Population structure of Drosophila on quantitative measure traits and its seasonal changes

In the Drosophila population the individuals of medium, small and large size prevail in spring, summer and autumn correspondingly. Such differences are genetically determined. Small individuals are characterized by the highest modification variability while the large individuals--by the lowest one. Genotypic heterogeneity of the population was also found in the body size of individuals, which is the highest in summer and the lowest in autumn. Dynamics of the individual sizes and of genetic heterogeneity is determined by radical seasonal changes in genotypic composition of the population. Such reconstruction influences the substantial part of gene pool, which defines the dimension of individuals. One of the possible reasons of this process is an integral selective action of the population density and temperature. It provides the regulation of variations in numbers and adaptation to the seasonal changes in life conditions.     

Population structure in Sardinia on the basis of quantitative palmar dermatoglyphic traits

We describe the genetic structure of twenty Sardinian subpopulations using quantitative palmar dermatoglyphic traits (a-b, and A-d counts, atd angle value, coefficient of Turpin and Lejeune, main line index, mean of A-, B-, C, and D-line terminations) of 3777 subjects (2043 males and 1734 females). The twenty subdivisions represent sixteen historical-geographic areas of the island in which people speaks Sardinian language, Sassarese and Gallura areas in which people speaks two Italian dialects, and the two communities of Alghero (Catalan speaking) and Carloforte (Ligurian speaking). Analysis was carried out for both hands and both sexes combined and using R-matrix technique and the extension of the Harpending-Ward model to quantitative traits according to Relethford & Blangero (1990). Multivariate minimum F_st value (0.0127) is higher than that of most. Mediterranean populations and shows the importance of isolation and genetic drift as evolutive forces at the basis of microdifferentiation among the Sardinian subpopulations considered. However, when the four populations not speaking Sardinian language are removed from the analysis, the value of F_st decreases to 0.008. The regression of mean genetic variance on distances from the centroid (r_ii values) states the marked effect of the genetic drift for Nuorese and Barbagia di Ollolai subdivisions (placed in the inner and mountainous areas of Sardinia) and reveals considerable levels of admixture for Carloforte subdivision. The contemporary genetic structure of these groups reflects their historical, linguistic and geographic characteristics. On the whole, our analysis confirms the usefulness of quantitative dermatoglyphic traits in studying genetic population structure.     

Evolution of sexual preferences in quantitative characters

An analysis of equilibria and dynamics of the means, variances, and covariances of female mating preference for a quantitative male secondary sexual character following a Gaussian model is presented. For many combinations of viability and sexual selection parameters the evolving Gaussian distribution of phenotypes can diverge. The results on the cases of convergence and their limiting forms suggest some reinterpretations of Fisher's "runaway" process of sexual selection. One possibility is to interpret Fisher's postulated "initial advantage not due to female preference" as a shift in viability selection where runaway evolution occurs if the mean preferred trait evolves beyond its new viability optimum (due to sexual selection). This definition is contrasted with situations in which the new viability optimum is undershot. The quantitative and qualitative conclusions differ from models that approximate genetic covariance evolution involving a constant covariance.     

On the quantitative genetics of mixture characters

Finite mixture models are helpful for uncovering heterogeneity due to hidden structure. Quantitative genetics issues of continuous characters having a finite mixture of Gaussian components as statistical distribution are explored in this article. The partition of variance in a mixture, the covariance between relatives under the supposition of an additive genetic model, and the offspring-parent regression are derived. Formulas for assessing the effect of mass selection operating on a mixture are given. Expressions for the genetic and phenotypic correlations between mixture and Gaussian traits and between two mixture traits are presented. It is found that, if there is heterogeneity in a population at the genetic or environmental level, then genetic parameters based on theory treating distributions as homogeneous can lead to misleading interpretations. Some peculiarities of mixture characters are: heritability depends on the mean values of the component distributions, the offspring-parent regression is nonlinear, and genetic or phenotypic correlations cannot be interpreted devoid of the mixture proportions and of the parameters of the distributions mixed.     

Population structure and eigenanalysis

Current methods for inferring population structure from genetic data do not provide formal significance tests for population differentiation. We discuss an approach to studying population structure (principal components analysis) that was first applied to genetic data by Cavalli-Sforza and colleagues. We place the method on a solid statistical footing, using results from modern statistics to develop formal significance tests. We also uncover a general “phase change” phenomenon about the ability to detect structure in genetic data, which emerges from the statistical theory we use, and has an important implication for the ability to discover structure in genetic data: for a fixed but large dataset size, divergence between two populations (as measured, for example, by a statistic like F_ST) below a threshold is essentially undetectable, but a little above threshold, detection will be easy. This means that we can predict the dataset size needed to detect structure.     

Population and quantitative genetics of regulatory networks

I evolved boolean regulatory networks in a computer simulation. I varied mutation, recombination, the size of the network, and the number of connections per node. I measured the performance of networks and the heritability and epistasis of genetic effects. Networks of intermediate connectivity performed best. The distinction between metabolic and quantitative genetic additivity explained some of the variation in performance. Metabolic additivity describes the interaction between changes in a single network, whereas quantitative genetic additivity measures the consistency of phenotypic effect caused by gene substitution in randomly chosen members of the population. I analysed metabolic additivity by the distribution of epistatic effects of pairs of mutations in individual networks. I measured quantitative genetic additivity by heritability. Highly connected networks had greater metabolic additivity for perturbations to individual networks, but had lower additivity when measured by the average effect of a gene substitution (heritability). The lower heritability of highly connected nets appeared to reduce the effectiveness of recombination in searching evolutionary space.     

Simulation of quantitative characters from qualitatively acting genes

Summary The phenotypes associated with the nine genotypes in a quantitative genetic system consisting of two loci, each having two alleles can be described in terms of nine parameters, giving a system of nine linear equations. Populations with desired magnitudes and known nature of intra- and interlocus interactions are obtained by the use of this linear combination model. The total sums of squares for genotypes in these populations are partitioned into orthogonal components denoting additive and dominance effects of the two loci and the four types of nonallelic interactions between them. In most cases, the relative magnitudes of dominance and epistatic variances are found to be considerably smaller than the actual proportions of these genetic effects. Duplicate interaction produces larger epistatic variance than complementary type of gene interaction. At the higher levels of epistasis, dominant epistasis yields much larger epistatic variance than recessive epistasis. No epistatic variance is produced in the absence of epistatic effects. But, appreciable contributions of additive and dominance gene actions to the total genotypic variability are obtained even in the complete absence of these effects, if additive × dominance and dominance × dominance epistatic effects, respectively, are present. It is concluded that in elucidating the nature of gene action in simplified genetic systems, the estimates of first degree parameters obtained from the linear combination model are more useful than the orthogonal components of genotypic sum of squares.

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Zusammenfassung Die in einem quantitativ-genetischen System mit je 2 Allelen an 2 Loci möglichen 9 Phänotypen, die mit den entsprechenden Genotypen assoziiert sind, können durch einen Satz von 9 linearen Gleichungen beschrieben werden. Mit Hilfe dieses Modells der linearen Kombination wurden Populationen mit willkürlich gewählter Dimension und Art der Interaktion innerhalb der und zwischen den Loci konstruiert. Die Gesamtsummen der Abweichungsquadrate für die Genotypen derartiger Populationen werden in orthogonale Komponenten zerlegt, die den additiven und den Dominanz-Effekten bzw. den vier Arten der nichtallelen Interaktion der beiden Loci zugeschrieben werden können. In der Mehrzahl der Fälle sind die relativen Größenordnungen der Dominanz- und Epistasie-Varianzen wesentlich kleiner als die tatsächlichen Anteile dieser Effekte. Eine gegenseitige Vertretbarkeit nichtalleler Gene (duplicate gene action, 15:1-Spaltung) führt zu einer größeren Epistasievarianz als komplementäre Genwirkung (9:7-Spaltung). Bei stark ausgeprägter Epistasie führt die sog. dominante Epistasie (12:3:1-Spaltung) zu einer wesentlich größeren Epistasievarianz als die rezessive Epistasie (9:3:4-Spaltung). In Abwesenheit epistatischer Effekte wird keine Epistasievarianz beobachtet. Jedoch werden bemerkenswerte Beiträge additiver und dominanter Genwirkungen zur genotypischen Gesamtvariabilität auch bei völliger Abwesenheit derartiger Wirkungen beobachtet, wenn Interaktionen des Typs additiv × dominant bzw. dominant × dominant vorliegen. Hieraus wird geschlossen, daß die Aufklärung der Art der Genwirkung in einfachen genetischen Systemen gezeigt hat, daß die Schätzwerte der Parameter 1. Grades, die aus dem zitierten Modell mit linearer Kombination erhalten werden können, brauchbarer sind als die orthogonalen Kombinationen der genotypischen Summe der Abweichungsquadrate.

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The investigation was supported by the grant number A6221 of the National Research Council of Canada.     

Index selection for genetic improvement of quantitative characters

Summary This paper reviews the basic theory and summarizes various modifications of the selection index. The limitations of selection index are discussed in four parts: (1) changes of parameters due to selection. (2) sampling errors of parameter estimation. (3) evaluation of relative economic weights and (4) internal deterrents to index selection.     

Population niche structure

Summary The response of twenty maternal families of the annual Abutilon theophrasti to two resource gradients, nutrient and light, was investigated. The structure of the population niche for both biomass and reproductive output was found to be quite different on the two gradients. On the light gradient there was a great diversity of responses among the families while on the nutrient gradient the families responded in a similar manner. On both gradients the plants showed a significant genotype/environment interaction. Three strategies for the production of seed variation have been proposed-all offspring are adapted to the same restricted environment, each offspring of an individual is adapted to a particular environment somewhat different thant that of its siblings, and all the offspring are able to grow in a wide range of environments. We found evidence for all three of these strategies amongst the families. The range of responses seen amongst families (of the same species) in this study was as broad as that found in previous studies among species of the old field annual community to which Abutilon theophrasti belongs. This has significant implications to the nature of competitive interactions and to the evolution of differential resource use in plant populations.     

Population structure and evolutionary progress

Wright's shifting-balance theory is discussed as an example of a process that can cause species to evolve combinations of characters that could not evolve under natural selection alone. A review of the existing theory of peak shifts indicates that the conditions of extreme isolation that are necessary to permit genetic drift to alter the outcome of natural selection in local populations would make gene flow too weak to spread a new combination of genes to other populations in a reasonable time. Instead, it seems likely that major demographic changes must occur in a species for the shifting-balance process to work. A discussion of direct and indirect studies of gene flow in natural populations suggests that the current genetic structure of many species is likely to reflect past demographic events rather than ongoing gene flow. It is possible then that demographic processes could be responsible for spreading new traits in a species, but that would be true whether those new traits evolved only owing to natural selection or owing in addition to genetic drift and other forces.     

A genetic model of interpopulation variation and covariation of quantitative characters

Evolutionary consequences of natural selection, migration, genotype-environment interaction, and random genetic drift on interpopulation variation and covariation of quantitative characters are analysed in terms of a selection model that partitions natural selection into directional and stabilizing components. Without migration, interpopulation variation and covariation depend mainly on the pattern and intensities of selection among populations and the harmonic mean of effective population sizes. Both transient and equilibrium covariance structures are formulated with suitable approximations. Migration reduces the differentiation among populations, but its effect is less with genotype-environment interaction. In some special cases of genotype-environment interaction, the equilibrium interpopulation variation and covariation is independent of migration.     

Transposable elements and the penetrance of quantitative characters in Drosophila melanogaster

We wanted to determine whether there is a correlation between the quantitative character, the penetrance of the loss of humeral bristles in scute lines, and the distribution of transposable genetic elements in their genomes. We derived 18 isogenic lines with penetrance ranging between 2.8% and 92.0% from six mutant lines. The localization of the transposable elements (TEs) P, mdg1, Dm412, copia, gypsy and B104 was determined in all isogenic derivatives by in situ hybridization. The total number of the TE sites over all lines was 180. A comparison of the distribution of the TEs in the isogenic lines revealed the location of sites typical of lines with similar penetrance, no matter which parental line was involved. The results obtained suggest that such typical sites appear to tag the genome regions where the polygenes affecting the character in question are most likely to be found.