- Gloucester Point, Virginia, United States
Jan McDowell
Virginia Institute of Marine Science, Fisheries Science, Faculty Member
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Page 1. Marine Biology (1995) 122:193-203 9 Springer-Verlag 1995 JE Graves 9 JR McDowell Inter-ocean genetic divergence of istiophorid billfishes Received: 25 October 1994/Accepted: 30 November 1994 Abstract Restriction ...
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Research Interests: Genetics, Zoology, Phylogeography, Biology, Ecology, and 13 morePopulation structure, Mitochondrial DNA, Population, Genetic Structure, Microsatellite, Fisheries Sciences, Analysis of Molecular Variance, Genetic Differentiation, Control Region, Pacific ocean, Genetic variation, microsatellite loci, and mtDNA control region
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Restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA) was used to determine the magnitude of intraspecific genetic differentiation among samples of striped marlin (Tetrapturus audax). Composite genotypes,... more
Restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA) was used to determine the magnitude of intraspecific genetic differentiation among samples of striped marlin (Tetrapturus audax). Composite genotypes, representing information from 11 restriction endonucleases, were generated for samples of approximately 40 striped marlin each from Mexico, Ecuador, Australia, and Hawaii. Genotypic variation was relatively high in all samples: the pooled nucleon diversity was 0.82 (sample range 0.69–0.84), and the pooled nucleotide sequence diversity was 0.30% (sample range 0.20–0.32%). Values of corrected mean nucleotide sequence divergence between samples were low (0.01–0.06%, mean 0.03%). Despite low intersample divergences, significant heterogeneity was observed in the distribution of composite mtDNA genotypes among samples. Each location was characterized by the presence of unique genotypes at elevated frequencies. Samples taken from the same location 1 yr apart demonstrated temporal stability of the distribution of genotypes at that site. These results demonstrate significant spatial partitioning of genetic variation within striped marlin, a situation that sharply contrasts with similar studies of several species of tuna. Management should focus on units smaller than those currently used to conserve unique genetic variation within the species.
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Different classes of molecular markers occasionally yield discordant views of population structure within a species. Here, we examine the distribution of molecular variance from 14 polymorphic loci comprising four classes of molecular... more
Different classes of molecular markers occasionally yield discordant views of population structure within a species. Here, we examine the distribution of molecular variance from 14 polymorphic loci comprising four classes of molecular markers within ≈ 400 blue marlin individuals (Makaira nigricans). Samples were collected from the Atlantic and Pacific Oceans over 5 years. Data from five hypervariable tetranucleotide microsatellite loci and restriction fragment length polymorphism (RFLP) analysis of whole molecule mitochondrial DNA (mtDNA) were reported and compared with previous analyses of allozyme and single‐copy nuclear DNA (scnDNA) loci. Temporal variance in allele frequencies was nonsignificant in nearly all cases. Mitochondrial and microsatellite loci revealed striking phylogeographic partitioning among Atlantic and Pacific Ocean samples. A large cluster of alleles was present almost exclusively in Atlantic individuals at one microsatellite locus and for mtDNA, suggesting that, if gene flow occurs, it is likely to be unidirectional from Pacific to Atlantic oceans. Mitochondrial DNA inter‐ocean divergence (FST) was almost four times greater than microsatellite or combined nuclear divergences including allozyme and scnDNA markers. Estimates of Neu varied by five orders of magnitude among marker classes. Using mathematical and computer simulation approaches, we show that substantially different distributions of FST are expected from marker classes that differ in mode of inheritance and rate of mutation, without influence of natural selection or sex‐biased dispersal. Furthermore, divergent FST values can be reconciled by quantifying the balance between genetic drift, mutation and migration. These results illustrate the usefulness of a mitochondrial analysis of population history, and relative precision of nuclear estimates of gene flow based on a mean of several loci.
Research Interests: Genetics, Biology, Medicine, Molecular Ecology, Biological Sciences, and 15 moreMolecular, mtDNA, Mitochondrial DNA, Computer Simulation, Haplotypes, Atlantic Ocean, Animals, Fishes, Microsatellite, Biological evolution, Molecular Marker, Linkage Disequilibrium, Genetic variation, Microsatellite DNA, and Genetic Markers
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In the Unites States, blue marlin, Makaira nigricans, is managed as separate Atlantic and Indo-Pacific stocks. To facilitate the discrimination of the two stocks for enforcement purposes, we developed 10 new microsatellite markers and... more
In the Unites States, blue marlin, Makaira nigricans, is managed as separate Atlantic and Indo-Pacific stocks. To facilitate the discrimination of the two stocks for enforcement purposes, we developed 10 new microsatellite markers and screened collections of 20 individuals each from Ghana and Hawai’i. Observed heterozygosities (HO) ranged from 0.700 to 1.000 among loci at the two collections. The number
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Large pelagic fishes are often broadly distributed and capable of long-distance movements. These factors can promote gene flow that makes it difficult to disentangle intra- vs. inter-specific levels of genetic differentiation. Here, we... more
Large pelagic fishes are often broadly distributed and capable of long-distance movements. These factors can promote gene flow that makes it difficult to disentangle intra- vs. inter-specific levels of genetic differentiation. Here, we assess the relationship of two istiophorid billfishes, white marlin (Kajikia albida) and striped marlin (K. audax), presently considered sister species inhabiting separate ocean basins. Previous studies report levels of genetic differentiation between these species that are smaller than those observed among populations of other istiophorid species. To determine whether white marlin and striped marlin comprise separate species or populations of a single globally distributed species, we surveyed 2520 single nucleotide polymorphisms (SNPs) in 62 white marlin and 242 striped marlin across the Atlantic, Pacific, and Indian oceans. Multivariate analyses resolved white marlin and striped marlin as distinct groups, and a species tree composed of separate line...
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Globally, tunas are among the most valuable fish stocks, but are also inherently difficult to monitor and assess. Samples of larvae of Western Atlantic bluefin tuna Thunnus thynnus (Linnaeus, 1758) from standardized annual surveys in the... more
Globally, tunas are among the most valuable fish stocks, but are also inherently difficult to monitor and assess. Samples of larvae of Western Atlantic bluefin tuna Thunnus thynnus (Linnaeus, 1758) from standardized annual surveys in the northern Gulf of Mexico provide a potential source of “offspring” for close-kin mark-recapture (CKMR) estimates of abundance. However, the spatial patchiness and highly skewed numbers of larvae per tow suggest sampled larvae may come from a small number of parents, compromising the precision of CKMR. We used high throughput genomic profiling to study sibship within and among larval tows from the 2016 standardized Gulf-wide survey compared to targeted sampling carried out in 2017. Full- and half-siblings were found within both years, with 12% of 156 samples in 2016 and 56% of 317 samples in 2017 having at least one sibling. There were also two pairs of cross cohort half-siblings. Targeted sampling increased the number of larvae collected per sampling...
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The Echeneoidea comprise three families of cosmopolitan tropical/subtropical marine fishes: the Echeneidae (remoras), Coryphaenidae (dolphinfishes), and Rachycentridae (cobia). Complete nucleotide sequences from the mitochondrial 12S... more
The Echeneoidea comprise three families of cosmopolitan tropical/subtropical marine fishes: the Echeneidae (remoras), Coryphaenidae (dolphinfishes), and Rachycentridae (cobia). Complete nucleotide sequences from the mitochondrial 12S rRNA, 16S rRNA, protein-coding ND2, and nuclear ITS-1 gene regions were used to reconstruct the phylogenetic history of these fishes. Parsimony, maximum likelihood, and Bayesian analyses of combined data sets resolved trees of similar topology. Congruent with evolutionary hypotheses based on larval morphology, a monophyletic Rachycentridae + Coryphaenidae was resolved with high support. Within a monophyletic Echeneidae, the subfamilies Echeneinae and Remorinae were monophyletic. In agreement with recent morphological analyses, the genus Remora was paraphyletic based on the position of Remorina albescens Temminck and Schlegel, 1850. Consistent resolution within the Remorinae using parsimony, maximum likelihood, and Bayesian inference was not achieved wit...
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SUMMARY The genetic basis of stock structure of Atlantic sailfish (Istiophorus platypterus) was investigated using analyses of the mitochondrial control region and three nuclear microsatellite loci. Relatively robust samples (n= 293) from... more
SUMMARY The genetic basis of stock structure of Atlantic sailfish (Istiophorus platypterus) was investigated using analyses of the mitochondrial control region and three nuclear microsatellite loci. Relatively robust samples (n= 293) from 6 locations throughout the Atlantic were screened for variation at these four hypervariable gene regions. While considerable variation was revealed, almost all of the variation was present in individual samples. The null hypothesis that samples were drawn from a common gene pool could not be rejected when each sample location was considered separately, or when collections were combined into eastern and western Atlantic samples. The apparent genetic homogeneity reinforces recent reports on the distribution of sailfish across the Atlantic and calls to question the current application of eastern and western Atlantic stocks. RESUME
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Independent molecular markers based on mitochondrial and nuclear DNA were developed to provide positive identification of istiophorid and xiphiid billfishes (marlins, spearfishes, sailfish, and swordfish). Both classes of markers were... more
Independent molecular markers based on mitochondrial and nuclear DNA were developed to provide positive identification of istiophorid and xiphiid billfishes (marlins, spearfishes, sailfish, and swordfish). Both classes of markers were based on amplification of short segments (<1.7 kb) of DNA by the polymerase chain reaction and subsequent digestion with informative restriction endonucleases. Candidate markers were evaluated for their ability to discriminate among the different species and the level of intraspecific variation they exhibited. The selected markers require no more than two restriction digestions to allow unambiguous identification, although it was not possible to distinguish between white marlin and striped marlin with any of the genetic characters screened in our study. Individuals collected from throughout each species’ range were surveyed with the selected markers demonstrating low levels of intraspecific character variation within species. The resulting keys prov...
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Nucleotide sequences from the hypervariable mitochondrial control region were used to investigate phylogeographic structuring in the marlinsucker, Remora osteochir (Cuvier, 1829). Complete DNA sequences were isolated from 71 individuals... more
Nucleotide sequences from the hypervariable mitochondrial control region were used to investigate phylogeographic structuring in the marlinsucker, Remora osteochir (Cuvier, 1829). Complete DNA sequences were isolated from 71 individuals collected from seven geographically distant sample locations (5 Atlantic and 2 Pacific). Analyses of molecular variance (AMOVA) and ?st resolved significant levels of population structuring among collections from the Atlantic and Pacific oceans, whereas negligible levels of population structuring were resolved among collections from within the Atlantic or Pacific oceans. Cluster analysis of haplotypes based upon inter-haplotype divergences resolved two distinct evolutionary lineages, one composed of haplotypes that only occurred in the Atlantic, and one composed of haplotypes that occurred in both Atlantic and Pacific samples. Observations are consistent with vicariant isolation of Atlantic and Pacific marlinsucker, followed by geologically recent se...
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Escolar (Lepidocybium Xavobrunneum) is a large, mesopelagic Wsh that inhabits tropical and temperate seas throughout the world, and is a common bycatch in pelagic longline Wsheries that target tuna and swordWsh. Few studies have explored... more
Escolar (Lepidocybium Xavobrunneum) is a large, mesopelagic Wsh that inhabits tropical and temperate seas throughout the world, and is a common bycatch in pelagic longline Wsheries that target tuna and swordWsh. Few studies have explored the biology and natural history of escolar, and little is known regarding its population structure. To evaluate the genetic basis of population structure of escolar throughout their range, we surveyed genetic variation over an 806 base pair fragment of the mitochondrial control region. In total, 225 individuals from six geographically distant locations throughout the Atlantic (Gulf of Mexico, Brazil, South Africa) and PaciWc (Ecuador, Hawaii, Australia) were analyzed. A neighbor-joining tree of haplotypes based on maximum likelihood distances revealed two highly divergent clades ( = 4.85%) that were predominantly restricted to the Atlantic and Indo-PaciWc ocean basins. All Atlantic clade individuals occurred in the Atlantic Ocean and all but four Pa...
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Billfishes are genetically and morphologically distinct enough from scombroids to merit placement in a separate suborder, Xiphioidei. Two extant families are usually recognized: Xiphiidae (swordfish, Xiphias) and Istiophoridae, currently... more
Billfishes are genetically and morphologically distinct enough from scombroids to merit placement in a separate suborder, Xiphioidei. Two extant families are usually recognized: Xiphiidae (swordfish, Xiphias) and Istiophoridae, currently containing three genera, Istiophorus (sailfishes), Makaira (marlins), and Tetrapturus (spear fishes, white, and striped marlins). Phylogenetic analyses of molecular data from mi tochondrial and nuclear gene sequences (mitochondrial control region, ND2, 12S, and nuclear MN 32 regions) show a different picture of relationships. Makaira is not monophyletic: blue marlin cluster with sailfish and placement of black marlin is un stable. Accepting the molecular phylogeny gives two possible classifications: (1) two genera: blue marlin + sailfish (as Istiophorus) and all the rest (as Tetrapturus), or (2) five genera: blue marlin (), sailfish (), black marlin ( Istiompax), striped and white marlin (Kajikia), and four spearfishes ( Tetrapturus). We prefer the ...
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SUMMARY Investigations of the stock structures of blue marlin (Makaira nigricans) and white marlin (Tetrapturus albidus) within the Atlantic Ocean using analyses of mitochondrial (mt) DNA, single copy nuclear (scn) DNA, and microsatellite... more
SUMMARY Investigations of the stock structures of blue marlin (Makaira nigricans) and white marlin (Tetrapturus albidus) within the Atlantic Ocean using analyses of mitochondrial (mt) DNA, single copy nuclear (scn) DNA, and microsatellite DNA are summarized. The levels of variation revealed by the different molecular methodologies varied between species and across molecular markers. In general, variation was very high for both mtDNA and the microsatellite loci. ScnDNA loci were less variable, but sufficiently polymorphic for analyses of population structure. With one exception, analyses of samples from the same location taken in different years did not reveal significant heterogeneity for any of the molecular markers, and allowed us to pool temporal samples, thereby increasing the power of subsequent analyses of spatial heterogeneity. Significant heterogeneity in the distribution of allelic variants among Atlantic sampling locations of either species was not detected for any of the ...
The identification of larval istiophorid billfishes from the western North Atlantic Ocean has long been problematic. In the present study, a molecular technique was used to positively identify 27 larval white marlin (Tetrapturus albidus),... more
The identification of larval istiophorid billfishes from the western North Atlantic Ocean has long been problematic. In the present study, a molecular technique was used to positively identify 27 larval white marlin (Tetrapturus albidus), 96 larval blue marlin (Makaira nigricans), and 591 larval sailfish (Istiophorus platypterus) from the Straits of Florida and the Bahamas. Nine morphometric measurements were taken for a subset of larvae (species known), and lower jaw pigment patterns were recorded on a grid. Canonical variates analysis (CVA) was used to reveal the extent to which the combination of morphometric, pigment pattern, and month of capture information was diagnostic to species level. Linear regression revealed species-specific relationships between the ratio of snout length to eye orbit diameter and standard length (SL). Confidence limits about these relationships served as defining characters for sailfish >10 mm SL and for blue and white marlin >17 mm SL. Pigment p...
1SUMMARY The recent validation of the roundscale spearfish (Tetrapturus georgii) from the western North Atlantic indicates that historical landings of the morphologically similar white marlin (Kajikia albida) likely include misclassified... more
1SUMMARY The recent validation of the roundscale spearfish (Tetrapturus georgii) from the western North Atlantic indicates that historical landings of the morphologically similar white marlin (Kajikia albida) likely include misclassified roundscale spearfish. The objective of this study was to use genetic characters to determine interannual variation in the proportion of roundscale spearfish and white marlin from archived tissue samples collected at a fishing tournament held in Cape May, New Jersey (USA) during the third week of August each year for the past 19 years. Overall, roundscale spearfish comprised 21.8% of the two species combined but there was significant variation in the proportion of the two species over the course of the time series. Roundscale spearfish comprised only 2.1% of the combined species from 1992-2000, but represented 33.3% of the combined species from 2002-2010. There was also considerable yearto-year variation in the relative proportion of the two species ...
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The billfishes are a group of highly migratory fishes comprising two extant families. The family Xiphiidae is monotypic and contains only the broadbill swordfish (Xiphias gladius Linnaeus, 1758), while the nine-other species of billfishes... more
The billfishes are a group of highly migratory fishes comprising two extant families. The family Xiphiidae is monotypic and contains only the broadbill swordfish (Xiphias gladius Linnaeus, 1758), while the nine-other species of billfishes constitute the family Istiophoridae. The outcomes of past phylogenetic investigations using partial nuclear and mitochondrial (mtDNA) sequences recommended a restructuring of Istiophoridae, by expanding the three existing genera into five: Makaira, Istiophorus, Kajikia, Istiompax, and Tetrapturus. However, several aspects of the Istiophoridae phylogeny remain unresolved, including the position of the genus Istiompax. In this investigation, we sequenced the whole mitogenomes for all istiophorid billfish species. Phylogenetic analyses supported the presence of five genera within Istiophoridae and resolved the placement of Istiompax as the sister taxon to Kajikia + Tetrapturus. Genetic identity between the striped marlin [Kajikia audax (Philippi, 1887)] and the white marlin [Kajikia albida (Poey, 1860)] was found to be higher than between Indo-Pacific and Atlantic clades of both the blue marlin (Makaira nigricans Lacepede, 1802) and sailfish [Istiophorus platypterus (Shaw, 1792)], suggesting recent isolation or ongoing genetic connectivity between K. audax and K. albida. An assessment of mtDNA gene regions identified NADH dehydrogenase subunit 4 (ND4) as best reflecting the phylogenetic reconstruction using the whole mitogenomes, while cytochrome b (Cytb) was the only gene region able to separate all species of billfishes in the present study. Our investigation increased the phylogenetic resolution of istiophorid billfishes and underscores the need for further investigations to fully resolve the complex genetic relationships within Tetrapturus and Kajikia.
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In the Unites States, blue marlin, Makaira nigricans, is managed as separate Atlantic and Indo-Pacific stocks. To facilitate the discrimination of the two stocks for enforcement purposes, we developed 10 new microsatellite markers and... more
In the Unites States, blue marlin, Makaira nigricans, is managed as separate Atlantic and Indo-Pacific stocks. To facilitate the discrimination of the two stocks for enforcement purposes, we developed 10 new microsatellite markers and screened collections of 20 individuals each from Ghana and Hawai’i. Observed heterozygosities (HO) ranged from 0.700 to 1.000 among loci at the two collections. The number of alleles per locus ranged from 14 to 24, there were no significant deviations from the expectations of Hardy–Weinberg equilibrium, and there was no evidence of linkage disequilibrium among the loci. These new markers will provide more power to assign marketed blue marlin products to ocean origin.
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Atlantic menhaden (Brevoortia tyrannus), through landings, support one of the largest commercial fisheries in the United States. Recent consolidation of the once coast-wide reduction fishery to waters within and around Chesapeake Bay has... more
Atlantic menhaden (Brevoortia tyrannus), through landings, support one of the largest commercial fisheries in the United States. Recent consolidation of the once coast-wide reduction fishery to waters within and around Chesapeake Bay has raised concerns over the possibility of the loss of unique genetic variation resulting from concentrated fishing pressure. To address this question, we surveyed variation at the mitochondrial cytochrome c oxidase subunit I (COI) gene region and seven nuclear microsatellite loci to evaluate stock structure of Atlantic menhaden. Samples were collected from up to three cohorts of Atlantic menhaden at four geographic locations along the U.S. Atlantic coast in 2006 and 2007, and from the closely related Gulf menhaden (B. patronus) in the Gulf of Mexico. Genetic divergence between Atlantic menhaden and Gulf menhaden, based on the COI gene region sequences and microsatellite loci, was more characteristic of conspecific populations than separate species. Hi...
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We studied the effects of metabolic cold adaptation (MCA) in two populations of a eurythermal species, spotted seatrout (Cynoscion nebulosus) along the U.S. East Coast. Fish were captured from their natural environment and acclimated at... more
We studied the effects of metabolic cold adaptation (MCA) in two populations of a eurythermal species, spotted seatrout (Cynoscion nebulosus) along the U.S. East Coast. Fish were captured from their natural environment and acclimated at control temperatures 15 °C or 20 °C. Their oxygen consumption rates, a proxy for metabolic rates, were measured using intermittent flow respirometry during acute temperature decrease or increase (2.5 °C per hour). Mass-specific standard metabolic rates (SMR) were higher in fish from the northern population across an ecologically relevant temperature gradient (5 °C to 30 °C). SMR were up to 37% higher in the northern population at 25 °C and maximum metabolic rates (MMR) were up to 20% higher at 20 °C. We found evidence of active metabolic compensation in the southern population from 5 °C to 15 °C (Q10 < 2), but not in the northern population. Taken together, our results indicate differences in metabolic plasticity between the northern and southern ...
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Highly migratory, cosmopolitan oceanic sharks often exhibit complex movement patterns influenced by ontogeny, reproduction, and feeding. These elusive species are particularly challenging to population genetic studies, as representative... more
Highly migratory, cosmopolitan oceanic sharks often exhibit complex movement patterns influenced by ontogeny, reproduction, and feeding. These elusive species are particularly challenging to population genetic studies, as representative samples suitable for inferring genetic structure are difficult to obtain. Our study provides insights into the genetic population structure one of the most abundant and wide-ranging oceanic shark species, the blue shark Prionace glauca, by sampling the least mobile component of the populations, i.e., young-of-year and small juveniles (<2 year; N = 348 individuals), at three reported nursery areas, namely, western Iberia, Azores, and South Africa. Samples were collected in two different time periods (2002-2008 and 2012-2015) and were screened at 12 nuclear microsatellites and at a 899-bp fragment of the mitochondrial control region. Our results show temporally stable genetic homogeneity among the three Atlantic nurseries at both nuclear and mitocho...