Genetic structure of the flounders Platichthys flesus and P. stellatus at different geographic scales

The genetic structure of the flounders Platichthys flesus L. and P. stellatus Pallas was investigated on different spatial scales through analysis of allozyme variation at 7 to 24 polymorphic loci in samples collected from different regions (Baltic Sea, North Sea, Brittany, Portugal, western Mediterranean, Adriatic Sea, Aegean Sea and Japan) in 1984 to 1987. No geographic variation was evident within a region. Some pattern of differentiation by distance was inferred within the Atlantic, while the Mediterranean comprised three geographically isolated populations and was itself geographically isolated from the Atlantic (fixed allele differences at up to three loci were found among P. flesus populations from the Atlantic, the western Mediterranean, the Adriatic Sea, the Aegean Sea and also P. stellatus from the coast of Japan). Sea temperature during the reproductive period probably acts as a barrier to gene flow between populations. Genetic distances among European flounder populations (P. flesus) were higher than, or of the same magnitude as, the genetic distance between Pacific (P. stellatus) and European flounder populations, suggesting that P. flesus is paraphyletic and/or there is no phylogenetic basis to recognising P. stellatus as a different species. The divergence between P. flesus and P. stellatus was thus inferred to be more recent than the divergence between the present P. flesus populations from the NE Atlantic and eastern Mediterranean. The eastern Mediterranean populations are thought to originate from the colonisation of the Mediterranean by a proto-P. flesus/P. stellatus ancestor, whereas the present western Mediterranean population has undergone a more recent colonisation event by P. flesus. Patterns of mitochondrial DNA variation, established on a smaller array of P. flesus samples, were in accordance with the geographic patterns inferred from the allozyme survey. In addition, they supported the hypothesis of a two-step colonisation of the western Mediterranean. These results contribute to our understanding of the biogeography of the Mediterranean marine fauna, especially the group of boreal remnants to which P. flesus belongs. Received: 7 February 1997 / Accepted: 26 March 1997     

Population structure of the common sole (<Emphasis Type="Italic">Solea solea</Emphasis>) in the Northeastern Atlantic and the Mediterranean Sea: revisiting the divide with EPIC markers

Spatial and temporal population genetic structures of the common sole, Solea solea, were studied in Northeastern Atlantic and Mediterranean Sea populations, using three polymorphic exon-primed intron-crossing (EPIC) markers. Results demonstrated significant multilocus differentiation among Eastern Mediterranean and a group composed by Western Mediterranean and Atlantic populations (θ = 0.150, P < 0.001), but also suggested unrecorded genetic differentiation of the Adriatic Sea population. No pattern of isolation-by-distance was recorded across the range covered by sampling, from the Kattegat to the Aegean Sea. Conversely to genetically structured Mediterranean populations, Atlantic populations ranging from Denmark to Portugal could be considered as representative of the same panmictic unit (θ = 0.009, not significant). Results further demonstrated stability of multilocus genetic structure among temporarily replicated cohort samples [0+, 1+, subadults] from several coastal and estuarine locations from Bay of Biscay, excepted for the amylase locus Am2B3-2 at one location (Pertuis d’Antioche). Despite coherence of such observed patterns of multilocus differentiation with previous allozymic surveys in sole, and with patterns generally obtained for other marine fish species, single-locus results from EPICs indicated divergent coalescence schemes supporting a complex response to ecology and history of sole’s populations. Results stress the use of nuclear genes such as EPIC markers to investigate population structure, but also historical, demographic, and possibly selective processes in marine fishes.     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999     

Allozyme variation in European populations of the oyster Ostrea edulis

Variations at 22 enzyme coding loci were surveyed in 11 populations of the oyster Ostrea edulis L., which were sampled between 1988 and 1990 along the Atlantic and Mediterranean coasts of Europe. Atlantic oyster beds suffered a steady decline during the last century, and restocking of beds with oysters of foreign origin has probably resulted in a high degree of interbreeding of natural oyster stocks from all Atlantic Europe. Our study confirms the low levels of genetic variability previously reported for the oyster populations from the Atlantic coasts, and extends it to the Mediterranean coasts. The locus arginine-kinase (ARK ^*) exhibited a high degree of interpopulation differentiation (F _ST=0.289), resulting from extensive variation in gene frequencies along a geographical cline. However, the overall genetic differentiation between populations was slight, and similar to that reported for other local populations of bivalves (mean genetic distance between populations is 0.010, mean F _ST=0.062). A general pattern of increasing differentiation along the coastline in an Atlantic-mediterranean direction emerged; but genetic differentiation among the Atlantic populations was not significantly lower than that observed among the Mediterranean populations. This and other results suggest that the effects of extensive transplantation of oysters among various areas in Europe are detectable only in some particular localities. The geographical distribution of low-frequency alleles suggests a restriction to gene flow outwards from the Mediterranean Sea, across the Straits of Gibraltar.     

Microsatellite analysis of albacore tuna (<Emphasis Type="Italic">Thunnus alalunga</Emphasis>): population genetic structure in the North-East Atlantic Ocean and Mediterranean Sea

Stock heterogeneity was investigated in albacore tuna (Thunnus alalunga, Bonnaterre 1788), a commercially important species in the North Atlantic Ocean and Mediterranean Sea. Twelve polymorphic microsatellite loci were examined in 581 albacore tuna from nine locations, four in the north-east Atlantic Ocean (NEA), three in the Mediterranean Sea (MED) and two in the south-western Pacific Ocean (SWP). Maximum numbers of alleles per locus ranged from 9 to 38 (sample mean, 5.2–22.6 per locus; overall mean, 14.2 ± 0.47 SE), and observed heterozygosities per locus ranged from 0.44 to 1.00 (overall mean: 0.79 ± 0.19 SE). Significant deficits of heterozygotes were observed in 20% of tests. Multilocus F _ST values were observed ranging from 0.00 to Θ = 0.036 and Θ′ = 0.253, with a mean of Θ = 0.013 and Θ′ = 0.079. Pairwise F _ST values showed that the SWP, NEA and MED stocks were significantly distinct from one another, thus corroborating findings in previous studies based on mitochondrial DNA, nuclear DNA (other than microsatellites) and allozyme analyses. Heterogeneity was observed for the first time between samples within the Mediterranean Sea. GENELAND indicated the potential presence of three populations across the NEA and two separate populations in the Mediterranean Sea. Observed genetic structure may be related to migration patterns and timing of movements of subpopulations to the feeding grounds in either summer or autumn. We suggest that a more intensive survey be conducted throughout the entire fishing season to ratify or refute the currently accepted genetic homogeneity within the NEA albacore stock.     

Allozyme variation in global populations of striped mullet, Mugil cephalus (Pisces: Mugilidae)

The striped mullet, Mugil cephalus Linnaeus, 1758, is one of the few species of marine shore fish with a worldwide circumtropical distribution. Because of this distribution and the dependency of M. cephalus on coastal waters during various phases of its life cycle, as well as nearshore living habits, questions have been raised regarding levels of genetic divergence and gene flow among transoceanic populations. To cast more light on this, allozyme variation at 27 presumptive gene loci was investigated in ten globally diverse populations. The observed heterozygosity ranged from 0.018 (Hawaii) to 0.081 (Florida), averaging 0.050. The proportion of polymorphic loci showed a similar trend. Several populations were characterised by fixed allelic differences. Estimated gene diversities were very high, the allele frequency variation among populations was found to be 68%; genetic distances reached 0.242, with an average of 0.117. Estimated rates of gene flow were high among Mediterranean populations (Nm = 7.26), and between Mediterranean and East Atlantic populations (Nm= 2.86), but extremely low between non-contiguous populations within the Indian, Pacific and Atlantic Oceans, where Nm ranged from 0.03 to 0.05. Received: 7 April 1997 / Accepted: 4 February 1998     

Phylogeographic patterns of the mysid Mesopodopsis slabberi (Crustacea, Mysida) in Western Europe: evidence for high molecular diversity and cryptic speciation

The phylogeographic patterns among populations of Mesopodopsis slabberi (Crustacea, Mysida), an ecological important mysid species of marine and estuarine habitats, were analysed by means of DNA sequencing of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) and the 16S ribosomal RNA genes. Samples of M. slabberi collected from five Atlantic and two Western Mediterranean populations were investigated. Very high levels of within-population molecular diversity were observed in all samples (mean h=0.807 and π=0.0083), with exception of the Mediterranean Ebro population which contained only one haplotype. Differentiation among populations was high, and a clear phylogeographic break was observed between the Atlantic and Mediterranean populations. Moreover, a strong differentiation was detected between both populations in the Western Mediterranean Sea (Alicante and Ebro delta), while two divergent lineages occurred in sympatry within the Atlantic Mondego estuary. The high congruence between both the COI and 16S rRNA sequence data, the reciprocal monophyly of the different mitochondrial clades and the levels of nucleotide divergence between them suggest the presence of a complex of cryptic species within M. slabberi. Estimations of divergence time between the different mitochondrial lineages indicate that a split occurred during the late Miocene/early Pliocene. Such a divergence could be concordant with vicariant events during sea-level drops within the Mediterranean region at that time. However, within the Mediterranean Sea, the potential of divergence through ecological diversification cannot be ruled out.     

Population differentiation in megrim (<Emphasis Type="Italic">Lepidorhombus whiffiagonis</Emphasis>) and four spotted megrim (<Emphasis Type="Italic">Lepidorhombus boscii</Emphasis>) across Atlantic and Mediterranean waters and implications for wild stock management

Megrim, Lepidorhombus whiffiagonis, and four spot megrim, Lepidorhombus boscii, are two marine fish species of high commercial interest. Despite their quite heavy exploitation little is known on the genetic structure of their populations. The present work aimed at characterizing the first seven microsatellites markers available for the two megrim species. These new markers were in a second step employed to describe the population structure of the two species among their almost entire habitat range (Atlantic and Mediterranean samples). Our study confirmed the existence of a strong genetic difference between Atlantic and Mediterranean megrim species already described in the literature for L. whiffiagonis on the basis of variations at ribosomal genes. Additionally our analysis gave the first evidences of a strong genetic differentiation among Atlantic populations in both megrim species (within Atlantic global F_ST in L. whiffiagonis and L. boscii were respectively 0.158 and 0.145). When describing megrim population structure, the comparison between allele-frequency-based tests (F_ST comparisons) and genotype-based inferences (Bayesian approach) gave evidences of a hierarchical structure of the populations. In conclusion, our work enlighten the existence of two different stocks within the Atlantic Ocean and one in the Mediterranean Sea that will clearly need to be managed separately. As the present results do not fully support the current megrim stock boundaries they will surely help to rethink megrim management policies in the future.     

Genetic variability and relationships among geographically widely separated populations of Petitia amphophthalma (Polychaeta: Syllidae). Results from RAPD-PCR investigations

An analysis of the population genetics of the meiofaunal polychaete Petitia amphophthalma Siewing, 1956, in which the RAPD-PCR method was applied to 103 individuals from eight populations, some of them very far apart (Atlantic: Florida, Tenerife, France; Mediterranean: two Greek islands, Tunisia; Red Sea: Egypt), gave closely reproducible results. In the band patterns produced with 13 decamer primers, a total of 195 genetic characters was detected. The data were evaluated by a number of methods, including the cluster programs UPGMA, WPGMA and neighbour-joining. The detected genetic distances between the populations vary between 58.9 and 66.6, but 97% of the genetic characters, although polymorphic, are found in at least two populations and usually in all the others as well. Phenograms of the analyses find four population clusters [Florida, France (Atlantic), the Mediterranean and Tenerife]. They are, however, not completely congruent and show low bootstrap values at the junction points of the clusters (with the exception of the Tenerife cluster). Mediterranean P. amphophthalma form a cohesive population, although within it the genetic distances are graded in parallel with the geographic distances between the sites. The colonization of Tenerife, an island of relatively recent volcanic origin, can be taken as evidence that this meiofaunal species can become dispersed not only along coastlines but also across expanses of open water. However, the severely restricted variability of these populations implies that in this case a founder effect has operated, and that transport over open water is not a routine event but extremely rare. The absence of the species on the Australian coast and, for instance, on the Galapagos Islands indicates that there has been no continuous gene flow across the oceans. The idea that all the populations investigated belong to one cosmopolitan species is discussed. Received: 17 October 1997 / Accepted: 15 April 1998     

Phylogeny and geographic differentiation of Atlanto–Mediterranean species of the genus Xantho (Crustacea: Brachyura: Xanthidae) based on genetic and morphometric analyses

The crab genus Xantho Leach, 1814 is restricted to the northeastern Atlantic Ocean and the Mediterranean Sea. It consists of four species, Xantho hydrophilus (Herbst, 1790), X. poressa (Olivi, 1792), X. pilipes A. Milne-Edwards, 1867, and X. sexdentatus (Miers, 1881). X. hydrophilus has been divided into two geographic forms, of which one, X. h. granulicarpus (Forest, 1953), is postulated to be endemic to the Mediterranean Sea. In this study, we reconstruct phylogenetic relationships of the genus Xantho and related genera from the Atlantic Ocean or Mediterranean Sea and compare different geographic populations of Xantho hydrophilus and, to a lesser extent, of X. poressa by means of population genetic and morphometric analyses. The molecular phylogeny is based on two mitochondrial genes (large subunit rRNA and cytochrome oxidase I) and indicates that X. poressa, X. hydrophilus and X. sexdentatus form a monophyletic group, the latter two species sharing identical haplotypes. On the other hand, X. pilipes shows affinities to Xanthodius denticulatus. Population genetics based on the COI gene reveal genetic differentiation within X. hydrophilus. Morphometric results also give evidence for distinct geographic forms in X. hydrophilus with a clear discrimination. In comparison, morphometric discrimination between different geographic populations of X. poressa is less clear, but still significant. We therefore suggest a recent/ongoing morphological and genetic differentiation within Xantho hydrophilus, restricted gene flow between its Atlantic and Mediterranean populations (not allowing subspecific differentiation) and possible mtDNA introgression between the species X. hydrophilus and X. sexdentatus.     

Molecular phylogeny of vestimentiferans collected around Japan, revealed by the nucleotide sequences of mitochondrial DNA

The phylogenetic relationships among local populations of undescribed vestimentiferan species that belong to the genera Lamellibrachia and Escarpia and had been collected at six sites around Japan were analyzed on the basis of the partial (1023 bp) nucleotide sequences of the mitochondrial gene for cytochrome oxidase I (COI), using a pogonophoran and a polychaete as outgroups. The identical amino acid sequence was deduced from the nucleotide sequence obtained from each of the vestimentiferans analyzed. The strong similarity among deduced amino acid sequences of COI suggested a close relationship between vestimentiferans and pogonophorans. On the basis of the phylogenetic relationships, the analyzed vestimentiferan populations were classified tentatively as five species. The genetic differentiation of vestimentiferans was suggested to occur bathymetrically as well as being a consequence of horizontal segregation. Two of these tentatively identified species inhabit both a hydrothermal area and a cold seep area, as is the case for some species of bivalves that belong to the genera Bathymodiolus and Calyptogena. Received: 28 August 1996 / Accepted: 2 October 1996     

Genetic and taxonomic relationships among Northeastern Atlantic and Mediterranean populations of the soft coral Alcyonium coralloides

Species boundaries among taxa of colonial marine organisms are often obscured by intraspecific morphological and ecological variation; genetic comparisons of recognized “ecotypes” frequently reveal them to be reproductively isolated species. Based on morphological similarities, it has been proposed that the Mediterranean soft coral Alcyonium (=Parerythropodium) coralloides Pallas, 1766 and its Atlantic congener A. hibernicum belong to one highly variable and geographically widespread species, A. coralloides. I collected A. coralloides from ten Atlantic and three Mediterranean locations in 1990 and 1994, and used differences in colony form, substrate use and color to separate them into five distinct morphotypes. Two occur sympatrically in the Mediterranean (M1, M2) and three have overlapping distributions in the Atlantic (A1, A2, A3). I used allozyme electrophoresis to compare morphotypes genetically at 14 enzyme loci. Where two morphotypes occurred sympatrically, fixed allelic differences at 4 to 6 loci indicated reproductive isolation. In all but one pairwise comparison (M1 and A2), morphotypes whose ranges did not overlap were also separated by large genetic distances. From these results I suggest that the five morphotypes represent four distinct species. A. coralloides comprises two morphotypes (M1, A2) with relatively high genetic identity. A. hibernicum (=A1) is reproductively isolated from A. coralloides and should be retained as a valid species; levels of genetic diversity and heterozygosity within populations support the absence of outcrossing in this reportedly asexual species. Morphotypes M2 and A3 are also reproductively isolated from A. coralloides; they are taxonomically distinct from but belong to the same phylogenetic clade as A. hibernicum. Although preliminary observations suggest that differences in reproductive timing maintain species boundaries in sympatry, wider geographic sampling will be required to elucidate the events leading to speciation within this species complex. Received: 8 May 1998 / Accepted: 8 October 1998     

Population differentiation of the Atlantic spotted dolphin (Stenella frontalis) in the western North Atlantic, including the Gulf of Mexico

Information about the genetic population structure of the Atlantic spotted dolphin [Stenella frontalis (G. Cuvier 1829)] in the western North Atlantic would greatly improve conservation and management of this species in USA waters. To this end, mitochondrial control region sequences and five nuclear microsatellite loci were used to test for genetic differentiation of Atlantic spotted dolphins in the western North Atlantic, including the Gulf of Mexico (n=199). Skin tissue samples were collected from 1994–2000. Significant heterozygote deficiencies in three microsatellite loci within samples collected off the eastern USA coast prompted investigation of a possible Wahlund effect, resulting in evidence for previously unsuspected population subdivision in this region. In subsequent analyses including three putative populations, two in the western North Atlantic (n=38, n=85) and one in the Gulf of Mexico (n=76), significant genetic differentiation was detected for both nuclear DNA (R _ST=0.096, P≤0.0001) and mitochondrial DNA (Φ _ST=0.215, P≤0.0001), as well as for all pair-wise population comparisons for both markers. This genetic evidence for population differentiation coupled to known biogeographic transition zones at Cape Hatteras, North Carolina and Cape Canaveral, Florida, USA, evidence of female philopatry, and preliminary support for significant genetic differences between previously documented morphotypes of Atlantic spotted dolphins in coastal and offshore waters all indicate that the biology and life history of this species is more complex than previously assumed. Assumptions of large, panmictic populations might not be accurate in other areas where S. frontalis is continuously distributed (e.g., eastern Atlantic), and could have a detrimental effect on long-term viability and maintenance of genetic diversity in this species in regions where incidental human-induced mortality occurs.

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Genetic stock structure of the swordfish (Xiphias gladius ) inferred by PCR-RFLP analysis of the mitochondrial DNA control region

Restriction fragment length polymorphism (RFLP) analysis was performed on PCR amplified DNA fragments containing the control region of the swordfish (Xiphias gladius Linnaeus, 1758) mitochondrial DNA. A total of 456 individuals comprising 13 local samples (six Pacific, three Atlantic, the Mediterranean Sea, two Indian Ocean and the Cape of Good Hope) were surveyed with four endonucleases (Alu I, Dde I, Hha I and Rsa I), yielding a total of 52 composite genotypes. Within-sample genotypic diversity (H) was high ranging from 0.702 to 0.962 with a value of 0.922 for the pooled sample. Significant geographic variation in the frequencies of genotypes and restriction patterns was revealed. The Mediterranean sample was highly distinct from all other samples. Further, Rsa I digestion revealed high levels of polymorphism in all but the Mediterranean samples, indicating that exogenous swordfishes rarely enter that body of water. Heterogeneity between the North and South Atlantic samples was significant, both of which differed from those of the Pacific. In contrast, the Indian Ocean samples were not significantly different from the samples of South Atlantic and Pacific. Genetic differentiation among the Pacific samples was low. The results indicate that the world-wide swordfish population is genetically structured not only among, but also within ocean basins and suggest that gene flow is restricted despite the absence of geographic barriers. Received: 28 August 1996 / Accepted: 2 October 1996     

Genetic structure of Patagonian toothfish (Dissostichus eleginoides) populations on the Patagonian Shelf and Atlantic and western Indian Ocean Sectors of the Southern Ocean

The genetic structure of Patagonian toothfish populations in the Atlantic and western Indian Ocean Sectors of the Southern Ocean (SO) were analysed using partial sequences of the mitochondrial 12S rRNA gene and seven microsatellite loci. Both haplotype frequency data (F _ST>0.906, P<0.01) and microsatellite genotype frequency data (F _ST=0.0141–0.0338, P<0.05) indicated that populations of toothfish from around the Falkland Islands were genetically distinct from those at South Georgia (eastern Atlantic Sector SO), around Bouvet Island (western Atlantic Sector SO) and the Ob Seamount (western Indian Ocean Sector of the SO). Genetic differentiation between these populations is thought to result from hydrographic isolation, as the sites are separated by two, full-depth, ocean-fronts and topographic isolation, as samples are separated by deep water. The South Georgia, Bouvet and Ob Seamount samples were characterised by an identical haplotype. However, microsatellite genotype frequencies showed genetic differentiation between South Georgia samples and those obtained from around Bouvet Island and nearby seamounts (F _ST=0.0037, P<0.05). These areas are separated by large geographic distance and water in excess of 3,000 m deep, below the distributional range of toothfish (<2,200 m). No significant genetic differentiation was detected between samples around Bouvet Island and the Ob Seamount although comparisons may have been influenced by low sample size. These localities are linked by topographic features, including both ridges and seamounts, that may act as oceanic “stepping stones” for migration between these populations. As for other species of deep-sea fish, Patagonian toothfish populations are genetically structured at the regional and sub-regional scales.     

Population genetic structure in brooding sea anemones (Epiactis spp.) with contrasting reproductive modes

Effects of dispersal and mating systems on the genetic structure of populations were evaluated by comparing five sea anemones: four Epiactis species that brood their offspring to the juvenile stage and one Anthopleura species that broadcasts gametes and has pelagic, planktotrophic larvae. The anemones were sampled at sites ranging from British Columbia to southern California between 1988 and 1992 and were analyzed by enzyme electrophoresis and by multilocus DNA-fingerprinting. Results were only partially consistent with expectations. While all four brooding species had lower observed heterozygosities than the broadcasting species, not all brooding species had greater population subdivision than the broadcasting species. The self-fertile E. prolifera had the expected evidence of intense local inbreeding ( f = 0.955); unexpectedly, the cross-fertile E. lisbethae and E. ritteri also had similar departures from random mating ( f = 0.957 and 0.831, respectively) probably due to biparental inbreeding among near neighbors in small, highly subdivided populations. Received: 24 May 1996 / Accepted: 12 July 1996     

Genetic analysis of tuna populations,<Emphasis Type="Italic"> Thunnus thynnus thynnus</Emphasis> and<Emphasis Type="Italic"> T</Emphasis>.<Emphasis Type="Italic"> alalunga</Emphasis>

The genetic population structures of Atlantic northern bluefin tuna ( Thunnus thynnus thynnus) and albacore ( T. alalunga) were examined using allozyme analysis. A total of 822 Atlantic northern bluefin tuna from 18 different samples (16 Mediterranean, 1 East Atlantic, 1 West Atlantic) and 188 albacore from 5 samples (4 Mediterranean, 1 East Atlantic) were surveyed for genetic variation in 37 loci. Polymorphism and heterozygosity reveal a moderate level of genetic variability, with only two highly polymorphic loci in both Atlantic northern bluefin tuna ( FH* and SOD- 1*) and albacore ( GPI- 3* and XDH*). The level of population differentiation found for Atlantic northern bluefin tuna and albacore fits the pattern that has generally been observed in tunas, with genetic differences on a broad rather than a more local scale. For Atlantic northern bluefin tuna, no spatial or temporal genetic heterogeneity was observed within the Mediterranean Sea or between the East Atlantic and Mediterranean, indicating the existence of a single genetic grouping on the eastern side of the Atlantic Ocean. Very limited genetic differentiation was found between West Atlantic and East Atlantic/Mediterranean northern bluefin tuna, mainly due to an inversion of SOD- 1* allele frequencies. Regarding albacore, no genetic heterogeneity was observed within the Mediterranean Sea or between Mediterranean and Azores samples, suggesting the existence of a single gene pool in this area.     

Genetic diversity in two barnacle species, <Emphasis Type="Italic">Chthamalus stellatus</Emphasis> and <Emphasis Type="Italic">Tesseropora atlantica</Emphasis> (Crustacea,Cirripedia), with different larval dispersal modes in the archipelago of the Azores

ISSRs (Inter Simple Sequence Repeats) were employed to compare the genetic structure of Chthamalus stellatus and Tesseropora atlantica in the Azores. The two barnacles differ as regards life cycle: the former conducting pelagic life for about 22 days, the latter for only 24 h. Thirty individuals of each species were analysed from four locations of three Azorean islands. Five ISSR primers produced, respectively, 117 and 79 polymorphic bands in C. stellatus and T. atlantica. Estimates of among-samples genetic diversity suggested extensive connectivity in the former species and isolation in the latter. AMOVA further supported these results by attributing 2% of genetic variance to the among-islands component of C. stellatus and 30% to T. atlantica. Results are consistent with expectations based on species life history and corroborate the importance of pelagic stages in determining the degree of genetic structuring in benthic marine invertebrates.