Genetic divergence of Maldivian and Micronesian demes of the damselfishes Stegastes nigricans,Chrysiptera biocellata,C. glauca and C. leucopoma (Pomacentridae)

Samples representing the damselfishes (Perciformes: Pomacentridae) Stegastes nigricans, Chrysiptera biocellata, C. glauca, and C. leucopoma from North Male Atoll, Republic of Maldives, and Guam, Southern Marianas, were surveyed electrophoretically. A sample representing C. leucopoma from Ngemelis Island, Republic of Palau, was included in the analysis to provide a within-Micronesia comparison. On average, products of 23 presumptive protein-coding loci were examined in each sample. In comparisons of Maldivian and Guamanian samples, absolute or virtually fixed allelic differences were detected at: (1) ADA ^* and PEPS ^* for Stegastes nigricans; (2) ADA ^*, AAT-1 ^*, AAT-2 ^*, and mMDH ^* for C. biocellata, and; (3) ADA ^*, AAT-1 ^*, and AAT-2 ^* for C. glauca. At three polymorphic loci, ADA ^*, sMDH-1 ^*, and sMDH-2 ^*, similar allele frequencies were found in Palauan and Guamanian samples representing C. leucopoma, suggesting that these samples share a gene pool. Significantly different allele frequencies at ADA ^* and sMDH-1 ^* and a virtually fixed allelic difference at sMDH-2 ^* were detected between the Maldivian samples and each of the Micronesian samples representing C. leucopoma. The observed patterns of allele frequency differentiation suggest that Maldivian and Micronesian samples of each of the four study species represent separate demes.     

Genetic population structure of southern bluefin tuna (Thunnus maccoyii )

Samples of southern bluefin tuna, Thunnus maccoyii (Castelnau), taken from off the coasts of South Africa, Western Australia, South Australia and Tasmania from 1992 to 1994 were analysed for six polymorphic allozyme loci (ADA ^*, GDA ^*, GPI-A ^*, MPI ^*, PGDH ^* and PGM-1 ^*, n = 595 to 733 per locus) and for mitochondrial DNA variants revealed by three restriction enzymes (Bam HI, Bcl I and Eco RI) detecting polymorphic cut sites (n = 555). No significant spatial heterogeneity was detected. There were no sex-related differences in allele or mtDNA haplotype frequencies. Juveniles (30 to 35 cm and 46 to 54 cm) from what are thought to be two temporally-separated spawning peaks showed no significant genetic differentiation. There were also no significant differences in allele or haplotype frequencies between fish smaller than 70 cm and those larger than 70 cm. These data are consistent with the null hypothesis of a single unit stock of southern bluefin tuna, with a single spawning area. This is located to the south of Java and off the north-west coast of Australia. Received: 28 August 1996 / Accepted: 30 September 1996     

Allozyme and mitochondrial DNA variation in orange roughy,Hoplostethus atlanticus (Teleostei: Trachichthyidae): little differentiation between Australian and North Atlantic populations

Allozyme and mitochondrial DNA (mtDNA) genetic variation was compared in orange roughy (Hoplostethus atlanticus Collett) collected from waters off southern Australia and from waters about 22 000 km away in the North Atlantic west of Scotland. Samples were screened for 11 polymorphic allozyme loci and with 9 restriction enzymes. Significant heterogeneity between the two areas was detected for three allozyme loci (ADA ^*, CK ^* and GPI-1 ^*), and the overall G _ST (gene-diversity statistic) value of 1% was small but significant. Significant mtDNA haplotype heterogeneity was observed after ²- of haplotype frequencies but not after a G _ST analysis. Nucleotide sequence-diversity analysis showed very low net divergence (0.0023%) between the two samples. The Australian orange roughy had a lower allozyme heterozygosity and a lower mitochondrial DNA nucleon diversity than the North Atlantic sample. The very limited, although significant, allozyme and mitochondrial DNA heterogeneity between these areas suggests that there is some gene flow between these two populations. The species appears to be widespread, with its presence reported from the southern Pacific, southern Indian, and northern and southern Atlantic Oceans, and it is likely that gene flow between the antipodes is mediated by stepping-stone exchange between adjacent populations rather than by direct migration.     

Fixed allele frequency differences among Palauan and Okinawan populations of the damselfishes Chrysiptera cyanea and Pomacentrus coelestis

A survey of intraspecific allozymic variation among samples of the damselfishes Chrysiptera cyanea and Pomacentrus coelestis collected from Palau and Okinawa revealed levels of genetic divergence far in excess of estimates reported previously for populations of coral reef fishes. Absolute or nearly fixed differences in allele frequencies were detected at ADA ^*, sMDH-2 ^*, MEP-1 ^*, PEPB ^*, PEP-LT ^* and sSOD ^*, and at sAAT ^*, GPI-A ^*, LDH-1 ^*, and PEPB ^*, among the geographic samples of C. cyanea and P. coelestis, respectively. Examination of allele frequencies at most other loci (26 for C. cyanea; 24 for P. coelestis) revealed slight differentiation or identical fixation among the geographic samples. The observed patterns of allele frequency distribution suggest the existence of localized demes of these fishes: these demes may be cryptic and/or incipient species. Whether or not speciation is incipient, the observed patterns of allozymic and isozymic variability suggest that natural selection is a factor in the maintenance of population substructuring of the study species. Pronounced allelic differences were highly concentrated at a small number of loci: strongly bimodal frequency distributions of loci relative to their genetic identities (I) were observed for among-population comparisons in both study species. Allozymes encoded by the PEPB ^* locus in samples of the noncongeners from Palau exhibited identical electrophoretic mobilities. In Palauan P. coelestis, mSOD ^* is expressed but sSOD ^* apparently is not, whereas in Okinawan P. coelestis, both mSOD ^* and sSOD ^* are expressed.     

Mitochondrial DNA variation in king mackerel (Scomberomorus cavalla) from the western Atlantic Ocean and Gulf of Mexico

King mackerel (Scomberomorus cavalla Cuvier) collected in 1992 and 1993 from 13 localities along the Atlantic coast of the southeastern USA and in the northern Gulf of Mexico were surveyed for variation in mitochondrial (mt)DNA and a nuclear-encoded dipeptidase locus (PEPA-2). Both polymorphic and fixed mtDNA restriction sites were identified and mapped using conventional and polymerase chain-reaction (PCR)-based methods. Heterogeneity in mtDNA haplotype frequencies was found only in comparisons of pooled haplotypes from Atlantic localities versus pooled haplotypes from Gulf localities. This finding indicates weak genetic divergence between king mackerel from the Atlantic and those from the Gulf. Frequencies of two PEPA-2 alleles essentially paralleled previous findings: one allele (PEPA-2a) was common among samples from western Gulf localities, whereas the other allele (PEPA-2b) was common among samples from Atlantic and eastern Gulf localities. There was considerable variation in PEPA-2 allele frequencies within broadly-defined regions. Variation in mtDNA haplotypes and PEPA-2 genotypes was independent, as was variation in mtDNA haplotypes with sex or age of individuals. Variation in PEPA-2 genotypes was not independent of sex or age of individuals. The latter result suggests that frequencies of PEPA-2 alleles in samples of king mackerel may stem, in part, from sex and age distributions of individuals within samples, and indicates that caution should be exercised in using allelic variation at PEPA-2 as a measure of population (stock) structure in king mackerel. The discordance in spatial patterning of mtDNA haplotypes versus PEPA-2 alleles across the Gulf (i.e. homogeneity in mtDNA haplotype frequencies versus heterogeneity in PEPA-2 allele frequencies) may be due to either female excess at several localities, sex-biased migration, or both. Observed patterns of genetic variation also are consistent with the hypothesis that king mackerel in the western Atlantic may have been subdivided during Pleistocene glaciation, and that the current distribution of PEPA-2 alleles may be a historical artefact. Received: 17 December 1996 / Accepted: 2 April 1997     

Population structure of red porgy, <Emphasis Type="Italic">Pagrus pagrus</Emphasis>, in the Atlantic Ocean

The red porgy, Pagrus pagrus (L.), is a protogynous sparid associated with reefs and hard bottom habitat throughout the warm-temperate Atlantic Ocean. In this study, the degree of geographic population differentiation in Atlantic populations was examined with microsatellite and mitochondrial DNA markers (mtDNA). Six microsatellite loci were amplified and scored in 690 individuals from the eastern North Atlantic (Crete, Madeira, and Azores), western North Atlantic (North Carolina to Florida, and the eastern Gulf of Mexico), and Brazil. At two loci, fixed allelic differences were found among the three major geographic areas, while frequency differences were observed at three other loci. The DNA of 371 individuals was amplified at the mtDNA control region, and 526 bp were sequenced. Tamura–Nei’s D was used as a measure of nucleotide diversity and divergence: diversity averaged 0.011 within samples, while the corrected divergence averaged 0 between samples within the same area and 0.061 between samples from different areas. Transversion haplotype minimum spanning networks, nucleotide divergence, and F _ST values all show that the western Atlantic samples were more closely related to each other than any was to samples from the eastern North Atlantic. Within the western North Atlantic, no significant population differentiation was observed, and within the eastern North Atlantic, only the Azores sample showed detectable differences from Crete and Madeira. These data indicate general homogeneity within large areas, and deep divisions between these areas. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Geographical patterns of variability at allozyme loci in the European oyster Ostrea edulis

The variability of 14 enzyme-coding genes has been analysed in samples from 19 populations of the oyster Ostrea edulis L., collected along the Atlantic and Mediterranean coasts of Europe. We found an abundance of clines, which appeared at 8 loci, including the most polymorphic (AP-2 ^*, ARK ^*, EST-4 ^*, MDH-2 ^*, ME-1 ^*, 6PGH ^*, PGI ^* and PGM ^*). Another 6 loci (ALDH ^*, EST-3 ^*, EST-5 ^*, IDH-2 ^*, MDH-1 ^*, ME-2 ^*) exhibited V-shaped patterns of gene-frequency variation, with clines at one or both sides of the Straits of Gibraltar. The observation of coincident clines at many loci can be explained by a model of secondary intergradation. The geographical location of the midpoints of the clines and V-shaped patterns suggests the existence of two ancient Atlantic and Mediterranean oyster stocks which became differentiated in allopatry and subsequently merged. Clines observed along Atlantic and/or Mediterranean coasts at the loci with V-shaped patterns must have arisen independently. The large heterogeneity observed in the levels of gene differentiation (G _ST ) across loci (G _ST ranged from 0.008 to 0.290) and important differences in estimates of gene flow obtained by different methods suggest that the populations of O. edulis are not in genetic equilibrium. Lack of population equilibrium can be due to natural selection and/or restrictions to gene flow. The average among-population variability was higher than in other oyster species that do not show incubatory habits, and represented 8.8% of the total heterozygosity. Levels of intrapopulation variability were lowest in populations from the North Atlantic, suggesting low population sizes in that area.     

Genetic structure and phylogeography of the lined shore crab, Pachygrapsus crassipes, along the northeastern and western Pacific coasts

Marine invertebrates with high larval dispersal capacity typically exhibit low degrees of population differentiation, which reflects both contemporary and historical processes. We sampled 346 individuals from seven populations of the lined shore crab, Pachygrapsus crassipes Randall, along the northeastern Pacific Coast and Korea during summer 2003. DNA sequence analysis of 613 bp of the mitochondrial COI gene showed that overall gene diversity (h) was high (0.92±0.01), whereas overall nucleotide diversity (π) was low (0.009±0.005). A total of 154 mtDNA haplotypes were identified; however, 114 were present in only one individual. Analysis of molecular variance revealed significant genetic structuring at Point Conception, CA, USA, that is likely due to the oceanographic circulation patterns, which result in asymmetrical migration of haplotypes. However, genetic variation among eastern Pacific populations was generally low, probably because of high contemporary gene flow and recent common ancestry of haplotypes. Mismatch analysis and nested clade analysis suggested that the population history of this region is characterized by two contiguous northwards range expansions, which are congruent with Late Pleistocene glacial cycles. Highly significant genetic differentiation was detected between eastern Pacific populations and Korea, indicating transpacific gene flow is restricted. Time of divergence between the two transpacific lineages was estimated between 0.8 and 1.2 Myrs ago. The small, recently founded population of P. crassipes at Bamfield, BC, Canada, did not appear to have undergone a founder effect.     

Population structure of the wreckfish Polyprion americanus determined with microsatellite genetic markers

 We examined population structure in the wreckfish, Polyprion americanus, by assaying six microsatellite loci in 422 individuals collected throughout the geographic range. Eighteen hapuku, P. oxygeneios, were assayed at the same loci for use as an outgroup. Expected heterozygosities ranged from 0.49 to 0.88 and averaged 0.66. Allele-frequency distributions at those loci indicated that samples from the eastern North Atlantic, western North Atlantic and the Mediterranean were genetically similar, confirming the pattern seen in a previous analysis of mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs). Both mtDNA and microsatellite studies differentiated northern and southern wreckfish stocks. However, in contrast to the mtDNA studies, allelic variation at microsatellite loci clearly differentiated wreckfish from two Southern Hemisphere locations, Brazil and the South Pacific. Far more genetic variation was observed at microsatellite loci than with mtDNA RFLPs (haplotype diversity averaged 0.01), and we saw more evidence of population structure with the microsatellite loci. The differentiation between southern and northern wreckfish supports the distribution records, which indicate that wreckfish do not occur in the tropics. Temperature profiles and current patterns throughout the southern oceans apparently also prevent significant gene flow between the South Pacific and Brazilian samples. Received: 29 January 2000 / Accepted: 27 June 2000     

Distribution and diversity of mtDNA lineages among southern right whales (Eubalaena australis) from Australia and New Zealand

Using a biopsy dart system, samples of skin tissue were collected from southern right whales (Eubalaena australis) in 1995 on two wintering grounds, southwest Australia (n = 20) and the Auckland Islands of New Zealand (n = 20); and on offshore feeding grounds at Latitudes 40 to 43°, south of Western Australia (n = 5). A variable section of the mitochondrial DNA control-region (289 nucleotides) was amplified and sequenced from these 45 individuals (21 males, 20 females and 4 of unknown sex), distinguishing a total of seven unique sequences (i.e. mtDNA haplotypes). Two haplotypes were found on both wintering grounds (including a common type representing 45% of each sample), and five types were unique to only one wintering ground. An analysis of variance adapted for molecular information revealed significant genetic differentiation between the two wintering grounds (p = 0.017). The feeding-ground sample was too small for statistical comparison with the wintering grounds, but included two haplotypes found only in the Auckland Islands as well as the common haplotype found on both wintering grounds. The nucleotide diversity and differentiation of mtDNA among the right whales was similar to that among humpback whales (Megaptera novaeangliae) from the same regions (Baker et al. 1998), but haplotype diversity was significantly reduced, perhaps as a result of more intensive hunting during the last century and continued illegal hunting during this century. Received: 16 March 1998 / Accepted: 18 December 1998     

Molecular genetic variation in tarpon (<Emphasis Type="Italic">Megalops atlanticus</Emphasis> Valenciennes) in the northern Atlantic Ocean

The tarpon (Megalops atlanticus) is a highly valued game fish and occasional food fish in the eastern and western Atlantic Ocean. Tarpon have a high capacity for dispersal, but some regional biological differences have been reported. In this study we used two molecular genetic techniques—protein electrophoresis of nuclear DNA loci, and restriction fragment length polymorphism analysis of the mitochondrial DNA (mtDNA)—to assess this species population genetic structure in the eastern (coastal waters off Gabon and Sierra Leone, Africa) and western (coastal waters off Florida, Caribbean Sea) Atlantic Ocean north of the equator. Genetic differentiation was observed between tarpon from Africa and tarpon from the western Atlantic Ocean. A unique allele and haplotype, significant differences in allozyme allele and mtDNA haplotype frequencies between the African and western Atlantic samples, and significant F_ST analyses suggest that levels of gene flow between tarpon from these two regions is low. Among the western Atlantic Ocean collections, genetic diversity values and allele and haplotype frequencies were similar. AMOVA analyses also showed a degree of genetic relatedness among most of the western Atlantic Ocean collections: however, some significant population structuring was detected in the allozyme data. A regional jackknifed F_ST analysis indicated the distinction of the Costa Rica population from the other western Atlantic populations and, in pairwise analyses, F_ST values tended to be higher (i.e., genetic relatedness was lower) when the Costa Rican sample was paired with any of the other western Atlantic samples. These data suggest that Costa Rican tarpon could be partially isolated from other western Atlantic tarpon populations. Ultimately, international cooperation will be essential in the management of this species in both the eastern and western Atlantic Ocean.Communicated by P.W. Sammarco, Chauvin     

Population genetics of the nurse shark (Ginglymostoma cirratum) in the western Atlantic

The nurse shark, Ginglymostoma cirratum, inhabits shallow, tropical, and subtropical waters in the Atlantic and the eastern Pacific. Unlike many other species of sharks, nurse sharks are remarkably sedentary. We assayed the mitochondrial control region and eight microsatellite loci from individuals collected primarily in the western Atlantic to estimate the degree of population subdivision. Two individuals from the eastern Atlantic and one from the Pacific coast of Panama also were genotyped. Overall, the mtDNA haplotype (h = 48 ± 5%) and nucleotide (π = 0.08 ± 0.06%) diversities were low. The microsatellite data mirror the mitochondrial results with the average number of alleles ([`(N)]<sub>A</sub> \bar{N}_{A}  = 9) and observed heterozygosity ([`(H)]_O \bar{H}_{O}  = 0.58) both low. The low levels of diversity seen in both the mtDNA and the microsatellite may be due to historical sea level fluctuations and concomitant loss of shallow water habitat. Eight of the 10 pair-wise western Atlantic F _ST estimates for mtDNA indicated significant genetic subdivision. Pair-wise F _ST values for the microsatellite loci indicated a similar pattern as the mtDNA. The western Atlantic population of nurse sharks is genetically subdivided with the strongest separation seen between the offshore islands and mainland Brazil, likely due to deep water acting as a barrier to dispersal. The eastern and western Atlantic populations were closely related. The eastern Pacific individual is quite different from Atlantic individuals and may be a cryptic, sister species.     

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.     

Phylogeography of the Southwestern Atlantic menhaden genus <Emphasis Type="Italic">Brevoortia</Emphasis> (Clupeidae,Alosinae)

Among pelagic fish, the Southwestern Atlantic menhaden genus Brevoortia (Clupeidae, Alosinae) constitutes an important species model to investigate the patterns of genetic differentiation. It is abundant in the Río de la Plata estuary and in the Atlantic coastal lagoons system from Uruguay and Southern Brazil. To access in the taxa discrimination and population structure in Brevoortia we perform a phylogeographic approach based on mitochondrial cytochrome b (cyt-b) sequences including 240 individuals from 16 collecting sites. Among the 720 bp cyt-b sequenced, 199 correspond to variables and 88 to phylogenetically informative sites. High values of haplotype diversity (h = 1.000) and nucleotide diversity (π = 0.061), as well as an average of 0.084 polymorphic segregating sites and 46 different haplotypes were found. Maximum likelihood analysis based on the GTR + I + G model and Bayesian inference strongly support the idea that B. aurea is the only species of the genus inhabiting the Southwestern Atlantic region. Our analyses revealed a complex population pattern characterized by the existence of long-term highly structured genetic assemblages of mixed stocks. Each monophyletic entity included individuals from different collecting sites, different age groups and collected in different years. Our data also suggest that the recruitment of unrelated mtDNA haplotypes carried out by individuals within schools could be occurring in the same nursery areas revealing the existence of many different maternal lineages. A scenario where different simultaneously and successively mixed mtDNA lineages remain historically connected through basal haplotypes among different clades could explains more accurately the complex and ordered metapopulation dynamic found in this pelagic fish.     

Low levels of genetic variation in mtDNA sequences over the western Mediterranean and Atlantic range of the sponge<Emphasis Type="Italic"> Crambe crambe</Emphasis> (Poecilosclerida)

Crambe crambe is a common encrusting sponge found in the Mediterranean and Atlantic littoral. An analysis of a partial sequence (535 bp) of the mitochondrial DNA (mtDNA) gene cytochrome oxidase subunit I (COI) was conducted in an attempt to determine population structure in this species. This is the first study of population genetics using this kind of marker in the phylum. Samples (N=86) were taken in eight populations separated by distances from 20 to 3,000 km, spanning from the western Mediterranean to the Atlantic. Low variability of this gene was found, as only two haplotypes were identified, along with low nucleotide diversity (=0.0006). However, the different frequencies found among populations revealed genetic structure and low gene flow between close populations, as expected from the dispersal biology of the species. The low variability found in sponges is in agreement with reports on cnidarians and points to a high conservation of mtDNA in diploblastic phyla.     

Phylogeography of two Atlantic squirrelfishes (Family Holocentridae): exploring links between pelagic larval duration and population connectivity

Genetic surveys of reef fishes have revealed high population connectivity within ocean basins, consistent with the assumption that pelagic larvae disperse long distances by oceanic currents. However, several recent studies have demonstrated that larval retention and self-recruitment may be higher than previously expected. To assess connectivity in tropical reef fishes, we contribute range-wide mtDNA surveys of two Atlantic squirrelfishes (family Holocentridae). The blackbar soldierfish, Myripristis jacobus, has a pelagic juvenile phase of about 58 days, compared to about 71 days (~22% longer) in the longjaw squirrelfish, Holocentrus ascensionis. If the pelagic duration is guiding dispersal ability, M. jacobus should have greater population genetic structure than H. ascensionis. In comparisons of mtDNA cytochrome b sequences from 69 M. jacobus (744 bp) and 101 H. ascensionis (769 bp), both species exhibited a large number of closely related haplotypes (h=0.781 and 0.974, π=0.003 and 0.006, respectively), indicating late Pleistocene coalescence of mtDNA lineages. Contrary to the prediction based on pelagic duration, M. jacobus has much less population structure (φ_ST=0.008, P=0.228) than H. ascensionis (φ_ST=0.091, P<0.001). Significant population partitions in H. ascensionis were observed between eastern, central and western Atlantic, and between Brazil and the Caribbean in the western Atlantic. These results, in combination with the findings from 13 codistributed species, indicate that pelagic larval duration is a poor predictor of population genetic structure in Atlantic reef fishes. A key to understanding this disparity may be the evolutionary depth among corresponding taxonomic groups of “reef fishes”, which extends back to the mid-Cretaceous and encompasses enormous diversity in ecology and life history. We should not expect a simple relationship between pelagic larval duration and genetic connectivity, among lineages that diverged 50–100 million years ago.     

Microgeographic heterogeneity in spatial distribution and mtDNA variability of gray mouse lemurs (<Emphasis Type="Italic">Microcebus murinus</Emphasis>, Primates: Cheirogaleidae)

The objective of our study was to investigate the spatial distribution and genetic structure of a solitary primate at the microgeographical scale of adjacent local populations. We obtained spatial data and tissue samples for mtDNA analysis from 205 gray mouse lemurs (Microcebus murinus) captured along transects and within 3 grid systems within a 12.3 km² area in Kirindy Forest, western Madagascar. Our capture data revealed that, even though the forest was continuous, gray mouse lemurs were not evenly distributed, and that daily and maximum dispersal distances were significantly greater in males. The frequency distribution of 22 mtDNA D-loop haplotypes was highly skewed. Nine haplotypes were unique to males, indicating male-mediated gene flow from surrounding areas. The geographic distribution of haplotypes revealed that males were also more dispersed than females. Females with the same haplotype showed a tendency towards spatial aggregation, and the correlation between genetic and geographic distances was higher in females. In several areas of the forest, however, spatially clustered females were not of the same haplotype, and females were not always found in clusters. Hence, in contrast to suggestions from previous studies, matrilineal clustering is not the only way females are socially organized. In addition, our study revealed heterogeneity and patterns in population structure that were not evident at smaller spatial scales, some of which may be relevant for designing conservation strategies.Communicated by C. Nunn     

Inter-ocean genetic divergence of istiophorid billfishes

Restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA) was used to investigate the taxonomic status of the following species-pairs of Atlantic and Indo-Pacific istiophorid billfishes: Atlantic blue marlin Makaira nigricans (Lacépède) and Indo-Pacific blue marlin M. mazara (Jordan and Snyder); Atlantic sailfish Istiophorus albicans (Latreille) and Indo-Pacific sailfish I. platypterus (Shaw and Nodder); and white marlin Tetrapturus albidus Poey and striped marlin T. audax (Phillippi). Tissue samples were collected from 1990 to 1992. Several mtDNA haplotypes were common to Atlantic and Indo-Pacific samples of blue marlin and sailfish, although there were significant differences in the distribution of haplotypes between samples from different oceans. For both blue marlin and sailfish, a single group of closely related mtDNA haplotypes was found among all indo-Pacific and some Atlantic individuals, while the remaining Atlantic specimens exhibited mtDNA haplotypes that differed by several consistent restriction site changes from the common haplotype. No restriction site differences were found to discriminate white marlin from striped marlin, and the mtDNA haplotypes of both species were very similar although significant differences were found in the distribution of haplotypes between the two species. Two of 26 haplotypes were shared between white and striped marlin, and the corrected mean nucleotide sequence divergence between species (0.06%) was not much greater than that observed between geographically distant samples of striped marlin from the Pacific Ocean (mean 0.03%). The presence of identical haplotypes in samples from both oceans for each of the three species-pairs of istiophorid billfishes suggests that specific status may not be warranted for any of the Atlantic and Indo-Pacific populations. The significant difference in the distributions of mtDNA haplotypes between Atlantic and Indo-Pacific populations, which contrasts sharply with the homogeneity reported for several species of tunas, indicates considerable population structuring within the highly vagile billfishes.     

Genetic evidence of cryptic speciation within hammerhead sharks (Genus Sphyrna)

Surveys of genetic variation within cosmopolitan marine species often uncover deep divergences, indicating historical separation and potentially cryptic speciation. Based on broad geographic (coastal eastern North America, Gulf of Mexico, western Africa, Australia, and Hawaii) and temporal sampling (1991–2003), mitochondrial (control region [CR] and cytochrome oxidase I [COI]) and nuclear gene (lactate dehydrogenase A intron 6 [LDHA6]) variation among 76 individuals was used to test for cryptic speciation in the scalloped hammerhead, Sphyrna lewini (Griffith and Smith). CR and COI gene trees confirmed previous evidence of divergence between Atlantic and Indo-Pacific scalloped hammerhead populations; populations were reciprocally monophyletic. However, the between-basin divergence recorded in the mtDNA genome was not reflected in nuclear gene phylogenies; alleles for LDHA6 were shared between ocean basins, and Atlantic and Indo-Pacific populations were not reciprocally monophyletic. Unexpectedly, CR, COI, and LDHA6 gene trees recovered a deep phylogenetic partition within the Atlantic samples. For mtDNA haplotypes, which segregated by basin, average genetic distances were higher among Atlantic haplotypes (CR: D _HKY=0.036, COI: D _GTR=0.016) than among Indo-Pacific haplotypes (CR: D _HKY=0.010, COI: D _GTR=0.006) and approximated divergences between basins for CR (D _HKY=0.036 within Atlantic; D _HKY=0.042 between basins). Vertebral counts for eight specimens representing divergent lineages from the western north Atlantic were consistent with the genetic data. Coexistence of discrete lineages in the Atlantic, complete disequilibrium between nuclear and mitochondrial alleles within lineages and concordant partitions in genetic and morphological characters indicates reproductive isolation and thus the occurrence of a cryptic species of scalloped hammerhead in the western north Atlantic. Effective management of large coastal shark species should incorporate this important discovery and the inference from sampling that the cryptic scalloped hammerhead is less abundant than S. lewini, making it potentially more susceptible to fishery pressure.     

Phylogeography of the ocean quahog (Arctica islandica): influences of paleoclimate on genetic diversity and species range

The ocean quahog, Arctica islandica (Linnaeus, 1767), is a commercially important bivalve found on continental shelves throughout much of the North Atlantic. To assess genetic subdivision in this species, we sequenced 385 nucleotides of the mitochondrial cytochrome b (cyt b) gene from 83 specimens collected from 12 localities between September 1998 and July 1999 (based on preliminary data, the Internal Transcribed Spacers, ITS, of the nuclear ribosomal repeat were not useful). The cyt b data delimited 11 haplotypes with 0.26 to 8.1% nucleotide difference (coded by 36 variable nucleotide positions) among them. Only three haplotypes were detected in 39 specimens collected along the USA coastline, compared to five haplotypes from nine Icelandic individuals. The western Atlantic populations ranging from Penobscot Bay (Maine, USA) to southern Virginia showed relatively low diversity and appeared genetically similar in that region. Based on the presence of shared haplotypes, AMOVA analyses, and phylogenetic reconstructions, Icelandic populations appear to be more genetically similar to western Atlantic populations than eastern Atlantic populations. Specimens from the Faroe Islands (n=4) show mixed affinities. These data are consistent with the hypothesis that a warm Holocene climatic optimum (ca. 7,500 years BP), and not glacial refugia, shaped the present-day genetic structure in A. islandica. Received: 18 January 2000 / Accepted: 26 June 2000