Microgeographic population structure of green swordail fish: genetic differentiation despite abundant migration

Swordtails (Xiphophorus; Poeciliidae) have figured prominently in research on fish mating behaviours, sexual selection, and carcinogenesis, but their population structures and dispersal patterns have been relatively neglected. Using nine microsatellite loci, we estimated genetic differentiation in Xiphophorus helleri within and between adjacent streams in Belize. The genetic data were complemented by a tagging study of movement within one stream. In the absence of physical dispersal barriers (waterfalls), population structure followed an isolation by distance (IBD) pattern. Genetic differentiation (F_ST up to 0.07) was significant between and within creeks, despite high dispersal in the latter as judged by the tagging data. Such heterogeneity apparently was a result of genetic drift in local demes, due to small population sizes and highly skewed paternity. The IBD pattern was interrupted by waterfalls, boosting F_ST above 0.30 between adjacent samples across these barriers. Overall, our results are helpful in understanding the interplay of evolutionary forces and population dynamics in a small fish living in a changeable habitat.     

POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE MEDITERRANEAN LAND SNAIL ALBINARIA CORRUGATA (PULMONATA: CLAUSILIIDAE)

The amount of gene flow among local populations partly determines the relative importance of genetic drift and natural selection in the differentiation of such populations. Land snails, because of their limited powers for dispersal, may be particularly likely to show such differentiation. In this study, we directly estimate gene flow in Albinaria corrugata, a sedentary, rock-dwelling gastropod from Crete, by mark-recapture studies. In the same area, 23 samples were taken and studied electrophoretically for six polymorphic enzyme loci. The field studies indicate that the population structure corresponds closely to the stepping-stone model: demes are present on limestone boulders that are a few meters apart, and dispersal takes place mainly between adjacent demes. Average deme size (N) is estimated at 29 breeding individuals and the proportion of migrants per generation at 0.195 (Nm = 5.7). We find no reason to assume long-distance dispersal, apart from dispersal along occasional stretches of suitable habitat. Genetic subdivision of the population, as derived from F_ST values, corresponds to the direct estimate only at the lowest spatial level (distance between sample sites < 10 m), where values for Nm of 5.4 and 17.6 were obtained. In contrast, at the larger spatial scales, F_ST values give gene-flow estimates that are incompatible with the expected amount of gene flow at these scales. We explain these discrepancies by arguing that gene flow is in fact extremely limited, making correct estimates of Nm from F_ST impossible at the larger spatial scales. In view of these low levels of gene flow, it is concluded that both genetic drift and natural selection may play important roles in the genetic differentiation of this species, even at the lowest spatial scales.     

Spatial genetic structure of a tropical understory shrub,PSYCHOTRIA OFFICINALIS (RuBIACEAE)

Analyses of fine-scale and macrogeographic genetic structure in plant populations provide an initial indication of how gene flow, natural selection, and genetic drift may collectively influence the distribution of genetic variation. The objective of our study is to evaluate the spatial dispersion of alleles within and among subpopulations of a tropical shrub, Psychotria officinalis (Rubiaceae), in a lowland wet forest in Costa Rica. This insect-pollinated, self-incompatible understory plant is dispersed primarily by birds, some species of which drop the seeds immediately while others transport seeds away from the parent plant. Thus, pollination should promote gene flow while at least one type of seed dispersal agent might restrict gene flow. Sampling from five subpopulations in undisturbed wet forest at Estación Biologíca La Selva, Costa Rica, we used electrophoretically detected isozyme markers to examine the spatial scale of genetic structure. Our goals are: 1) describe genetic diversity of each of the five subpopulations of Psychotria officinalis sampled within a contiguous wet tropical forest; 2) evaluate fine-scale genetic structure of adults of P. officinalis within a single 2.25-ha mapped plot; and 3) estimate genetic structure of P. officinalis using data from five subpopulations located up to 2 km apart. Using estimates of coancestry, statistical analyses reveal significant positive genetic correlations between individuals on a scale of 5 m but no significant genetic relatedness beyond that interplant distance within the studied subpopulation. Multilocus estimates of genetic differentiation among subpopulations were low, but significant (F_st = 0.095). Significant F_st estimates were largely attributable to a single locus (Lap-2). Thus, multilocus estimates of F_st may be influenced by microgeographic selection. If true, then the observed levels of IBD may be overestimates.     

THE MAINTENANCE OF OBLIGATE SEX IN FINITE,STRUCTURED POPULATIONS SUBJECT TO RECURRENT BENEFICIAL AND DELETERIOUS MUTATION

Although there is no known general explanation as to why sexual populations resist asexual invasion, previous work has shown that sexuals can outcompete asexuals in structured populations. However, it is currently unknown whether costly sex can be maintained with the weak structure that is commonly observed in nature. We investigate the conditions under which obligate sexuals resist asexual invasion in structured populations subject to recurrent mutation. We determine the level of population structure needed to disfavor asexuals, as calculated using the average F_st between all pairs of demes. We show that the critical F_st needed to maintain sex decreases as the population size increases, and approaches modest levels as observed in many natural populations. Sex is maintained with lower F_st if there are both advantageous and deleterious mutation, if mutation rates are sufficiently high, and if deleterious mutants have intermediate selective strengths, which maximizes the effect of Muller’s ratchet. Additionally, the critical F_st needed to maintain sex is lower when there are a large number of subpopulations. Lower F_st values are needed to maintain sex when demes vary substantially in their pairwise distances (e.g., when arrayed along one dimension), although this effect is often modest, especially if some long‐distance dispersal is present.     

Life‐stage differences in spatial genetic structure in an irruptive forest insect: implications for dispersal and spatial synchrony

Dispersal determines the flux of individuals, energy and information and is therefore a key determinant of ecological and evolutionary dynamics. Yet, it remains difficult to quantify its importance relative to other factors. This is particularly true in cyclic populations in which demography, drift and dispersal contribute to spatio‐temporal variability in genetic structure. Improved understanding of how dispersal influences spatial genetic structure is needed to disentangle the multiple processes that give rise to spatial synchrony in irruptive species. In this study, we examined spatial genetic structure in an economically important irruptive forest insect, the spruce budworm (Choristoneura fumiferana) to better characterize how dispersal, demography and ecological context interact to influence spatial synchrony in a localized outbreak. We characterized spatial variation in microsatellite allele frequencies using 231 individuals and seven geographic locations. We show that (i) gene flow among populations is likely very high (F_st ≈ 0); (ii) despite an overall low level of genetic structure, important differences exist between adult (moth) and juvenile (larvae) life stages; and (iii) the localized outbreak is the likely source of moths captured elsewhere in our study area. This study demonstrates the potential of using molecular methods to distinguish residents from migrants and for understanding how dispersal contributes to spatial synchronization. In irruptive populations, the strength of genetic structure depends on the timing of data collection (e.g. trough vs. peak), location and dispersal. Taking into account this ecological context allows us to make more general characterizations of how dispersal can affect spatial synchrony in irruptive populations.     

Linking seed dispersal and genetic structure of trees: a biogeographical approach

Aim Natural and human‐induced differences in frugivore assemblages can influence the seed dispersal distances of trees. An important issue in seed dispersal systems is to understand whether differences in seed dispersal distances also affect the genetic structure of mature trees. One possible approach to test for a relationship between seed dispersal and the genetic structure of mature trees is to compare the genetic structure of two closely related tree species between two biogeographical regions that differ in frugivore assemblages and seed dispersal distances. Previous studies on two Commiphora species revealed that Commiphora guillauminii in Madagascar has a much lower seed dispersal distance than Commiphora harveyi in South Africa. We tested whether the lower seed dispersal distance might have caused decreased gene flow, resulting in a stronger genetic structure in Madagascar than in South Africa. Location Madagascar and South Africa. Methods Using amplified fragment length polymorphism markers we investigated the genetic structure of 134 trees in Madagascar and 158 trees in South Africa at a local and a regional spatial scale. Results In concordance with our hypothesis, kinship analysis suggests that gene flow was restricted mostly to 3 km in Madagascar and to 30 km in South Africa. At the local spatial scale, the genetic differentiation among groups of trees within sample sites was marginally significantly higher in Madagascar (F_ST = 0.069) than in South Africa (F_ST = 0.021). However, at a regional spatial scale genetic differentiation was lower in Madagascar (F_ST = 0.053) than in South Africa (F_ST = 0.163). Main conclusions Our results show that lower seed dispersal distances of trees were linked to higher genetic differentiation of trees only at a local spatial scale. This suggests that seed dispersal affects the genetic population structure of trees at a local, but not at a regional, spatial scale.     

Dispersal ability and habitat requirements determine landscape‐level genetic patterns in desert aquatic insects

Species occupying the same geographic range can exhibit remarkably different population structures across the landscape, ranging from highly diversified to panmictic. Given limitations on collecting population‐level data for large numbers of species, ecologists seek to identify proximate organismal traits—such as dispersal ability, habitat preference and life history—that are strong predictors of realized population structure. We examined how dispersal ability and habitat structure affect the regional balance of gene flow and genetic drift within three aquatic insects that represent the range of dispersal abilities and habitat requirements observed in desert stream insect communities. For each species, we tested for linear relationships between genetic distances and geographic distances using Euclidean and landscape‐based metrics of resistance. We found that the moderate‐disperser Mesocapnia arizonensis (Plecoptera: Capniidae) has a strong isolation‐by‐distance pattern, suggesting migration–drift equilibrium. By contrast, population structure in the flightless Abedus herberti (Hemiptera: Belostomatidae) is influenced by genetic drift, while gene flow is the dominant force in the strong‐flying Boreonectes aequinoctialis (Coleoptera: Dytiscidae). The best‐fitting landscape model for M. arizonensis was based on Euclidean distance. Analyses also identified a strong spatial scale‐dependence, where landscape genetic methods only performed well for species that were intermediate in dispersal ability. Our results highlight the fact that when either gene flow or genetic drift dominates in shaping population structure, no detectable relationship between genetic and geographic distances is expected at certain spatial scales. This study provides insight into how gene flow and drift interact at the regional scale for these insects as well as the organisms that share similar habitats and dispersal abilities.     

High gene flow between populations of Macrotermes michaelseni (Isoptera, Termitidae)

Termite alates are thought to be poor active flyers, and this should lead to considerable genetic differentiation on small spatial scales. However, using four microsatellite loci for the termite Macrotermes michaelseni we found low values of genetic differentiation (F_ST) across a spatial scale of even more than 50 km. Genetic differentiation between populations increased with spatial distance up to 50 km. Furthermore, up to this distance, the scatter around the linear regression of genetic differentiation versus spatial distance increased with spatial distance. This suggests that across such spatial distances gene flow and genetic drift are of about equal importance, and near equilibrium. Using a regional F_ST as well as the distance between populations with non-significant F_ST-values (up to 25 km), gene flow is sufficiently high so that populations may be regarded as panmictic on spatial scales of 25 to 50 km. The apparent contradiction between dispersal distances observed in the field and estimates of gene flow from genetic markers may be due to the masses of swarming alates. Assuming a leptokurtic distribution of dispersal distances, atleast some alates are expected to travel considerable distances, most likely by passive drift. Received 25 January 2005; revised 11 April 2005; accepted 26 April 2005.     

Comparative genetic structure reflects underlying life histories of three sympatric small mammal species in continuous forest of south-eastern Australia

There are strong links between habitat fragmentation, population size and genetic structure. However, to fully understand the long‐term effects of fragmentation on population viability, it is necessary to first understand the relationship between life history traits and genetic characteristics in un‐fragmented habitats. This is best done through comparing patterns of genetic diversity in sympatric species, since relative data may be maximally informative. We compared genetic structure and diversity among three sympatric small mammals – of which two were marsupial species – over a small spatial scale (<4 km) using microsatellite genotypic data from individuals sampled from four grids spaced in a linear fashion. Expected heterozygosity was high for all three species (average H_e range: 0.781–0.886), but the smallest species had significantly higher genetic diversity (both heterozygosity and allelic diversity) than the two larger species, possibly due to it having; 1) large effective population size and little overlap in generations, and/or 2) high mutation rates in large alleles. Genotypic isolation by distance (measured as relatedness) was detected only at the smallest distance of 750 m, for any species. In the two marsupial species F_st, relatedness and percentage of individuals assigned to site of capture were significantly lower in males than in females, corroborating previous reports of male‐biased dispersal. However, sex‐biased dispersal was not detected for the third species (the native bush rat Rattus fuscipes), and presence of significant heterozygous deficits suggested this resulted in inbreeding within kin‐structured demes. We speculate that habitat fragmentation will have differing effects on population dynamics, social organisation and mating systems for the two marsupial species compared to the native murid rodent, due to their differing population structure and divergent inbreeding avoidance mechanisms.     

Geographic Influence on Genetic Structure in the Widespread Neotropical Tree Simarouba amara (Simaroubaceae)

This study aimed to assess the population genetic structure of a widespread Neotropical tree species, Simarouba amara, at local, regional and continental spatial scales. We used five microsatellite loci to examine genetic variation in 14 natural populations (N?=?478 individuals) of this vertebrate-dispersed rain forest tree species in Panama, Ecuador, and French Guiana. Estimates of genetic differentiation (F _st and R _st) were significant among all but one population pair and global differentiation was moderate (F _st?=?0.25, R _st?=?0.33) with 94% of genetic variation ascribed to differences among three main geographic regions (Central America, Western Ecuador, Amazon basin). There was no evidence of isolation by distance within regions. Allele-size mutations contributed significantly (R _ST > F _ST) to the divergences between cis- and trans-Andean populations, highlighting the role of the northern Andean cordilleras as an important geographic barrier for this species.     

Gene flow does not prevent personality and morphological differentiation between two blue tit populations

Understanding the causes and consequences of population phenotypic divergence is a central goal in ecology and evolution. Phenotypic divergence among populations can result from genetic divergence, phenotypic plasticity or a combination of the two. However, few studies have deciphered these mechanisms for populations geographically close and connected by gene flow, especially in the case of personality traits. In this study, we used a common garden experiment to explore the genetic basis of the phenotypic divergence observed between two blue tit (Cyanistes caeruleus) populations inhabiting contrasting habitats separated by 25 km, for two personality traits (exploration speed and handling aggression), one physiological trait (heart rate during restraint) and two morphological traits (tarsus length and body mass). Blue tit nestlings were removed from their population and raised in a common garden for up to 5 years. We then compared adult phenotypes between the two populations, as well as trait‐specific Q_st and F_st. Our results revealed differences between populations similar to those found in the wild, suggesting a genetic divergence for all traits. Q_st–F_st comparisons revealed that the trait divergences likely result from dissimilar selection patterns rather than from genetic drift. Our study is one of the first to report a Q_st–F_st comparison for personality traits and adds to the growing body of evidence that population genetic divergence is possible at a small scale for a variety of traits including behavioural traits.     

Contrasting effects of habitat discontinuity on three closely related fungivorous beetle species with diverging host‐use patterns and dispersal ability

Understanding how landscape structure influences biodiversity patterns and ecological processes are essential in ecological research and conservation practices. Forest discontinuity is a primary driver affecting the population persistence and genetic structure of forest‐dwelling species. However, the actual impacts on populations are highly species‐specific. In this study, we tested whether dispersal capability and host specialization are associated with susceptibility to forest discontinuity using three closely related, sympatric fungivorous ciid beetle species (two host specialists, Octotemnus assimilis and O. crassus; one host generalist, O. kawanabei). Landscape genetic analyses and the estimation of effective migration surfaces (EEMS) method consistently demonstrated contrasting differences in the relationships between genetic structure and configuration of forest land cover. Octotemnus assimilis, one of the specialists with a presumably higher dispersal capability due to lower wing loading, lacked a definite spatial genetic structure in our study landscape. The remaining two species showed clear spatial genetic structure, but the results of landscape genetic analyses differed between the two species: while landscape resistance appeared to describe the spatial genetic structure of the specialist O. crassus, genetic differentiation of the generalist O. kawanabei was explained by geographic distance alone. This finding is consistent with the prediction that nonforest areas act more strongly as barriers between specialist populations. Our results suggest that differences in host range can influence the species‐specific resistance to habitat discontinuity among closely related species inhabiting the same landscape.     

Dendritic connectivity shapes spatial patterns of genetic diversity: a simulation‐based study

Landscape features notoriously affect spatial patterns of biodiversity. For instance, in dendritic ecological networks (such as river basins), dendritic connectivity has been proposed to create unique spatial patterns of biodiversity. Here, we compared genetic datasets simulated under a lattice‐like, a dendritic and a circular landscape to test the influence of dendritic connectivity on neutral genetic diversity. The circular landscape had a level of connectivity similar to that of the dendritic landscape, so as to isolate the influence of dendricity on genetic diversity. We found that genetic diversity and differentiation varied strikingly among the three landscapes. For instance, the dendritic landscape generated higher total number of alleles and higher global F_st than the lattice‐like landscape, and these indices also varied between the dendritic and the circular landscapes, suggesting an effect of dendricity. Furthermore, in the dendritic landscape, allelic richness was higher in highly connected demes (e.g. confluences in rivers) than in low‐connected demes (e.g. upstream and downstream populations), which was not the case in the circular landscape, hence confirming the major role of dendricity. This led to bell‐shaped distributions of allelic richness along an upstream–downstream gradient. Conversely, genetic differentiation (F_st) was lower in highly than in low‐connected demes (which was not observed in circular landscape), and significant patterns of isolation by distance (IBD) were also observed in the dendritic landscape. We conclude that in dendritic networks, the combined influence of dendricity and connectivity generates unique spatial patterns of neutral genetic diversity, which has implications for population geneticists and conservationists.     

Determinants of Genetic Structure in a Nonequilibrium Metapopulation of the Plant Silene latifolia

Population genetic differentiation will be influenced by the demographic history of populations, opportunities for migration among neighboring demes and founder effects associated with repeated extinction and recolonization. In natural populations, these factors are expected to interact with each other and their magnitudes will vary depending on the spatial distribution and age structure of local demes. Although each of these effects has been individually identified as important in structuring genetic variance, their relative magnitude is seldom estimated in nature. We conducted a population genetic analysis in a metapopulation of the angiosperm, Silene latifolia, from which we had more than 20 years of data on the spatial distribution, demographic history, and extinction and colonization of demes. We used hierarchical Bayesian methods to disentangle which features of the populations contributed to among population variation in allele frequencies, including the magnitude and direction of their effects. We show that population age, long-term size and degree of connectivity all combine to affect the distribution of genetic variance; small, recently-founded, isolated populations contributed most to increase F _ST in the metapopulation. However, the effects of population size and population age are best understood as being modulated through the effects of connectivity to other extant populations, i.e. F _ST diminishes as populations age, but at a rate that depends how isolated the population is. These spatial and temporal correlates of population structure give insight into how migration, founder effect and within-deme genetic drift have combined to enhance and restrict genetic divergence in a natural metapopulation.     

Development of genetic diversity,differentiation and structure over 500 years in four ponderosa pine populations

Population history plays an important role in shaping contemporary levels of genetic variation and geographic structure. This is especially true in small, isolated range‐margin populations, where effects of inbreeding, genetic drift and gene flow may be more pronounced than in large continuous populations. Effects of landscape fragmentation and isolation distance may have implications for persistence of range‐margin populations if they are demographic sinks. We studied four small, disjunct populations of ponderosa pine over a 500‐year period. We coupled demographic data obtained through dendroecological methods with microsatellite data to discern how and when contemporary levels of allelic diversity, among and within‐population levels of differentiation, and geographic structure, arose. Alleles accumulated rapidly following initial colonization, demonstrating proportionally high levels of gene flow into the populations. At population sizes of approximately 100 individuals, allele accumulation saturated. Levels of genetic differentiation among populations (F_ST and Jost's D_est) and diversity within populations (F_IS) remained stable through time. There was no evidence of geographic genetic structure at any time in the populations' history. Proportionally, high gene flow in the early stages of population growth resulted in rapid accumulation of alleles and quickly created relatively homogenous genetic patterns among populations. Our study demonstrates that contemporary levels of genetic diversity were formed quickly and early in population development. How contemporary genetic diversity accumulates over time is a key facet of understanding population growth and development. This is especially relevant given the extent and speed at which species ranges are predicted to shift in the coming century.     

Local dynamics and dispersal in a structured population of the whirligig beetle Deneutus assimilis

The study illustrates the ecological determinants and evolutionary consequences of dispersal in the pond-living water beetle Dineutus assimilis (Coleoptera: Gyrinidae). Over 2 years, local populatiopn dynamics were studied in 51 ponds within a 60-km² study area. In most of the 31 occupied ponds, and even in large populations, abundances changed dramatically from one year to the next. Nine extinction and nine colonisation events were observed. These temporal patterns show no sign of spatial autocorrelation. Such a habitat distribution should favour high dispersal rates. Indeed, D. assimilis was found to be a very effective coloniser of newly available sites (mean propagule size: 23). A mark-recapture study showed that most dispersal occurred after diapause and over distances ranging from 100 m to at least 20 km. Yet despite frequent movement, the local variability in environmental conditions maintiins a large variance in average reproductive success per pond. Furthermore, immigration rates vary widely within a season. The apparent lack of correlation between these two sources of variation should greatly strengthen the role of drift in this system. A companion paper (Nürnberger and Harrison 1995) documents a non-random distribution of mitochondrial haplotypes due to recent population bottlenecks.     

Spatial Scales of Pollen and Seed-Mediated Gene Flow in Tropical Rain Forest Trees

Gene flow via seed and pollen is a primary determinant of genetic and species diversity in plant communities at different spatial scales. This paper reviews studies of gene flow and population genetic structure in tropical rain forest trees and places them in ecological and biogeographic context. Although much pollination is among nearest neighbors, an increasing number of genetic studies report pollination ranging from 0.5–14 km for canopy tree species, resulting in extensive breeding areas in disturbed and undisturbed rain forest. Direct genetic measures of seed dispersal are still rare; however, studies of fine scale spatial genetic structure (SGS) indicate that the bulk of effective seed dispersal occurs at local scales, and we found no difference in SGS (Sp statistic) between temperate (N?=?24 species) and tropical forest trees (N?=?15). Our analysis did find significantly higher genetic differentiation in tropical trees (F _ST?=?0.177; N?=?42) than in temperate forest trees (F _ST?=?0.116; N?=?82). This may be due to the fact that tropical trees experience low but significant rates of self-fertilization and bi-parental inbreeding, whereas half of the temperate tree species in our survey are wind pollinated and are more strictly allogamous. Genetic drift may also be more pronounced in tropical trees due to the low population densities of most species.     

Microsatellite variation and population structure of a recovering Tree frog (Hyla arborea L.) metapopulation

Numbers and sizes of populations of the European tree frog in The Netherlands have dramatically decreased in the second half of the last century due to extensive habitat destruction and fragmentation. We have studied the genetic structure of a slowly recovering meta-population. Strong genetic differentiation, estimated at eight microsatellite loci, was found between clusters of populations (F _st-values above 0.2). Within clusters, consisting of ponds within a radius of about 5 km, European tree frog populations were less differentiated (F _st<0.08) and exact tests showed that most of the ponds within clusters were not significantly differentiated. Although local population sizes have been increasing since 1989, and some new ponds have been colonised in the direct vicinity of ponds that have been populated continuously, little evidence for gene flow between clusters of ponds was found (only one exception). Furthermore, levels of genetic diversity were low compared to populations in comparable areas elsewhere in Europe. Therefore, a continuous conservation effort is needed to prevent any further loss of genetic diversity. The alleviation of important barriers to dispersal between the clusters should be given a high priority for the restoration of the meta-population as a whole.     

Distance,dams and drift: what structures populations of an endangered,benthic stream fish?

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