Scutellaria indica is a perennial herb with both chasmogamous (CH) and cleistogamous (CL) flowers on the same plant in some populations, and only CL flowers in other populations. Actual seed production by CH and CL flowers was investigated in populations of S. indica. The average seed set of CL flowers was 19 times higher than CH flowers, indicating much greater fertilization success. The CL seeds were also significantly heavier than the CH seeds. However, the resource cost of producing a CH flower was much higher than that of producing a CL flower. The CH flower was approximately seven times larger, and its pollen/ovule ratio was approximately five times higher than flowers. The level and pattern of genetic diversity at both allozyme and random amplified polymorphic DNA (RAPD) levels were consistent with a predominantly selfing system in the species. The average amount of within-population genetic variation was extremely low (A = 1.025, P = 2.36%, HO = 0.001 and HE = 0.008 based on allozyme data, and P = 8.94% and HE = 0.03 based on RAPD data). At the species level, the estimates of total gene diversity (HT) were 0. 101 based on allozyme data and 0.139 based on RAPD data. A very high level of genetic differentiation occurred between populations (allozyme GST = 0.92 and RAPD GST = 0.81). Genetic drift coupled with predominant cleistogamous selfing apparently played the major role in determining the population genetic structure in S. indica. Although the features associated with CH and CL flower and seed production seem to be sufficient for the evolution of complete cleistogamy in S. indica, random fixation of alternative alleles for dimorphic or complete cleistogamy in small populations could maintain the multiple strategy of chasmogamous and cleistogamous reproduction in the species.
In this study, we investigate the relative role of historical factors and evolutionary forces in promoting population differentiation in a new case of sympatric dwarf and normal ecotypes of the rainbow smelt (Osmerus mordax Mitchill) in Lac Saint-Jean (Québec, Canada). Our first objective was to test the hypothesis that the evolution of sympatric smelt ecotypes in Lac Saint-Jean has been contingent upon the secondary contact between two evolutionary lineages in postglacial times. Secondly, the QST method was applied to test the null hypothesis that the extent of phenotypic differences relative to that of neutral marker variation would be similar in comparisons involving populations within and among ecotypes. Thirdly, we applied a quantitative-genetic method as an exploratory assessment as to whether the amount of gene flow observed between populations could affect divergence in adaptive traits under specific conditions. This study revealed a unique situation of dwarf and normal smelt ecotypes that are, respectively, characterized by selmiparous and iteroparous life histories and the occurrence in each of two genetically distinct populations that synchronously use the same spawning habitat in two tributaries. Historical contingency has apparently played little role in the origin of these populations. In contrast, an important role of divergent natural selection in driving their phenotypic divergence was suggested. While divergent selection has apparently been strong enough to maintain phenotypic differentiation in the face of migration, this study suggests that gene flow has been sufficiently important to modulate the extent of adaptive differentiation being achieved between ecotypes, unless the extent of stabilizing selection acting on smelt ecotypes is much more pronounced than usually reported in natural populations.
The identification of signatures of natural selection in genomic surveys has become an area of intense research, stimulated by the increasing ease with which genetic markers can be typed. Loci identified as subject to selection may be functionally important, and hence (weak) candidates for involvement in disease causation. They can also be useful in determining the adaptive differentiation of populations, and exploring hypotheses about speciation. Adaptive differentiation has traditionally been identified from differences in allele frequencies among different populations, summarised by an estimate of FST. Low outliers relative to an appropriate neutral population-genetics model indicate loci subject to balancing selection, whereas high outliers suggest adaptive (directional) selection. However, the problem of identifying statistically significant departures from neutrality is complicated by confounding effects on the distribution of FST estimates, and current methods have not yet been tested in large-scale simulation experiments. Here, we simulate data from a structured population at many unlinked, diallelic loci that are predominantly neutral but with some loci subject to adaptive or balancing selection. We develop a hierarchical-Bayesian method, implemented via Markov chain Monte Carlo (MCMC), and assess its performance in distinguishing the loci simulated under selection from the neutral loci. We also compare this performance with that of a frequentist method, based on moment-based estimates of FST. We find that both methods can identify loci subject to adaptive selection when the selection coefficient is at least five times the migration rate. Neither method could reliably distinguish loci under balancing selection in our simulations, even when the selection coefficient is twenty times the migration rate.
An understanding of how ecological traits influence past species response to environmental change can aid our future predictions of species persistence. We used ancient DNA and serial coalescent modelling in a hypothesis-testing framework to reveal differences in temporal genetic variation over 10,000 years for two species of subterranean rodents that currently differ in rarity (abundance, range size and habitat specificity) and mating system, but that reside in the same volcanically active region. Comparative phylochronologic analyses indicated little genetic change and suggest genetic stability in the solitary widespread Ctenomys haigi over thousands of years. In contrast, we found a pattern of haplotypic turnover in the rare and currently endangered Ctenomys sociabilis. Serial coalescent modelling indicated that the best-fit models of microevolutionary change included gene flow between isolated populations for this species. Although C. haigi and C. sociabilis are congeners that share many life history traits, they have behavioural, habitat-preference and population-size differences that may have resulted in contrasting patterns of temporal variation during periods of environmental change.
A Geographical Information System (GIS) is used to analyse allelic information of 13 sequenced loci of natural populations of Arabidopsis thaliana and to identify geographical structures. GIS provides tools for visualization and analysis of geographical population structures using molecular data. The geographical distribution of the number of variable positions in the alignments, the distribution of recombinant sequence blocks, and the distribution of a newly defined measure, the differentiation index, are studied. The differentiation index is introduced to measure the sequence divergence among individual plants sampled from various geographical localities. The numbers of variable positions and the differentiation index are also used for a metadata analysis covering about 26 kb of the genome. This analysis reveals, for the first time, differences in DNA sequence structures of geographically different populations of A. thaliana. The broadly defined west Mediterranean region consists of accessions with the highest numbers of polymorphic positions followed by the west European region. The GIS technology Kriging is used to define Arabidopsis specific diversity zones in Europe. The highest genetic variability is observed along the Atlantic coast from the western Iberian Peninsula to southern Great Britain, while lowest variability is found in central Europe.
The Hsp100/ClpB heat shock protein family is ancient and required for high temperature survival, but natural variation in expression and its phenotypic effects is unexplored in plants. In controlled environment experiments, we examined the effects of variation in the Arabidopsis cytosolic AtHsp101 (hereafter Hsp101). Ten wild-collected ecotypes differed in Hsp101 expression responses across a 22 to 40 degrees C gradient. Genotypes from low latitudes expressed the least Hsp101. We tested fitness and pleiotropic consequences of varying Hsp101 expression in 'control' vs. mild thermal stress treatments (15/25 degrees C D/N vs. 15/25 degrees D/N plus 3 h at 35 degrees C 3 days/week). Comparing wild type and null mutants, wt Columbia (Col) produced approximately 33% more fruits compared to its Hsp101 homozygous null mutant. There was no difference between Landsberg erecta null mutant NIL (Ler) and wt Ler; wt Ler showed very low Hsp101 expression. In an assay of six genotypes, fecundity was a saturating function of Hsp101 content, in both experimental treatments. Thus, in addition to its essential role in acquired thermal tolerance, Hsp101 provides a substantial fitness benefit under normal growth conditions. Knocking out Hsp101 decreased fruit production, days to germination and days to bolting, total dry mass, and number of inflorescences; it increased transpiration rate and allocation to root mass. Root : total mass ratio decayed exponentially with Hsp101 content. This study shows that Hsp101 expression is evolvable in natural populations. Our results further suggest that Hsp101 is primarily an emergency high-temperature tolerance mechanism, since expression levels are lower in low-latitude populations from warmer climates. Hsp101 expression appears to carry an important trade-off in reduced root growth. This trade-off may select for suppressed expression under chronically high temperatures.
In conventional phylogeographic studies, historical demographic processes are elucidated from the geographical distribution of individuals represented on an inferred gene tree. However, the interpretation of gene trees in this context can be difficult as the same demographic/geographical process can randomly lead to multiple different genealogies. Likewise, the same gene trees can arise under different demographic models. This problem has led to the emergence of many statistical methods for making phylogeographic inferences. A popular phylogeographic approach based on nested clade analysis is challenged by the fact that a certain amount of the interpretation of the data is left to the subjective choices of the user, and it has been argued that the method performs poorly in simulation studies. More rigorous statistical methods based on coalescence theory have been developed. However, these methods may also be challenged by computational problems or poor model choice. In this review, we will describe the development of statistical methods in phylogeographic analysis, and discuss some of the challenges facing these methods.
This study compares the properties of dominant markers, such as amplified fragment length polymorphisms (AFLPs), with those of codominant multiallelic markers, such as microsatellites, in reconstructing parentage. These two types of markers were used to search for both parents of an individual without prior knowledge of their relationships, by calculating likelihood ratios based on genotypic data, including mistyping. Experimental data on 89 oak trees genotyped for six microsatellite markers and 159 polymorphic AFLP loci were used as a starting point for simulations and tests. Both sets of markers produced high exclusion probabilities, and among dominant markers those with dominant allele frequencies in the range 0.1-0.4 were more informative. Such codominant and dominant markers can be used to construct powerful statistical tests to decide whether a genotyped individual (or two individuals) can be considered as the true parent (or parent pair). Gene flow from outside the study stand (GFO), inferred from parentage analysis with microsatellites, overestimated the true GFO, whereas with AFLPs it was underestimated. As expected, dominant markers are less efficient than codominant markers for achieving this, but can still be used with good confidence, especially when loci are deliberately selected according to their allele frequencies.
The merger of two or more divergent genomes within an allopolyploid nucleus can facilitate speciation and adaptive evolution in flowering plants. Widespread changes to gene expression have been shown to result from interspecific hybridisation and polyploidy in a number of plant species, and attention has now shifted to determining the epigenetic processes that drive these changes. We present here an analysis of cytosine methylation patterns in triploid F(1) Senecio (ragwort) hybrids and their allohexaploid derivatives. We observe that, in common with similar studies in Arabidopsis, Spartina and Triticum, a small but significant proportion of loci display nonadditive methylation in the hybrids, largely resulting from interspecific hybridisation. Despite this, genome duplication results in a secondary effect on methylation, with reversion to additivity at some loci and novel methylation status at others. We also observe differences in methylation state between different allopolyploid generations, predominantly in cases of additive methylation with regard to which parental methylation state is dominant. These changes to methylation state in both F(1) triploids and their allohexaploid derivatives largely mirror the overall patterns of nonadditive gene expression observed in our previous microarray analyses and may play a causative role in generating those expression changes. These similar global changes to DNA methylation resulting from hybridisation and genome duplication may serve as a source of epigenetic variation in natural populations, facilitating adaptive evolution. Our observations that methylation state can also vary between different generations of polyploid hybrids suggests that newly formed allopolyploid species may display a high degree of epigenetic diversity upon which natural selection can act.
To elucidate the co-evolutionary relationships between phloem-feeding insects and their secondary, or facultative, bacterial symbionts, we explore the distributions of three such microbes--provisionally named the R-type (or PASS, or S-sym), T-type (or PABS), and U-type--across a number of aphid and psyllid hosts through the use of diagnostic molecular screening techniques and DNA sequencing. Although typically maternally transmitted, phylogenetic and pairwise divergence analyses reveal that these bacteria have been independently acquired by a variety of unrelated insect hosts, indicating that horizontal transfer has helped to shape their distributions. Based on the high genetic similarity between symbionts in different hosts, we argue that transfer events have occurred recently on an evolutionary timescale. In several instances, however, closely related symbionts associate with related hosts, suggesting that horizontal transfer between distant relatives may be rarer than transmission between close relatives. Our findings on the prevalence of these symbionts within many aphid taxa, along with published observations concerning their effects on host fitness, imply a significant role of facultative symbiosis in aphid ecology and evolution.
Birds have for long been popular study objects in speciation research. Being easy to observe in the field, they have traditionally been particularly important in studies of behavioural and ecological factors in speciation, whereas the genetic aspects of the process have been studied in other organisms, such as Drosophila. More recently, however, a stronger genetic focus has been placed on speciation research also in birds. Here, we review ecological, behavioural and genetic studies on speciation in the pied flycatcher (Ficedula hypoleuca) and the collared flycatcher (Ficedula albicollis). These well-studied birds provide among the few proposed examples of the process of reinforcement of premating isolation, and the evidence for reinforcement is strong. They are further characterized by having strong intrinsic postzygotic barriers (female hybrid sterility), yet the two species appear to be very similar ecologically. This is in stark contrast to another well-studied bird complex, Darwin's finches, in which the species differ vastly in ecologically important traits but have no developmental problems arising from genetic incompatibilities, and where no evidence for reinforcement is found. In the flycatchers, sex chromosome linkage of genes affecting traits associated with both pre- and postzygotic barriers to gene exchange is likely to facilitate reinforcement. We discuss whether such sex-linkage may be common in birds. The contrast between flycatchers and Darwin's finches indicate that speciation in birds cannot always be understood mainly as a result of divergent natural selection ('ecological speciation'), and generalizations from one system may lead us astray. We discuss to what extent insight from research on the flycatchers may point to fruitful avenues for future research on bird speciation and specifically call for a more systematic effort to simultaneously investigate ecology, behaviour and genetics of birds caught in the process of speciation.
Genotypes are frequently used to identify parentage. Such analysis is notoriously vulnerable to genotyping error, and there is ongoing debate regarding how to solve this problem. Many scientists have used the computer program CERVUS to estimate parentage, and have taken advantage of its option to allow for genotyping error. In this study, we show that the likelihood equations used by versions 1.0 and 2.0 of CERVUS to accommodate genotyping error miscalculate the probability of observing an erroneous genotype. Computer simulation and reanalysis of paternity in Rum red deer show that correcting this error increases success in paternity assignment, and that there is a clear benefit to accommodating genotyping errors when errors are present. A new version of CERVUS (3.0) implementing the corrected likelihood equations is available at http://www.fieldgenetics.com.
Microarray technology provides a new tool with which molecular ecologists and evolutionary biologists can survey genome-wide patterns of gene expression within and among species. New analytical approaches based on analysis of variance will allow quantification of the contributions of among individual variation, genotype, sex, microenvironment, population structure, and geography to variation in gene expression. Applications of this methodology are reviewed in relation to studies of mechanisms of adaptation and divergence; delineation of developmental and physiological pathways and networks; characterization of quantitative genetic parameters at the level of transcription ('quantitative genomics'); molecular dissection of parasitism and symbiosis; and studies of the diversification of gene content. Establishment of microarray resources is neither prohibitively expensive nor technologically demanding, and a commitment to development of gene expression profiling methods for nonmodel organisms could have a tremendous impact on molecular and genetic research at the interface of organismal and population biology.
Using a molecular barcode, derived from single-specimen polymerase chain reaction (PCR) and sequencing of the 5' segment of the small subunit ribosomal RNA (SSU) gene, we have developed a molecular operational taxonomic unit (MOTU) scheme for soil nematodes. Individual specimens were considered to belong to the same MOTU when the sequenced segment of 450 bases was > 99.5% identical. A Scottish upland Agrostis-Festuca grassland soil was sampled, using both culture-based and random selection methods. One hundred and sixty-six cultured isolates were sequenced, and clustered into five MOTU. From 74 randomly sampled individuals across the study site, 19 MOTU were defined. A subsequent sample of 18 individuals from a single subplot contained eight MOTU, four of which were unique to the single subplot sample. Interestingly, seven of these MOTU were not present in the culture-independent sampling. Overall, a total of 23 MOTU were defined from only 240 sequences. Many MOTU could readily be assigned to classical, morphologically defined taxonomic units using a database of SSU sequences from named nematode species. The MOTU technique allows a rapid assessment of nematode taxon diversity in soils. Correlation with a database of sequences from known species offers a route to application of the technique in ecological surveys addressing biological as well as genetic diversity.
Dengue haemorrhagic fever emerged in the 1950s and has become a major public health concern in most Asian countries. In Vietnam, little is known about the intraspecific variation of the vector and its consequences on vectorial capacity. Here we report the use of microsatellite markers to differentiate Aedes aegypti populations in Ho Chi Minh City, a typical, overcrowded Asian city. Six microsatellite loci, with 5-14 alleles per locus, were scored in 20 mosquito samples collected in 1998 in Ho Chi Minh City. We found substantial differentiation among Ae. aegypti populations from the outskirts, whereas populations from the centre of the city showed less differentiation. These results are consistent with the hypothesis that populations of Ae. aegypti in central Ho Chi Minh City are panmictic because there are abundant larval breeding sites and an abundance of humans for adults to feed upon. In contrast, populations on the outskirts become differentiated largely through the processes of genetic drift because larval breeding sites are not as abundant. These findings implicate human activities associated with urbanization, as factors shaping the genetic structure of Ae. aegypti populations.
Microsatellites, or tandem simple sequence repeats (SSR), are abundant across genomes and show high levels of polymorphism. SSR genetic and evolutionary mechanisms remain controversial. Here we attempt to summarize the available data related to SSR distribution in coding and noncoding regions of genomes and SSR functional importance. Numerous lines of evidence demonstrate that SSR genomic distribution is nonrandom. Random expansions or contractions appear to be selected against for at least part of SSR loci, presumably because of their effect on chromatin organization, regulation of gene activity, recombination, DNA replication, cell cycle, mismatch repair system, etc. This review also discusses the role of two putative mutational mechanisms, replication slippage and recombination, and their interaction in SSR variation.
DNA was extracted from an 11,700-year-old rodent midden from the Atacama Desert, Chile and the chloroplast and animal mitochondrial DNA (mtDNA) gene sequences were analysed to investigate the floral environment surrounding the midden, and the identity of the midden agent. The plant sequences, together with the macroscopic identifications, suggest the presence of 13 plant families and three orders that no longer exist today at the midden locality, and thus point to a much more diverse and humid climate 11,700 years ago. The mtDNA sequences suggest the presence of at least four different vertebrates, which have been putatively identified as a camelid (vicuna), two rodents (Phyllotis and Abrocoma), and a cardinal bird (Passeriformes). To identify the midden agent, DNA was extracted from pooled faecal pellets, three small overlapping fragments of the mitochondrial cytochrome b gene were amplified and multiple clones were sequenced. These results were analysed along with complete cytochrome b sequences for several modern Phyllotis species to place the midden sequence phylogenetically. The results identified the midden agent as belonging to an ancestral P. limatus. Today, P. limatus is not found at the midden locality but it can be found 100 km to the north, indicating at least a small range shift. The more extensive sampling of modern Phyllotis reinforces the suggestion that P. limatus is recently derived from a peripheral isolate.
Common ash is a temperate forest tree with a colonizing behaviour, a discontinuous spatial distribution and a peculiar and poorly known mating system. Microsatellite markers were used to study the genetic structure in natural populations of common ash. Twelve populations located in northeastern France were analysed at five loci. Levels of genetic variability within and among stands were estimated for the seedling and adult stages. As expected for a forest tree, our results reveal high levels of intrapopulation diversity and a low genetic differentiation between stands. However, a general and significant heterozygote deficiency was found, with a mean F(IS) of 0.163 for the seedlings and of 0.292 for the adult trees. The different explanations for such an excess homozygosity are discussed: a nonMendelian inheritance of alleles, the presence of null alleles, a Wahlund effect and assortative mating.
We investigated population dynamics, genetic diversity and spatial structure in the aphid species Macrosiphoniella tanacetaria, a specialist herbivore feeding on tansy, Tanacetum vulgare. Tansy plants (genets) consist of many shoots (ramets), and genets are grouped in sites. Thus, aphids feeding on tansy can cluster at the level of ramets, genets and sites. We studied aphid population dynamics in 1997 and 2001 and found that within sites: (i). at any time, aphids used only a fraction of the available ramets and genets; (ii). at the level of ramets, most aphid colonies survived only one week; (iii). at the level of genets, mean survival time was less than 4 weeks; and (iv). colonization and extinction events occurred throughout the season. We sampled aphids in seven sites in the Alsace region, France (4-45 km apart) and two sites in Germany in 1999 to study genetic structure within and between populations. Genetic analyses using nine microsatellite loci showed that: (i). genotypic variability was high, (ii). none of the populations was in Hardy-Weinberg equilibrium, (iii). heterozygote deficits and linkage disequilibria were frequent, and (iv). all populations were genetically differentiated, even at a small geographical scale. Renewed sampling of the Alsace sites in 2001 showed that three populations had become extinct and significant genetic changes had occurred in the remaining four populations. The frequencies of extinction and colonization events at several spatial scales suggest a hierarchical metapopulation structure for M. tanacetaria. Frequent population turnover and drift are likely causes for the genetic differentiation of M. tanacetaria populations.
The distribution of genetic diversity at 10 highly polymorphic microsatellite loci within the European freshwater fish, Cottus gobio, L. was examined. The sampling range comprised a large geographical scale including lineages known to be highly divergent at both mitochondrial DNA (mtDNA) and allozymes. An analysis of genetic variability within populations showed that expected heterozygosity and allelic richness could be explained largely by current effective population sizes. Evidence was found, however, that historical processes predating the last major glaciation affected allelic richness. In addition to confirming the large-scale patterns from earlier studies, the microsatellite data revealed new insights into recent processes by analysing genetic structure within ancient lineages defined by mtDNA data. Stepwise mutation model (SMM) and nonSMM-based methods demonstrated a clear genetic structuring within the Northwestern European lineage comprising populations from Britain and Belgium, and within the Central European lineage populations from the rivers Danube, Elbe and Main. Supported by an analysis of genetic variability within populations these results showed that the bullhead populations most probably persisted throughout the last major glaciation within the British Isles and within the drainages of the rivers Elbe and Main. Such observations provide the first genetic evidence for a glacial refugium in such close proximity to the European glacial margins.
This study investigates patterns of genetic connectivity among 11 co-distributed tropical rainforest tree species from the genus Elaeocarpus across a biogeographic barrier, the Black Mountain Corridor (BMC) in the Australian Wet Tropics (AWT). We analysed a combination of allelic and flanking region sequence data from microsatellite markers, and evaluated the relative influence of environmental preferences and functional traits on genetic diversity and gene flow. The results indicate that only in three species geographic structuring of haplotype distribution reflects a north vs. south of the BMC pattern. Environmental factors linked with altitude were recognized as affecting genetic trends, but the selective processes operating on upland species appear to be associated with competitiveness and regeneration opportunities on poor soil types rather than climate variables alone. In contrast to previous observations within southeastern Australian rainforests, genetic differentiation in the AWT appears to be associated with small-fruited rather than large-fruited species, highlighting how external factors can influence the dispersal dimension. Overall, this study emphasizes the importance of considering functional and environmental factors when attempting generalizations on landscape-level patterns of genetic variation. Understanding how plant functional groups respond to environmental and climatic heterogeneity can help us predict responses to future change.
The application of molecular genetic techniques has revolutionized our view of avian mating systems. Contrary to prior expectations, birds are only very rarely sexually monogamous, with 'extra-pair offspring' found in approximately 90% of species. Even among socially monogamous species, over 11% of offspring are, on average, the result of extra-pair paternity (EPP). Based on over 150 molecular genetic studies of EPP in birds, we review two topical areas: (i) ecological explanations for interspecific variation in the rate of EPP; and (ii) evidence bearing on the adaptive function of EPP. We highlight the remaining challenges of understanding the relative roles of genes and ecology in determining variation between taxa in the rate of extra paternity, and testing for differences between extra-pair offspring and those sired within-pair.
Microsatellite markers are routinely used to investigate the genetic structuring of natural populations. The knowledge of how genetic variation is partitioned among populations may have important implications not only in evolutionary biology and ecology, but also in conservation biology. Hence, reliable estimates of population differentiation are crucial to understand the connectivity among populations and represent important tools to develop conservation strategies. The estimation of differentiation is c from Wright's FST and/or Slatkin's RST, an FST -analogue assuming a stepwise mutation model. Both these statistics have their drawbacks. Furthermore, there is no clear consensus over their relative accuracy. In this review, we first discuss the consequences of different temporal and spatial sampling strategies on differentiation estimation. Then, we move to statistical problems directly associated with the estimation of population structuring itself, with particular emphasis on the effects of high mutation rates and mutation patterns of microsatellite loci. Finally, we discuss the biological interpretation of population structuring estimates.
Molecular markers derived from polymerase chain reaction (PCR) amplification of genomic DNA are an important part of the toolkit of evolutionary geneticists. Random amplified polymorphic DNA markers (RAPDs), amplified fragment length polymorphisms (AFLPs) and intersimple sequence repeat (ISSR) polymorphisms allow analysis of species for which previous DNA sequence information is lacking, but dominance makes it impossible to apply standard techniques to calculate F-statistics. We describe a Bayesian method that allows direct estimates of FST from dominant markers. In contrast to existing alternatives, we do not assume previous knowledge of the degree of within-population inbreeding. In particular, we do not assume that genotypes within populations are in Hardy-Weinberg proportions. Our estimate of FST incorporates uncertainty about the magnitude of within-population inbreeding. Simulations show that samples from even a relatively small number of loci and populations produce reliable estimates of FST. Moreover, some information about the degree of within-population inbreeding (FIS) is available from data sets with a large number of loci and populations. We illustrate the method with a reanalysis of RAPD data from 14 populations of a North American orchid, Platanthera leucophaea.
In the last few years microsatellites have become one of the most popular molecular markers used with applications in many different fields. High polymorphism and the relative ease of scoring represent the two major features that make microsatellites of large interest for many genetic studies. The major drawback of microsatellites is that they need to be isolated de novo from species that are being examined for the first time. The aim of the present paper is to review the various methods of microsatellite isolation described in the literature with the purpose of providing useful guidelines in making appropriate choices among the large number of currently available options. In addition, we propose a fast and easy protocol which is a combination of different published methods.
Three primary hypotheses currently prevail for correlations between heterozygosity at a set of molecular markers and fitness in natural populations. First, multilocus heterozygosity-fitness correlations might result from selection acting directly on the scored loci, such as at particular allozyme loci. Second, significant levels of linkage disequilibrium, as in recently bottlenecked-and-expanded populations, might cause associations between the markers and fitness loci in the local chromosomal vicinity. Third, in partially inbred populations, heterozygosity at the markers might reflect variation in the inbreeding coefficient and might associate with fitness as a result of effects of homozygosity at genome-wide distributed loci. Despite years of research, the relative importance of these hypotheses remains unclear. The screening of heterozygosity at polymorphic DNA markers offers an opportunity to resolve this issue, and relevant empirical studies have now emerged. We provide an account of the recent progress on the subject, and give suggestions on how to distinguish between the three hypotheses in future studies.
Some species of parasites occur on a wide range of hosts while others are restricted to one or a few host species. The host specificity of a parasite species is determined, in part, by its ability to disperse between host species. Dispersal limitations can be studied by exploring the genetic structure of parasite populations both within a single species of host and across multiple host species. In this study we examined the genetic structure in the mitochondrial cytochrome oxidase I (COI) gene of two genera of lice (Insecta: Phthiraptera) occurring on multiple sympatric species of doves in southern North and Central America. One genus, Columbicola, is generally less host-specific than the other, Physconelloides. For both genera we identified substantial genetic differentiation between populations of conspecific lice on different host species, generally 10-20% sequence divergence. This level of divergence is in the range of that often observed between species of these two genera. We used nested clade analysis to explore fine scale genetic structure within species of these feather lice. We found that species of Physconelloides exhibited more genetic structure, both among hosts and among geographical localities, than did species of Columbicola. In many cases, single haplotypes within species of Columbicola are distributed on multiple host species. Thus, the population genetic structure of species of Physconelloides reveals evidence of geographical differentiation on top of high host species specificity. Underlying differences in dispersal biology probably explain the differences in population genetic structure that we observed between Columbicola and Physconelloides.
Many genetic distances have been developed to summarize allele frequency differences between populations. I review the evolutionary and statistical properties of three popular genetic distances: DS, DA, and theta;, using computer simulation of two simple evolutionary histories: an isolation model of population divergence and an equilibrium migration model. The effect of effective population size, mutation rate, and mutation mechanism upon the parametric value between pairs of populations in these models explored, and the unique properties of each distance are described. The effect of these evolutionary parameters on study design is also investigated and similar results are found for each genetic distance in each model of evolution: large sample sizes are warranted when populations are relatively genetically similar; and loci with more alleles produce better estimates of genetic distance.
Cyclical parthenogens, including aphids, are attractive models for comparing the genetic outcomes of sexual and asexual reproduction, which determine their respective evolutionary advantages. In this study, we examined how reproductive mode shapes genetic structure of sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) populations of the aphid Rhopalosiphum padi by comparing microsatellite and allozyme data sets. Allozymes showed little polymorphism, confirming earlier studies with these markers. In contrast, microsatellite loci were highly polymorphic and showed patterns very discordant from allozyme loci. In particular, microsatellites revealed strong heterozygote excess in asexual populations, whereas allozymes showed heterozygote deficits. Various hypotheses are explored that could account for the conflicting results of these two types of genetic markers. A strong differentiation between reproductive modes was found with both types of markers. Microsatellites indicated that sexual populations have high allelic polymorphism and heterozygote deficits (possibly because of population subdivision, inbreeding or selection). Little geographical differentiation was found among sexual populations confirming the large dispersal ability of this aphid. In contrast, asexual populations showed less allelic polymorphism but high heterozygosity at most loci. Two alternative hypotheses are proposed to explain this heterozygosity excess: allele sequence divergence during long-term asexuality or hybrid origin of asexual lineages. Clonal diversity of asexual lineages of R. padi was substantial suggesting that they could have frozen genetic diversity from the pool of sexual lineages. Several widespread asexual genotypes were found to persist through time, as already seen in other aphid species, a feature seemingly consistent with the general-purpose genotype hypothesis.
We present a new approach for defining groups of populations that are geographically homogeneous and maximally differentiated from each other. As a by-product, it also leads to the identification of genetic barriers between these groups. The method is based on a simulated annealing procedure that aims to maximize the proportion of total genetic variance due to differences between groups of populations (spatial analysis of molecular variance; samova). Monte Carlo simulations were used to study the performance of our approach and, for comparison, the behaviour of the Monmonier algorithm, a procedure commonly used to identify zones of sharp genetic changes in a geographical area. Simulations showed that the samova algorithm indeed finds maximally differentiated groups, which do not always correspond to the simulated group structure in the presence of isolation by distance, especially when data from a single locus are available. In this case, the Monmonier algorithm seems slightly better at finding predefined genetic barriers, but can often lead to the definition of groups of populations not differentiated genetically. The samova algorithm was then applied to a set of European roe deer populations examined for their mitochondrial DNA (mtDNA) HVRI diversity. The inferred genetic structure seemed to confirm the hypothesis that some Italian populations were recently reintroduced from a Balkanic stock, as well as the differentiation of groups of populations possibly due to the postglacial recolonization of Europe or the action of a specific barrier to gene flow.
The spatial genetic structure of the neotropical, clustered tree species Vouacapoua americana (Aublet) was studied in two natural forest stands (Paracou and Nouragues) in French Guiana. Using eight microsatellite loci, V. americana is characterized by a marked genetic structure at small spatial distances (under 30-60 m), in agreement with the limited seed dispersal by rodent species. Gene flow through pollen is also shown to be mainly restricted to less than 100 m. This result suggests that most pollination events (mediated through small insects) are probably limited to within-patches of individuals, which might explain the high genetic differentiation among patches (F(ST) = 0.11) separated by less than 2 km. We also assume that stronger genetic structure in Paracou is likely to be due to lower seed dispersal by rodents, large spatial distances separating patches, or a recent recolonization event.
Metrosideros bartlettii (Myrtaceae) is a distinctive and extremely rare tree, endemic to New Zealand, first discovered in 1975. Prior to this study, a total of 19 adult individuals of the species had been reported; these are located in three small forest remnants in the far north of the North Island of New Zealand. Here we describe a total of 31 adult M. bartlettii at the three sites, including 12 individuals newly discovered by us. We analyse the genetic diversity of the species, using microsatellites to examine the chloroplast genome and amplified fragment length polymorphisms (AFLPs) to monitor nuclear variation. The results clearly demonstrate that M. bartlettii is a unique species, distinct from its two closest relatives M. robusta and M. excelsa. Analysis of genetic diversity within the 31 remaining individuals of M. bartlettii showed an average heterozygosity (< H >) of 0.18 and a proportion of polymorphic genes (< P >) of 0.44. Population structure, as shown by 286 AFLP loci, varied between the three geographical sites; the site with fewest individuals, containing two trees, showed some separation from the populations at the other two locations. These two latter sites, by contrast, had highly overlapping AFLP population diversity profiles. The implications of these results for conservation of the species are discussed.
Forest tree species provide many examples of well-studied adaptive differentiation, where the search for the underlying genes might be possible. In earlier studies and in our common conditions in a greenhouse, northern populations set bud earlier than southern ones. A difference in latitude of origin of one degree corresponded to a change of 1.4 days in number of days to terminal bud set of seedlings. Earlier physiological and ecological genetics work in conifers and other plants have suggested that such variation could be governed by phytochromes. Nucleotide variation was examined at two phytochrome loci (PHYP and PHYO, homologues of the Arabidopsis thaliana PHYB and PHYA, respectively) in three populations: northern Finland, southern Finland and northern Spain. In our samples of 12-15 sequences (2980 and 1156 base pairs at the two loci) we found very low nonsynonymous variation; pi was 0.0003 and 0.0002 at PHYP and PHYO loci, respectively. There was no functional differentiation between populations at the photosensory domains of either locus. The overall silent variation was also low, only 0.0024 for the PHYP locus. The low estimates of silent variation are consistent with the estimated low synonymous substitution rates between Pinus sylvestris and Picea abies at the PHYO locus. Despite the low level of nucleotide variation, haplotypic diversity was relatively high (0.42 and 0.41 for fragments of 1156 nucleotides) at the two loci.
Detecting the signature of adaptation on nucleotide variation is often difficult in species that like Arabidopsis thaliana might have a complex demographic history. Recent re-sequencing surveys in this species provided genome-wide information that would mainly reflect its demographic history. We have used a large empirical data set (LED) as well as multilocus coalescent simulations to analyse sequence variation at loci involved in the phenylpropanoid pathway of this species. We surveyed and examined DNA sequence variation at nine of these loci (about 19.7 kb) in 23 accessions of A. thaliana and one accession of its closely related species Arabidopsis lyrata. Nucleotide variation was lower at nonsynonymous sites than at silent sites in all loci, indicating generalized functional constraint at the protein level. No association between variation and position in the metabolic pathway was detected. When the data were contrasted against the standard neutral model, significant deviations for silent variation were detected with Tajima's D, Fu's F(S) and Fay and Wu's H multilocus test statistics. These deviations were in the same direction than in previous large-scale multilocus analyses, suggesting a genome-wide effect. When the nine-locus data set was contrasted against the large empirical data set, the level (Watterson's theta) and pattern of variation (Tajima's D) detected in these loci did not deviate either at the single-locus or multilocus level from the corresponding empirical distributions. These results would support an important role of the demographic history of A. thaliana in shaping nucleotide variation at the nine studied phenylpropanoid loci. The potential and limitations of the empirical distribution approach are discussed.
A recent region-wide study determined that the central California coyote (Canis latrans) population was genetically subdivided according to habitat bioregions, supporting the hypothesis that coyotes exhibit a dispersal bias toward their natal habitat type. Here, we further investigated this hypothesis using radio-collared coyotes captured on a 150-km(2) study site on the border of (i.e. overlapping) two bioregions (Great Valley and Cascade Mountains). As predicted, most coyotes were assigned (based on a priori genetic criteria) to genetic clusters corresponding to one of these two bioregions. All of those assigned to the Great Valley genetic cluster were caught in (and for the most part, remained in) the Great Valley bioregion. However, contrary to expectations, the coyotes assigned to the Cascades genetic cluster occurred commonly in both bioregions. Nearly all resident individuals on the study site, regardless of the particular bioregion, were assigned to the Cascades genetic cluster, whereas a sizable fraction of nonresident (transient or dispersing) coyotes caught in the Great Valley bioregion were assigned to the Great Valley cluster. Even among resident coyotes, interrelatedness of packs was greater within than between bioregions, and packs with territories overlapping both bioregions were more closely related to those with territories completely within the Cascades bioregion than territories completely within the Great Valley bioregion. Finally, direct estimates indicated that gene flow was twice as high from the Cascades bioregion to the Great Valley bioregion than in the reverse direction. Collectively, these findings reveal the anatomy of the genetic subdivision as beginning abruptly at the bioregion boundary and ending diffusely within the Great Valley bioregion.
High levels of inbreeding are expected to cause a strong reduction in levels of genetic variability, effective recombination rates and in adaptation compared with related outcrossing populations. We examined patterns of DNA polymorphism at five nuclear loci and one chloroplast locus within and between four populations of the outcrossing plant Arabidopsis lyrata, a close relative of the highly self-fertilizing model species A. thaliana. The observed patterns are compared with species-wide polymorphism at orthologous loci, as well as within- and between-population patterns at other studied loci in A. thaliana. In addition to evidence for much higher average within-population diversity, species-wide levels of silent polymorphism are generally higher in A. lyrata than in A. thaliana, unlike the results from a previous study of the ADH locus. However, polymorphism is also low in the North American A. lyrata subspecies lyrata compared with the European subspecies petraea, suggesting either a population bottleneck in North American populations or recent admixture involving diverged European populations. Differentiation between the two subspecies is strong, although there are few fixed differences, suggesting that their isolation is recent. Estimates of intralocus recombination rates and analysis of haplotype structure in European A. lyrata populations indicate lower recombination than predicted based on the variability together with physical recombination rates estimated from A. thaliana. This may be due to strong population subdivision, or to recent departures from demographic equilibrium such as a bottleneck or population admixture. Alternatively, there may be consistently lower recombination rates in the outcrossing species. In contrast, estimates of recombination rates from species-wide samples of A. thaliana are close to the values expected assuming a high rate of self-fertilization. Complex population histories in both A. thaliana and A. lyrata complicate theoretical predictions and empirical tests of the effects of inbreeding on polymorphism and molecular evolution.
In an effort to characterize further the patterns of selection and adaptive evolution at the methuselah locus in Drosophila species, we extended an analysis of geographical variation to include single nucleotide polymorphisms (SNPs) in adjacent genes on either side of the mth locus, and examined the molecular variation in a neighbouring methuselah paralogue (mth2). An analysis of 13 SNPs spanning a region of nearly 19 kilobases surrounding the mth locus demonstrated that a clinal pattern associated with the most common mth haplotype does not extend to adjacent gene loci, providing compelling evidence that the clinal pattern results from selection on as yet unidentified sites associated with the functional mth locus. mth2 exhibited a significant pattern of adaptive divergence among D. melanogaster, D. simulans and D. yakuba similar to that seen at mth. However, Ka : Ks ratios indicate a difference in levels of functional constraint at the two methuselah, loci with mth2 exhibiting a five- to six-fold reduction in levels of amino acid divergence relative to mth.
The brown bears of coastal Alaska have been recently regarded as comprising from one to three distinct genetic groups. We sampled brown bears from each of the regions for which hypotheses of genetic uniqueness have been made, including the bears of the Kodiak Archipelago and the bears of Admiralty, Baranof and Chichagof (ABC) Islands in southeast Alaska. These samples were analysed with a suite of nuclear microsatellite markers. The 'big brown bears' of coastal Alaska were found to be part of the continuous continental distribution of brown bears, and not genetically isolated from the physically smaller 'grizzly bears' of the interior. By contrast, Kodiak brown bears appear to have experienced little or no genetic exchange with continental populations in recent generations. The bears of the ABC Islands, which have previously been shown to undergo little or no female-mediated gene flow with mainland populations, were found not to be genetically isolated from mainland bears. The data from the four insular populations indicate that female and male dispersal can be reduced or eliminated by water barriers of 2-4 km and 7 km in width, respectively.
Bacterial endosymbionts are common in insects and can have dramatic effects on their host's evolution. So far, the only heritable symbionts found in Drosophila have been Wolbachia and Spiroplasma. While the incidence and effects of Wolbachia have been studied extensively, the prevalence and significance of Spiroplasma infections in Drosophila are less clear. These small, gram-positive, helical bacteria infect a diverse array of plant and arthropod hosts, conferring a variety of fitness effects. Male-killing Spiroplasma are known from certain Drosophila species; however, in others, Spiroplasma appear not to affect sex ratio. Previous studies have identified different Spiroplasma haplotypes in Drosophila populations, although no extensive surveys have yet been reported. We used a multilocus sequence analysis to reconstruct a robust Spiroplasma endosymbiont phylogeny, assess genetic diversity, and look for evidence of recombination. Six loci were sequenced from over 65 Spiroplasma-infected individuals from nine different Drosophila species. Analysis of these sequences reveals at least five separate introductions of four phylogenetically distinct Spiroplasma haplotypes, indicating that more extensive sampling will likely reveal an even greater Spiroplasma endosymbiont diversity. Patterns of variation in Drosophila mitochondrial haplotypes in Spiroplasma-infected and uninfected flies imply imperfect vertical transmission in host populations and possible horizontal transmission.
We evaluated the population genetic structure of seven microsatellite loci for old growth and second growth populations of eastern white pine (Pinus strobus). From each population, located within Hartwick Pines State Park, Grayling, Michigan, USA, 120-122 contiguous trees were sampled for genetic analysis. Within each population, genetic diversity was high and inbreeding low. When comparing these populations, there is a significant, but small (less than 1%), genetic divergence between populations. Spatial distance between populations or timber harvest at the second growth site were reasonable explanations for the observed minor differences in allele frequencies between populations. Spatial autocorrelation analysis suggested that, for the old growth population, weak positive structuring at 15 m fits the isolation by distance model for a neighbourhood size of about 100 individuals. In comparison, genotypes were randomly distributed in the second growth population. Thus, logging may have decreased spatial structuring at the second growth site, suggesting that management practices may be used to alter natural spatial patterns. In addition, the amount of autocorrelation in the old growth population appears to be lower for some of the microsatellites, suggesting higher numbers of rare alleles and that higher mutation rates may have directly affected spatial statistics by reducing structure.
Variation in pigmentation is common in marine invertebrates, although few studies have shown the existence of genetic differentiation of chromatic forms in these organisms. We studied the genetic structure of a colonial ascidian with populations of different colour morphs in the northwestern Mediterranean. A fragment of the c oxidase subunit 1 (COI) mitochondrial gene was sequenced in seven populations of Pseudodistoma crucigaster belonging to three different colour morphs (orange, yellow and grey). Maximum likelihood analyses showed two well-supported clades separating the orange morph from the yellow-grey morphotypes. Genetic divergence between these clades was 2.12%, and gamma(ST) values between populations of the two clades were high (average 0.936), pointing to genetic isolation. Nested clade and coalescence analyses suggest that a past fragmentation event may explain the phylogeographical origin of these two clades. Non-neutral mtDNA evolution is observed in our data when comparing the two clades, showing a significant excess of nonsynonymous polymorphism within the yellow-grey morphotype using the McDonald-Kreitman test, which is interpreted as further support of reproductive isolation. We conclude that the two clades might represent separate species. We compare the population genetic differentiation found with that estimated for other colonial and solitary ascidian species, and relate it to larval dispersal capabilities and other life-history traits.
For captive breeding to play a significant role in conservation, ex situ populations must be scientifically managed to meet objective goals for retaining representative genetic variation. Imperfect genealogical information requires fundamental assumptions to be made that may bias downstream measures of genetic importance, upon which management decisions are based. The impacts of such assumptions are most pronounced within breeding programmes characterized by a high proportion of individuals of unknown ancestry, as exemplified by the large captive population of the St Vincent parrot (Amazona guildingii). The degree to which microsatellite-based estimates of relatedness may improve upon the assumptions of conventional pedigree-based management was investigated using genotypic data collected at eight microsatellite loci and two marker-based relatedness estimators. The measure, rxyLR, was found to explain the highest amount of variation in true relatedness. Integration of pairwise estimates of founder relatedness with studbook data transformed current understanding of the relatedness structure of the A. guildingii population from two subgroups characterized by a high and low degree of relatedness, respectively, to a situation where all 72 individuals are prioritized for breeding according to their estimated mean kinships. Furthermore, the discovery of opposing, directional bias exhibited by rxyLR and rxyQG in assigning dyads to a given relationship category suggests that an approach that utilizes a combination of pairwise relatedness estimators may provide the most genetic information for balancing the dual considerations of maximizing gene diversity and minimizing inbreeding in developing breeding recommendations.
Using four highly polymorphic microsatellite markers (12-28 alleles), we gentoyped workers from 63 colonies of Pogonomyrmex occidentalis. Colonies have a single, multiply mated queen, and an average number of 6.3 patrilines per colony. Colony growth was measured over an 8-year period in the study population. Intracolonial relatedness and colony growth are correlated negatively, indicating a substantial fitness benefit to multiple mating.
The use of noninvasively collected samples greatly expands the range of ecological issues that may be investigated through population genetics. Furthermore, the difficulty of obtaining reliable genotypes with samples containing low quantities of amplifiable DNA may be overcome by designing optimal genotyping schemes. Such protocols are mainly determined by the rates of genotyping errors caused by false alleles and allelic dropouts. These errors may not be avoided through laboratory procedure and hence must be quantified. However, the definition of genotyping error rates remains elusive and various estimation methods have been reported in the literature. In this paper we proposed accurate codification for the frequencies of false alleles and allelic dropouts. We then reviewed other estimation methods employed in hair- or faeces-based population genetics studies and modelled the bias associated with erroneous methods. It is emphasized that error rates may be substantially underestimated when using an erroneous approach. Genotyping error rates may be important determinants of the outcome of noninvasive studies and hence should be carefully computed and reported.
The red-legged frog, Rana aurora, has been recognized as both a single, polytypic species and as two distinct species since its original description 150 years ago. It is currently recognized as one species with two geographically contiguous subspecies, aurora and draytonii; the latter is protected under the US Endangered Species Act. We present the results of a survey of 50 populations of red-legged frogs from across their range plus four outgroup species for variation in a phylogenetically informative, approximately 400 base pairs (bp) fragment of the mitochondrial cytochrome b gene. Our mtDNA analysis points to several major results. (1) In accord with several other lines of independent evidence, aurora and draytonii are each diagnosably distinct, evolutionary lineages; the mtDNA data indicate that they do not constitute a monophyletic group, but rather that aurora and R. cascadae from the Pacific northwest are sister taxa; (2) the range of the draytonii mtDNA clade extends about 100 km further north in coastal California than was previously suspected, and corresponds closely with the range limits or phylogeographical breaks of several codistributed taxa; (3) a narrow zone of overlap exists in southern Mendocino County between aurora and draytonii haplotypes, rather than a broad intergradation zone; and (4) the critically endangered population of draytonii in Riverside County, CA forms a distinct clade with frogs from Baja California, Mexico. The currently available evidence favours recognition of aurora and draytonii as separate species with a narrow zone of overlap in northern California.
Mitochondrial DNA (mtDNA) has been used to study molecular ecology and phylogeography for 25 years. Much important information has been gained in this way, but it is time to reflect on the biology of the mitochondrion itself and consider opportunities for evolutionary studies of the organelle itself and its ecology, biochemistry and physiology. This review has four sections. First, we review aspects of the natural history of mitochondria and their DNA to show that it is a unique molecule with specific characteristics that differ from nuclear DNA. We do not attempt to cover the plethora of differences between mitochondrial and nuclear DNA; rather we spotlight differences that can cause significant bias when inferring demographic properties of populations and/or the evolutionary history of species. We focus on recombination, effective population size and mutation rate. Second, we explore some of the difficulties in interpreting phylogeographical data from mtDNA data alone and suggest a broader use of multiple nuclear markers. We argue that mtDNA is not a sufficient marker for phylogeographical studies if the focus of the investigation is the species and not the organelle. We focus on the potential bias caused by introgression. Third, we show that it is not safe to assume a priori that mtDNA evolves as a strictly neutral marker because both direct and indirect selection influence mitochondria. We outline some of the statistical tests of neutrality that can, and should, be applied to mtDNA sequence data prior to making any global statements concerning the history of the organism. We conclude with a critical examination of the neglected biology of mitochondria and point out several surprising gaps in the state of our knowledge about this important organelle. Here we limelight mitochondrial ecology, sexually antagonistic selection, life-history evolution including ageing and disease, and the evolution of mitochondrial inheritance.
The genealogical process for a sample from a metapopulation, in which local populations are connected by migration and can undergo extinction and subsequent recolonization, is shown to have a relatively simple structure in the limit as the number of populations in the metapopulation approaches infinity. The result, which is an approximation to the ancestral behaviour of samples from a metapopulation with a large number of populations, is the same as that previously described for other metapopulation models, namely that the genealogical process is closely related to Kingman's unstructured coalescent. The present work considers a more general class of models that includes two kinds of extinction and recolonization, and the possibility that gamete production precedes extinction. In addition, following other recent work, this result for a metapopulation divided into many populations is shown to hold both for finite population sizes and in the usual diffusion limit, which assumes that population sizes are large. Examples illustrate when the usual diffusion limit is appropriate and when it is not. Some shortcomings and extensions of the model are considered, and the relevance of such models to understanding human history is discussed.