Altered genotypic and phenotypic frequencies of aphid populations under enriched CO2 and O3 atmospheres

Environmental change is anticipated to negatively affect both plant and animal populations. As abiotic factors rapidly change habitat suitability, projections range from altered genetic diversity to wide-spread species loss. Here, we assess the degree to which changes in atmospheric composition associated with environmental change will influence not only the abundance, but also the genotypic/phenotypic diversity, of herbivore populations. Using free-air CO₂ and O₃ enrichment (FACE) technology, we assess numerical responses of pea aphids (Acyrthosiphon pisum) exhibiting a pink–green genetic polymorphism and an environmentally determined wing polyphenism on broad bean plants (Vicia faba) under enriched CO₂ and/or O₃ atmospheres, over multiple generations. We show that these two greenhouse gases alter not only aphid population sizes, but also genotypic and phenotypic frequencies. As the green genotype was positively influenced by elevated CO₂ levels, but the pink genotype was not, genotypic frequencies (pink morph : green morph) ranged from 1 : 1 to 9 : 1. These two genotypes also displayed marked differences in phenotypic frequencies. The pink genotype exhibited higher levels of wing induction under all atmospheric treatments, however, this polyphenism was negatively influenced by elevated O₃ levels. Resultantly, frequencies of winged phenotypes (pink morph : green morph) varied from 10 : 1 to 332 : 1. Thus, atmospheric conditions associated with environmental change may alter not just overall population sizes, but also genotypic and phenotypic frequencies of herbivore populations, thereby influencing community and ecosystem functioning.     

The recent spread of <Emphasis Type="Italic">Artemia parthenogenetica</Emphasis> in Western Australia

In Western Australia, populations of Artemia parthenogenetica in coastal salt lakes at Rottnest Island and Lake Hayward, and in salterns at Port Hedland and Shark Bay, are widely accepted to have been introduced by humans. Further, within the past 10 years, populations of A. parthenogenetica have been found in inland playa salt lakes in the wheatbelt of south-west Western Australia, where none had been recorded in previous salt lake studies. Here we hypothesise that birds act as transport vectors for Artemia cysts both within Australia and between the Asian and Australian continents. Allozyme analysis was used to identify clonal types (multi-locus genotypes), clonal frequencies, genotypic diversities and genotypic identity of six populations (three coastal, three inland). Overall, the inland populations displayed almost identical genotypic structure to the coastal population from Lake Hayward, indicating that Lake Hayward could be the major source for dispersal and colonisation of inland populations. Results support the hypothesis of dispersal inland by nomadic bird species. Furthermore, evidence suggests that the inland and Lake Hayward populations may be an example of a metapopulation. The greater variety of genotypes present in the Rottnest population indicates that this population has received a large number of small immigrations, or that it received one large introduction. The former may indicate a long period of suitable salinities, providing a greater time-span over which migration and succession of clonal types could occur in comparison to other populations. While we cannot rule out the possibility of human introduction of A. parthenogenetica to Rottnest, the hypothesis of cyst dispersal along the Austral-Asian flyway remains possible. Guest Editor: John M. Melack Saline Waters and their Biota     

THE GENETIC STRUCTURE OF LARGE-LAKE DAPHNIA POPULATIONS

Cyclical parthenogenesis allows study of the genetic and evolutionary characteristics of groups exhibiting both asexual and sexual reproduction. The cladoceran genus Daphnia contains species which vary with respect to the relative incidence of sexual reproduction; pond species tend to undergo sexual reproduction more regularly than species found in large lakes. Previous genetic studies have focused on pond populations, generating expectations about large-lake populations that have not been fully met by recent studies. The present study of the Palearctic species Daphnia galeata further examines the genetic structure of large-lake populations. Nine local populations, from lakes in northern Germany, are examined for genetic variation at seven enzyme loci. Populations exhibit similar allelic arrays and often similar allele frequencies at the five polymorphic loci; values of Nei's genetic distance (D) ranged from 0.002 to 0.239, with a mean of 0.084. F_ST values range from 0.012 to 0.257, and spatial autocorrelation coefficients range from -0.533 to 0.551, for the eight alleles analyzed. With few exceptions, within-population genotypic frequencies were in Hardy-Weinberg equilibrium. There was, however, significant heterogeneity in genotypic frequencies among populations. The number of coexisting clonal groups, as determined by three locus genotypes, is high within populations. Clonal groups are widely distributed among localities. The amount of genetic divergence observed among these large-lake populations is smaller than that previously observed among pond populations and suggests that different processes may be important in determining the genetic structure and subsequent phenotypic divergence of lake versus pond populations.     

Population genetics of the fissiparous holothurian Stichopus chloronotus (Aspidochirotida) on the Great Barrier Reef, Australia

 Population genetic structure was studied in one nearshore and two offshore populations of Stichopus chloronotus, a common holothurian species on Indo-Pacific coral reefs. Genetic variation at five polymorphic loci was examined using allozyme electrophoresis. The nearshore population consisted almost exclusively of male individuals, and more males than females were found in all populations studied. Deviations of heterozygosity from that predicted under Hardy-Weinberg equilibrium indicated that asexual reproduction occurred in all populations. Estimates of the level of asexual reproduction using the ratios of the number of sexually produced individuals to sample size, observed genotypic diversity to expected genotypic diversity, and number of genotypes to sample size confirmed that this reproductive mode was more important at the nearshore reef compared to the two offshore reefs. There were large differences in genotypic frequencies between males and females. F-statistics on clonal genotypic frequencies were not statistically significant between populations for neither females or males, suggesting high dispersal of larvae between reefs. A higher mortality of females during larval or early post-settlement stages, or reduced dispersal capability of female larvae are the most likely reasons for biased sex ratios. Accepted: 23 November 1998     

Genetic structure of Lake Magadi tilapia populations

The control region of the mitochondrial DNA haplotype frequencies were significantly different among the two separate lagoon populations of Oreochromis alcalicus grahami in Lake Magadi and of O. a. alcalicus from lake Natron, and DNA fingerprint similarity indices were significantly higher for intra-population comparisons of the two Magadi lagoon populations and the Lake Natron population than the inter-population similarity indices among these populations. A modified F _st measure indicated population sub-division and the phylogeographic partitioning of the VNTR fragments observed were unique to specific populations further indicating substantial genetic differentiation. The lagoon populations within Lake Magadi demonstrated the same degree of genetic differentiation as either of these populations did to the outgroup (the Lake Natron population). There appears to be limited gene flow between Lake Magadi tilapia populations and this population structure has important implications for protecting locally adapted populations within this unique ecosystem.     

CONTRIBUTIONS OF SEXUAL AND ASEXUAL REPRODUCTION TO POPULATION STRUCTURE IN THE CLONAL SOFT CORAL,ALCYONIUM RUDYI

Numerous studies of population structure in sessile clonal marine invertebrates have demonstrated low genotypic diversity and nonequilibrium genotype frequencies within local populations that are monopolized by relatively few, highly replicated genets. All of the species studied to date produce planktonic sexual propagules capable of dispersing long distances; despite local genotypic disequilibria, populations are often panmictic over large geographic areas. The population structure paradigm these species represent may not be typical of the majority of clonal invertebrate groups, however, which are believed to produce highly philopatric sexual propagules. I used allozyme variation to examine the population structure of the temperate soft coral, Alcyonium rudyi, a typical clonal species whose sexually produced larvae and asexually produced ramets both have very low dispersal capabilities. Like other clonal plants and invertebrates, the local population dynamics of A. rudyi are dominated by asexual reproduction, and recruitment of new sexually produced genets occurs infrequently. As expected from its philopatric larval stage, estimates of genetic differentiation among populations of A. rudyi were highly significant at all spatial scales examined (mean θ = 0.300 among 20 populations spanning a 1100-km range), suggesting that genetic exchange seldom occurs among populations separated by as little as a few hundred meters. Mapping of multilocus allozyme genotypes within a dense aggregation of A. rudyi ramets confirmed that dispersal of asexual propagules is also very limited: members of the same genet usually remain within < 50 cm of one another on the same rock surface. Unlike most previously studied clonal invertebrates, populations of A. rudyi do not appear to be dominated by a few widespread genets: estimates of genotypic diversity (G_o) within 20 geographically distinct populations did not differ from expectations for outcrossing, sexual populations. Despite theoretical suggestions that philopatric dispersal combined with typically small effective population sizes should promote inbreeding in clonal species, inbreeding does not appear to contribute significantly to the population structure of A. rudyi. Genet genotype frequencies conformed to Hardy-Weinberg expectations in all populations, and inbreeding coefficients (f) were close to zero. In general, the population structure of A. rudyi did not differ significantly from that observed among outcrossing sexual species with philopatric larval dispersal. Age estimates suggest, however, that genets of A. rudyi live for many decades. Genet longevity may promote high genotypic diversity within A. rudyi populations and may be the most important evolutionary consequence of clonal reproduction in this species and the many others that share its dispersal characteristics.     

Isolation and characterization of polymorphic microsatellite markers in the grasshopper <Emphasis Type="Italic">Mioscirtus wagneri</Emphasis> (Orthoptera: Acrididae)

Eight polymorphic microsatellite markers were developed for the grasshopper Mioscirtus wagneri. Polymorphism at these loci was evaluated in 25 individuals from Central Spain. The number of alleles per locus ranged from 3 to 9 and their observed and expected heterozygosities ranged from 0.28 to 0.80 and 0.25 to 0.84, respectively. All genotypic frequencies conformed to Hardy–Weinberg equilibrium expectations, with no evidence of genotypic linkage disequilibrium between any pair of loci. These loci will be highly useful for the study of the population genetic structure and diversity of this grasshopper species forming highly fragmented populations of great conservation concern.     

Allozyme variation among natural populations of Holopedium gibberum (Crustacea; Cladocera)

• 1 Holopedium gibberum, from twenty lakes in Rhode Island and Maine, were examined for allozyme variation at five loci to determine the pattern and degree of generic variation among sites and the genetic structure within individual lakes.
• 2 There were significant differences in allele frequencies among sites. Most populations were fixed for a particular allele at each locus. Only five lakes had polymorphic populations.
• 3 Polymorphic populations showed significant deviation from expected Hardy-Weinberg genotype frequencies. In each case, there was an excess of homozygotes.
• 4 Two lakes were examined for intra-lake allele frequency differences. In one lake there were no differences. The other lake exhibited significant allele frequency differences between stations at the north and south ends of the lake.
• 5 Populations were examined for the frequency distribution of composite genotypes over three loci. Most populations were dominated by one or two genotypes.
• 6 The results suggest sporadic sexual recruitment and a high degree of genetic isolation among these populations of H. gibberum. In these respects they resemble the permanent pond populations of Daphnia magna examined by Hebert (1974a).

     

Ten polymorphic tetranucleotide microsatellite markers isolated from the Anolis roquet series of Caribbean lizards

The Anolis roquet series of Caribbean lizards provides natural replicates with which to examine the role of historical contingency and ecological determinism in shaping evolutionary patterns. Here, we describe 10 polymorphic tetranucleotide microsatellites to facilitate studies on population differentiation and gene flow. All loci successfully amplified in several species from this series. Genotyping 96 individuals from two A. roquet populations demonstrated the markers’ suitability as population genetic markers: genetic diversity was high (9–22 alleles per locus); there were no instances of linkage disequilibrium; and, with one exception, all genotypic frequencies conformed to Hardy–Weinberg equilibrium expectations.     

Reproductive strategies and isolation‐by‐demography in a marine clonal plant along an eutrophication gradient

Genetic diversity in clonal organisms includes two distinct components, (i) the diversity of genotypes or clones (i.e. genotypic richness) in a population and (ii) that of the alleles (i.e. allelic and gene diversity within populations, and differentiation between populations). We investigated how population differentiation and genotypic components are associated across a gradient of eutrophication in a clonal marine plant. To that end, we combined direct measurements of sexual allocation (i.e. flower and seed counts) and genotypic analyses, which are used as an estimator of effective sexual reproduction across multiple generations. Genetic differentiation across sites was also modelled according to a hypothesis here defined as isolation‐by‐demography, in which we use population‐specific factors, genotypic richness and eutrophication that are hypothesized to affect the source‐sink dynamics and thus influence the genetic differentiation between a pair of populations. Eutrophic populations exhibited lower genotypic richness, in agreement with lower direct measurements of sexual allocation and contemporaneous gene flow. Genetic differentiation, while not explained by distance, was best predicted by genotypic richness and habitat quality. A multiple regression model using these two predictors was considered the best model (R² = 0.43). In this study, the relationship between environment and effective sexual–asexual balance is not simply (linearly) predicted by direct measurements of sexual allocation. Our results indicate that population‐specific factors and the isolation‐by‐demography model should be used more often to understand genetic differentiation.     

Clonal diversity and turnover in an overwintering Daphnia pulex population,and the effect of fish predation

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Evaluation of a remnant lake sturgeon population’s utility as a source for reintroductions in the Ohio River system

The selection of an appropriate source population may be crucial to the long-term success of reintroduction programs. Appropriate source populations often are those that originate from the same genetic lineage as native populations. However, source populations also should exhibit high levels of genetic diversity to maximize their capacity to adapt to variable environmental conditions. Finally, it is preferable if source populations are genetically representative of historical lineages with little or no contamination from non-native or domesticated stocks. Here, we use nuclear (microsatellite) and cytoplasmic (mitochondrial control region) markers to assess the genetic suitability of a potential source population inhabiting the White River in Indiana: the last extant lake sturgeon population in the Ohio River drainage. The White River population exhibited slightly lower levels of genetic diversity than other lake sturgeon populations. However, the population’s two private microsatellite alleles and three private haplotypes suggest a unique evolutionary trajectory. Population assignment tests revealed only two putative migrants in the White River, indicating the population has almost completely maintained its genetic integrity. Additionally, pairwise F _ST estimates indicated significant levels of genetic divergence between the White River and seven additional lake sturgeon populations, suggesting its genetic distinctiveness. These data indicate that the White River population may be the most suitable source population for future lake sturgeon reintroductions throughout the Ohio River drainage. Furthermore, the White River population appears to be a reservoir of unique genetic information and reintroduction may be a necessary strategy to ensure the persistence of this important genetic lineage.     

Phenotypic and genotypic variation of Phragmites australis: Comparison of populations in two human-made lakes of different age and history

Populations of Phragmites australis (CAV.) TRIN. ex STEUD. were studied in the littoral zones of two man-made lakes located in the Třeboň Basin (South Bohemia, the Czech Republic): (1) Opatovický fishpond, a shallow artificial lake constructed in 1510–1514 by damming a shallow valley and used since for carp production, and (2) Halámky sand pit, a new lake formed by sand extraction in 1970–1994. Phenotypic variability was assessed on the basis of shoot morphological and growth characteristics, measured at the time of seasonal maximum aboveground biomass. Genotypic variability was detected using RAPDs, which demonstrated a high clonal diversity in both habitats. The clonal diversity would be strongly underestimated if it were based only on morphological differences. Higher genotypic variability was found in the fishpond reed, not corresponding with low variability in its phenotype performance. Based on analysis of 160 samples, four patterns of genotypic variation were detected: (1) Some stands were genetically uniform and were therefore considered to be monoclonal in both populations studied. (2) Some stands consisted of several clones at the Halámky sand pit. However, these clones showed more similarity within the particular stands than with clones of adjacent stands. (3) In the Opatovický fishpond population, multiclonal stands consisted of clones with a low degree of similarity. (4) Identical clones were detected in several neighbouring stands separated by gaps in the Opatovický fishpond population. The findings support a model of colonization postulating that populations initiated by seeds are initially genetically diverse and over time become dominated by a few clones as a result of competition and selection. These processes then decrease both genetic and morphological variability.     

Fitness of allozyme variants in Drosophila pseudoobscura III. Possible factors contributing to the maintenance of polymorphisms in nature

Experiments have been performed to investigate the mechanisms maintaining enzyme polymorphisms in natural populations. We have measured effects on fitness of genotypic variants at three loci, Est-5, Odh, and Mdh-2, in D. pseudoobscura. Significant differences exist among the genotypes in the rate of development from egg-to-adult; there is also indication of differences in larval survival. In a population segregating for allelic variants at all three loci, there is indication that segregation distortion at meiosis or some form of gametic selection might be involved. The relative fitnesses of alternative genotypes are reversed when either different fitness components are considered, or the genotypic frequencies are changed, or the larval density is increased. These fitness reversals may contribute to the maintenance of the polymorphisms, and may account for cyclical oscillations of allozyme frequencies observed in natural populations.Research supported by U.S. Public Health Service Research Fellowship (1F05 TWO 1991-01) to D.M. and by contract AT(04-3)34 with the U.S. Atomic Energy Commission. Adress reprint requests from Europe to D.M.; from elsewhere to F.J.A.     

Variation in life history and genetic traits of Hawaiian mosquitofish populations

Mosquitofish (Gambusia affinis) were collected from 17 reservoirs on three islands in Hawaii, USA. Genetic and life history traits for adult females from these populations were used to evaluate hypotheses concerning short-term evolutionary divergence of populations recently established from a common ancestral source. The effects of founder events and drift on genetic variability and population differentiation were also examined. Significant differences in life history characteristics, allele frequencies, and multi-locus heterozygosities (H) were found among fish populations collected from different reservoirs and between reservoirs classified as stable or fluctuating on the basis of temporal fluctuation in water level. Females from stable reservoirs exhibited greater standard length (35.1 vs 32.8 mm), lower fecundity (11.9 vs 15.2 embryos), lower reproductive allocation (18.2% vs 22.8%), but larger mean embryo size (1.95 vs 1.67 mg) than females from fluctuating reservoirs. Consistency in means among replicates of each reservoir class and concordance in direction and magnitude of differences reported here and results of sampling conducted from these same locations 10 years previously (Stearns, 1983a) suggest that ecological factors intrinsic to these two environments are important in determing population life history traits. Females from stable reservoirs exhibited lower heterozygosity than females from fluctuating reservoirs (0.134 vs 0.158, respectively). Levels and direction od differences in heterozygosity, the high proportion of polymorphic loci and lack of fixation of alternative alleles argue against a purely stochastic explanation for genetic and life history variation among reservoir populations. Levels of genetic variability and interpopulation differentiation were similar to those observed in mainland populations of this species. A high proportion of the genetic diversity was apportioned between populations and within populations due to differences between juveniles and adults. Significant genotypic differences between adult and juvenile age classes suggest that the genetic divergence of local populations may occur over short periods of time.     

Unilateral incompatibility as a major cause of skewed segregation in the cross between Lycopersicon esculentum and L. pennellii

Skewed segregations are frequent events in segregating populations derived from different interspecific crosses in tomato. To determine a basis for skewed segregations in the progeny of the cross between Lycopersicon esculentum and L. pennellii, monogenic segregations of 16 isozyme loci were analyzed in an F₂ and two backcross populations of this cross. In the F₂, 9 loci mapping to chromosomes 1, 2, 4, 9, 10 and 12 exhibited skewed segregations and in all cases there was an excess of L. pennellii homozygotes. The genotypic frequencies at all but one locus were at Hardy-Weinberg equilibria. In the backcross populations, all except two loci exhibited normal Mendelian segregations. No post-zygotic selection model could statistically or biologically explain the observed segregation patterns in the F₂ and backcross populations. A pre-zygotic selection model, assuming selective elimination of the male gametophytes during pollen function (i.e., from pollination to karyogamy), could adequately explain the observed segregations in all three populations. The direction of the skewed segregations in the F₂ population was consistent with that expected based on the effects of unilateral incompatibility reactions between the two species. In addition, the chromosomal locations of 5 of the 9 markers that exhibited skewed segregations coincided with the locations of several known compatibility-related genes in tomato. Multigenic unilateral incompatibility reactions between L. esculentum pollen and the stigma or style of L. pennellii (or its hybrid derivatives) are suggested to be the major cause of the skewed segregations in the F₂ progeny of this cross.     

Inferring ancient Agave cultivation practices from contemporary genetic patterns

Several Agave species have played an important ethnobotanical role since prehistory in Mesoamerica and semiarid areas to the north, including central Arizona. We examined genetic variation in relict Agave parryi populations northeast of the Mogollon Rim in Arizona, remnants from anthropogenic manipulation over 600 years ago. We used both allozymes and microsatellites to compare genetic variability and structure in anthropogenically manipulated populations with putative wild populations, to assess whether they were actively cultivated or the result of inadvertent manipulation, and to determine probable source locations for anthropogenic populations. Wild populations were more genetically diverse than anthropogenic populations, with greater expected heterozygosity, polymorphic loci, effective number of alleles and allelic richness. Anthropogenic populations exhibited many traits indicative of past active cultivation: fixed heterozygosity for several loci in all populations (nonexistent in wild populations); fewer multilocus genotypes, which differed by fewer alleles; and greater differentiation among populations than was characteristic of wild populations. Furthermore, manipulated populations date from a period when changes in the cultural context may have favoured active cultivation near dwellings. Patterns of genetic similarity among populations suggest a complex anthropogenic history. Anthropogenic populations were not simply derived from the closest wild A. parryi stock; instead they evidently came from more distant, often more diverse, wild populations, perhaps obtained through trade networks in existence at the time of cultivation.     

Variability of the Ramularia collo‐cygni Population in Central Europe

The genetic structure of the fungal barley pathogen Ramularia collo‐cygni (Rcc) population in Central Europe involving the isolates from the Czech Republic, the Slovak Republic, Germany and Swiss was determined using amplified fragment length polymorphism (AFLP) analysis. One hundred and eighty‐four markers were chosen to determine genetic and genotypic diversity and to test the hypothesis of random mating and population differentiation of Rcc isolates. Among the 337 isolates collected, the overall gene diversity was moderate ( = 0.216). The level of multilocus genotypic diversity was higher within populations than among them. All individuals had unique multilocus genotypes. Genetic differentiation was significant among populations in localities, but at a moderate level (θ = 0.12; P < 0.001), suggesting that gene flow is occurring among populations. The isolates from all twelve clusters produced by Structure were found in all local populations, although at different frequencies. Therefore, the inferred clusters did not represent geographical populations. Although the null hypothesis of random mating in Rcc populations was rejected, the high level of genotypic diversity suggests that the Rcc population structure appears to be generated by a mixed reproductive system including both asexual and sexual reproduction, along with a rather high migration rate.     

Population structure, genetic diversity and host specificity of the parasitic weedStriga hermonthica (Scrophulariaceae) in Sahel

Seed proteins of individual plants from 14 populations ofStriga hermonthica growing on sorghum, millet, maize and wild grasses in Burkina Faso, Mali and Niger were studied using gel electrophoresis in order to assess the population structure and genetic diversity of the parasitic weed. The relative intensity of the different bands within a pattern was used to depict the genotypic constitution of each sample. Genotypic frequencies conformed to Hardy-Weinberg expectations in 13 populations out of 14 for the two loci that were interpreted. Heterozygote deficiencies could be the result of the Wahlund effect. The genetic divergence between populations appears to be low for bothAdh andGot loci. Thus, the physiological specialization for a particular host could be a recent phenomenon. A low host specificity ofS. hermonthica populations could affect the efficiency of introducing new resistant cultivars as a control measure against the parasitic weed.     

Polymorphisms of DNA repair genes <Emphasis Type="Italic">ERCC</Emphasis>2 and <Emphasis Type="Italic">XRCC</Emphasis>1 in populations of Russia

We have conducted a comparative study of allele frequencies of single nucleotide polymorphisms (SNPs) rs1799793 and rs13181 of the ERCC2 gene as well as rs1799782 and rs25487 of the XRCC1 gene in population samples from European regions of Russia as well as in populations of Izhemski and Priluzski Komi and Yakuts. Significant differences in the distribution of polymorphic variants of the ERCC2 gene were demonstrated between populations of Yakuts and populations of Russians and Komi. In case of XRCC1 gene Izhemski Komi population exhibited dissimilar allele frequencies compared to other populations.