Microgeographic Scale Research Articles

Genetic structure of an introduced pest, grape phylloxera (<i>Daktulosphaira vitifoliae</i> Fitch), in Europe

A model for the genetic structure of grape phylloxera populations in Europe was developed using hierarchical sampling techniques and AFLP-PCR (amplified fragment length polymorphism--polymerase chain reaction) methodology. One-hundred three European and 6 North American phylloxera populations were studied. An additional European sampling set comprising 60 samples was analyzed to study regional subdivision. The populations grouped into two clusters loosely correlated with collection site location. Phylloxera populations collected from northern (above lat 43 degrees) geographic regions were significantly different from southern (below 43 degrees) populations. The northern cluster was more heterogeneous than the southern cluster, possibly reflecting holocyclic versus anholocyclic reproduction. Microgeographic scales of phylloxera genetic structure displayed as much variation within as among host plants. The host plant did not affect the genetic structure of European phylloxera as revealed in two independent experiments.

The genetic heterogeneity of deer mouse populations ( Peromyscus maniculatus ) in an insular landscape

A survey of the genetic variability in deer mouse populations was performed using specimens collected from six different islands on a lake covering approximately 50 km2. Random amplified polymorphic DNA (RAPD) was used to measure the extent of the genetic differences in this insular system. An analysis of molecular variance (AMOVA) revealed that populations are clearly separated at this microgeographic scale (Fst = 0.13863; P < 0.001). The homogeneity of molecular variance test (HOMOVA) indicated that within-population levels vary greatly (Bp = 0.76831; P < 0.001). The within-population molecular variance was found to be mainly correlated with the accessibility of the islands, computed as the inverse of the geographic distance separating an island from the lakeshore (r = 0.916; P < 0.003).

Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France.

Aphis gossypii is an aphid species that is found throughout the world and is extremely polyphagous. It is considered a major pest of cotton and cucurbit species. In Europe, A. gossypii is assumed to reproduce exclusively by apomictic parthenogenesis. The present study investigates the genetic diversity of A. gossypii in a microgeographic, fragmented habitat consisting of eight glasshouses of cucurbit crops. This analysis, which was based on the results from seven microsatellite loci, has confirmed that A. gossypii populations in southern France are primarily asexual, as only 12 nonrecombinant genotypic classes (clones) were identified from 694 aphids. Moreover, a high proportion of the aphids (87%) had one of three common genotypes. No significant correlation was found between genotypic class and host plant species. Within a glasshouse population of A. gossypii, a significant reduction in clonal diversity was observed as the spring/summer season progressed. The final predominance of a clone could result from interclonal competition. At the microgeographic level (i.e. glasshouses within a 500-m radius), significant genetic subdivision was detected and could be attributed to founder effects and the limitation of gene flow imposed by the enclosed nature of the glasshouse structure. Finally, the three common clones of A. gossypii detected in 1996 reappeared in spring 1997 following the winter extinction, together with rare clones that had not previously been seen. The probability that A. gossypii overwinters within refuges at a microgeographic scale from which populations are renewed each spring is discussed.

Dispersal and Distribution of the Tick Ixodes uriae within and among Seabird Host Populations: The Need for a Population Genetic Approach

The aim of this study was to characterize the spatial distribution of the tick Ixodes uriae within and among populations of its seabird hosts and to consider the potential insight that could be gained by a population genetic approach to the issue of dispersal of this tick. Analyses of data collected around the Avalon Peninsula, Newfoundland, indicated that both the prevalence and mean abundance of ticks varied significantly among sample locations. Whereas ticks were found on all 4 host species examined (Rissa tridactyla, Uria aalge, Alca torda, Fratercula arctica), infestation prevalence and mean abundance differed among the species. On R. tridactyla, ticks were significantly aggregated at the among-nest scale and nestling infestation was spatially autocorrelated. Conversely, ticks were not aggregated among chicks within nests. These results enabled us to make a priori predictions regarding tick dispersal and host specificity and suggest there may be spatial structure of Ixodes uriae populations at both macro- and microgeographic scales. Investigating the population genetic structure of ticks within and among populations of hosts with different breeding biologies should provide direct insight into the metapopulation dynamics of such a spatially structured system.

Microsatellite gene diversity analysis in anadromous arctic char,Salvelinus alpinus, from Labrador, Canada

We analysed six loci among 257 Arctic char (Salvelinus alpinus) representing seven locations in Labrador and Newfoundland to provide a first assessment of microsatellites gene diversity in anadromous char and to determine the geographic scale of population structuring within the species. The number of alleles per locus varied between 9 and 48, and gene diversity ranged from 0.190 to 0.968. Significant FSTand differences in allele frequencies were observed among most samples, as well as heterozygous deficiency, which was indicative of a Wahlund's effect. These results implied the existence of genetically distinct populations on a microgeographic scale (less than 10 km) and that our samples represented an admixture of char from those populations that interchange among rivers for owerwintering, in congruence with tagging investigations. These results indicate that microsatellites potentially offer more sensitivity than allozymes and mitochondrial DNA to infer fine-scale population structure in anadromous arctic char.

Brief communication. Characterization of DNA microsatellite loci from a threatened snake: the eastern massasauga rattlesnake (Sistrurus c. catenatus and their use in population studies

We describe the isolation and genetic characterization of six microsatellite DNA loci from a threatened snake species, the eastern massasauga rattlesnake (Sistrurus c. catenatus and document the use of these loci for a variety of population-level analyses. Based on data from more than 70 adults from a single population in Killbear Provincial Park, Ontario, these loci: (1) are highly variable, with numbers of detected alleles per locus ranging between 4 and 12 and expected heterozygosities between 0.15 and 0.85, although observed heterozygosities are lower; (2) generally segregate in a Mendelian fashion based on comparisons between genotypes of mothers and their offspring in two litters; (3) may not evolve via a stepwise mutation model as based on frequency distributions of allele sizes; (4) are sufficiently variable for accurate determination of paternity in this species with a significant probability (P = .979) of detecting multiple paternity in single broods; and (5) amplify similar size fragments in other species of snakes. Further analyses demonstrate a significant heterozygote deficiency at all loci in this population which, for at least three of the six loci, is not likely due to the presence of null alleles. This pattern suggests that this population has experienced local inbreeding and/or is subdivided on a microgeographic scale. These markers will be ideal for further analyses of population variation and differentiation in this species, which will provide useful information for conservation and evolutionary studies of these snakes.

Isolation and characterization of microsatellite loci in Swietenia humilis (Meliaceae): an endangered tropical hardwood species

AbstractMicrosatellites are now firmly established as an informative marker system, with increasing popularity as a tool amongst molecular ecologists. We have developed a method of constructing an enriched microsatellite library for the tropical tree species Swietenia humilis Zucc. (Meliaceae). This method is based on a precloning enrichment of SSRs using synthetic oligonucleotide probes, bound to magnetic beads and hybridizing to complementary microsatellite core sequences in digested genomic DNA. Here we describe the isolation and characterization of 10 microsatellite loci that have been used to survey the genetic diversity within a natural population of S. humilis. A total of 97 alleles were identified with an average of 9.7 alleles over all loci. Very high levels of allelic polymorphism were detected at individual loci, with 23 alleles observed at the most variable. The mean observed heterozygosity was 0.415 (range 0.038‐0.815) exceeding levels of diversity detected in related species which used isozymes as the marker system. Subpopulation differentiation at a microgeographical scale was low (FST= 0.036) and the values of Nm, calculated from the allelic frequencies, were greater than 1 thus reflecting the extent of gene flow occurring between individual trees.

Morphological and Vocal Variation among Subspecies of the Black-Faced Sheathbill

The Black-faced Sheathbill (Chionis minor) is a sedentary and polytypic species. Four allopatric subspecies are known, each breeding on one archipelago in the Southern Indian Ocean. To evaluate the degree of isolation of these four subspecies, morphometrics and vocalizations of adult birds of Iles Kerguelen and Crozet were compared with those of the other localities (Prince Edward and Heard Islands). Two groups were distinguished (Prince Edward-Crozet and Kerguelen-Heard) on geographic and morphological criteria. In the eastern group (Kerguelen-Heard), corresponding to higher latitudes, sheathbills were larger and heavier, following Bergmann's Rule. The sheathbills from Iles Kerguelen also had a lower-pitched voice than those from Iles Crozet, consistent with their larger body size. Moreover, the birds from the southernmost locality (Heard Island) had a shorter culmen, consistent with Allen's Rule, but longer tarsi and deeper sheaths. Within the western group (Prince Edward-Crozet), and at Iles Kerguelen, there also was variability on a microgeographical scale. Differences between subspecies of Black-faced Sheathbill therefore could be due not only to environmental correlates of latitude, but also to possible genetic drift. The four subspecies are allopatric and do not differ in their breeding schedule or in their general behavior and diet, suggesting that differentiation may be recent and mainly due to geographical isolation.

Assessing the nucleotide diversity of three aphid species by RAPD

AbstractA method is presented for the estimation of nucleotide diversity and genetic structure of populations from RAPD (random amplified polymorphic DNA) data. It involves a modification of the technique developed by Lynch and Crease (1990) for the case of restriction sites as survey data. As new elements the method incorporates (i) dominance correction, (ii) values of asexual reproduction of the populations sampled, and (iii) an analytical variance of the number of nucleotide substitutions per site.Sampling was carried out at two geographic scales for three aphid species. At a macrogeographic scale, populations of Rhopalosiphum padi did not show statistical genetic differentiation. Aphis gossypii and Myzus persicae, which were sampled at a microgeographic scale, showed a higher genetic differentiation than R. padi, it being statistically significant in M. persicae. The major sources of sampling variance within‐ and between‐populations were found to be nucleotide (i.e., the number of alleles used as a function of the number of primers used) and population (i.e., sample size) sampling.Extremely low estimates of nucleotide diversity were obtained for the species studied here. This result is consistent with previous reports on genetic diversity for the same or other aphid species which were based on allozyme polymorphism, mitochondrial DNA variation and qualitative analyses of RAPDs.

Oligonucleotide Fingerprinting Detects Genetic Variability at Different Levels in Nigerian Mycosphaerella fijiensis

AbstractDNA fingerprinting with synthetic simple repetitive oligonucleotides such as (CA)8 or (CAA)5 detected polymorphisms between various isolates of the ascomycete Mycosphaerella fijiensis, the causal agent of the black Sigatoka disease of Musa. These microsatellite motifs are present at multiple chromosomal locations and in high copy numbers in the Mycosphaerella genome, generating informative fingerprints with low background. Variability exists on a macro‐ as well as a microgeographical scale: it occurred within one lesion, between lesions of one plant, between plants, cultivars, and geographic locations. Mathematical analysis of the data produced dendrograms that demonstrated the presence of different genetically related groups of Mycosphaerella fijiensis in Nigeria.

Lambornella clarki (Ciliophora:Tetrahymenidae) resistance in Aedes sierrensis (Diptera:Culicidae):population differentiation in a quantitative trait and allozyme loci.

Heritable variation for resistance to the parasite Lambornella clarki (Corliss & Coats) by its host Aedes sierrensis (Ludlow) and neutral genetic variation among local Oregon mosquito populations were assessed by selection and allozyme frequencies. L. clarki resistance was significantly greater in a population of mosquitoes normally exposed to this parasite than in an unexposed population. Despite this difference between populations, there was no significant increase in heritable resistance to L. clarki within populations of Ae. sierrensis after 1 generation of laboratory selection. Differentiation estimated from allozyme electrophoresis did not coincide with differentiation in L. clarki resistance in Ae. sierrensis populations at the microgeographic scale. Ae. sierrensis has the capacity to respond to foreign clones of the parasite, but electrophoretic surveys may fail to indicate underlying differences in the parasite resistance of mosquito populations.

MOLECULAR GENETIC ANALYSIS OF A STEPPED MULTILOCUS CLINE IN THE AMERICAN OYSTER (CRASSOSTREA VIRGINICA).

Gulf of Mexico versus Atlantic populations of several coastal species in the southeastern United States are known to differ sharply in genetic composition, but most transitional zones have not previously been examined in detail. Here we employ molecular markers from mitochondrial and nuclear loci to characterize cytonuclear genetic associations at meso- and microgeographic scales along an eastern Florida transitional zone between genetically distinct Atlantic and Gulf populations of the American oyster, Crassostrea virginica. The single- and multilocus cytonuclear patterns display: (1) a cline extending along 340 km of the east Florida coastline; (2) a pronounced step in the cline centered at Cape Canaveral (shifts in allelic frequencies by 50-75% over a 20 km distance); (3) a close agreement of observed genotypic frequencies with Hardy-Weinberg expectations within locales; and (4) mild or nonexistent nuclear and cytonuclear disequilibria in most local population samples. These results imply: (1) considerable restrictions to interpopulational gene flow along the eastern Florida coastline; (2) within locales, free interbreeding (as opposed to mere population admixture) between Gulf and Atlantic forms of oysters; and (3) localized population recruitment in the transition zone localities. These findings demonstrate that marine organisms with high dispersal potential via long-lived pelagic larvae can nonetheless display pronounced spatial population genetic structure, and more generally they exemplify the utility of pronounced genetic transition zones for the study of population level processes.

MICROGEOGRAPHIC PATTERNS OF GENETIC AND MORPHOLOGICAL VARIATION IN SAVANNAH SPARROWS (PASSERCULUS SANDWICHENSIS).

Surveys of genetic population structure are often limited to large geographic scales because geographically close populations are indistinguishable. Genetic uniformity across adjacent demes can be interpreted as evidence for cohesion (panmixia) or recent divergence. However, poor genetic resolution at microgeographic scales can also arise from the use of overly conservative (slowly evolving) markers. This study examines the ability of hypervariable, minisatellite loci to discriminate among geographically close populations of Savannah sparrows (Passerculus sandwichensis) and to track morphological differentiation at a microgeographic scale (interregional distance < 55 km). Savannah sparrows breeding at five island and two mainland sites in the Bay of Fundy, New Brunswick, Canada, show concordant patterns of variation in external morphology (seven characters) and multilocus DNA fingerprinting profiles (Sxy ): island sparrows are phenotypically larger and genetically more similar to each other than they are to mainland sparrows. This pattern of variation is consistent with both adaptive (natural selection) and nonadaptive (genetic drift) mechanisms of population divergence. Based on minisatellite diversity, the effective size of both island and mainland populations is 37, an estimate substantially lower than census population sizes. These data are discordant with observations of sparrow vagility and abundance and suggest a closer examination of microgeographic patterns in avian systems.

Genetic variation among populations of Mytilus spp. in eastern Newfoundland

Limited sampling of Mytilus spp. in eastern Newfoundland has suggested that M. trossulus Gould and M. edulis L. are distributed as pure populations or mixtures of both species. In 1989 and 1990 we collected a wider series of samples of Mytilus spp. from 16 sites in eastern Newfoundland in order to determine the genetic structure based on four polymorphic enzyme loci (Est, Pgm, Lap, Ap). Associations among loci (particularly Pgm and Est), and a bimodal distribution of individual principal-component scores (based on fourlocus genotypes) provided evidence for the presence of both M. trossulus and M. edulis. Most of the sites consisted of a mixture of the two species. The relative frequency of each species in the samples varied on both a macro- and microgeographic scale. Two wave-exposed sites had the highest frequency (>90%) of M. trossulus. The distribution of hybrid index scores, based on three partially diagnostic loci (Est, Pgm, Lap), provided no evidence for hybridization.

Coexistence and weak amensalism of congeneric gall‐forming aphids on the Japanese elm

AbstractClosely related species of gall‐forming aphids are often associated with a single host species. Six Tetraneura species coexist on the Japanese elm, Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro‐ and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species. Tetraneura radicicola and sorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity of radicicola and sorini, but had a negative influence on that of sp. O. On average, radicicola and sorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence in not so strong as to exclude sp. O competitively from the Tetraneura community.

Genetic differentiation of deep-sea hydrothermal vent alvinellid populations (Annelida: Polychaeta) along the East Pacific Rise

The alvinellid polychaetes, which live in the hottest part of the deep-sea hydrothermal environment, have a nested island-like distribution and locally are subjected to extinctions. They are sedentary and exhibit a peculiar reproductive behaviour and a development which may result in little or no planktonic stage (i.e. larval dispersal). The genetic variation within and among populations of the three main species (Alvinella pompejana, Alvinella caudata and Paralvinella grasslei) inhabiting vents along the East Pacific Rise was examined at a hierarchy of spatial scales using allozyme electrophoresis. The genetic diversity of P. grasslei is high (Ho = 0.24), about twice that of both the Alvinella species (Ho = 0.10). The three species show a strong tendency towards a heterozygote deficiency which systematically occurs at the same loci in nearly all the populations. These structures are particularly obvious in the genus Alvinella and might be explained by differential allozyme fitness. Populations display considerable genetic differentiation at the microgeographical scale, which could be explained by repeated founder effects in populations, but it varies from species to species according to their possible ability to be transported by crabs from vent to vent. However, the genetic variation among populations separated by at least 1000 km is of the same magnitude as that found within the 13°N/EPR segment. These results demonstrate that each species maintains its genetic identity along the oceanic rifts despite the evidence for founder effects. To explain this phenomenon, we hypothesize that in such a harsh environment, genetic drift in alvinellid populations could be balanced by a uniform selective pressure stemming from the vent chemistry.

Predation risk and alternative mating tactics in male Trinidadian guppies (Poecilia reticulata).

In the guppy (Poecilia reticulata), males have two alternative mating tactics. Individual males may either display to a receptive female prior to attempting to copulate with her or attempt to quickly sneakcopulate with a female without first displaying to her or without a prior receptive response from her. In this study, I experimentally investigated the effects of simulated local increases in the risk of predation (in the form of a cichlid fish predator model in situ) on the mating tactics used by free-ranging male guppies in two typical macrohabitats (riffle and pool) of a Trinidadian river. Focal male guppies displayed to females significantly less often on average, and conversely attempted sneak copulations more often, in the presence of the predator model than in its absence; this pattern was similar for both habitats. These fish therefore performed a lower proportion of sigmoid displays and increased their sneaky mating attempts when the apparent risk of predation had increased locally. This predator-mediated response is consistent with a trade-off between mating success and risk of mortality due to predation. The results are the first to confirm risk-sensitive mating behaviour in free-ranging male guppies within a population, and demonstrate the potential importance of predators in influencing the relative use of alternative mating tactics in this species on a microgeographical scale in the wild.

Changes in Vegetation and Soil Fertility along a Predictable Snowmelt Gradient in the Mosquito Range, Colorado, U. S. A.

Changes in edaphic conditions and vegetation along snowmelt gradients are well known in many alpine areas, but very few studies have used multivariate techniques to document these changes at a microgeographic scale and to measure predictability of snowmelt patterns. This paper describes concordant changes in microenvironmental factors and plant species abundances among >200 evenly spaced microsites in a large alpine snowbed in the Mosquito Range, Colorado. The snowmelt gradient was highly consistent: averaged over all microsites, the mean date of snowmelt varied by only 1 wk from 1989 to 1992, and relative date of snowmelt among microsites was highly predictable (R2 > 90% among years). Soil in later melting microsites had reduced organic content, water content, nitrogen, phosphorous, and acidity. Total soil nitrogen and water content decreased through the growing season, while pH increased significantly. Species richness and total vegetation cover were significantly greater in early melting microsites, and soil disturbance by pocket gophers and erosion peaked in moderately latemelting areas. Canonical correspondence analysis (CCA) showed that microgeographic changes in the plant community were associated with four environmental factors: snowmelt date, rock cover, soil disturbance, and soil organic content. Plant species that are common in the vegetation surrounding the snowbed tended to decrease in abundance in later melting snowbed sites. Other species attained peak abundance in the less productive late melting sites, and a few appeared to be associated with soil disturbance. Plant community differences between paired sampling units (quadrats) were determined more by differences in their snowmelt date than by their spatial separation, suggesting that plant species distributions along this alpine gradient are determined principally by ecological interactions and physiological tolerances, rather than by historical accident and limited dispersal.

The local distribution of highly divergent mitochondrial DNA haplotypes in toque macaques Macaca sinica at Polonnaruwa, Sri Lanka

Surveys of mitochondrial DNA (mtDNA) variation in macaque monkeys have revealed extremely high levels of intraspecific divergence among haplotypes. One consistent pattern that has emerged from these studies is that divergent haplotypes are geographically segregated so that sampling a few matrilines from a given region shows them to be identical, or a closely related subset of haplotypes. Geographically structured mtDNA variation has also been commonly observed in other taxa. In this study, haplotype variation and distribution are studied in detail within a local population of toque macaques. The results show that highly divergent haplotypes, differing by 3.1% in their nucleotide sequences, coexist in this population and that they may be spatially segregated even on this micro-geographic scale. Furthermore, these differences are maintained between social groups that exchange male migrants, and thus nuclear genes, frequently.

Multilocus DNA fingerprinting detects population differentiation in the outbred and abundant fish species Poecilia latipinna

AbstractSeveral workers have suggested that multilocus multilocus variable number of tandem repeat (VNTR) based ‘DNA fingerprints’ are not useful in detecting differentiation among outbred populations. They suggest that the extremely high mutation rates and complexity associated with multilocus VNTR fragments make detection of interpopulation differences against a background of extremely high intrapopulation variation unlikely. This paper shows that DNA fingerprinting with the multilocus VNTR probes (GACA)4 and (CT)9 reveal significant population differences in VNTR frequencies between Florida and Georgia populations of the outbred, abundant and vagile fish species Poecilia latipinna. Differences in mutation rates among some VNTR loci may account for the ability to detect interpopulation differentiation with these probes. These results suggest that appropriate species/probe combinations would allow investigations of population structure on a microgeographical scale even in outbred species with multilocus VNTR probes where less‐sensitive techniques have failed.