Levels Of Interpopulational Gene Flow Research Articles

Intraspecific evolutionary relationships among peregrine falcons in western North American high latitudes.

Subspecies relationships within the peregrine falcon (Falco peregrinus) have been long debated because of the polytypic nature of melanin-based plumage characteristics used in subspecies designations and potential differentiation of local subpopulations due to philopatry. In North America, understanding the evolutionary relationships among subspecies may have been further complicated by the introduction of captive bred peregrines originating from non-native stock, as part of recovery efforts associated with mid 20th century population declines resulting from organochloride pollution. Alaska hosts all three nominal subspecies of North American peregrine falcons–F. p. tundrius, anatum, and pealei–for which distributions in Alaska are broadly associated with nesting locales within Arctic, boreal, and south coastal maritime habitats, respectively. Unlike elsewhere, populations of peregrine falcon in Alaska were not augmented by captive-bred birds during the late 20th century recovery efforts. Population genetic differentiation analyses of peregrine populations in Alaska, based on sequence data from the mitochondrial DNA control region and fragment data from microsatellite loci, failed to uncover genetic distinction between populations of peregrines occupying Arctic and boreal Alaskan locales. However, the maritime subspecies, pealei, was genetically differentiated from Arctic and boreal populations, and substructured into eastern and western populations. Levels of interpopulational gene flow between anatum and tundrius were generally higher than between pealei and either anatum or tundrius. Estimates based on both marker types revealed gene flow between augmented Canadian populations and unaugmented Alaskan populations. While we make no attempt at formal taxonomic revision, our data suggest that peregrine falcons occupying habitats in Alaska and the North Pacific coast of North America belong to two distinct regional groupings–a coastal grouping (pealei) and a boreal/Arctic grouping (currently anatum and tundrius)–each comprised of discrete populations that are variously intra-regionally connected.

Stay at home aphids: comparative spatial and seasonal metapopulation structure and dynamics of two specialist tansy aphid species studied using microsatellite markers

2 ) using polymorphic microsatellite markers. Both species were found in approximately 60% of sites formerly known to harbour the aphids, although, generally when they did occur, they occurred singly (MA ~50%; ME ~60%) and rarely together on the same plant at the same time (approximately 10%) and then usually only in the early part of the growing season. This difference may be a result of quasi-apparent competition effects elicited by ants farming ME aphids, and preferentially actively eliminating or disturbing MA aphids. In terms of population genetics, both aphids showed extreme genetic heterogeneity within a metapopulation structure, with ME more than MA (i.e. higher FST values, approximately 0.4 versus 0.15, respectively), and limited levels of interpopulation gene flow. Subpopulations often deviated from Hardy-Weinberg equilibrium and showed linkage disequilibria, as expected in animals with extended parthenogenetic reproduction, and had positive FIS values for most large samples, suggesting inbreeding, and possibly philopatry, certainly in ME. Hierarchical analysis (allele range and number per locus, analysis of molecular variance and FST) strongly suggested that the plant rather than site governs the level of genetic variation. Bayesian clustering analysis revealed that both species had heterogeneous historical genetic patterning, with K (number of subgroups) in the range 3-7. Evidence is also provided from isolation-by-distance and private allele analyses indicating that, in MA, the presence of winged autumn males, absent in ME where males are wingless, influences comparative population genetic structuring, such that ME subpopula- tions are comparatively more inbred and genetically differentiated than MA subpopulations. Lastly, additional spatial arrangement (ALLELES-IN-SPACE) analysis showed that, in both species, certain subpopulations were genetically isolated from the remainder, probably as a result of geographical barriers, including intervening buildings and woods. As such, the biology of these tansy aphids living in semi-natural habitats is very different from many pest aphid species examined within agro-ecosystems and infesting ephemeral crops. This is because the former appear to be much more reluctant to fly and hence show contrastingly much higher levels of interpopulation divergence, even at small spatial scales as investigated in the present study. Indeed, the number of genotypic clusters found for tansy aphids using Bayesian approaches is similar to that globally for the major pest, the peach-potato aphid, Myzus persicae. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 838-865. ADDITIONAL KEYWORDS: gene flow - genetic variation - historical patterns - inbreeding - migration - population divergence.

NATURAL SELECTION ALONG AN ENVIRONMENTAL GRADIENT: A CLASSIC CLINE IN MOUSE PIGMENTATION

We revisited a classic study of morphological variation in the oldfield mouse (Peromyscus polionotus) to estimate the strength of selection acting on pigmentation patterns and to identify the underlying genes. We measured 215 specimens collected by Francis Sumner in the 1920s from eight populations across a 155-km, environmentally variable transect from the white sands of Florida's Gulf coast to the dark, loamy soil of southeastern Alabama. Like Sumner, we found significant variation among populations: mice inhabiting coastal sand dunes had larger feet, longer tails, and lighter pigmentation than inland populations. Most striking, all seven pigmentation traits examined showed a sharp decrease in reflectance about 55 km from the coast, with most of the phenotypic change occurring over less than 10 km. The largest change in soil reflectance occurred just south of this break in pigmentation. Geographic analysis of microsatellite markers shows little interpopulation differentiation, so the abrupt change in pigmentation is not associated with recent secondary contact or reduced gene flow between adjacent populations. Using these genetic data, we estimated that the strength of selection needed to maintain the observed distribution of pigment traits ranged from 0.0004 to 21%, depending on the trait and model used. We also examined changes in allele frequency of SNPs in two pigmentation genes, Mc1r and Agouti, and show that mutations in the cis-regulatory region of Agouti may contribute to this cline in pigmentation. The concordance between environmental variation and pigmentation in the face of high levels of interpopulation gene flow strongly implies that natural selection is maintaining a steep cline in pigmentation and the genes underlying it.

A comparison of the genetic diversity in Dipteronia sinensis Oliv. and Dipteronia dyeriana Henry

Dipteronia is an endemic genus to China and includes only two species, Dipteronia sinensis and D. dyeriana. Based on random amplified polymorphic DNA (RAPD) markers, a comparative study of the genetic diversity and genetic structure of Dipteronia was performed. In total, 128 and 103 loci were detected in 17 D. sinensis populations and 4 D. dyeriana populations, respectively, using 18 random primers. These results showed that the proportions of polymorphic loci for the two species were 92.97% and 81.55%, respectively, indicating that the genetic diversity of D. sinensis was higher than that of D. dyeriana. Analysis, based on similarity coefficients, Shannon diversity index and Nei gene diversity index, also confirmed this result. AMOVA analysis demonstrated that the genetic variation of D. sinensis within and among populations accounted for 56.89% and 43.11% of the total variation, respectively, and that of D. dyeriana was 57.86% and 42.14%, respectively. The Shannon diversity index and Nei gene diversity index showed similar results. The abovementioned characteristics indicated that the genetic diversity levels of these two species were extremely similar and that the interpopulational genetic differentiation within both species was relatively high. Analysis of the genetic distance among populations also supported this conclusion. Low levels of interpopulational gene flow within both species were believed to be among the leading causes for the above-mentioned phenomenon. The correlation analysis between genetic and geographical distances showed the existence of a remarkably significant correlation between the genetic distance and the longitudinal difference among populations of D. sinensis (p < 0.01), while no significant correlation was found between genetic and geographical distances among populations of D. dyeriana. This indicated that genetic distance was correlated with geographical distances on a large scale rather than on a small scale. This result may be related to differences in the selection pressure on species by their habitats with different distribution ranges. We suggest that in situ conservation efforts should focus on establishing more sites to protect the natural populations and their habitats. Ex situ conservation efforts should focus on enhancing the exchange of seeds and seedlings among populations to facilitate gene exchange and recombination, and to help conserve genetic diversity.

Conserving marginal populations of the food plant ( Impatiens noli-tangere) of an endangered moth ( Eustroma reticulatum) in a changing climate

Impatiens noli-tangere is scarce in the UK and probably only native to the Lake District and Wales. It is the sole food plant for the endangered moth Eustroma reticulatum. Significant annual fluctuations in the size of I. noli-tangere populations endanger the continued presence of E. reticulatum in the UK. In this study, variation in population size was monitored across native populations of I. noli-tangere in the English Lake District and Wales. In 1998, there was a crash in the population size of all metapopulations in the Lake District but not of those found in Wales. A molecular survey of the genetic affinities of samples in 1999 from both regions and a reference population from Switzerland was performed using AFLP and ISSR analyses. The consensus UPGMA dendrogram and a PCO scatter plot revealed clear differentiation between the populations of I. noli-tangere in Wales and those in the Lake District. Most of the genetic variation in the UK ( H T =0.064) was partitioned between ( G ST=0.455) rather than within ( H S=0.034) regions, inferring little gene flow occurs between regions. There was similar bias towards differentiation between metapopulations in Wales, again consistent with low levels of interpopulation gene flow. This contrasts with far lower levels of differentiation in the Lake District which suggests modest rates of gene flow may occur between populations. It is concluded that in the event of local extinction of sites or populations, reintroductions should be restricted to samples collected from the same region. We then surveyed climatic variables to identify those most likely to cause local extinctions. Climatic correlates of population size were sought from two Lake District metapopulations situated close to a meteorological station. A combination of three climatic variables common to both sites explained 81–84% of the variation in plant number between 1990 and 2001. Projected trends for these climatic variables were used in a Monte Carlo simulation which suggested an increased risk of I. noli-tangere population crashes by 2050 at Coniston Water, but not at Derwentwater. Implications of these findings for practical conservation strategies are explored.

Dispersal barriers and isolation among deep-sea mussel populations (Mytilidae: Bathymodiolus) from eastern Pacific hydrothermal vents.

Deep-sea hydrothermal vent species are widely dispersed among habitat islands found along the global mid-ocean ridge system. We examine factors that affect population structure, gene flow and isolation in vent-endemic mussels of the genus Bathymodiolus from the eastern Pacific Ocean. Mussels were sampled from localities including the Galapagos Rift (GAR, 0 degrees 48' N; 86 degrees 10' W) and the East Pacific Rise (EPR, 13 degrees N to 32 degrees S latitude) across a maximum distance of 4900 km. The sampled range crossed a series of topographical features that interrupt linear aspects of the ridge system, and it encompassed regions of strong cross-axis currents that could impede along-axis dispersal of mussel larvae. Examinations of mitochondrial DNA sequences and allozyme variation revealed significant barriers to gene flow along the ridge axis. All populations from the GAR and EPR from 13 degrees N to 11 degrees S were homogeneous genetically and appeared to experience unimpeded high levels of interpopulational gene flow. In contrast, mussels from north and south of the Easter Microplate were highly divergent (4.4%), possibly comprising sister-species that diverged after formation of the microplate approximately 4.5 Ma. Strong cross-axis currents associated with inflated bathymetry of the microplate region may reinforce isolation across this region.

Gene diversity and demographic turnover in central and peripheral populations of the perennial herbGypsophila fastigiata

Within‐population gene diversity (HS) was estimated (using allozyme markers) for 16 populations of the perennial, outcrossing plant,Gypsophila fastigiata, on the Baltic island of Öland. The populations were characterized by data on extent, density, life‐stages, and habitat diversity. Populations were classed as central or peripheral in relation to the distribution of “alvar” (habitats with shallow, calcareous soils on limestone bedrock) on southern Öland. Three minimal adequate models were used to explain HSand the proportions of juveniles and dead adults. In the first model, HSwas significantly lower in peripheral populations and there were no significant additional effects of other explanatory variables. The lower diversity in peripheral populations can be explained by a combination of genetic drift (in populations that vary in size in response to habitat fragmentation) and lower levels of interpopulation gene flow than in central populations. In the two life‐stage models, peripheral populations had significantly larger proportions of both juveniles and dead adults – indicating a greater demographic turnover than in the central populations. There were also significant effects of HSand species diversity on the proportion of juveniles. The central or peripheral position of populations is the strongest predictor of both within‐population gene diversity and life‐stage dynamics in ÖlandG. fastigiata.

Genetic variation among populations of Pythium irregulare in southern Australia

Isolates of Pythium irregulare were sampled from seven cereal crops throughout South Australia to determine the extent of genetic diversity within this pathogen and the scale of genetic differentiation among populations. Data derived from 29 individual restriction fragment length polymorphism (RFLP) loci differentiated 54 DNA fingerprints among the 92 isolates analysed. Some isolates had two alleles at several RFLP loci and were scored as heterozygous. One such isolate was selfed in vitro and segregation ratios in the progeny were not significantly different from those expected for allelic variation in a diploid. These data provided evidence that outcrossing occurs within P. irregulare and may contribute to the high level of genetic variation within the species (DT = 0·502). Allelic frequencies were significantly different among all seven populations and GST values showed significant genetic differentiation between populations. The average genetic identity among populations was low and hierarchical cluster analysis provided no clear evidence that populations formed geographically related groups. These analyses indicate low levels of interpopulation gene flow within P. irregulare and imply that population differentiation results from genetic drift.

Levels and Patterns of Genetic Variation in Isoetes karstenii with Observations on I. palmeri

Genetic variation was surveyed in eleven populations from throughout the range of the diploid Isoetes karstenii, as well as in two populations of a closely related tetraploid, I. pal- meri, using allozyme electrophoresis. Genetic variation was high in I. karstenii and appears to be highly structured, as evidenced by analyses of F-statistics and genetic vs. geographic distance. No fixed unique alleles were found that could distinguish between I. karstenii and I. palmeri, although one allele was observed that was unique to I. palmeri. These analyses confirm earlier observations suggesting low levels of interpopulational gene flow and have implications for sampling strategies. Isoetes karstenii A. Braun was first described based on a collection from the high-altitude paramos of Merida, Venezuela (Braun, 1862). Since that time it has variously been accepted or synonymized by subsequent researchers, yet no detailed discussions of its variation or distribution have been published. As part of continuing research aimed at characterizing the neotropical species of Isoetes, we performed a survey of genetic variation in I. karstenii from throughout its range, as well as in a pair of populations from a single locality of a related species, I palmeri H.P. Fuchs, using allozyme electrophoresis. A taxonomic revision of the species complex to which these taxa belong is cur- rently in preparation (Small and Hickey, in prep.).

Genetic variation within and among populations of the narrow endemic, delphinium viridescens (Ranunculaceae)

Delphinium viridescens Leiberg (Ranunculaceae) is a narrow endemic, known from only 17 populations in the Wenatchee Mountains of central Washington. Electrophoretic data for ten loci and approximately 50 plants from each of the 17 populations indicate high levels of variability both within and among populations. High levels of heterozygosity, coupled with a general conformance of genotype frequencies to Hardy‐Weinberg expectations, suggest that D. viridescens is an outcrossing species, and outcrossing rates range from 0.391 to 1.454. Although all populations share a suite of common alleles, interpopulational differences in allele frequency were detected. Nei's genetic identities range from 0.883 to 1.000, with a mean of 0.964, suggesting only slight differentiation among populations and at least moderate levels of interpopulational gene flow. However, FST values indicate greater interpopulational heterogeneity. The data from this study are intended for use in developing a management plan for D. viridescens, to maintain the viability of this species, and to assess the need for its listing as threatened or endangered.

Genetic Variation within and Among Populations of the Narrow Endemic, Delphinium viridescens (Ranunculaceae)

Delphinium viridescens Leiberg (Ranunculaceae) is a narrow endemic, known from only 17 populations in the Wenatchee Mountains of central Washington. Electrophoretic data for ten loci and approximately 50 plants from each of the 17 populations indicate high levels of variability both within and among populations. High levels of heterozygosity, coupled with a general conformance of genotype frequencies to Hardy-Weinberg expectations, suggest that D. viridescens is an outcrossing species, and outcrossing rates range from 0.391 to 1.454. Although all populations share a suite of common alleles, interpopulational differences in allele frequency were detected. Nei's genetic identities range from 0.883 to 1.000, with a mean of 0.964, suggesting only slight differentiation among populations and at least moderate levels of interpopulational gene flow. However, FST values indicate greater interpopulational heterogeneity. The data from this study are intended for use in developing a management plan for D. viridescens, to maintain the viability of this species, and to assess the need for its listing as threatened or endangered.

Patterns of isozyme variation in relation to population size, isolation, and phytogeographic history in royal catchfly (Silene regia; Caryophyllaceae)

The distribution of genetic variation within and among plant populations is influenced by both contemporary and historical factors. I used isozyme analysis of band phenotypes to examine genetic structure in the rare prairie forb Silene regia. Relationships between current‐day population size, isolation, and phenotypic variation were assessed for 18 populations in two regions with differing postglacial history. Western populations from unglaciated southern Missouri and Arkansas were more genetically diverse based on the Shannon‐Weaver index (H) and a polymorphic index than were more eastern populations. These differences may be due to loss of variation with repeated founding of new populations in previously glaciated sites in Indiana and Ohio. Within the western region, population size was not significantly correlated with genetic variation. In the east, size was correlated with Shannon‐Weaver diversity. There was no relationship between variation and isolation in either region, but eastern populations were slightly more differentiated. Greater among‐population differentiation and the demonstrated connection between population size and variation in the eastern sites may reflect lower levels of interpopulation gene flow in the fragmented remnant prairies of Indiana and Ohio.

Patterns of Isozyme Variation in Relation to Population Size, Isolation, and Phytogeographic History in Royal Catchfly (Silene regia; Caryophyllaceae)

The distribution of genetic variation within and among plant populations is influenced by both contemporary and historical factors. I used isozyme analysis of band phenotypes to examine genetic structure in the rare prairie forb Silene regia. Relationships between current-day population size, isolation, and phenotypic variation were assessed for 18 populations in two regions with differing postglacial history. Western populations from unglaciated southern Missouri and Arkansas were more genetically diverse based on the Shannon-Weaver index (H) and a polymorphic index than were more eastern populations. These differences may be due to loss of variation with repeated founding of new populations in previously glaciated sites in Indiana and Ohio. Within the western region, population size was not significantly correlated with genetic variation. In the east, size was correlated with Shannon-Weaver diversity. There was no relationship between variation and isolation in either region, but eastern populations were slightly more differentiated. Greater among-population differentiation and the demonstrated connection between population size and variation in the eastern sites may reflect lower levels of interpopulation gene flow in the fragmented remnant prairies of Indiana and Ohio.

Genetic diversity, mating systems, and interpopulation gene flow in neotropical Hemionitis palmata L. (Adiantaceae)

Molecular data from isozyme analyses were used to characterize levels and patterns of genetic diversity in populations of the neotropical cheilanthoid fern Hemionitis palmata. All populations exhibited lower levels of genetic diversity than most other fern species previously studied. Variability in fixation indices and estimated intragametophytic selfing rates suggest that the predominant mating system varies from outcrossing to moderate levels of selfing among populations. Populations were significantly genetically differentiated from each other and estimated levels of interpopulation gene flow were generally low (i.e. Nm < 1.0). The wide geographical distribution of H. palmata may be accounted for by the ability of founding individuals to inbreed. Such an ability, along with founder effects, may explain the observed high levels of genetic differentiation among populations.

MATING SYSTEM AND DISTRIBUTION OF GENETIC VARIATION IN GYMNOCARPIUM DRYOPTERIS SSP. DISJUNCTUM

Levels and distribution of genetic variation were analyzed in 15 populations of the homosporous fern Gymnocarpium dryopteris ssp. disjunctum to evaluate the mating system and the genetic structure of this taxon. Estimated rates of intragametophytic selling were 0.000 in all populations, signifying that all sporophytes examined arose via intergametophytic matings. Furthermore, values of F, the fixation index, ranged from –0.171 to 0.038, indicating that all populations approach Hardy‐Weinberg equilibrium. Therefore, not only is intragametophytic selling rare in G. dryopteris ssp. disjunctum, but intergametophytic selling is also uncommon. Factors promoting outcrossing in G. dryopteris ssp. disjunctum include an antheridiogen system, an ontogenetic sequence that favors intergametophytic matings, and high levels of genetic load. Analysis of F‐statistics indicated statistically significant genetic differentiation among populations despite a mean genetic identity of 0.973 and high levels of interpopulational gene flow (Nm = 3.41 to 4.09). These patterns may reflect levels of gene flow that prevent significant interpopulational divergence while permitting slight genetic differentiation among populations.

ESTIMATES OF INTRAGAMETOPHYTIC SELFING AND INTERPOPULATIONAL GENE FLOW IN HOMOSPOROUS FERNS

Relatively little information is available on mating systems and interpopulational gene flow in species of homosporous pteridophytes. Because of the proximity of antheridia and archegonia on the same thallus, it has long been maintained that intragametophytic selling is the predominant mode of reproduction in natural populations of homosporous ferns and other homosporous plants. Furthermore, quantitative estimates of interpopulational gene flow via spore dispersal are lacking. In this paper, we examine five species of homosporous ferns (Botrychium virginianum, Polystichum munitum, P. imbricans, Blechnum spicant, and Dryopteris expansa) and present estimates of 1) rates of intragametophytic selling, 2) levels of interpopulational gene flow, and 3) interpopulational genetic differentiation (FST). Our data demonstrate that mating systems vary among species of ferns, just as they do among species of seed plants. The data also suggest that levels of interpopulational gene flow are generally high. The FST values indicate little genetic divergence among populations for all species except Dryopteris expansa, which exhibits significant levels of interpopulational genetic differentiation. Patterns of genetic diversity in the five species examined are related to the mating system and rate of interpopulational gene flow in each species. The FST values for all species except Botrychium virginianum are in close agreement with those predicted for an island model of population structure.

Polyploidy and Breeding Systems in Homosporous Pteridophyta: A Reevaluation

Despite extremely high chromosome numbers, homosporous Pteridophyta are genetically diploid. There is no genetic evidence from enzyme electrophoresis to support the claim that these organisms are highly polyploid. Because these data suggest that homosporous ferns are diploid organisms without large numbers of duplicated genes, there also is no genetic evidence that these plants maintain heterozygosity through homoeologous chromosome pairing. Instead, homosporous ferns appear to maintain genetic variation in the same manner as do other diploid organisms, through various levels of outcrossing. Although homosporous ferns may have bisexual gametophytes, there is no evidence that these gametophytes characteristically self-fertilize. In contrast, electrophoretic data indicate that intergametophytic matings occur frequently in homosporous Pteridophyta. Although a few species exhibit high levels of intragametophytic selfing, for most species analyzed, nearly all matings appear to be intergametophytic. Like seed plants, homosporous ferns exhibit a diversity of breeding systems, from inbreeding through mixed to outcrossing. Gene-flow estimates indicate that high levels of interpopulational gene flow may occur in species of homosporous ferns. In contrast to earlier investigations of homosporous ferns, which emphasized that dramatic differences in polyploidy, breeding system, selfing rates, and gene flow existed between these plants and all heterosporous plants, available data indicate striking overall similarities. However, if homosporous ferns are truly diploid organisms initiated with high chromosome numbers, fundamental differences in genome organization could exist between these plants and heterosporous plants. These plants may possess, for example, fewer active genes per chromo-some, lower levels of gene linkage, and large amounts of inactive DNA.

Contrasting patterns of geographic variation in the cosmopolitan sibling species Drosophila melanogaster and Drosophila simulans.

An electrophoretic study was carried out to compare the geographic pattern of genetic variation in Drosophila simulans with that of its sibling species, Drosophila melanogaster. An identical set of 32 gene-protein loci was studied in four geographically distant populations of D. simulans and two populations of D. melanogaster, all originating from Europe and Africa. The comparison yielded the following results: tropical populations of D. simulans were, in terms of the number of unique alleles, average heterozygosity per locus, and percentage of loci polymorphic, more variable than conspecific-temperate populations; some loci in both species showed interpopulation differences in allele frequencies that suggest latitudinal clines; and temperate-tropical genetic differentiation between populations was much less in D. simulans than in D. melanogaster. Similar differences between these two species have previously been shown for chromosomal, quantitative, physiological, and middle-repetitive DNA variation. Estimates of Nm (number of migrants per generation) from the spatial distribution of rare alleles suggest that both species have similar levels of interpopulation gene flow. These observations lead us to propose two competing hypotheses: the low level of geographic differentiation in D. simulans is due to its evolutionarily recent worldwide colonization and, alternatively, D. simulans has a narrower niche than D. melanogaster. Geographic variation data on different genetic elements (e.g., mitochondrial DNA, two-dimensional proteins, etc.) are required before these hypotheses can be adequately tested.