On the degree of genetic divergence in Drosophila ananassae populations transferred to laboratory conditions

Twelve natural populations of Drosophila ananassae were sampled and laboratory populations were derived. All the populations were maintained in food bottles in the laboratory for ten generations by transferring fifty flies (females and males in equal number) in each generation. After ten generations they were analysed chromosomally to determine the frequency of different chromosome arrangements. The results show that there is significant variation in the frequencies of chromosome arrangements and in the level of inversion heterozygosity. Although some of the populations became mo-nomorphic for certain inversions, in general all populations remained polymorphic even after ten generations. The degree of genetic differentiation in the populations after they were transferred to laboratory conditions has been estimated by calculating genetic identity and distance between the initial and final populations based on the differences in chromosome arrangement frequencies. The estimates of I and D suggest that there is considerable variation in the degree of genetic divergence in D. ananassae populations. Some populations have remained unchanged while others have diverged to a considerable extent.     

Population genetic structure of Cydia pomonella: a review and case study comparing spatiotemporal variation

Analysis of population genetic structure is a key aspect to understand insect pest population dynamics in agricultural scenarios. Here the role of geography, hosts and time on the population genetic structure of codling moth Cydia pomonella (Linnaeus) (Lep., Tortricidae) populations is described. Temporal variation was examined in two French orchards among each of three adult flights during two successive years. Analyses were conducted using two insecticide resistance markers (variation at the sodium channel gene and enzymatic activity of cytochrome P450 oxidases) and three microsatellite loci. Levels of genetic variation among temporal populations were not significant based on variation in the sodium channel gene and microsatellite loci. However, P450 oxidase activity differed significantly during both flights and years, decreasing during the three flights of the first year and increasing during the second. These results suggest that phytosanitary measures are among the factors shaping the genetic structure of C. pomonella populations over temporal and geographical scales. We discuss the relative importance of natural and passive dispersal related to anthropogenic activities affecting C. pomonella population genetics and highlight population genetic research needs in order to design more efficient pest management practices.     

Genetic variation and co-variation for fitness between intra-population and inter-population backgrounds in the red flour beetle, Tribolium castaneum

Hybrids from crosses between populations of the flour beetle, Tribolium castaneum, express varying degrees of inviability and morphological abnormalities. The proportion of allopatric population hybrids exhibiting these negative hybrid phenotypes varies widely, from 3% to 100%, depending upon the pair of populations crossed. We crossed three populations and measured two fitness components, fertility and adult offspring numbers from successful crosses, to determine how genes segregating within populations interact in inter-population hybrids to cause the negative phenotypes. With data from crosses of 40 sires from each of three populations to groups of five dams from their own and two divergent populations, we estimated the genetic variance and covariance for breeding value of fitness between the intra- and inter-population backgrounds and the sire × dam population interaction variance. The latter component of the variance in breeding values estimates the change in genic effects between backgrounds owing to epistasis. Interacting genes with a positive effect, prior to fixation, in the sympatric background but a negative effect in the hybrid background cause reproductive incompatibility in the Dobzhansky-Muller speciation model. Thus, the sire × dam population interaction provides a way to measure the progress towards speciation of genetically differentiating populations on a trait by trait basis using inter-population hybrids.     

Niche variation and the maintenance of variation in body size in a burying beetle

1. In burying beetles (Nicrophorinae), body size is known to provide both a fecundity advantage (in females) and successful resource defence (in males and females). Despite this, considerable variation in body sizes is observed in natural populations. 2. A possible explanation for the maintenance of this variation, even with intra‐ and inter‐specific resource competition, is that individuals might assort according to body size on different‐sized breeding resources. 3. We tested prediction that ‘bigger is always better’, in the wild and in the laboratory, by experimentally manipulating combinations of available breeding‐resource size (mouse carcasses) and competitor's body size in Nicrophorus vespilloides (Herbst 1783). 4. In the field, large female beetles deserted small carcasses, without breeding, more often than they did larger carcasses, but small females used carcasses indiscriminately with respect to size. In the laboratory, large beetles reared larger broods (with more offspring) on larger carcasses than small beetles, but on small carcasses small beetles had a reproductive advantage over large ones. Offspring size covaried with carcass size independently of parental body size. 5. The present combined results suggest breeding resource value depends on an individual's body size, and variation in body size is environmentally induced: maintained by differences in available carcass sizes. This produces a mechanism by which individual specialisation leads to an increase in niche variation via body size in these beetles.     

Allozyme and microsatellite genetic variation in natural samples of zebrafish,Danio rerio

In this paper, we report the results from allozyme and microsatellite markers in natural populations of the zebrafish, a species of great significance in biological studies. Four zebrafish wild samples from West Bengal, India, were analysed in a preliminary survey of levels and patterns of genetic variation. Results indicate high levels of genetic variability and weak genetic structure, although the latter is consistent with the geographical features of the area under study, sampling sites being located in the Ganges and Brahmaputra delta region, which is characterized by high waterways connectivity.     

Nonadditive genetic components in resistance of the red flour beetle Tribolium castanaeum against parasite infection

Genetically coupled antagonistic coevolution between host and parasites can select for the maintenance of recombination in the host. Mechanistically, maintenance of recombination relies on epistatic interactions between resistance genes creating linkage disequilibria (LD). The role of epistasis in host resistance traits is however only partly understood. Therefore, we applied the joint scaling principle to assess epistasis and other nonadditive genetic components of two resistance traits, survival, and parasite spore load, in population crosses of the red flour beetle Tribolium castanaeum under infections with the microsporidian Nosema whitei. We found nonadditive components only in infected populations but not in control populations. The genetic architecture underlying survival under parasite infection was more complex than that of spore load. Accordingly, the observed negative correlation between survival and spore load was mainly based on a correlation between shared additive components. Breakdown of resistance was especially strong in F2 crosses between resistant lines indicating that multiple epistatic routes can lead to the same adaptation. In general, the wide range of nonoverlapping genetic components between crosses indicated that parasite resistance in T. castanaeum can be understood as a multi peaked fitness landscape with epistasis contributing substantially to phenotypic differentiation in resistance.     

Living on the edge: the role of geography and environment in structuring genetic variation in the southernmost populations of a tropical oak

Understanding the factors determining genetic diversity and structure in peripheral populations is a long‐standing goal of evolutionary biogeography, yet little empirical information is available for tropical species. In this study, we combine information from nuclear microsatellite markers and niche modelling to analyse the factors structuring genetic variation across the southernmost populations of the tropical oak Quercus segoviensis. First, we tested the hypothesis that genetic variability decreases with population isolation and increases with local habitat suitability and stability since the Last Glacial Maximum (LGM). Second, we employed a recently developed multiple matrix regression with randomisation (MMRR) approach to study the factors associated with genetic divergence among the studied populations and test the relative contribution of environmental and geographic isolation to contemporary patterns of genetic differentiation. We found that genetic diversity was negatively correlated with average genetic differentiation with other populations, indicating that isolation and limited gene flow have contributed to erode genetic variability in some populations. Considering the relatively small size of the study area (<120 km), analyses of genetic structure indicate a remarkable inter‐population genetic differentiation. Environmental dissimilarity and differences in current and past climate niche suitability and their additive effects were not associated with genetic differentiation after controlling for geographic distance, indicating that local climate does not contribute to explain spatial patterns of genetic structure. Overall, our data indicate that geographic isolation, but not current or past climate, is the main factor determining contemporary patterns of genetic diversity and structure within the southernmost peripheral populations of this tropical oak.     

Temporal patterns of genetic variation for resistance and infectivity in a Daphnia-microparasite system

Theoretical studies have indicated that the population genetics of host-parasite interactions may be highly dynamic. with parasites perpetually adapting to common host genotypes and hosts evolving resistance to common parasite genotypes. The present study examined temporal variation in resistance of hosts and infectivity of parasites within three populations of Daphnia magna infected with the sterilizing bacterium Pasteuria ramosa. Parasite isolates and host clones were collected in each of two years (1997, 1998) from one population; in two other populations, hosts were collected from both years, but parasites from only the first year. We then performed infection experiments (separately for each population) that exposed hosts to parasites from the same year or made combinations involving hosts and parasites from different years. In two populations, patterns were consistent with the evolution of host resistance: either infectivity or the speed with which parasites sterilized hosts declined from 1997 to 1998. In another population, infectivity, virulence, and parasite spore production did not vary among host-year or parasite-year. For this population, we also detected strong within-population genetic variation for resistance. Thus, in this case, genetic variability for fitness-related traits apparently did not translate into evolutionary change. We discuss a number of reasons why genetic change may not occur as expected in parasite-host systems, including negative correlations between resistance and other traits, gene flow, or that the dynamic process itself may obscure the detection of gene frequency changes.     

Evolution of phenotypic plasticity: Genetic differentiation and additive genetic variation for induced plant defence in wild arugula Eruca sativa

Phenotypic plasticity is the primary mechanism of organismal resilience to abiotic and biotic stress, and genetic differentiation in plasticity can evolve if stresses differ among populations. Inducible defence is a common form of adaptive phenotypic plasticity, and long‐standing theory predicts that its evolution is shaped by costs of the defensive traits, costs of plasticity and a trade‐off in allocation to constitutive versus induced traits. We used a common garden to study the evolution of defence in two native populations of wild arugula Eruca sativa (Brassicaceae) from contrasting desert and Mediterranean habitats that differ in attack by caterpillars and aphids. We report genetic differentiation and additive genetic variance for phenology, growth and three defensive traits (toxic glucosinolates, anti‐nutritive protease inhibitors and physical trichome barriers) as well their inducibility in response to the plant hormone jasmonic acid. The two populations were strongly differentiated for plasticity in nearly all traits. There was little evidence for costs of defence or plasticity, but constitutive and induced traits showed a consistent additive genetic trade‐off within each population for the three defensive traits. We conclude that these populations have evolutionarily diverged in inducible defence and retain ample potential for the future evolution of phenotypic plasticity in defence.     

Spring phenology shows genetic variation among and within populations in seedlings of Scots pine (Pinus sylvestris L.) in the Scottish Highlands

Background: Genetic differentiation in phenotypic traits is often observed among forest tree populations, but less is known about patterns of adaptive variation within populations. Such variation is expected to enhance the survival likelihood of extant populations under climate change.

Aims: Scots pine (Pinus sylvestris) occurs over a spatially and temporally heterogeneous landscape in Scotland. Our goal was to examine whether populations had differentiated genetically in timing of bud flush in response to spatial heterogeneity and whether variation was also maintained within populations.

Methods: Two common-garden studies, involving maternal families of seedlings from 21 native pinewoods, were established and variation in the trait was measured at the beginning of the second growing season.

Results: Populations showed genetic differences in the trait correlated with the length of growing season at their site of origin, but the majority of variation was observed within populations. Populations also differed in their levels of variation in the trait; a pattern that may be influenced by spatial variation in the extent of temporal climate variability.

Conclusions: Our findings suggest that populations have adapted to their home environments and that they also have substantial ability to adapt in situ to changes in growing season length.     

Resistance and tolerance in a host plant-holoparasitic plant interaction: genetic variation and costs

Host organisms are believed to evolve defense mechanisms (i.e., resistance and/or tolerance) under selective pressures exerted by natural enemies. A prerequisite for the evolution of resistance and tolerance is the existence of genetic variation in these traits for natural selection to act. However, selection for resistance and/or tolerance may be constrained by negative genetic correlations with other traits that affect host fitness. We studied genetic variation in resistance and tolerance against parasitic infection and the potential fitness costs associated with these traits using a novel study system, namely the interaction between a flowering plant and a parasitic plant. In this system, parasitic infection has significant negative effects on host growth and reproduction and may thus act as a selective agent. We conducted a greenhouse experiment in which we grew host plants, Urtica dioica, that originated from a single natural population and represented 20 maternal families either uninfected or infected with the holoparasitic dodder, Cuscuta europaea. that originated from the same site. We calculated correlations among resistance, tolerance, and host performance to test for costs of resistance and tolerance. We measured resistance as parasite performance (quantitative resistance) and tolerance as the slopes of regressions relating the vegetative and reproductive biomass of host plants to damage level (measured as parasite biomass). We observed significant differences among host families in parasite resistance and in parasite tolerance in terms of reproductive biomass, a result that suggests genetic variation in these traits. Furthermore, we found differences in resistance and tolerance between female and male host plants. In addition, the correlations indicate costs of resistance in terms of host growth and reproduction and costs of tolerance in terms of host reproduction. Our results thus indicate that host tolerance and resistance can evolve as a response to infection by a parasitic plant and that costs of resistance and tolerance may be one factor maintaining genetic variation in these traits.     

不同地理种群银杏大蚕蛾COI基因序列变异与遗传分化

银杏大蚕蛾Caligula japonica是亚洲东部的特有种, 既是一种重要的林业害虫, 也是一种珍贵的野生蚕类资源。为了揭示银杏大蚕蛾地理种群间的内在联系, 测定了我国分布的12个地理种群的线粒体细胞色素氧化酶C亚基I（COI）基因部分序列（GenBank登录号: FJ358506-FJ358517）, 对地理种群间的序列变异和遗传分化进行了分析。结果表明: 银杏大蚕蛾地理种群间的COI序列同源性高达99%~100%, 显示出比较小的遗传差异。序列对准后从供试COI序列中仅鉴定出9个变异位点和6个单元型, 其中3种是共享单元型。系统发育分析结果表明种群间已经按地理位置形成了一定的地理格局, AMOVA分析显示北方组和南方组之间已经具有明显的遗传分化（FST=0.478, P<0.001）。综合分析, 我们认为北方组和南方组之间的遗传分化可能与差异巨大的生态条件有关。研究结果为银杏大蚕蛾的种群遗传学和生态学研究提供了一个基本的分子生物学线索。     

Non-gradual variation in colour morphs of the strawberry poison frog Dendrobates pumilio: genetic and geographical isolation suggest a role for selection in maintaining polymorphism

The relative roles that geographical isolation and selection play in driving population divergence remain one of the central questions in evolutionary biology. We approached this question by investigating genetic and morphological variation among populations of the strawberry poison frog, Dendrobates pumilio, in the Bocas del Toro archipelago, Panama. We found significant population genetic structure and isolation by distance based on amplified fragment length polymorphism markers. Snout vent length (SVL), coloration and the extent and size of dorsal black spots showed large variation among the studied populations. Differences in SVL correlated with genetic distance, whereas black spot patterns and other coloration parameters did not. Indeed, the latter characters were observed to be dramatically different between contiguous populations located on the same island. These results imply that neutral divergence among populations may account for the genetic patterns based on amplified fragment length polymorphism markers and SVL. However, selective pressures need to be invoked in order to explain the extraordinary variation in spot size and coverage, and coloration. We discuss the possibility that the observed variation in colour morphs is a consequence of a combination of local variation in both natural selection on an aposematic signal towards visual predators and sexual selection generated by colour morph-specific mate preferences.     

Resistance to thermal stress in adult Drosophila buzzatii: acclimation and variation among populations

Survival after heat stress was examined in adult Drosophila buzzatii from seven localities. Males and females were conditioned by a non-lethal high temperature before exposure to a severe thermal stress. Variable times elapsed between conditioning, either as adults or pupae, and exposure to the stress were used, and experimental times to the stress ranged from 2 to 96 hours. Survival after stress varied among populations, and differences generally were consistent across conditioning treatments and across experiments. Resistance to thermal stress was much higher following one conditioning bout 2–4 hours before exposure to a severe stress than when the time elapsed between conditioning and exposure was increased to 24 or more hours. Significantly more adults survived the stress if conditioned 4 days before exposure to the thermal stress, either as adults or as pupae, than if not conditioned. The rank order of resistance roughly followed that predicted from the climatic conditions of the localities of origin.     

A preliminary examination of genetic variation in a peripheral population of Blanding's turtle, Emydoidea blandingii

Random amplified polymorphic DNA (RAPD) was used to compare the Nova Scotia population of Blanding's turtle (Emydoidea blandingii) with several populations from the species' main range. The Nova Scotia population is believed to have been isolated from the main range for 4000-8000 years. Cluster analysis using a neighbour-joining algorithm produced a dendrogram showing the Nova Scotia population clustering separately from those populations in the main range. Analysis of molecular variance shows 34.28% of total variance to be accounted for between the Nova Scotia population and populations in the main range. While this study is preliminary, the results suggest that the Nova Scotia population of Blanding's turtle may be important to the maintenance of genetic diversity in the species.     

The evolution of male genitalia: patterns of genetic variation and covariation in the genital sclerites of the dung beetle Onthophagus taurus

Three main hypotheses, have been invoked to explain divergent genital evolution, the lock and key, pleiotropy, and sexual selection hypotheses, each of which make different predictions about how genital traits are inherited. Here we used a half-sib breeding design to examine the patterns of genetic variation and covariation between male genital sclerites, and their covariance with general body morphology in the dung beetle Onthophagus taurus. We found CV(A)'s and CV(P)'s were similar for both genital and general morphological traits and that CV(R)'s were large for both trait types. We found that male genital sclerites were negatively genetically correlated with general morphological traits. Variation in male genital morphology has direct implications for a male's fertilization success and the resulting sexual selection acting on male genitalia is predicted to maintain high levels of additive genetic variance. Contrary to this prediction, we found that individual genital sclerites all had low levels of additive genetic variance and large maternal and environmental sources of variation. Our data suggest that the genital sclerites in O. taurus are not inherited independently but as a genetically integrated unit. More importantly, the way the different sclerites function to influence male fertilization success reflects this genetic integration. Even though levels of V(A) in individual genital sclerites may be low, there may still be sufficient V(A) in multivariate trait space for selection to generate evolutionary change in the overall morphology of male genitalia.     

Low levels of genetic variation within and high levels of genetic differentiation among populations of species of Abies from southern Mexico and Guatemala

Populations of Abies in southern Mexico and Guatemala (A. flinckii, A. guatemalensis, A. hickeli, and A. religiosa) have a patchy distribution. This pattern is particularly clear in A. guatemalensis. Genetic diversity within populations, measured by average heterozygosity at 16 isozyme loci, is lower than the range reported for most conifers (mean H(o) ranging from 0.069 in A. guatemalensis to 0.113 in A. flinckii), while differentiation among populations is higher than that observed in most conifer species studied (θ = F(st) ranging from 0.073 in A. hickeli to 0.271 in A. flinckii). Estimated levels of gene flow are low (ranging from 0.672 in A. flinckii to 3.17 in A. hickeli). Populations in most cases had an excess of homozygosity over that expected under Hardy-Weinberg equilibrium, suggesting some inbreeding (F(is) ranging from 0.074 in A. flinckii to 0.235 in A. guatemalensis). A significant relationship between gene flow and geographic distance was observed in A. religiosa, but not in the other three taxa studied. The patterns of genetic variation appear to have been influenced by the distributions and histories of these species. Paleoclimatic evidence suggests that the ranges of these species retreated upwards during the Pleistocene glaciation and became fragmented during the warming period that followed. The populations could have passed through genetic bottlenecks that reduced genetic variation and led to interpopulation differentiation.     

Latitudinal divergence in a widespread amphibian: Contrasting patterns of neutral and adaptive genomic variation

Stochastic effects from demographic processes and selection are expected to shape the distribution of genetic variation in spatially heterogeneous environments. As the amount of genetic variation is central for long‐term persistence of populations, understanding how these processes affect variation over large‐scale geographical gradients is pivotal. We investigated the distribution of neutral and putatively adaptive genetic variation, and reconstructed demographic history in the moor frog (Rana arvalis) using 136 individuals from 15 populations along a 1,700‐km latitudinal gradient from northern Germany to northern Sweden. Using double digest restriction‐site associated DNA sequencing we obtained 27,590 single nucleotide polymorphisms (SNPs), and identified differentiation outliers and SNPs associated with growing season length. The populations grouped into a southern and a northern cluster, representing two phylogeographical lineages from different post‐glacial colonization routes. Hybrid index estimation and demographic model selection showed strong support for a southern and northern lineage and evidence of gene flow between regions located on each side of a contact zone. However, patterns of past gene flow over the contact zone differed between neutral and putatively adaptive SNPs. While neutral nucleotide diversity was higher along the southern than the northern part of the gradient, nucleotide diversity in differentiation outliers showed the opposite pattern, suggesting differences in the relative strength of selection and drift along the gradient. Variation associated with growing season length decreased with latitude along the southern part of the gradient, but not along the northern part where variation was lower, suggesting stronger climate‐mediated selection in the north. Outlier SNPs included loci involved in immunity and developmental processes.