Population genetic structure in Atlantic and Pacific Ocean common murres (Uria aalge): natural replicate tests of post‐Pleistocene evolution

Understanding the factors that influence population differentiation in temperate taxa can be difficult because the signatures of both historic and contemporary demographics are often reflected in population genetic patterns. Fortunately, analyses based on coalescent theory can help untangle the relative influence of these historic and contemporary factors. Common murres (Uria aalge) are vagile seabirds that breed in the boreal and low arctic waters of the Northern Hemisphere. Previous analyses revealed that Atlantic and Pacific populations are genetically distinct; however, less is known about population genetic structure within ocean basins. We employed the mitochondrial control region, four microsatellite loci and four intron loci to investigate population genetic structure throughout the range of common murres. As in previous studies, we found that Atlantic and Pacific populations diverged during the Pleistocene and do not currently exchange migrants. Therefore, Atlantic and Pacific murre populations can be used as natural replicates to test mechanisms of population differentiation. While we found little population genetic structure within the Pacific, we detected significant east–west structuring among Atlantic colonies. The degree that population genetic structure reflected contemporary population demographics also differed between ocean basins. Specifically, while the low levels of population differentiation in the Pacific are at least partially due to high levels of contemporary gene flow, the east–west structuring of populations within the Atlantic appears to be the result of historic fragmentation of populations rather than restricted contemporary gene flow. The contrasting results in the Atlantic and Pacific Oceans highlight the necessity of carefully considering multilocus nonequilibrium population genetic approaches when reconstructing the demographic history of temperate Northern Hemisphere taxa.     

Land barriers and open oceans: effects on gene diversity and population structure in Avicennia germinans L. (Avicenniaceae)

Avicennia germinans L. is a widespread mangrove species occupying the west coast of Africa and the Atlantic and Pacific coasts of the Americas from the Bahamas to Brazil and Baja California to Peru. An amplified fragment length polymorphism (AFLP) molecular analysis was carried out to assess genetic architecture within this species and to evaluate the effects of the Atlantic Ocean and the Central American Isthmus (CAI) on population and regional genetic diversity and differentiation. In total, 349 polymorphic AFLP fragments were identified among 144 individuals from 14 populations from the east Atlantic, west Atlantic and east Pacific. Levels of genetic diversity varied considerably among populations, but were generally higher in populations from the east Atlantic. Regional differentiation between the Pacific coast and Atlantic populations was greater than between east and west Atlantic populations, suggesting that the CAI has had an important influence on population genetic structure in this species. The lower level of divergence of east Atlantic from west Atlantic populations suggests some dispersal across the Atlantic Ocean, although migration rates are probably low; Nm from GST equal to 0.41 and accumulation of private and rare alleles in the east Atlantic. Population differentiation did not appear to follow an isolation by distance model and has probably resulted from complex patterns of population bottlenecks, and founder events due to landscape changes during the Pleistocene, particularly in the west Atlantic. The molecular data provide no support for the treatment of east Atlantic populations as a separate species A. africana.     

The Kuroshio Current influences genetic diversity and population genetic structure of a tropical seagrass,Enhalus acoroides

Information on genetic diversity and differentiation of seagrass populations is essential for the conservation of coastal ecosystems. However, little is known about the seagrasses in the Indo‐West Pacific Ocean, where the world's highest diversity of seagrasses occurs. The influence of sea currents on these populations is also unknown. We estimated the genetic diversity and population genetic structure and identified reproductive features in Enhalus acoroides populations from the Yaeyama Islands, Hainan Island and the Philippines. The Philippines are situated at the centre of the E. acoroides range, Yaeyama and Hainan are peripheral populations, and the Yaeyama population is at the northern limit of the species range. The powerful Kuroshio Current flows from the Philippines to Yaeyama. Genetic analyses using nine microsatellite markers indicated that reproduction of E. acoroides is mostly sexual. Clonal diversity does not decrease in northern populations, although genetic diversity does. However, the genetic diversity of the Yaeyama populations is greater than that of the Hainan populations. Significant genetic differentiation among most populations was evident; however, the Yaeyama and north‐east Philippines populations were genetically similar, despite being separated by ~1100 km. An assignment test suggested that recruitment occurs from the north‐east Philippines to Yaeyama. The strong current in this region is probably responsible for the extant genetic diversity and recruitment patterns.     

Mitochondrial DNA reveals multiple Northern Hemisphere introductions of Caprella mutica (Crustacea, Amphipoda)

Caprella mutica (Crustacea, Amphipoda) has been widely introduced to non-native regions in the last 40 years. Its native habitat is sub-boreal northeast Asia, but in the Northern Hemisphere, it is now found on both coasts of North America, and North Atlantic coastlines of Europe. Direct sequencing of mitochondrial DNA (cytochrome c oxidase subunit I gene) was used to compare genetic variation in native and non-native populations of C. mutica . These data were used to investigate the invasion history of C. mutica and to test potential source populations in Japan. High diversity (31 haplotypes from 49 individuals), but no phylogeographical structure, was identified in four populations in the putative native range. In contrast, non-native populations showed reduced genetic diversity (7 haplotypes from 249 individuals) and informative phylogeographical structure. Grouping of C. mutica populations into native, east Pacific, and Atlantic groups explained the most among-region variation (59%). This indicates independent introduction pathways for C. mutica to the Pacific and Atlantic coasts of North America. Two dominant haplotypes were identified in eastern and western Atlantic coastal populations, indicating several dispersal routes within the Atlantic. The analysis indicated that several introductions from multiple sources were likely to be responsible for the observed global distribution of C. mutica , but the pathways were least well defined among the Atlantic populations. The four sampled populations of C. mutica in Japan could not be identified as the direct source of the non-native populations examined in this study. The high diversity within the Japan populations indicates that the native range needs to be assessed at a far greater scale, both within and among populations, to accurately assess the source of the global spread of C. mutica .     

A comparison of biallelic markers and microsatellites for the estimation of population and conservation genetic parameters in Atlantic salmon (Salmo salar)

Biallelic markers such as single nucleotide polymorphisms (SNPs) and insertion/deletion polymorphisms have become increasingly popular markers for various population genetics applications. However, the effort required to develop biallelic markers in nonmodel organisms is still substantial. In this study, we compared the estimation of various population genetic parameters (genetic divergence and structuring, isolation-by-distance, genetic diversity) using a limited number of biallelic markers (in total 7 loci) to those estimated with 14 microsatellite loci in 21 Atlantic salmon (Salmo salar) populations from northern Europe. Pairwise FST values were significantly correlated between biallelic loci and microsatellite datasets, as was overall heterozygosity when both anadromous and nonanadromous populations were analyzed together. However, when the anadromous and nonanadromous samples were analyzed separately, only genetic divergence correlations remained significant. Biallelic markers alone were not sufficient for reliable neighbor-joining clustering of populations but gave highly similar isolation-by-distance signals when compared with microsatellites. Finally, although several population prioritization measures for conservation exhibited significant correlation between different marker types, the specific populations highlighted as being most valuable for conservation purposes varied depending on the marker type and conservation criteria applied. This study demonstrates that a relatively small set of biallelic markers can be sufficient for obtaining concordant results in most of the analyses compared with microsatellites, although estimates of genetic distance are generally more concordant than estimates of genetic diversity. This suggests that a relatively small number of biallelic markers can provide useful information for various population genetic applications. However, we emphasize that the use of much higher number of loci is preferable, especially when the genetic differences between populations are subtle or individual multilocus genotype-based analyses are to be performed.     

Gene flow, historical population dynamics and genetic diversity within French Guianan populations of a rainforest tree species, Vouacapoua americana

Both gene flow and historical events influence the genetic diversity of natural populations. One way to understand their respective impact is to analyze population genetic structure at large spatial scales. We studied the distribution of genetic diversity of 17 populations of Vouacapoua americana (Caesalpiniaceae) in French Guiana, using nine microsatellite loci. Low genetic diversity was observed within populations, with a mean allelic richness and gene diversity of 4.1 and 0.506, respectively, which could be due to low effective population size and/or past bottlenecks. Using the regression between Fst/(1-Fst), estimated between pairs of populations, and the logarithm of the geographical distance, the spatial genetic structure can partly be explained by isolation-by-distance and limited gene flow among populations. This result is in agreement with the species' biology, including seed and pollen dispersal by rodents and insects, respectively. In contrast, no clear genetic signal of historical events was found when examining genetic differentiation among populations in relation to biogeographical hypotheses or by testing for bottlenecks within populations. Our conclusion is that nuclear spatial genetic structure of V. americana, at the geographic scale of French Guiana, is better explained by gene flow rather than by historical events.     

Microsatellite variation and genetic structure of brook trout (Salvelinus fontinalis) populations in Labrador and neighboring Atlantic Canada: evidence for ongoing gene flow and dual routes of post-Wisconsinan colonization

In conservation genetics and management, it is important to understand the contribution of historical and contemporary processes to geographic patterns of genetic structure in order to characterize and preserve diversity. As part of a 10-year monitoring program by the Government of Newfoundland and Labrador, Canada, we measured the population genetic structure of the world's most northern native populations of brook trout (Salvelinus fontinalis) in Labrador to gather baseline data to facilitate monitoring of future impacts of the recently opened Trans-Labrador Highway. Six-locus microsatellite profiles were obtained from 1130 fish representing 32 populations from six local regions. Genetic diversity in brook trout populations in Labrador (average H(E)= 0.620) is within the spectrum of variability found in other brook trout across their northeastern range, with limited ongoing gene flow occurring between populations (average pairwise F(ST)= 0.139). Evidence for some contribution of historical processes shaping genetic structure was inferred from an isolation-by-distance analysis, while dual routes of post-Wisconsinan recolonization were indicated by STRUCTURE analysis: K= 2 was the most likely number of genetic groups, revealing a separation between northern and west-central Labrador from all remaining populations. Our results represent the first data from the nuclear genome of brook trout in Labrador and emphasize the usefulness of microsatellite data for revealing the extent to which genetic structure is shaped by both historical and contemporary processes.     

Phylogeography of the catfish Hatcheria macraei reveals a negligible role of drainage divides in structuring populations

Southern South America provides a set of unusual geographic features that make it particularly interesting for studying phylogeography. The Andes Mountains run along a north-to-south axis and act as a barrier to gene flow for much of the biota of this region, with southern portions experiencing extensive historical glaciation. Geological data reveal a series of drainage reversals, shifting from Pacific Ocean outlets to Atlantic Ocean outlets because of glacier formation that dammed and reversed rivers. Once glaciers melted around 13 000 years ago, drainages returned to the Pacific Ocean. This geologic history predicts that aquatic organisms in Pacific rivers should have their closest relationships to their counterparts in Atlantic rivers immediately to their east. We tested this prediction in the trichomycterid catfish Hatcheria macraei from 38 locations using the mitochondrial cytochrome b gene. Our results show that most populations found in Pacific rivers were closely related to fish found in the adjacent Atlantic draining Río Chubut. Surprisingly, one documented drainage reversal (from Río Deseado into Río Baker) did not result in movement of H. macraei. Overall, we found the lowest levels of genetic structure between most Pacific rivers that are adjacent to the Atlantic draining Río Chubut. We also found low levels of population structuring among three of four contemporary river basins that drain to the Atlantic Ocean. Our findings suggest that drainage basin boundaries have historically not played an important long-term role in structuring between nine of 11 drainages, an unusual finding in freshwater biogeography.     

Microsatellite markers and mtDNA data indicate two distinct groups in dwarf galaxias, Galaxiella pusilla (Mack) (Pisces: Galaxiidae), a threatened freshwater fish from south-eastern Australia

Galaxiella pusilla is a small, non-migratory freshwater fish, endemic to south-eastern Australia and considered nationally threatened. To assist in the conservation of the species, microsatellite markers were developed and used to characterize genetic variation in 20 geographically distinct populations across its range. Substantial genetic differentiation was found between an eastern (Victoria east of the Otway Ranges and Tasmania) and western (South Australia and Victoria west of, and including, the Otway Ranges) region. This major separation was also observed in data from a mitochondrial gene and supports a previously proposed split. Populations from the eastern region had overall lower genetic diversity for both the microsatellite and mtDNA markers. There was substantial genetic differentiation between populations within the two regions, suggesting that gene flow is limited by the isolation of freshwater streams. Genetic structure, consistent with an isolation-by-distance model, was also evident in both regions. Patterns of genetic variation in this threatened species are compared to those obtained for other taxa across the same region. The need to consider separate conservation strategies for the two sets of populations is emphasized.     

Genetic diversity of the mangrove‐associated alga Bostrychia radicans/Bostrychia moritziana (Ceramiales,Rhodophyta) from southern Central America

Previous studies suggested that the biodiversity of the mangrove‐associated Bostrychia radicans/Bostrychia moritziana species complex on the Pacific coast of Central America, based on genetic and reproductive data, were low compared with similar areas on the Atlantic coast. Evolutionary scenarios were proposed based on either a recent introduction to the Pacific, or a more uniform environment leading to genetically connected populations and low differentiation between populations. We sampled more extensively in southern Mexico, Guatemala and El Salvador and sequenced the samples for the RuBisCo spacer. Our results show that genetic diversity is high in these populations. Many haplotypes retrieved are also found in the Atlantic Ocean (USA, Brazil), an observation not made before. Data suggest that populations are highly differentiated with little evidence of isolation‐by‐distance. The population at La Puntilla, El Salvador is highly differentiated from other populations. Data also suggest that diversity is reduced in a northerly direction, with only one haplotype, unique to Pacific Central America, found north of Chiapas, Mexico. This could be due to northern expansion of this unique genotype as sea surface temperatures ameliorated following the last glacial maximum. Our data do not support the previous proposition of low diversity in the east central Pacific and suggest that much of the Pacific Central America diversity is from before the closure of the Isthmus of Panama.     

Isolation by distance within a river system: genetic population structuring of Atlantic salmon, Salmo salar, in tributaries of the Varzuga River in northwest Russia

An important issue for designing any conservation programme aimed at preserving genetic diversity is estimation of the scale at which genetic structuring occurs. Additional relevant factors include distinguishing whether or not population structuring is expected to be stable as predicted by the member-vagrant hypothesis, or alternatively, whether populations are more prone to local extinction-recolonization processes, as predicted by the metapopulation evolutionary model. In this study, the population genetic structure of Atlantic salmon from 11 locations within or nearby the Varzuga River tributary system was assessed using 17 microsatellites. Mantel tests and spatial autocorrelation analyses revealed a significant isolation-by-distance signal within the tributary system as well as a negative association between the level of genetic diversity and waterway distance from the river mouth, indicating that dispersal is less likely to occur to populations deep in the tributary system. Individual-level spatial autocorrelation analyses indicated that the majority of migration occurred between populations situated within 20 km of each other. The relatively high level of genetic structuring and significant isolation-by-distance signal observed in the Varzuga tributaries are concordant with the predictions of the member-vagrant evolutionary model. However, one subpopulation in particular revealed signs of instability which may be due to its location in the tidal zone, or due to the fact that it is more affected by human impacts. The results suggest that preservation of a number of spawning sites spaced throughout the tributary system is recommendable for ensuring sustainable fishing tourism in the river.     

Recolonization history and large-scale dispersal in the open sea: the case study of the North Atlantic cod, Gadus morhua L.

Most studies of the genetic structure of Atlantic cod have focused on small geographical scales. In the present study, the genetic structure of cod sampled on spawning grounds in the North Atlantic was examined using eight microsatellite loci and the Pan I locus. A total of 954 cod was collected from nine different regions: the Baltic Sea, the North Sea, the Celtic Sea, the Irish Sea and Icelandic waters during spring 2002 and spring 2003, from Norwegian waters and the Faroe Islands (North and West spawning grounds) in spring 2003, and from Canadian waters in 1998. Temporal stability among spawning grounds was observed in Icelandic waters and the Celtic Sea, and no significant difference was observed between the samples from the Baltic Sea and between the samples from Faroese waters. F -statistics showed significant differences between most populations and a pattern of isolation-by-distance was described with microsatellite loci. The Pan I locus revealed the presence of two genetically distinguishable basins, the North-west Atlantic composed of the Icelandic and Canadian samples and the North-east Atlantic composed of all other samples. Permutation of allele sizes at each microsatellite locus among allelic states supported a mutational component to the genetic differentiation, indicating a historical origin of the observed variation. Estimation of the time of divergence was approximately 3000 generations, which places the origin of current genetic pattern of cod in the North Atlantic in the late Weichselian (Wisconsinian period), at last glacial maximum.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 315–329.     

Inter‐oceanic variation in patterns of host‐associated divergence in a seabird ectoparasite

Aim Parasites with global distributions and wide host spectra provide excellent models for exploring the factors that drive parasite diversification. Here, we tested the relative force of host and geography in shaping population structure of a widely distributed and common ectoparasite of colonial seabirds, the tick Ixodes uriae. Location Two natural geographic replicates of the system: numerous seabird colonies of the North Pacific and North Atlantic Ocean basins. Methods Using eight microsatellite markers and tick samples from a suite of multi‐specific seabird colonies, we examined tick population structure in the North Pacific and compare patterns of diversity and structure to those in the Atlantic basin. Analyses included population genetic estimations of diversity and population differentiation, exploratory multivariate analyses, and Bayesian clustering approaches. These different analyses explicitly took into account both the geographic distance among colonies and host use by the tick. Results Overall, little geographic structure was observed among Pacific tick populations. However, host‐related genetic differentiation was evident, but was variable among host types and lower than in the North Atlantic. Main conclusions Tick population structure is concordant with the genetic structure observed in seabird host species within each ocean basin, where seabird populations tend to be less structured in the North Pacific than in the North Atlantic. Reduced tick genetic structure in the North Pacific suggests that host movement among colonies, and thus tick dispersal, is higher in this region. In addition to information on parasite diversity and gene flow, our findings raise interesting questions about the subtle ways that host behaviour, distribution and phylogeographic history shape the genetics of associated parasites across geographic landscapes.     

Population Genetic Structure of the Magnificent Frigatebird Fregata magnificens (Aves,Suliformes) Breeding Colonies in the Western Atlantic Ocean

The Magnificent Frigatebird Fregata magnificens has a pantropical distribution, nesting on islands along the Atlantic and Pacific coasts. In the Caribbean, there is little genetic structure among colonies; however, the genetic structure among the colonies off Brazil and its relationship with those in the Caribbean are unknown. In this study, we used mtDNA and microsatellite markers to infer population structure and evolutionary history in a sample of F. magnificens individuals collected in Brazil, Grand Connétable (French Guyana), and Barbuda. Virtually all Brazilian individuals had the same mtDNA haplotype. There was no haplotype sharing between Brazil and the Caribbean, though Grand Connétable shared haplotypes with both regions. A Bayesian clustering analysis using microsatellite data found two genetic clusters: one associated with Barbuda and the other with the Brazilian populations. Grand Connétable was more similar to Barbuda but had ancestry from both clusters, corroborating its “intermediate” position. The Caribbean and Grand Connétable populations showed higher genetic diversity and effective population size compared to the Brazilian population. Overall, our results are in good agreement with an effect of marine winds in isolating the Brazilian meta-population.     

High genetic variation in marginal fragmented populations at extreme climatic conditions of the Patagonian Cypress Austrocedrus chilensis

Knowledge about current patterns of genetic structure of populations together with the evolutionary history of a species helps to understand and predict the adaptation of populations to future climate change. We assayed variation at nuclear microsatellite markers among peripheral vs. continuous populations of the temperate South American species Austrocedrus chilensis, to investigate the role of historical vs. demographical forces in shaping population genetic structure. This species occurs in continuous populations in the west and central distribution range, but becomes highly fragmented at the eastern limit, which comprised ice-free areas during Quaternary glaciations and has extreme climatic conditions at present times. Bayesian analysis methods identified two contrasting patterns of genetic structure; (I) populations from humid, mesic and peri-glacial regions formed a single deme with relatively low genetic differentiation and high admixture levels whereas (II) a highly heterogeneous genetic structure with low level of admixture was found in the steppe, towards the east and northeast limit of the distribution range. In the steppe, population fragmentation, restricted gene flow and isolation-by-distance were also inferred. In addition, several small steppe populations showed high genetic diversity and divergent gene pools, suggesting that they constitute ancient refuges from pre-Holocene glaciations with just a subgroup of them contributing significantly to post-glacial spread. These results are discussed in relation to patterns of genetic variation found for other temperate species and the contribution of the particular southern Andes topography and climate to post-glacial spread.     

Population structure of North Atlantic and North Pacific sei whales (<Emphasis Type="Italic">Balaenoptera borealis</Emphasis>) inferred from mitochondrial control region DNA sequences and microsatellite genotypes

Currently, three stocks of sei whales (Balaenoptera borealis) are defined in the North Atlantic; the Nova Scotian, Iceland-Denmark Strait and Eastern North Atlantic stocks, which are mainly based upon historical catch and sighting data. We analyzed mitochondrial control region DNA (mtDNA) sequences and genotypes from 7 to 11 microsatellite loci in 87 samples from three sites in the North Atlantic; Iceland, the Gulf of Maine and the Azores, and compared against the North Pacific using 489 previously published samples. No statistically significant deviations from homogeneity were detected among the North Atlantic samples at mtDNA or microsatellite loci. The genealogy estimated from the mtDNA sequences revealed a clear division of the haplotypes into a North Atlantic and a North Pacific clade, with the exception of one haplotype detected in a single sample from the Azores, which was included in the North Pacific clade. Significant genetic divergence between the North Atlantic and North Pacific Oceans was detected (mtDNA Φ_ST?=?0.72, microsatellite Weir and Cockerham’s ? = 0.20; p?<?0.001). The coalescent-based estimate of the population divergence time between the North Atlantic and North Pacific populations from the sequence variation among the mtDNA sequences was at 163,000 years ago. However, the inference was limited by an absence of samples from the Southern Hemisphere and uncertainty regarding mutation rates and generation times. The estimates of inter-oceanic migration rates were low (Nm at 0.007 into the North Pacific and at 0.248 in the opposite direction). Although estimates of genetic divergence among the current North Atlantic stocks were low and consistent with the extensive range of movement observed in satellite tagged sei whales, the high uncertainty of the genetic divergence estimates precludes rejection of multiple stocks in the North Atlantic.     

Genetic sex identification and the potential evolution of sex determination in Pacific halibut (Hippoglossus stenolepis)

The discovery of genetic markers linked to physiological traits in wild populations is increasingly desired for ecological and evolutionary studies, as well as to inform management decisions. However, identifying such markers often requires a large investment of both time and money. Serendipitously, in a recent microsatellite survey, we discovered three out of 16 microsatellite loci that were correlated to the female sex in Pacific halibut (Hippoglossus stenolepis). These three loci were screened in 550 Pacific halibut to determine their accuracy at identifying sex. Genetic assignment successfully identified sex in 92% of individuals from sample collections spanning 3,000 km and 9 years. All but two of 287 females had one copy of a characteristic allele for at least one of the three microsatellite loci, resulting in consistent heterozygote excess in females. This pattern is consistent with the hypothesis that females are the heterogametic sex in Pacific halibut, which thus may have a different sex-determination pattern than the closely related Atlantic halibut (Hippoglossus hippoglossus). A rapid divergence of sex-determining mechanisms could be either a cause or consequence of speciation between Pacific and Atlantic halibut. In either case, the ability to genetically identify sex in individual Pacific halibut provides a new tool for ecological studies, fisheries management, and insight into the evolution of sex determination in flatfish.     

Life on the edge: the long-term persistence and contrasting spatial genetic structure of distinct brown trout life histories at their ecological limits

The Atlantic Iberian brown trout is at the southwestern limit of its distribution. At this ecological edge, which was once a glacial refugia, anadromy becomes less common as increased water temperatures restricted populations closer to the headwaters. We examined 847 individuals from 20 populations from throughout this region and assessed spatial genetic structure using 11 protein and four microsatellite loci. The higher levels of heterozygosity and allelic diversity north of the southernmost limit of anadromy (SLA), as well as an isolation-by-distance model of population structure, likely influenced by the anadromous forms, suggest that more stable demographic conditions existed over time in this region. Populations south of the SLA were highly differentiated given the restricted size of the area (protein F(ST) = 0.16 in the north and 0.63 in the south of the SLA; microsatellite F(ST) = 0.18 in the north and 0.70 in the south of the SLA). The low levels of heterozygosity and the pattern of southward allele depletion in resident populations is indicative of fragmentation, caused by stressful ecological conditions that reduced the anadromy (restricting gene flow) and the effective population sizes (higher genetic drift), which, in combination, dramatically decreased within-population genetic variation and increased among-population genetic variation. The higher gene diversity north of the SLA does not reflect ancestry but rather the signature of a population size expansion, as evidence suggest the persistence of older populations (with several private alleles) south of the SLA. These data support a scenario that demonstrates how contemporary events (critical ecological conditions) can moderate historical influences, suggesting that careful interpretation of the evolutionary history of glacial refugia is necessary, especially where populations persisted for a long time but not always with optimal ecological conditions. These peripheral populations are of high conservation value and should be preserved to help conserve the future potential of the species.     

Molecular genetic variation of boll weevil populations in North America estimated with microsatellites: Implications for patterns of dispersal

The boll weevil (Anthonomus grandis Boheman) is an insect pest of cotton that underwent a well-documented range expansion across the southeastern U.S. from Mexico beginning about 110 years ago. Eleven microsatellite loci were surveyed to infer the magnitude and pattern of genetic differentiation among boll weevil populations from 18 locations across eight U.S. states and northeast Mexico. Estimates of genetic diversity (allelic diversity and heterozygosity) were greater in Southern than Northern populations, and were greater in the west than the east among Northern populations. Boll weevil populations were genetically structured as a whole across the geographic range sampled, with a global F _ST of 0.241. South-central populations exhibit classic isolation by distance, but evidence suggests that populations within the Eastern and Western regions have not yet reached genetic equilibrium. Gene flow appears to be relatively high among populations within the Eastern region. Population assignment data and estimates of gene flow indicate that migration between locations separated by < 300 km is frequent. The database of microsatellite genotypes generated in this study now makes it possible, through population assignment techniques, to identify the most likely geographic source of a boll weevil reintroduced to an eradication zone, which will help action agencies decide the most appropriate mitigation response. Kyung Seok Kim - Formerly: USDA-ARS, Areawide Pest Management Research Unit, 2771 F & B Rd., College Station,TX 77845, USA.     

Population structure and genetic diversity in tristylous Narcissus triandrus: insights from microsatellite and chloroplast DNA variation

We investigated cpDNA sequence and nuclear microsatellite variation among populations of the wild daffodil Narcissus triandrus to examine the role of historical vs. contemporary forces in shaping population structure, morphological differentiation and sexual-system evolution. This wide-ranging heterostylous species of the Iberian Peninsula is largely composed of two allopatric varieties (vars. cernuus and triandrus), and populations with either stylar trimorphism or dimorphism. Dimorphic populations only occur in var. triandrus, are mainly restricted to the northwestern portion of the species range, and uniformly lack the mid-styled morph (M-morph). Chloroplast DNA (cpDNA) sequence variation revealed strong geographical structuring and evidence for a fragmentation event associated with differentiation of the two varieties. In var. triandrus, population fragmentation, restricted gene flow and isolation-by-distance were also inferred. Significant differences in genetic diversity and population structure between the two varieties likely reflect historical and contemporary differences in demography and gene flow among populations. Discordance between cpDNA markers and both microsatellite and morphological variation indicate that hybridization has occurred between the two varieties at contact zones. There were no differences in genetic diversity or population structure between dimorphic and trimorphic populations, and chloroplast haplotypes were not associated with either sexual system, indicating transitions in morph structure within each maternal lineage. M-morph frequencies were positively correlated with differentiation at microsatellite loci, indicating that the evolutionary processes influencing these neutral markers also influence alleles controlling the style morphs.