Increase In Genetic Diversity Research Articles

Low among-population genetic differentiation in Chinese bisexual Artemia populations.

We studied the population genetic structure of nine bisexual Artemia sinica populations from the provinces of Inner Mongolia, Shanxi and Qinghai in China, using variation at nine allozyme loci (cellulose acetate electrophoresis). There is a clear-cut tendency for an increase in genetic variation, as measured by heterozygosity, with increasing habitat size. Although we observe a positive relationship between genetic differentiation and geographical distance, overall FST values are low: populations separated by approximately 1000 km show average FST values of 0.05-0.1, whereas populations separated by 100 km show no genetic differentiation at all.

Genetic variance in temperature dependent adult size deriving from physiological genetic variation at temperature boundaries.

An increase in genetic variation in body size has often been observed under stress; an increase in dominance variance and interaction variance as well as in additive genetic variance has been reported. The increase in genetic variation must be caused by physiological mechanisms that are specific to adverse environments. A model is proposed to explain the occurrence of an increase in genetic variation in body size in Drosophila at extreme temperatures. The model has parameters specific to the low- and high-temperature regions of the viable range. Additive genetic variation in the boundary temperatures leads to a marked increase in additive genetic variation in development rate and body size at extreme temperatures. Additive genetic variation in the temperature sensitivity in the low- and high-temperature regions adds non-additive genetic variation. Development rate shows patterns in additive genetic variation that differ from the patterns of genetic variation in body size; therefore, the genetic correlation between development rate and body size changes sign repeatedly as a function of temperature. The existence of dominance in the genetic variation in the boundary temperatures or in the low- and high-temperature sensitivities leads to a higher total genetic variance due to higher dominance and interaction variance, for both development rate and body size.

Clines in polygenic traits.

This article outlines theoretical models of clines in additive polygenic traits, which are maintained by stabilizing selection towards a spatially varying optimum. Clines in the trait mean can be accurately predicted, given knowledge of the genetic variance. However, predicting the variance is difficult, because it depends on genetic details. Changes in genetic variance arise from changes in allele frequency, and in linkage disequilibria. Allele frequency changes dominate when selection is weak relative to recombination, and when there are a moderate number of loci. With a continuum of alleles, gene flow inflates the genetic variance in the same way as a source of mutations of small effect. The variance can be approximated by assuming a Gaussian distribution of allelic effects; with a sufficiently steep cline, this is accurate even when mutation and selection alone are better described by the 'House of Cards' approximation. With just two alleles at each locus, the phenotype changes in a similar way: the mean remains close to the optimum, while the variance changes more slowly, and over a wider region. However, there may be substantial cryptic divergence at the underlying loci. With strong selection and many loci, linkage disequilibria are the main cause of changes in genetic variance. Even for strong selection, the infinitesimal model can be closely approximated by assuming a Gaussian distribution of breeding values. Linkage disequilibria can generate a substantial increase in genetic variance, which is concentrated at sharp gradients in trait means.

Sex and adaptation in a changing environment.

In this study we consider a mathematical model of a sexual population that lives in a changing environment. We find that a low rate of environmental change can produce a very large increase in genetic variability. This may help to explain the high levels of heritability observed in many natural populations. We also study asexuality and find that a modest rate of environmental change can be very damaging to an asexual population, while leaving a sexual population virtually unscathed. Furthermore, in a changing environment, the advantages of sexuality over asexuality can be much greater than suggested by most previous studies. Our analysis applies in the case of very large populations, where stochastic forces may be neglected.

Caracterização genética de populações de Egeria najas presentes no reservatório de Jupiá e rios afluentes

A Egeria najas é uma espécie aquática submersa nativa da bacia hidrográfica do rio Paraná. Com o represamento das águas do rio para geração de energia elétrica a espécie tem mudado seu comportamento de colonização dos leitos dos rios e ocorrido em grandes maciços dentro da represa de Jupiá e rios afluentes. Essa planta tem causado problemas por obstruir a passagem de água para as turbinas de geração de energia elétrica. Coletas de material vegetativo foram realizadas na represa e afluentes do rio Paraná para análise da variabilidade isoenzimática e de DNA. A análise isoenzimática mostrou haver somente quatro classes de diferentes biotipos. No entanto, utilizando-se os procedimentos de RAPD, observou-se que a espécie possui grande variabilidade genética. O represamento das águas também está permitindo o acumulo de variabilidade no local e promovendo um aumento de variabilidade por meio de possíveis cruzamentos entre genótipos diferentes. Os resultados também possibilitaram inferir sobre as possíveis rotas de migração de material genético para colonização da represa e rios afluentes.

Characterization of Phytophthora infestans Populations in Western Washington.

The first detection in the United States of isolates of Phytophthora infestans having metalaxyl insensitivity and complex pathotypes occurred in western Washington during the early 1990s. To determine the genetic structure of the current population in western Washington, a total of 115 isolates of P. infestans were obtained during 1996 from infected tubers or foliage of potato, tomato, nightshade, and bittersweet throughout the region. An additional 45 isolates were collected from a single field. Based on mating type, metalaxyl-insensitivity, and molecular markers (allozymes of glucose-6-phosphate isomerase, peptidase, and RG57 DNA fingerprint), all of the isolates were A1 mating type and had the US-11 multilocus genotype. Analyses of an additional 120 isolates collected during 1997 from potato, tomato, and nightshade were performed. As in 1996, US-11 was the predominant genotype detected on all three hosts. However, three additional A2 mating type genotypes were also detected: US-7, US-8, and US-14. These three genotypes represent the first A2 mating type isolates detected in western Washington. Most of a subset of 60 isolates infected 4 to 7 of the 10 potato differentials tested. This included 90% of the isolates collected in 1996 (all US-11), plus 72% of the US-11 and 100% of the US-8 and US-14 isolates collected during 1997. Virulence phenotypes in this region are complex even without the selection pressure of R-genes in the local commercial cultivars. The apparent increase in genetic variation observed in populations of P. infestans in western Washington from 1996 to 1997 most likely occurred by migration rather than by sexual recombination.

The influence of genetic factors and life stress on depression among adolescent girls.

The possible causes of greater depression among adolescent girls were investigated by examining variation in the influence of genetic and environmental risk factors among 182 prepubertal female, 237 prepubertal male, 314 pubertal female, and 171 pubertal male twin pairs from the Virginia Twin Study of Adolescent Behavioral Development. To compare the trajectory of depressive symptoms among boys and girls from childhood to adolescence; to analyze the role of genetic, shared, and unique environmental factors in depression among prepubertal and pubertal male and female twins; and to investigate a possible link between liability to depression and one salient index of the child's environment: past-year life events. Child-reported depression was assessed using the Child and Adolescent Psychiatric Interview and ratings of past-year life events and pubertal status obtained by maternal questionnaire and interview, respectively. The impact of life events on depression was particularly evident in the adolescent girls. The results from model fitting indicate increased heritability for depression in this group, and its long-term consistency was mediated primarily by latent genetic factors. Model fitting also showed that at least part of the liability to depression and to life events can be linked to a common set of genes in the adolescent girls, and there is a notable developmental increase in the genetic variance for life events. The greater heritability for depression in pubertal girls, its genetic mediation over time, and the increase in genetic variance for life events may be one possible explanation for the emergence of increased depression among pubertal girls and its persistence through adolescence.

Herança da estatura de planta e do comprimento da panícula principal no cruzamento entre Avena Sativa L. e Avena Sterilis L.

A espécie selvagem Avena sterilis L. tem sido amplamente utilizada para cruzamento com aveia cultivada, pela alta afinidade existente entre ambas, o que possibilita um aumento na variabilidade genética e transferência de genes de importância agronômica. Entretanto, caracteres indesejáveis, como a alta estatura e um maior comprimento de panícula, podem ser transferidos à progênie. Nesse sentido, com o objetivo de avaliar a variabilidade genética, bem como a herança em cruzamentos entre Avena sativa L. e Avena sterilis L., foram mensurados os caracteres estatura de planta e comprimento da panícula principal. As variâncias fenotípicas e genéticas foram altas e as variâncias de ambiente foram baixas, proporcionando uma alta herdabilidade no sentido amplo. O efeito gênico de aditividade foi o mais importante para explicar a variação genética em ambos os caracteres.

Allozyme variation in the diploid (A genome) populations ofScilla scilloides (Hyacinthaceae)

Allozyme variation at eleven loci encoding seven enzyme systems were examined in 20 populations of diploid (genome AA, 2n = 16)Scilla scilloides in China. In comparison with the average species of seed plants studied, populations of this species display a high amount of genetic variation (A = 2.0, P = 58.6%, Ho = 0.172, and He = 0.185). Allozyme variation pattern revealed predominant outcrossing within populations and considerable differentiation (FST = 0.314) among populations as well as between the subtropic and temperate regions. The wide distribution, long existence and outcrossing are presumably the main factors responsible for the high genetic diversity within populations. But the gravity dispersal of seeds and pollination by small insects set limits to the increase of genetic variation within populations and promote differentiation between populations and regions. In addition, allozyme variation does not distinguishS. scilloides var.albo-viridis and suggests that subtropic populations may be considered as a genetic entity.

Environmental effects on body size variation in Drosophila melanogaster and its cellular basis.

Eight isofemale lines of Drosophila melanogaster were raised at four temperatures and at four yeast concentrations in their food. Temperature and food show a significant interaction in determining wing length and thorax length, affecting mean size per line and genetic variation between lines. The combination of low temperature and poor food conditions leads to a sharp increase in the genetic variation over lines of both body size characters. The increase in genetic variation in wing length under less favourable conditions is due to an increase in genetic variation of both cell size and cell number. Changes in wing area in response to both temperature and food level follow a common cell size/cell number trajectory. Changes in wing size are obtained by line-specific changes in the cellular composition of the wing, rather than by changes specific for the environmental factor.

A CLADISTIC ANALYSIS OF PHEROMONE EVOLUTION IN IPS BARK BEETLES (COLEOPTERA: SCOLYTIDAE).

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Bottleneck effects on genetic variance for courtship repertoire.

Bottleneck effects on evolutionary potential in mating behavior were addressed through assays of additive genetic variances and resulting phenotypic responses to drift in the courtship repertoires of six two-pair founder-flush lines and two control populations of the housefly. A simulation addressed the complication that an estimate of the genetic variance for a courtship trait (e.g., male performance vigor or the female requirement for copulation) must involve assays against the background behavior of the mating partners. The additive "environmental" effect of the mating partner's phenotype simply dilutes the net parent-offspring covariance for a trait. However, it there is an interaction with this "environmental" component, negative parent-offspring covariances can result under conditions of high incompatibility between the population's distributions for male performance and female choice requirements, despite high levels of genetic variance. All six bottlenecked lines exhibited significant differentiation from the controls in at least one measure of the parent-offspring covariance for male performance or female choice (estimated by 50 parent-son and 50 parent-daughter covariances for 10 courtship traits per line) which translated to significant phenotypic drift. However, the average effect across traits or across lines did not yield a significant net increase in genetic variance due to bottlenecks. Concerted phenotypic differentiation due to the founder-flush event provided indirect evidence of directional dominance in a subset of traits. Furthermore, indirect evidence of genotype-environment interactions (potentially producing genotype-genotype effects) was found in the negative parent-offspring covariances predicted by the male-female interaction simulation and by the association of the magnitude of phenotypic drift with the absolute value of the parent-offspring covariance. Hence, nonadditive genetic effects on mating behavior may be important in structuring genetic variance for courtship, although most of the increases in genetic variance would be expected to reflect inbreeding depression with relatively rare situations representing the facilitation of speciation by bottlenecks.

Segregation variance after hybridization of isolated populations.

We develop a model to predict the increase in genetic variance of a quantitative character in a hybrid population produced by crossing two previously isolated populations of the same species. The increase in variance in the F2 hybrids, the 'segregation variance', is caused by differences in the average allelic effects at each locus and by linkage disequilibrium among loci. We focus on the case in which the character is additively based and the average value of the character does not differ in the two populations. In that case the predicted segregation variance depends strongly on what is assumed about the genetic basis of the character. If the genetic variance of the character in each population is attributable to loci with numerous alleles of small effect that are in moderate frequency, as in Lande's (1975) model, the segregation variance should increase linearly with time since the populations were isolated, at a rate determined by the inverse of the effective population size. If the genetic variance is attributable to loci with alleles in very low frequency, as in Turelli's (1984) house-of-cards model or in Barton's (1990) model of pleiotropic, deleterious alleles, then the segregation variance in the hybrid population increases at a much lower rate.

GENETIC VARIABILITY, QUEEN NUMBER, AND POLYANDRY IN SOCIAL HYMENOPTERA.

Several hypotheses have been put forward to explain the adaptive significance of interspecific variation in mating frequencies by eusocial hymenopteran queens. Four of these hypotheses assert that polyandry is advantageous to queens because of the resultant increase in genetic variability within colonies (referred to as the "GV" hypotheses). Here we compare the frequency of polyandry between monogynous (single-queen) and polygynous (multiple-queen) ant species to test the hypotheses that (1) multiple mating functions primarily to increase intracolonial genetic variability, and (2) mating has costs (such as increased energetic losses or risk of predation or venereal disease). If one of the GV hypotheses is true and mating is costly, the frequency of polyandry should be lower in polygynous species (in which the presence of multiple queens results in low relatedness among workers) than in monogynous species. As predicted by the GV hypotheses, polyandry is less common among polygynous than among monogynous species. Furthermore, it seems that the causal relationship underlying this association is that the number of matings by queens depends on the number of queens present in the colony (rather than the number of queens being influenced by the number of matings), which also supports the GV hypotheses together with the assumption that mating has costs.

Genetic Variation within a Lotic Population of Janthinobacterium lividum

An understanding of the genetic variation within and between populations should allow scientists to address many problems, including those associated with endangered species and the release of genetically modified organisms into the environment. With respect to microorganisms, the release of genetically engineered microorganisms is likely to increase dramatically given the current growth in the bioremediation industry. In this study, genetic variation within a lotic, bacterial population of Janthinobacterium lividum was measured with restriction fragment length polymorphism analysis. Chromosomal DNA from 10 Kettle Creek (Hawk Mountain Sanctuary, Kempton, Pa.) J. lividum isolates was digested with six restriction endonucleases and probed with a 7.5-kb pKK3535 fragment containing the E. coli rrnB rRNA operon. Genetic variation, as measured in terms of nucleotide diversity, was high within the population. The 0.0781 value for genetic variation was especially high given the conservative nature of the genetic probe. The average percent similarity among isolates within the population was 67.25%. Pairwise comparisons of nucleotide diversity values (pi) and similarity coefficients (F) yielded values ranging from 0.0032 to 0.1816 and 0.3363 to 0.9808, respectively. Putative clonemates were not present within the group of isolates; however, all isolates shared 14 fragments across a spectrum of six restriction enzymes. The presence of these common fragments indicates that restriction fragment length polymorphism analysis may provide population- or species-specific diagnostic markers for J. lividum. Data that suggest a plume effect with respect to the downstream movement of J. lividum are also presented. An increase in genetic variation within groups of isolates along the longitudinal gradient of Kettle Creek is also suggested.

Genetic differences in learning behavior in honeybees (Apis mellifera capensis)

Workers from colonies of Cape honeybees show marked phenotypic differences in performance in proboscis extension reflex (PER) conditioning. Analysis of these differences using parthenogenetic offspring groups permitted the estimation of genotypic values and revealed a high degree of genetic variability that is evident among related as well as unrelated bees. The results obtained from related groups are of particular importance, since they demonstrated the existence of strong genetic variability among individuals of the same colony. Quantitative analysis yielded high estimates of additive genetic effects and low estimates of dominance effects. Selection of individual workers resulted in an explicit increase in genetic variance of the next generation (G1). However, selection of bees from the parthenogenetic G1 generation, which was done to obtain parthenogenetic G2 offspring, did not lead to further improvement in selection. This observation suggests that recombination of linked genes underlying proboscis extension reflex was negligible during selection in parthenogenetic groups. Taken together with further behavioral analysis (Brandes and Menzel, 1990; Brandes et al., 1988), results from these quantitative genetic experiments suggest that additive genetic factors contribute significantly to variability among individuals for associative learning.

Increase in Genetic Variance Following a Population Bottleneck

D'apres les donnees d'une population experimentale, la reduction de la taille de la population peut parfois etre accompagnee d'une augmentation plutot qu'une diminution de la variance genetique. Si elle est suivie par un renouvellement et un reajustement de la selection naturelle, un important changement dans l'organisation du genome peut s'ensuivre

Increase of genetic variability by asymmetric cell hybridization and isolated chromosome transfer

Increase of genetic variability by asymmetric cell hybridization and isolated chromosome transfer

Heritable variation and heterozygosity under a balance between mutations and stabilizing selection.

SummaryA model of the balance between mutations and stabilizing selection affecting a quantitative character is developed and analysed. This model is essentially a discretized version of the continuum-of-alleles models analysed previously by Kimura, Lande, Turelli and others, and is formally similar to the stepwise mutation models used to interpret electrophoretic data. The complete model cannot be solved even for a haploid species, but there are useful approximations for most parameter values of interest. The ‘house-of-cards’ approximation can be used when selection is strong relative to mutation, and a normal approximation can be used when selection is relatively weak. For intermediate levels of selection a new ‘five-allele’ approximation provides accurate results over a wide range of parameter values. The house-of-cards and five-allele approximations applied to recessive alleles in a diploid population show that, for a given mutation rate, a somewhat larger genetic variance is maintained at equilibrium than in a comparable model of additive alleles. Under directional selection, the increase in genetic variance is largest for alleles of large effect and is much smaller for alleles of intermediate or small effect. At an equilibrium under stabilizing selection, homozygotes would tend to have a higher average fitness than heterozygotes when each mutation has a relatively large effect (the house-of-cards approximation), with the reverse if each mutation has a small effect (the normal approximation).

Perspectives for Molecular Genetics Research and Application in Poultry

Genetic engineering of microrganisms to produce more efficient vaccines, immunizing polypeptides, monoclonal antibodies, microbes with enhanced biodegradable capabilities, and pharmaceuticals have potential usefulness for poultry producers and researchers. Molecular technology can be used for chromosome mapping. Application of gene transfer technology to poultry requires an integrated program, including molecular genetics expertise, cell biology-embryology knowledge, and breeding capabilities. A successful gene transfer in poultry promises an increase in genetic variability. A gene theoretically can be inserted directly into selected stocks without extraneous genetic material. There is the exciting potential for inserting genes from other species. The recombinant deoxyribonucleic acid (DNA) technology for isolating and cloning a gene sequence can be readily applied to poultry. Cloned DNA for a desired gene sequence requires a delivery system to get the sequence into the germ line. A retrovirus vector that includes the sequence appears to be the system of choice for oviparous species at this time. Germ line target cells are primordial germ cells, spermatozoa, ova, and early embryonic cells that will give rise to gonads that have a chromosome with the inserted sequence. Evaluation of gene transfers at the molecular level will determine if insertion is accomplished, as well as the number and location.A major portion of a molecular genetics program will include breeding evaluation for expression, stability, and usefulness, including all the conventional procedures for multiplication, selection and structuring of populations for crossing, and field testing.