We assessed genetic variation, using microsatellite markers, in 14 populations of northern pike (Esox lucius) in the North Central United States and in six populations from Quebec, Alaska, Siberia, and Finland. Eight of 13 loci examined were polymorphic in at least one population with an average heterozygosity at all loci and across all populations of 0.14. The R st and F st values indicated differentiation among populations (R st = 0.61, F st = 0.42). Although microsatellite variation found in northern pike was much lower than that found in sympatric and other fish species, the polymorphisms differentiated populations of greater geographical proximity than was possible in prior studies using allozymes and mitochondrial DNA. We generated UPGMA-clustering phenograms based on five genetic distance measures with 2000 bootstrap replicates per measure. All measures yielded highly repeatable population structure between continents (supporting values = 92.4-100%) and within Finland (42.3-98%). Four measures differentiated the Alaskan population and Young Lake (Great Lakes drainage) from other North American populations (56.6-87.7%). Relationships among other North Central United States populations were unclear, as indicated by low supporting values. Results support the hypotheses of one refugium in the North Central United States and more than one refugium in Europe during the last glaciation.Résumé : Nous avons évalué, au moyen de marqueurs microsatellites, la variation génétique dans 14 populations de grand brochet du centre-nord des États-Unis et dans six populations du Québec, de l'Alaska, de la Sibérie et de la Finlande. Huit des 13 loci examinés étaient polymorphes dans au moins une population, et l'hétérozygotie moyenne pour l'ensemble des loci de toutes les populations était de 0,14. Les valeurs de R st et F st indiquaient une différenciation parmi les populations (R st = 0,61, F st = 0,42). Alors que la variation des microsatellites observée chez le grand brochet était de beaucoup inférieure à celle observée chez des espèces sympatriques et chez d'autres espèces, notre examen des polymorphismes a permis de différencier des populations plus rapprochées géographiquement que celles différenciées dans des études antérieures utilisant des allozymes et l'ADN mitochondrial. Nous avons produit des phénogrammes par groupement UPGMA à partir de cinq mesures de la distance génétique, avec 2 000 réplicats bootstrap par mesure. Toutes les mesures ont donné une structure de population très constante entre continents (valeurs du bootstrap = 92,4-100%) et à l'intérieur de la Finlande (42,3-98%). Quatre mesures ont différencié les populations de l'Alaska et du lac Young (bassin des Grands Lacs) des autres populations nord-américaines (56,6-87,7%). Les relations entre les autres populations du centre-nord des États-Unis n'étaient pas claires, comme l'indiquent les basses valeurs du bootstrap. Nos résultats laissent croire qu'il y a eu un seul refuge dans le centre-nord des États-Unis et plus d'un refuge en Europe durant la derni...
We supplemented physical‐tagging data with genetic data to provide evidence for spawning‐site and natal‐site fidelity by two spawning populations of northern pike Esox lucius in Kabetogama Lake, Minnesota. A mark–recapture study supported previous reports that individual northern pike and other esocids tend to return to the spawning grounds that they used in previous years. Of 1,900 northern pike marked at two spawning sites in the first year of the study, 23% were recaptured on spawning grounds one to three times over the next 4 years. Of these recaptured fish, only 1.3% and 4.8%, respectively, of those marked at the two spawning sites were recaptured at the other site. Tag returns from anglers showed that the year‐round ranges for fish from the two sites overlapped, so that lack of dispersal could not completely explain the high fidelity to spawning sites. Significant allele frequency differences at five microsatellite DNA loci between the 1983 (P = 0.03) and 1985 (P = 0.002) spawning populations indicated low levels of gene flow between the populations. This reproductive isolation would only be expected if most individuals first spawn at the site of their own birth and subsequently return to that site. We therefore conclude that northern pike in Kabetogama Lake exhibit both natal‐site and spawning‐site fidelity. Management on the basis of discrete spawning populations within lakes may thus be appropriate for a larger number of species and locations than commonly practiced.
Conserving genetic diversity within and among populations of northern pike Esox lucius is important for maintaining their short‐term fitness and long‐term evolutionary potential. Northern pike have consistently shown low within‐population variation, as detected by allozymes, mitochondrial DNA, and randomly amplified polymorphic DNA. Microsatellite DNA loci reveal considerably greater variation in northern pike, yet microsatellite variation tends to be less than that in other fish species. Low genetic variation within populations may result from compounding factors of low effective population size and bottlenecks. Allozyme, randomly amplified polymorphic DNA, and microsatellite markers have revealed significant differentiation of populations located in different continents and major drainages, but only microsatellites have differentiated populations on finer geographic scales. Within the north‐central United States, analyses with microsatellites have detected genetic differences among most populations but have inconsistently detected structure, or genetic relationships, among populations. Microsatellites did reveal fine‐scale structure between freshwater and brackish‐water populations in Finland, showing that population structure does develop in this species. The lack of structure in the north‐central United States may be the result of repopulation from a common glacial refugium, but stock transfers also may have played a role in obscuring past structure. Allozyme studies indicated that western North American populations might have recolonized from a second glacial refugium. Northern pike biologists should consider how management actions affect within‐ and among‐population variation. Stock transfers are the most common means by which genetically differentiated populations are mixed. Genetic data support avoiding transfers of northern pike across continents and between populations in western and central North America. Management actions such as harvest and habitat manipulations can also reduce genetic variation within populations by altering demographic factors that determine effective population size.
The giant river prawn (Macrobrachium rosenbergii) is one of the most farmed freshwater crustaceans in the world. Its global production has been stalling in the past decade due to the inconsistent quality of broodstock and hatchery-produced seeds. A better understanding of the role of nutrition in maturation diets will help overcome some of the production challenges. Arachidonic acid (20:4 n-6, ARA) is a fatty acid precursor of signaling molecules important for crustacean reproduction, prostaglandins E and F of the series II (PGE2 and PGF2α), and is often lacking in maturation diets of shrimp and prawns. We examined the effects of ARA in a combination of different fish oil (FO) and soybean oil (SO) blends on females’ reproductive performance and larval quality. Adult females (15.22 ± 0.13 g and 11.12 ± 0.09 cm) were fed six isonitrogenous and isolipidic diets containing one of two different base compositions (A or B), supplemented with one of three levels of Mortierella alpine-derived ARA (containing 40% active ARA): 0, 1 or 2% by ingredient weight. The two base diets differed in the percentages of (FO and SO with diet A containing 2% SO and 2% FO and diet B containing 2.5% SO and 1.5% FO, resulting in differences in proportional contents of dietary linoleic acid (18:2n-6, LOA) and docosahexaenoic acid (22:6n-3, DHA)). After the eight-week experiment, prawns fed diet B with 1 and 2% ARA supplement (B1 and B2) exhibited the highest gonadosomatic index (GSI), hepatosomatic index (HSI), egg clutch weight, fecundity, hatching rate, number of larvae, and reproductive effort compared to those fed other diets (p ≤ 0.05). Larvae from these two dietary treatments also had higher tolerance to low salinity (2 ppt). The maturation period was not significantly different among most treatments (p ≥ 0.05). ARA supplementation, regardless of the base diet, significantly improved GSI, HSI, egg clutch weight and fecundity. However, the diets with an enhanced ARA and LOA (B1 and B2) resulted in the best reproductive performance, egg hatchability and larval tolerance to low salinity. These dietary treatments also allow for effective accumulation of ARA and an n-3 lcPUFA, DHA in eggs and larvae.
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