Stability of population structure and genetic diversity across generations assessed by microsatellites among sympatric populations of landlocked Atlantic salmon (Salmo salar L.)

It may often be necessary to perform genetic analyses of temporal replicates to estimate the significance of spatial variation independently from that of temporal variation in order to ensure the reliability of estimates of a defined population structure. Nevertheless, temporal studies of genetic diversity remain scarce in the literature relative to the plethora of empirical studies of population structure. In vertebrates, a limited number of studies have specifically assessed the temporal stability of population structure for more than one generation. In this study, we performed a microsatellite analysis of DNA obtained from archived scales to compare the population structure among four sympatric landlocked populations of Atlantic salmon ( Salmo salar ) over a time frame of three to five generations. The same patterns of allele frequency distribution, θ, R _ST and genetic distance estimates were observed among populations for two time periods, confirming the temporal stability of the population structure. Despite population declines and stocking during this period, no statistically significant changes in intrapopulation genetic diversity were apparent. This study illustrates the feasibility and usefulness of microsatellite analysis of temporal samples, not only to infer changes of intrapopulation genetic diversity, but also to assess the stability of population structure over a time frame of several generations.     

Rapid changes in plasticity across generations within an expanding cedar forest

We investigated the inter‐individual variation of phenotypic plasticity and its evolution across three generations within an expanding forest. Plasticity was assessed in situ from dendrochronological data as the response of radial growth to summer rainfall. A linear mixed model was used to account for spatial effects (environment and stand structure), temporal factors (stand dynamics) and the variation with age. Beyond these effects, our results reveal a significant inter‐individual variance of growth and plasticity within each generation. We also show that the mean values and variances of growth and plasticity changed significantly across generations, with different patterns for both traits. The possible environmental and genetic drivers of these changes are discussed. Contrasting with the trade‐off between stress tolerance and plasticity generally observed among populations, we detected a positive covariance at the individual level, which does not support the cost of plasticity hypothesis in this case.     

Analysis of gene flow in North American face fly (Musca autumnalis) populations

Abstract. Vertical polyacrylamide gel electrophoresis was used to resolve enzymes at ten putative loci in face flies Musca autumnalis De Geer, a colonizing, Palaearctic species established in North America for at least 210 generations. Flies were sampled in 1991 from six locations in Iowa, two in Maryland, two in Minnesota, two in Tennessee, three in New York, and three in Missouri. Nondirectional temporal variation in gene frequencies over a 4-year interval was detected at farms in central Iowa. Heterogeneity in allele frequencies was detected among farms in Iowa, Maryland, New York and Minnesota but not in Tennessee and Missouri. There were no consistent departures from random mating. Partitioning variances in gene frequencies showed that 58% of the variation occurred in populations among states and 43% between populations within states. Mean reproducing immigrants per population per generation was estimated to be eighteen flies. No regional genetic differentiation was detected, and there were no barriers to gene flow within or among diverse populations in the six states. Earlier data bearing on gene flow among and between Nearctic and Palearctic face fly samples were analysed and significant differentiation was not detected.     

Genetic estimates of contemporary effective population size: to what time periods do the estimates apply?

Although most genetic estimates of contemporary effective population size (Ne) are based on models that assume Ne is constant, in real populations Ne changes (often dramatically) over time, and estimates (Ne) will be influenced by Ne in specific generations. In such cases, it is important to properly match Ne to the appropriate time periods (for example, in computing Ne/N ratios). Here I consider this problem for semelparous species with two life histories (discrete generations and variable age at maturity--the 'salmon' model), for two different sampling plans, and for estimators based on single samples (linkage disequilibrium, heterozygote excess) and two samples (temporal method). Results include the following. Discrete generations: (i) Temporal samples from generations 0 and t estimate the harmonic mean Ne in generations 0 through t - 1 but do not provide information about Ne in generation t; (ii) Single samples provide an estimate of Ne in the parental generation, not the generation sampled; (iii) single-sample and temporal estimates never provide information about Ne in exactly the same generations; (iv) Recent bottlenecks can downwardly bias estimates based on linkage disequilibrium for several generations. Salmon model: (i) A pair of single-cohort (typically juvenile) samples from years 0 and t provide a temporal estimate of the harmonic mean of the effective numbers of breeders in the two parental years (N b(0) and N b(t)), but adult samples are more difficult to interpret because they are influenced by Nb in a number of previous years; (ii) For single-cohort samples, both one-sample and temporal methods provide estimates of Nb in the same years (contrast with results for discrete generation model); (iii) Residual linkage disequilibrium associated with past population size will not affect single-sample estimates of Nb as much as in the discrete generation model because the disequilibrium diffuses among different years of breeders. These results lead to some general conclusions about genetic estimates of Ne in iteroparous species with overlapping generations and identify areas in need of further research.     

Geographical variation in age at menarche in Britain

The geographical distribution of age at menarche in great Britain is analysed in two successive generations. It appears that the variances for age at menarche are equal throughout the country. The means, however, show significant and complicated patterns of regional variation. In the southern and eastern parts of the country, menarche was earlier than in the northern parts in both generations. This patterning is not due to social class differentiation. Except for the sparsely populated areas in northern and western Scotland and for the extreme south-western parts of England and Wales all areas show evidence of a decline in age at menarche from the mothers' to the daughters' generation.     

Genotypic variation in the persistence of transgenerational responses to seasonal cues*

Phenotypes respond to environments experienced directly by an individual, via phenotypic plasticity, or to the environment experienced by ancestors, via transgenerational environmental effects. The adaptive value of environmental effects depends not only on the strength and direction of the induced response but also on how long the response persists within and across generations, and how stably it is expressed across environments that are encountered subsequently. Little is known about the genetic basis of those distinct components, or even whether they exhibit genetic variation. We tested for genetic differences in the inducibility, temporal persistence, and environmental stability of transgenerational environmental effects in Arabidopsis thaliana. Genetic variation existed in the inducibility of transgenerational effects on traits expressed across the life cycle. Surprisingly, the persistence of transgenerational effects into the third generation was uncorrelated with their induction in the second generation. Although environmental effects for some traits in some genotypes weakened over successive generations, others were stronger or even in the opposite direction in more distant generations. Therefore, transgenerational effects in more distant generations are not merely caused by the retention or dissipation of those expressed in prior generations, but they may be genetically independent traits with the potential to evolve independently.     

Inter-generational equity index for assessing environmental sustainability: An example on global warming

Inter-generational equity is essential for environmental sustainability. The current generation inherits an environment with a certain quality from the previous generation. The impact on the environment gradually exacerbates and accumulates over a period of time between two generations. However, currently there is no index available to assess inter-generational equity. Generally a typical environmental index is established to represent the environmental status in a specific year. This kind of index, although it presents the annual environmental variation, does not reflect the degree of change in environmental sustainability between two generations. Therefore, an inter-generational equity index (IGEI) and an endowment equation to examine the temporal trend of the changing environment are proposed for assessing inter-generational equity. To demonstrate the applicability of the endowment equation, an IGEI was established to assess the inter-generational equity of global warming. The global warming IGEI evaluates the status between two generations based on three sub-indexes; CO₂ emission, loss due to climate disasters, and the size of the existing forest area. The pressure–state–response (PSR) framework was adopted to explain the causal relationship between these three sub-indexes. According to the endowment rate determined by the proposed equation for each sub-index, the increase in CO₂ emission from 1980 to 2000 shows an obviously inequitable pattern between generations. Subsequently, the loss due to climate disasters between generations was also more serious. The size of the forest area, an important factor for reducing the impact of global warming, is unfortunately also decreasing significantly between generations. Using the endowment rate determined by the proposed endowment equation, the evaluation of the inter-generational equity is made possible and is demonstrated by the IGEI established for global warming.     

Impact of selection and breeding on the genetic diversity in Douglas-fir

Genetic changes following domestication of Douglas-fir were studied using isozyme data derived from two generations of seed orchards and their 49 wild progenitor populations. In addition, the breeding, production, and infusion populations used in the seed orchards were compared to their wild counterparts. Several parameters of gene diversity were measured (number of alleles per locus N _a, per cent of polymorphic loci PLP, and expected heterozygosity H, and population divergence D). These measures were similar or higher in the domesticated populations compared to their natural progenitors, indicating that early selection and breeding of a highly polymorphic species does not significantly reduce genetic variation. The two generations of seed orchards also did not differ, indicating that genetic variation may remain stable over future generations of forest plantations. Interestingly, compared to the natural populations, heterozygosity was higher in the seed orchards, probably due to pooling of widely distributed natural populations; however, rare localized or private alleles seemed to be less frequent in the domesticated populations. Differentiation values were not significant between the first generation orchards and the natural populations, but significant differences were observed between the second generation orchards and the wild progenitor populations, probably due to the interbreeding that forms the advanced generation seed orchards.     

Temporal dynamics of morphologic diversity of the Japanese scallop <Emphasis Type="Italic">Mizuhopecten Yessoensis</Emphasis> (Jay, 1856)

The temporal diversity of 11 morphological features of both shell valves in eight cultivated samples of the Japanese scallop M. yessoensis from Alexeev Bay (Popov Island, Sea of Japan) at different ages and from different generations was analyzed. The sample diversity with respect to each investigated feature was observed. The sample differences in the studied features as well as shell valve variability within the sample were demonstrated to be determined by both mollusk age and sample generation. This phenomenon is considered to be the result of differences in the environmental influence on each mollusk generation under constant technological conditions.     

Spatio-temporal analysis of the population dynamics of the oriental moth,Monema flavescens (Lepidoptera: Limacodidae)

The comparative analysis of life tables of the oriental moth, Monema flavescens, obtained in 6 patches for 8 generations in 4 years revealed the following:

1. The ratio of maximum to minimum of cocoon density for each patch ranged from 5.34 to 22.50, each value being more than 3.20, the ratio for the whole study area.
2. The density change from adult to cocoon in the next generation caused most of the spatial variation in density change per patch. The rate of adult-to-cocoon population change showed spatial density dependence in some generations but not in others. When the change rate lacked spatial density dependence, it was the key-factor for spatial variation in adult density for the following few generations till the change rate recovered spatial density dependence. This was because of flooding, which killed the spatial density dependence existing potentially in the adult-to-cocoon change rate and damaged the same patches during the few successive generations.
3. The rate of population change from overwintered generation adults (summer ones) to first generation cocoons was not only the key-factor for the rate of throughout-the-year change but temporally density dependent in each patch. Therefore, the density for the whole study area is considered to fluctuate within a range. However, the strong equilibrium seen in the cocoon density for the whole study area was due to the floods that happened to occur when the density was near and at its maximum, and it is considered that such a strong equilibrium does not always occur.
4. In the population change from summer adults to first generation cocoons, temporal density dependence was found in all the patches, but it was found only in one patch in the population change from autumn adults to second generation cocoons. This was because the spatial density dependence seen in the former corresponded to the absolute density of adults, while that in the latter corresponded to the relative density.

     

Environmental and genetic sources of diversification in the timing of seed germination: implications for the evolution of bet hedging

Environmental variation that is not predictably related to cues is expected to drive the evolution of bet-hedging strategies. The high variance observed in the timing of seed germination has led to it being the most cited diversification strategy in the theoretical bet-hedging literature. Despite this theoretical focus, virtually nothing is known about the mechanisms responsible for the generation of individual-level diversification. Here we report analyses of sources of variation in timing of germination within seasons, germination fraction over two generations and three sequential seasons, and the genetic correlation structure of these traits using almost 10,000 seeds from more than 100 genotypes of the monocarpic perennial Lobelia inflata. Microenvironmental analysis of time to germination suggests that extreme sensitivity to environmental gradients, or microplasticity, even within a homogeneous growth chamber, may act as an effective individual-level diversification mechanism and explains more than 30% of variance in time to germination. The heritability of within-season timing of germination was low (h(2) = 0.07) but significant under homogeneous conditions. Consistent with individual-level diversification, this low h(2) was attributable not to low additive genetic variance, but to an unusually high coefficient of residual variation in time to germination. Despite high power to detect additive genetic variance in within-season diversification, it was low and indistinguishable from zero. Restricted maximum likelihood detected significant genetic variation for germination fraction (h(2) = 0.18) under homogeneous conditions. Unexpectedly, this heritability was positive when measured within a generation by sibling analysis and negative when measured across generations by offspring-on-parent regression. The consistency of dormancy fraction over multiple delays, a major premise of Cohen's classic model, was supported by a strong genetic correlation (r = 0.468) observed for a cohort's germination fraction over two seasons. We discuss implications of the results for the evolution of bet hedging and highlight the need for further empirical study of the causal components of diversification.     

白背飞虱对水稻抗虫品种N_(22)的适应性研究

在室内连续用感虫品种TN1和抗虫品种N2 2 单管饲养白背飞虱 (Sogatellafurcifera)种群 ,研究它对抗虫水稻品种的适应性及其体内保护酶的变化 .结果表明 ,白背飞虱在感虫品种TN1和抗虫品种N2 2品种上饲养 1至 2代 ,其卵历期、若虫期和全世代历期均无明显差异 .从感虫品种TN1转移到抗虫品种N2 2 上饲养 1代 ,白背飞虱的若虫存活率、雌成虫寿命、体重、蜜露量、产卵量和内禀增长率等均低于在抗虫品种上连续饲养 2代 ,而后者又低于在感虫品种上饲养的指标 .白背飞虱在抗虫品种上连续饲养 2代后 ,体内保护酶中超氧化物岐化酶 (SOD)和过氧化氢酶 (CAT)活性逐渐接近于感虫品种上连续饲养的结果 ,说明白背飞虱从感虫品种转到抗虫品种在开始时并不适应 ,经过连续繁殖多代后白背飞虱逐渐适应 ,最后导致抗虫品种的抗性丧失     

白背飞虱对水稻抗虫品种N22的适应性研究

在室内连续用感虫品种TN1和抗虫品种N22单管饲养白背飞虱(Sogatella furcifera)种群,研究它对抗虫水稻品种的适应性及其体内保护酶的变化,结果表明,白背飞虱在感虫品种TN1和抗虫品种N22品种上饲养1至2代,其卵历期、若虫期和全世代历期均无明显差异,从感虫品种TN1转移到抗虫品种N22上饲养1代,白背飞虱的若虫存活率、雌成虫寿命、体重、蜜露量、产卵量和内禀增长率等均低于在抗虫品种上连续饲养2代,而后者又低于在感虫品种上饲养的指标,白背飞虱在抗虫品种上连续饲养2代后,体内保护酶中超氧化物岐化酶(SOD)和过氧化氢酶(CAT)活性逐渐接近于感虫品种上连续饲养的结果,说明白背飞虱从感虫品种转到抗虫品种在开始时并不适应,经过连续繁殖多代后白背飞虱逐渐适应,最后导致抗虫品种的抗性丧失。     

Impact of founder population, drift and selection on the genetic diversity of a recently translocated tree population

Recently established, temperate tree populations combine a high level of differentiation for adaptive traits, suggesting rapid genetic evolution, with a high level of genetic diversity within population, suggesting a limited impact of genetic drift and purifying selection. To study experimentally the evolutionary forces in a recently established population, we assessed the spatial and temporal patterns of genetic diversity within a disjunct population of Cedrus atlantica established 140 years ago in south-eastern France from a North African source. The population is expanding through natural regeneration. Three generations were sampled, including founder trees. We analysed 12 isozyme loci, three of which were previously found in tight association with selected genes, and quantitative traits. No bottleneck effect was detected in the founder generation, but a simple test of allelic association revealed an initial disequilibrium which disappeared in the following generations. The impact of genetic drift during secondary evolution was limited, as suggested by the weak temporal differentiation. The genetic load was not reduced after 3 generations, and the quantitative variation for adaptive traits did not change either. Thus, initial genetic changes first proceed from a rapid re-organisation of the diversity through mating and recombination, whereas genetic erosion through drift and selection is delayed due to temporal and spatial stochasticity. Two life-history traits of trees contribute to slowing down the processes of genetic erosion: perenniality and large spatial scale. Thus, one would expect recently established tree populations to have a higher diversity than older ones, which seems in accordance with experimental surveys.     

Temporal uniformity of the spontaneous mutational variance of quantitative traits in Drosophila melanogaster

Spontaneous mutations were allowed to accumulate over 209 generations in more than 100 lines, all of them independently derived from a completely homozygous population of Drosophila melanogaster and subsequently maintained under strong inbreeding (equivalent to full-sib mating). Traits scored were: abdominal (AB) and sternopleural (ST) bristle number, wing length (WL) and egg-to-adult viability (V). On two occasions--early (generations 93-122) and late (generations 169-209)--ANOVA estimates of the mutational variance and the mutational line x generation interaction variance were obtained. Mutational heritabilities of morphological traits ranged from 2 x 10(-4) to 2 x 10(-3) and the mutational coefficient of variation of viability was 0.01. For AB, WL and V, temporal uniformity of the mutational variance was observed. However, a fluctuation of the mutational heritability of ST was detected and could be ascribed to random genotype x environment interaction.     

Comparison of historical bottleneck effects and genetic consequences of re‐introduction in a critically endangered island passerine

Re‐introduction is an important tool for recovering endangered species; however, the magnitude of genetic consequences for re‐introduced populations remains largely unknown, in particular the relative impacts of historical population bottlenecks compared to those induced by conservation management. We characterize 14 microsatellite loci developed for the Seychelles paradise flycatcher and use them to quantify temporal and spatial measures of genetic variation across a 134‐year time frame encompassing a historical bottleneck that reduced the species to ~28 individuals in the 1960s, through the initial stages of recovery and across a second contemporary conservation‐introduction‐induced bottleneck. We then evaluate the relative impacts of the two bottlenecks, and finally apply our findings to inform broader re‐introduction strategy. We find a temporal trend of significant decrease in standard measures of genetic diversity across the historical bottleneck, but only a nonsignificant downward trend in number of alleles across the contemporary bottleneck. However, accounting for the different timescales of the two bottlenecks (~40 historical generations versus <1 contemporary generation), the loss of genetic diversity per generation is greater across the contemporary bottleneck. Historically, the flycatcher population was genetically structured; however, extinction on four of five islands has resulted in a homogeneous contemporary population. We conclude that severe historical bottlenecks can leave a large footprint in terms of sheer quantity of genetic diversity lost. However, severely depleted genetic diversity does not render a species immune to further genetic erosion upon re‐introduction. In some cases, the loss of genetic diversity per generation can, initially at least, be greater across re‐introduction‐induced bottlenecks.     

激光诱变小麦生育期性状和穗部性状的生物学效应研究

本试验采用N2激光和He—Ne激光辐照“绵阳21号”等四个小麦材料的干种子,采用随机区组设计,重复三次,利用生物统计和数量遗传学的方法,对L。和L：两代的抽穗期、开花期等4个生育期性状及穗长、小穗粒数等10个穗部性状的遗传变异进行研究。结果表明：辐射材料不同,诱变后代的变异差异明显;激光种类不同,后代变异差异不大;抽穗期穗长、小穗粒数的遗传力较高,早代单株选择的效果较好。     

Genetic variation in a heterogeneous environment. V. Spatial heterogeneity in finite populations

The spatial model of Levene (1953) was examined in a finite population and compared to a temporal model. The spatial model was much more effective in retaining genetic variation in a finite population. Furthermore, the haploid spatial model was more effective in retaining genetic variation than the analogous diploid absolute dominance model. This is the opposite from that found for the temporal model, where the diploid model was more effective than the haploid. Here is an example where diploidy (sexual reproduction) may be disadvantageous. A model that permitted both spatial and temporal variation to act in concert gave retention of genetic variation in situations where either spatial or temporal variation, separately, did not. The relationship between the amount of heterozygosity and the retardation factor was discussed. An example of how spatial or temporal variation affects the proportion of populations fixed after a certain number of generations was given. It seems that these models have biological analogues, several examples of which are mentioned.     

TEMPORAL VARIATION IN POPULATIONS OF THE MARINE ISOPOD EXCIROLANA: HOW STABLE ARE GENE FREQUENCIES AND MORPHOLOGY?

Excirolana braziliensis is a dioecious marine isopod that lives in the high intertidal zone on both sides of tropical America. It lacks a dispersal phase and displays a remarkable degree of genetic divergence even between localities less than 1 km apart. Nine populations of this nominal species from both sides of the Isthmus of Panama and one population of the closely allied species, Excirolana chamensis, from the eastern Pacific were studied for 2 yr for allozymic temporal variation in 13 loci and for 3 to 4 yr for morphological variation in nine characters. The genetic and morphological constitution of 9 out of 10 populations remained stable. Allele frequencies at two loci and overall morphology in a tenth beach occupied by E. braziliensis changed drastically and significantly between 1986 and 1988. The change in gene frequency is too great to explain by genetic drift occurring during a maximum of 14 generations regardless of assumed effective population size; drift is also unlikely to have caused observed changes in morphology. Selective survival of a previously rare genotype is more plausible but still not probable. The most credible explanation is that the resident population at this locality became extinct and that the beach was recolonized by immigrants from another locality. Such infrequent episodes of extinction and recolonization from a single source may account for the large amount of genetic divergence between local populations of E. braziliensis. However, the low probability of large temporal genetic change even in a species such as this, in which gene flow between local demes is limited and generation time is short, suggests that a single sample through time is usually adequate for reconstructing the genetic history of populations.     

Testing population differentiation in plant species – how important are environmental maternal effects

The maternal environment may contribute to population differentiation in offspring traits if growing conditions of mother plants are different. However, the magnitude of such environmental maternal effects compared with genetic differentiation is often not clear. We tested the importance of environmental maternal effects by comparing population differentiation in parental seed directly collected in the field and in F₁ seed grown under homogeneous conditions. The F₁ seeds were obtained by random crosses within populations. We used five populations in each of four plant species to analyse seed mass and growth chamber germination of both generations at the same time. In two species, we additionally tested offspring performance in the field. We found a significant population differentiation in all species and for nearly all measured traits. Population‐by‐generation interactions indicating environmental maternal effects were significant for germination (three species) and for seed mass (two species) but not for growth and reproduction. The significant interaction was partly due to a reduction of among‐population differentiation from the parental to the F₁ generation that can be explained by a decrease of maternal provisioning effects. However, in some species by trait combinations a change in population ranking and not a decrease of variation was responsible for significant population‐by‐generation interactions indicating environmental maternal effects beyond maternal provisioning. Fitting of seed mass as covariate was not successful in reducing environmental maternal effects on population differentiation in germination. We discuss alternative methods to account for environmental maternal effects in studies on genetic differentiation among populations.