Microgeographical Scales Research Articles

Dominance of species sorting over dispersal at microgeographical scales in polluted lotic ecosystems

Global rivers, particularly those in populated urban areas, are challenged by multiple stressors such as pollution from anthropogenic activities. Dissecting the relative role of each ecological process involved in structuring biotic communities is fundamental in both theoretical and applied ecology. The ecological niche-based species sorting and stochastic dynamics-based dispersal are two major competing processes in determining community structure. Studies have reached a common realization on the environmental gradient-geographical scale relationship (EGGSR), resulting in species sorting over dispersal in determining community structure at large geographical scales where significant environmental gradients often exist. However, this relationship has been recently challenged at fine geographical scales when significant environmental gradients are formed by local anthropogenic activities. Here, we used three receiving rivers of wastewater treatment plants (WWTPs) as the promising system to test the hypothesis that species sorting plays a dominant role over dispersal in structuring zooplankton communities at microgeographical scales (∼1.2 km). After WWTP effluent discharge, we consistently detected significant environmental changes in all three receiving rivers, leading to significant variation in both community structure and taxonomic co-occurrence networks. Variation partitioning showed that environmental variables explained higher proportions of community variation than spatial ones, supporting that species sorting played a dominant role over dispersal in structuring zooplankton communities. Thus, our findings here reject EGGSR, illustrating that the effect of species sorting has been overlooked in disturbed aquatic ecosystems at fine spatial scales. More importantly, all analyses in multiple rivers here validate the “microscale species sorting” hypothesis. The validation of such hypothesis provides a novel methodology for point source pollution management by assessing environment-community interactions and functional changes of biological communities. The differed variables underlying species sorting among three rivers illustrate that ecological management should be case-specific, with the full consideration of local water quality background and pollutant composition of each point pollution source.

Common microgeographical selection patterns revealed in four European conifers.

Microgeographical adaptation occurs when the effects of directional selection persist despite gene flow. Traits and genetic loci under selection can then show adaptive divergence, against the backdrop of little differentiation at other traits or loci. How common such events are and how strong the selection is that underlies them remain open questions. Here, we discovered and analysed microgeographical patterns of genomic divergence in four European and Mediterranean conifers with widely differing life-history traits and ecological requirements (Abies alba MIll., Cedrus atlantica [Endl.] Manetti, Pinus halepensis Mill. and Pinus pinaster Aiton) by screening pairs from geographically close forest stands sampled along steep ecological gradients. We inferred patterns of genomic divergence by applying a combination of divergence outlier detection methods, demographic modelling, Approximate Bayesian Computation inferences and genomic annotation to genomic data. Surprisingly for such small geographical scales, we showed that selection is strong in all species but generally affects different loci in each. A clear signature of selection was systematically detected on a fraction of the genome, of the order of 0.1%-1% of the loci depending on the species. The novel modelling method we designed for estimating selection coefficients showed that the microgeographical selection coefficient scaled by population size (Ns) was 2-30. Our results convincingly suggest that selection maintains within-population diversity at microgeographical scales in spatially heterogeneous environments. Such genetic diversity is likely to be a major reservoir of adaptive potential, helping populations to adapt under fluctuating environmental conditions.

Evidences for genetic differentiation within the highly endemic and endangered annual fish Austrolebias nigrofasciatus (Cyprinodontiformes: Rivulidae).

Samples of Austrolebias nigrofasciatus (n = 103), an endangered species of annual fish endemic to a small area of the Patos-Mirim lagoon system encompassing the São Gonçalo Channel lowlands, were collected from eight isolated temporary ponds, four located at the known distribution range of the species and four located along the Piratini River lowlands, where morphologically different individuals were found. In the laboratory, fragments of the mitochondrial cytochrome c oxidase I (coI), cytochrome b (cytb) and nuclear rhodopsin (rho) genes were amplified, purified and sequenced for 100, 99 and 58 of these individuals, respectively. Samples were further analysed using phylogenetic and phylogeographic methods to evaluate the patterns of genetic diversity and differentiation presented within and between populations, while assessing their evolutionary history, in order to guide the application of further conservation strategies. We found that the four new populations from the Piratini River lowlands encompass a different lineage of A. nigrofasciatus that diverged from that encountered in the São Gonçalo Channel at approximately 0.165 M years before present, during a population expansion and did not yet attain reciprocal monophyly. This divergence was associated with a glacial event that was preceded by an interglacial period putatively associated with the dispersal. Moreover, significant levels of genetic differentiation and a high number of exclusive haplotypes could be encountered even in micro-geographical scales, as in the comparisons between populations located within the same major lineage, indicating each of them may encompass independent management units. Conservation actions are certainly urgent, especially in the face of signs of a recent bottleneck.

Complex genetic diversity patterns of cryptic, sympatric brown trout (Salmo trutta) populations in tiny mountain lakes

Intraspecific genetic variation can have similar effects as species diversity on ecosystem function; understanding such variation is important, particularly for ecological key species. The brown trout plays central roles in many northern freshwater ecosystems, and several cases of sympatric brown trout populations have been detected in freshwater lakes based on apparent morphological differences. In some rare cases, sympatric, genetically distinct populations lacking visible phenotypic differences have been detected based on genetic data alone. Detecting such “cryptic” sympatric populations without prior grouping of individuals based on phenotypic characteristics is more difficult statistically, though. The aim of the present study is to delineate the spatial connectivity of two cryptic, sympatric genetic clusters of brown trout discovered in two interconnected, tiny subarctic Swedish lakes. The structures were detected using allozyme markers, and have been monitored over time. Here, we confirm their existence for almost three decades and report that these cryptic, sympatric populations exhibit very different connectivity patterns to brown trout of nearby lakes. One of the clusters is relatively isolated while the other one shows high genetic similarity to downstream populations. There are indications of different spawning sites as reflected in genetic structuring among parr from different creeks. We used >3000 SNPs on a subsample and find that the SNPs largely confirm the allozyme pattern but give considerably lower FST values, and potentially indicate further structuring within populations. This type of complex genetic substructuring over microgeographical scales might be more common than anticipated and needs to be considered in conservation management.

Epigenetic Variability across Human Populations: A Focus on DNA Methylation Profiles of the KRTCAP3, MAD1L1 and BRSK2 Genes.

Natural epigenetic diversity has been suggested as a key mechanism in microevolutionary processes due to its capability to create phenotypic variability within individuals and populations. It constitutes an important reservoir of variation potentially useful for rapid adaptation in response to environmental stimuli. The analysis of population epigenetic structure represents a possible tool to study human adaptation and to identify external factors that are able to naturally shape human DNA methylation variability. The aim of this study is to investigate the dynamics that create epigenetic diversity between and within different human groups. To this end, we first used publicly available epigenome-wide data to explore population-specific DNA methylation changes that occur at macro-geographic scales. Results from this analysis suggest that nutrients, UVA exposure and pathogens load might represent the main environmental factors able to shape DNA methylation profiles. Then, we evaluated DNA methylation of candidate genes (KRTCAP3, MAD1L1, and BRSK2), emerged from the previous analysis, in individuals belonging to different populations from Morocco, Nigeria, Philippines, China, and Italy, but living in the same Italian city. DNA methylation of the BRSK2 gene is significantly different between Moroccans and Nigerians (pairwise t-test: CpG 6 P-value = 5.2*10 −3; CpG 9 P-value = 2.6*10 −3; CpG 10 P-value = 3.1*10 −3; CpG 11 P-value = 2.8*10 −3). Comprehensively, these results suggest that DNA methylation diversity is a source of variability in human groups at macro and microgeographical scales and that population demographic and adaptive histories, as well as the individual ancestry, actually influence DNA methylation profiles.

Influence of ecological and geological features on rangewide patterns of genetic structure in a widespread passerine.

Geological and ecological features restrict dispersal and gene flow, leading to isolated populations. Dispersal barriers can be obvious physical structures in the landscape; however microgeographic differences can also lead to genetic isolation. Our study examined dispersal barriers at both macro- and micro-geographical scales in the black-capped chickadee, a resident North American songbird. Although birds have high dispersal potential, evidence suggests dispersal is restricted by barriers. The chickadee's range encompasses a number of physiological features which may impede movement and lead to divergence. Analyses of 913 individuals from 34 sampling sites across the entire range using 11 microsatellite loci revealed as many as 13 genetic clusters. Populations in the east were largely panmictic whereas populations in the western portion of the range showed significant genetic structure, which often coincided with large mountain ranges, such as the Cascade and Rocky Mountains, as well as areas of unsuitable habitat. Unlike populations in the central and southern Rockies, populations on either side of the northern Rockies were not genetically distinct. Furthermore, Northeast Oregon represents a forested island within the Great Basin; genetically isolated from all other populations. Substructuring at the microgeographical scale was also evident within the Fraser Plateau of central British Columbia, and in the southeast Rockies where no obvious physical barriers are present, suggesting additional factors may be impeding dispersal and gene flow. Dispersal barriers are therefore not restricted to large physical structures, although mountain ranges and large water bodies do play a large role in structuring populations in this study.

Evidence of population genetic effects in Peromyscus melanophrys chronically exposed to mine tailings in Morelos, Mexico

Effects of environmental chemical pollution can be observed at all levels of biological organization. At the population level, genetic structure and diversity may be affected by exposure to metal contamination. This study was conducted in Huautla, Morelos, Mexico in a mining district where the main contaminants are lead and arsenic. Peromyscus melanophrys is a small mammal species that inhabits Huautla mine tailings and has been considered as a sentinel species. Metal bioaccumulation levels were examined by inductively coupled plasma mass spectrometry and genetic analyses were performed using eight microsatellite loci in 100 P. melanophrys individuals from 3 mine tailings and 2 control sites. The effect of metal bioaccumulation levels on genetic parameters (population and individual genetic diversity, genetic structure) was analyzed. We found a tissue concentration gradient for each metal and for the bioaccumulation index. The highest values of genetic differentiation (Fst and Rst) and the lowest number of migrants per generation (Nm) were registered among the exposed populations. Genetic distance analyses showed that the most polluted population was the most genetically distant among the five populations examined. Moreover, a negative and significant relationship was detected between genetic diversity (expected heterozygosity and internal relatedness) and each metal concentration and for the bioaccumulation index in P. melanophrys. This study highlights that metal stress is a major factor affecting the distribution and genetic diversity levels of P. melanophrys populations living inside mine tailings. We suggest the use of genetic population changes at micro-geographical scales as a population level biomarker.

Microgeographical population structure and adaptation in Atlantic cod Gadus morhua: spatio-temporal insights from gene-associated DNA markers

Recent technical advances have stimulated studies on spatial scales of adaptive genetic variation in marine fishes. However, very few studies have combined spatial and temporal sampling to investigate adaptive genetic structuring at local and microgeographical scales, i.e. scales at which neutral genetic markers have typically revealed very limited levels of population structure. In the present study we analyzed 92 gene-associated single-nucleotide polymorphism (SNP) markers in Atlantic cod Gadus morhua from several sampling sites within the North Sea and adjacent areas. To test for temporal stability, we included longand short-term (i.e. from 24 to 38 and from 8 to 11 yr, respectively) temporally replicated samples from a subset of populations. As expected, we found very low levels of neutral genetic population structure (FST = 0.003). Three specific loci, however, showed highly elevated levels of genetic differentiation. Interestingly, these loci were identical to loci previously found to display signals of adaptive evolution on larger spatial scales. Analysis of historical samples revealed long-term temporally stable patterns of both neutral and adaptive divergence between some populations, indicating long-term temporal adaptive stability driven by strong local selection. In an environmentally dynamic area, on the other hand, patterns of genetic structuring were more variable. Overall, our results not only suggest separation of populations under both evolutionary and ecological paradigms, but also illustrate the usefulness of the spatio-temporal approach for making inferences about the dynamics and geographical distribution of adaptive genetic variation in natural populations.

Byssus secretion of Mytilus galloprovincialis: effect of site at macro- and micro-geographical scales within Ría de Vigo (NW Spain)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 435:125-140 (2011) - DOI: https://doi.org/10.3354/meps09200 Byssus secretion of Mytilus galloprovincialis: effect of site at macro- and micro-geographical scales within Ría de Vigo (NW Spain) Jose M. F. Babarro1,*, Emily Carrington2 1Instituto de Investigaciones Marinas CSIC, Eduardo Cabello 6, 36208 Vigo, Spain 2Friday Harbor Laboratories, Department of Biology, University of Washington, Friday Harbor, Washington 98250, USA *Email: jbabarro@iim.csic.es ABSTRACT: The effect of the abiotic environment on byssus tenacity and associated features was investigated for Mytilus galloprovincialis in the Ría de Vigo (NW Spain). The effect of site was examined at macro-geographical (outer exposed Cabo Estay vs. inner sheltered San Simón Ensenada sites) and micro-geographical (intertidal vs. subtidal locations) scales. Site significantly influenced byssus tenacity, shape and byssus thread diameter, whereas location did not. Qualitative analysis of the byssus corroborated the importance of site; mussels inhabiting the rougher outer Ría secreted stronger and stiffer threads regardless of location and had a higher potential to form cross-links or metal chelation in the byssal collagen to gain structural integrity when needed. When mussels were transplanted between exposed and sheltered sites, asymmetrical changes were observed in tenacity, endogenous indices, byssus morphology and mechanical properties after 3 mo. Individuals transferred from the sheltered to the exposed site shifted all variables, suggesting that mussels have a plastic response to rougher environments by increasing byssus size and mechanical integrity. In contrast, mussels transplanted from the exposed to the sheltered site shifted tenacity, endogenous indices and thread length but not thread diameter nor mechanical properties. In summary, we report the highly dynamic nature of the mussel ability to modify byssus tenacity when subjected to abrupt environmental changes. Mussels have the potential to change byssus diameter and mechanical properties to increase strength in stressful abiotic conditions, and can re-allocate energy for vital structures such as gonadal and soft tissue growth in more benign environments. KEY WORDS: Mytilus galloprovincialis · Tenacity · Field · Byssus secretion · Abiotic environment · Tidal exposure · Plasticity Full text in pdf format PreviousNextCite this article as: Babarro JMF, Carrington E (2011) Byssus secretion of Mytilus galloprovincialis: effect of site at macro- and micro-geographical scales within Ría de Vigo (NW Spain). Mar Ecol Prog Ser 435:125-140. https://doi.org/10.3354/meps09200 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 435. Online publication date: August 22, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Second-generation environmental sequencing unmasks marine metazoan biodiversity

Biodiversity is of crucial importance for ecosystem functioning, sustainability and resilience, but the magnitude and organization of marine diversity at a range of spatial and taxonomic scales are undefined. In this paper, we use second-generation sequencing to unmask putatively diverse marine metazoan biodiversity in a Scottish temperate benthic ecosystem. We show that remarkable differences in diversity occurred at microgeographical scales and refute currently accepted ecological and taxonomic paradigms of meiofaunal identity, rank abundance and concomitant understanding of trophic dynamics. Richness estimates from the current benchmarked Operational Clustering of Taxonomic Units from Parallel UltraSequencing analyses are broadly aligned with those derived from morphological assessments. However, the slope of taxon rarefaction curves for many phyla remains incomplete, suggesting that the true alpha diversity is likely to exceed current perceptions. The approaches provide a rapid, objective and cost-effective taxonomic framework for exploring links between ecosystem structure and function of all hitherto intractable, but ecologically important, communities.

Locating irregularly shaped clusters of infection intensity

Patterns of disease may take on irregular geographic shapes, especially when features of the physical environment influence risk. Identifying these patterns can be important for planning, and also identifying new environmental or social factors associated with high or low risk of illness. Until recently, cluster detection methods were limited in their ability to detect irregular spatial patterns, and limited to finding clusters that were roughly circular in shape. This approach has less power to detect irregularly-shaped, yet important spatial anomalies, particularly at high spatial resolutions. We employ a new method of finding irregularly-shaped spatial clusters at micro-geographical scales using both simulated and real data on Schistosoma mansoni and hookworm infection intensities. This method, which we refer to as the "greedy growth scan", is a modification of the spatial scan method for cluster detection. Real data are based on samples of hookworm and S. mansoni from Kitengei, Makueni district, Kenya. Our analysis of simulated data shows how methods able to find irregular shapes are more likely to identify clusters along rivers than methods constrained to fixed geometries. Our analysis of infection intensity identifies two small areas within the study region in which infection intensity is elevated, possibly due to local features of the physical or social environment. Collectively, our results show that the "greedy growth scan" is a suitable method for exploratory geographical analysis of infection intensity data when irregular shapes are suspected, especially at micro-geographical scales.

Recent volcanism and mitochondrial DNA structuring in the lizard Gallotia atlantica from the island of Lanzarote

The phylogeography of the lacertid lizard Gallotia atlantica from the small volcanic island of Lanzarote (Canary Islands) was analysed based on 1075 bp of mitochondrial DNA (mtDNA) sequence (partial cytochrome b and ND2) for 157 individuals from 27 sites (including three sites from neighbouring islets). Levels of sequence divergence were generally low, with the most distant haplotypes separated by only 14 mutational steps. MtDNA divergence appears to coincide with formation of the middle Pleistocene lowland that united formerly separate ancient islands to form the current island of Lanzarote, allowing rejection of a two-island model of phylogeographical structure. There was evidence of large-scale population expansion after island unification, consistent with the colonization of new areas. A nested clade phylogeographical analysis (NCPA) revealed significant phylogeographical structuring. Two-step and higher-level clades each had disjunct distributions, being found to the east and west of a common area with a north-south orientation that extends between coasts in the centre-east of the island (El Jable). Other clades were almost entirely restricted to the El Jable region alone. Bayesian Markov chain Monte Carlo analyses were used to separate ongoing gene flow from historical associations. These supported the NCPA by indicating recent (75,000-150,000 years ago) east-west vicariance across the El Jable region. Lava flows covered El Jable and other parts of the central lowland at this time and likely led to population extinctions and temporary dispersal barriers, although present-day evidence suggests some populations would have survived in small refugia. Expansion of the latter appears to explain the presence of a clade located between the eastern and western components of the disjunct clades. Direct relationships between mtDNA lineages and morphology were not found, although one of two morphological forms on the island has a disjunct distribution that is broadly concordant with east-west components of the phylogeographical pattern. This work demonstrates how recent volcanic activity can cause population fragmentation and thus shape genetic diversity on microgeographical scales.

Macrogeographic and microgeographic genetic structure of the Chagas’ disease vector Triatoma infestans (Hemiptera: Reduviidae) from Catamarca, Argentina

The genetic structure in populations of the Chagas' disease vector Triatoma infestans from six localities belonging to areas under the same insecticide treatment conditions of Catamarca province (Argentina) was examined at macrogeographical and microgeographical scales. A total of 238 insects were typed for 10 polymorphic microsatellite loci. The average observed and expected heterozygosities ranged from 0.319 to 0.549 and from 0.389 to 0.689, respectively. The present results confirm that populations of T. infestans are highly structured. Spatial genetic structure was detectable at macrogeographical and microgeographical levels. Comparisons of the levels of genetic variability between two temporal samples were carried out to assess the impact of the insecticide treatment. The genetic diversity of the population was not significantly affected after insecticide use since different genetic parameters (allele number, observed and expected heterozygosities) remained stable. However, loss of low frequency alleles and not previously found alleles were detected. The effective population size (N(e)) estimated was substantially lower in the second temporal sample than in the first; nevertheless, it is possible that the size of the remnant population after insecticide treatment was still large enough to retain the genetic diversity. Very few individuals did not belong to the local T. infestans populations as determined by assignment analyses, suggesting a low level of immigration in the population. The results of the assignment and first-generation migrant tests suggest male-biased dispersal at microgeographical level.

Environment Explorer: Spatial Support System for the Integrated Assessment of Socio-Economic and Environmental Policies in the Netherlands

Environment Explorer is a system developed to support spatial scientists, planners and decision makers at the regional and national levels in the Netherlands to help them analyse a wide range of social, economic and environmental policies and their associated temporal and spatial dynamics. The core of this system consists of linked dynamic spatial models operating at both the macro- and the micro-geographical scales. At the macro scale, the modelling framework integrates several component sub models, representing the natural, social, and economic sub-systems. At the micro level, cellular automata based models determine the fate of individual parcels of land, based on institutional, physical and environmental factors as well as on the type of activities in their immediate neighbourhoods. The approach chosen enables the straightforward integration of detailed physical, environmental, and institutional characteristics as well as the particulars of the transportation infrastructure, and permits a very detailed representation of the evolving spatial system. As part of the policy support system, the models are supplemented with dedicated tools for interactive design, analysis and evaluation of the policy interventions and scenarios to be tried out. The system covers the entire territory of the Netherlands and represents processes at the national, the regional (40 economic regions), and the cellular (25 ha cells) levels. It runs on top of detailed GIS information and generates future land use and land cover for the period 2000 till 2030. The quality of the policies tried out is expressed in some 40 economic, social and environmental indicators available in the model as dynamic maps. The application has been developed over the past 5 years. It has been used at the national and the provincial level for the preparation of spatial policy documents. Some conclusions relative to the development and the use of the system are presented.

GENETIC RESOURCES OF WILD EMMER, TRITICUM DICOCCOIDES, FOR WHEAT IMPROVEMENT IN THE THIRD MILLENNIUM

The genetic resources and diversities of wild emmer wheat, Triticum dicoccoides, the progenitor of almost all cultivated wheats, are reviewed, primarily based on the research program at the Institute of Evolution, University of Haifa, Israel, during the period 1979–2000. Diverse genetic polymorphisms have been described across Israel in the following systems: allozymes and DNA markers at both macro- and microgeographical scales; a and b amylases; photosynthetic yield; Cab (chlorophyll a/b binding proteins) circadian rhythms; herbicide response; genetic divergence of heat production; salt tolerance; Na22 uptake; drought and temperature resistance, storage proteins; amino acids; and disease resistances (leaf, stem, and stripe rusts; wheat soil-borne mosaic virus) and their ecological and allozyme and/or DNA marker correlates. Genetic mapping of resistance to chlorotoluron, yellow rust, and powdery mildew on chromosomes 6B, 1BS, and 5BS, respectively, was conducted as a prelude to cloning and transformation....

Microsatellite Diversity Correlated with Ecological-Edaphic and Genetic Factors in Three Microsites of Wild Emmer Wheat in North Israel

This study was conducted to test the effects of internal (genetic) and external factors on allelic diversity at 27 dinucleotide microsatellite (simple sequence repeat [SSR]) loci in three Israeli natural populations of Triticum dicoccoides from Ammiad, Tabigha, and Yehudiyya, north of the Sea of Galilee. The results demonstrated that SSR diversity is correlated with the interaction of ecological and genetic factors. Genetic factors, including genome (A vs. B), chromosome, motif, and locus, affected average repeat number (ARN), variance in repeat number (sigma), and number of alleles (NA) of SSRs, but the significance of some factors varied among populations. Genome effect on SSR variation may result from different motif types, particularly compound (or imperfect) versus perfect motifs, which may be related to different evolutionary histories of genomes A and B. Ecological factors significantly affected SSR variation. Soil-unique and soil-specific alleles were found in two edaphic groups dwelling on terra rossa and basalt soils across macro- and microgeographical scales. The largest contributions of genetic and ecological effects were found for diversity of ARN and NA, respectively. Multiple regression indicated that replication slippage and unequal crossing over could be important mutational mechanisms, but their significance varied among motifs. Edaphic stresses may affect the probability of replication errors and recombination intermediates and thus control diversity level and divergence of SSRs. The results may indicate that SSR diversity is adaptive, channeled by natural selection and influenced by both internal and external factors and their interactions.

Genetic diversity in wild cereals: regional and local studies and their bearing on conservation ex situ and in situ

The current alarming global crisis and extinction of biodiversity affect negatively the planet's biosphere. Conservation of biodiversity is one attempt to alleviate the pending extinction of the biosphere by humans. Genetic diversity, the basis of evolution by natural selection, is gravely threatened in the progenitors of cultivated plants and its exploration, evaluation, conservation in situ and ex situ is imperative to guarantee sustainable development. This is illustrated by the population genetics and ecology of two important progenitors of cereals wild, wheat and barley. The wild cereals are rich in adaptive genetic diversity in the Fertile Crescent, primarily in Israel, which is their center of origin and diversity. The 40–55% intrapopulation diversity level in the wild cereals contrasts sharply with the average of 80% in outbreeders. Genetic diversity in wild wheat and barley is structured, particularly in wild emmer wheat, as an 'archipelago' ecological and genetic structure. These include central, semi-isolated and ecologically peripheral and marginal isolated populations, where specific alleles and allele combinations predominate as coadapted blocks of genes, adaptive to diverse ecological stresses. These involve both physical (climatic and edaphic) and biotic (pathogens and parasites) stresses at macro- and microgeographical scales. Complementary in situ and ex situ conservation is imperative across the geographic range of these species, to safeguard their immensely important genetic resources for crop improvement.

Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization

We describe genetic structure at microgeographical scales in two social forms of the introduced fire ant Solenopsis invicta using nuclear and mitochondrial markers. Fire ant populations in northern Georgia, U.S.A. are structured in fundamentally different ways at the two genomes. Virtually all nuclear genetic variation resides within sampling sites; no differentiation was apparent between the social forms and only minimal differentiation was found among sites in only one social form (the polygyne form, which has nests with numerous queens). In contrast, substantial mtDNA variation exists at all levels; the forms are significantly differentiated and strong divergence was detected among sites in the polygyne form. No mtDNA differentiation was found among sites in the monogyne form (which has nests with a single queen). These results are predicted on the basis of the social biology of the ants. Monogyne queens and males of both forms presumably disperse widely, and polygyne queens commonly mate with monogyne males, thus explaining the uniformity of nuclear allele frequencies across sampling sites and between the forms. Polygyne queens display habits consistent with limited dispersal, and queens of both forms seem unable to establish themselves as reproductives in nests of the alternate form, thus explaining the strong microgeographical structure within the polygyne form and differentiation between the forms found using mtDNA markers. The mtDNA haplotypes are distributed among sites in a manner that suggests a mechanism for the spread of polygyny in introduced fire ants. Monogyne queens apparently mate occasionally with polygyne males and acquire a Mendelian nuclear factor allowing the expression of polygyny in their sexual daughters.

Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization

We describe genetic structure at microgeographical scales in two social forms of the introduced fire ant Solenopsis invicta using nuclear and mitochondrial markers. Fire ant populations in northern Georgia, U.S.A. are structured in fundamentally different ways at the two genomes. Virtually all nuclear genetic variation resides within sampling sites; no differentiation was apparent between the social forms and only minimal differentiation was found among sites in only one social form (the polygyne form, which has nests with numerous queens). In contrast, substantial mtDNA variation exists at all levels; the forms are significantly differentiated and strong divergence was detected among sites in the polygyne form. No mtDNA differentiation was found among sites in the monogyne form (which has nests with a single queen). These results are predicted on the basis of the social biology of the ants. Monogyne queens and males of both forms presumably disperse widely, and polygyne queens commonly mate with monogyne males, thus explaining the uniformity of nuclear allele frequencies across sampling sites and between the forms. Polygyne queens display habits consistent with limited dispersal, and queens of both forms seem unable to establish themselves as reproductives in nests of the alternate form, thus explaining the strong microgeographical structure within the polygyne form and differentiation between the forms found using mtDNA markers. The mtDNA haplotypes are distributed among sites in a manner that suggests a mechanism for the spread of polygyny in introduced fire ants. Monogyne queens apparently mate occasionally with polygyne males and acquire a Mendelian nuclear factor allowing the expression of polygyny in their sexual daughters.

Evolution of ribosomal DNA (rDNA) genetic structure in colonial Californian populations of Avena barbata.

DNA samples from 980 plants of Avena barbata from 48 ecologically diverse sites in California and Oregon were assayed to determine their genotype for two duplicated loci governing rDNA variants. More than 40 different rDNA genotypes were observed among which 5 made up 96% of our sample in environmentally homogeneous sites; predominant genotypes were less frequent and recombinant genotypes were more frequent in environmentally heterogeneous sites. The spatial distribution of each predominant rDNA genotype was nearly an exact overlay on both macro- and microgeographical scales of a distinctive habitat and also of the distribution of an eight-locus morphological-allozyme variant genotype. In all, seven different habitat-genotype combinations (ecotypes) were distinguishable on the basis of their morphological-allozyme-rDNA genotypes. None of these seven genotypes has been found in ancestral Spanish populations; thus the above predominant multilocus genotypes (ecotypes) of the colonial populations evidently evolved subsequent to the recent introduction (within 150-200 generations) of A. barbata to California. The precise associations of specific alleles and genotypes of the morphological allozyme and rDNA loci with different specifiable habitats leads us to the conclusion that natural selection favoring particular multilocus combinations of alleles in different habitats was the main guiding force in shaping the internal genetic structure of local populations as well as the overall adaptive landscape of A. barbata over California and Oregon.