with separate Atlantic and Pacific radiations. The inclusion of Cancellothyris within Terebratulina also shows that adult shell characters such as umbo, foramen and symphytium shape, whilst probably indispensible for the practical classification of fossils, are not reliable guides to genealogy.     

Extreme mitochondrial DNA divergence within populations of the deep-sea gastropod Frigidoalvania brychia

The deep sea supports a diverse and highly endemic invertebrate fauna, the origin of which remains obscure. Little is known about geographic variation in deep-sea organisms or the evolutionary processes that promote population-level differentiation and eventual speciation. Sequence variation at the 16 S rDNA locus was examined in formalin-preserved specimens of the common upper bathyal rissoid Frigidoalvania brychia (Verrill, 1884) to examine its population genetic structure. The specimens came from trawl samples taken over 30 years ago at depths of 457-1,102 m at stations in the Northwest Atlantic south of Woods Hole, Massachusetts, USA. Near the upper boundary of its bathymetric range (500 m), extremely divergent haplotypes comprising three phylogenetically distinct clades (average uncorrected sequence divergence among clades ~23%, ~3% within clades) were found at stations separated by a maximum distance of ~80 km, suggesting the presence of high levels of intraspecific divergence or the possibility of morphologically cryptic species. Only one of these clades was found at two stations in the mid- to lower part of F. brychia's depth distribution (800-1,100 m), suggesting lower clade diversity with increasing depth, although among-sample divergence, with a single exception, was minimal. One station was genetically divergent from all others sampled, containing a unique suite of haplotypes including two found only at this site. Steep vertical selective gradients, major oceanographic changes during the late Cenozoic, and habitat fragmentation by submarine canyons might have contributed to an upper bathyal region that is highly conducive to evolutionary change.     

Effects of habitat fragmentation on the genetic structure and connectivity of the black-lipped pearl oyster Pinctada margaritifera populations in French Polynesia

Habitat destruction leading to increased fragmentation is detrimental to species by reducing population size and genetic diversity and by restraining population connectivity. However, little is known about the effects of naturally fragmented habitats on wild populations, especially when it comes to marine benthic invertebrates with long pelagic larval duration. In this framework, we investigated the connectivity and genetic diversity variation among nine wild populations of the black-lipped pearl oyster, Pinctada margaritifera, throughout French Polynesia using ten microsatellite DNA markers. Despite the naturally fragmented habitat (South Pacific oceanic islands), we found high values of genetic diversity and population admixture, indicating connectivity at small and large spatial scales within sampled sites of the Tuamotu, and between the Society and Tuamotu Archipelagos. In the meantime, habitat geomorphology increased genetic drift in populations occurring in small, closed lagoons. Significant genetic structure not correlated to geographic distance was detected mainly between closed and open lagoons. The Marquesas Islands hosted the most divergent populations, likely a result of vicariance. Our results also highlight that migration patterns among lagoons are not symmetrical. Altogether, the general pattern of gene flow, nonsymmetrical migration rates among populations, absence of isolation by distance and absence of recent extinction events found in our study strongly suggest that P. margaritifera populations of French Polynesia follow an asymmetrical island model of dispersal.     

Analysis of the genetic structure of European eel (Anguilla anguilla) using microsatellite DNA and mtDNA markers

The spawning population of European eel (Anguilla anguilla L.) has been considered panmictic on the basis of genetic markers and morphometric studies. This hypothesis was tested by screening glass eel from five locations (Ireland, Italy, Morocco, Sweden and U.K.), belonging to two cohorts at the cytochrome b (cyt b) locus (392 bp) of the mitochondrion and at five nuclear microsatellite loci. Seventeen cyt b haplotypes were detected, of which ten were singletons; the most common haplotype occurred in 47% of all fish. Haplotype number increased significantly with latitude. Phylogeographical structure based on the cytoplasmic marker was weak (F_ST=0.014) and non-significant. Close similarity was revealed between British and Irish glass eel populations, and weak differentiation among the British/Irish, Atlantic Moroccan, Italian and Swedish Baltic populations, respectively. No hierarchical genetic structure was obvious. Levels of genetic variation detected with five microsatellites were much higher levels than found with allozymes in previous studies (mean number of alleles per locus=11.1; mean expected heterozygosity=0.68). Overall among-population microsatellite variance was low but significant (F_ST=0.004), and caused by the linked microsatellite loci Aan03 and Aan04. The Hardy-Weinberg-Castle equilibrium and the absence of gametic disequilibria at these loci in the Moroccan population might point to its genetic isolation, although the impact of just two out of five loci is puzzling. Given the weak differentiation typical for marine species and the limitations of our data, the results should be interpreted with caution. However, combined with recent evidence from a related study, the paradigm that the European eel constitutes a panmictic population becomes difficult to maintain.     

Sibling species or poecilogony in the polychaete<Emphasis Type="Italic"> Scoloplos armiger</Emphasis>?

In marine invertebrates multiple modes of development, or poecilogony, may occur in a single species. However, after close examination, many of such putative cases turned out to be sibling species. A case in point may be the cosmopolitan orbiniid polychaete Scoloplos armiger, which inhabits marine shallow sediments. In addition to the well-known direct, holobenthic development from egg cocoons, pelagic larvae have also been described. Our culture experiments revealed a spatially segregated source of the two developmental modes. All females of an intertidal population produced egg cocoons and no pelagic larvae. All but 2 out of 15 females of an adjacent subtidal population produced pelagic larvae and no egg cocoons. Based on these results we performed a molecular genetic analysis (RAPD-PCR) on three intertidal and four subtidal populations in the North Sea. Selected samples from all sites were analysed also by the AFLP method. We found significantly higher genetic diversity within subtidal than within intertidal populations. This is consistent with a wider dispersal by pelagic larvae and a smaller effective population size when development is holobenthic. Total genetic divergence is not related to distance but to the intertidal/subtidal division. We suggest that S. armiger actually represents two sibling species.