The use of canonical analysis for selection within a population of spring barley

A selection method is described involving the application of canonical analysis to data for a number of measured traits in a composite population of spring barley grown in two seasons. Lines were selected which fell within the Euclidean space defined by the range, for a number of standard cultivars, on each significant canonical variate. Alternatively, limits were fixed for an acceptable range for each trait and transformed to canonical variates. Lines were selected which fell nearest to the centroid of this acceptable space. The method might be used as an alternative to the selection index method where difficulty is often encountered in assigning appropriate economic weightings to the traits measured.     

The realized effect of postpollination sexual selection in a natural plant population

The ultimate importance of postpollination sexual selection has remained elusive, largely because of the difficulty of assigning paternity in the field. Here I use a powerful new molecular marker (AFLP) for paternity analysis in a natural population of the outcrossing angiosperm Persoonia mollis (Proteaceae) to assess male reproductive success following equal pollination of 15 pollen donors on each of 6310 pistils. These results were contrasted with male reproductive success of these same plants following natural mating. Following equal pollination, there was a significant departure from equal siring success, indicating a potential for postpollination sexual selection. The most successful pollen donor sired more than twice the expected number of seeds, and this was largely consistent across recipient plants. However, siring success following natural mating was significantly different from siring success following artificial pollination and showed that the reproductive gains to be made from superior pollen performance did not translate into increased reproductive success following natural mating. As the ecological context for post-pollination sexual selection is strong in P. mollis, I suggest that pollen competition may ultimately have only a weak effect on non-random male mating success under natural conditions because the realized opportunities for pollen competition within pistils are limited.     

Clinal variation and selection on MDH allozymes in honeybees in Chile

Evidence of clinal variation and selection on Mdh-1 locus was observed in 27 samples from 22 sites in a 2800 km north-south transect across Chile. We found a negative correlation among F allele normalized frequency and mean temperature, and minimum temperature of January and July, as well as a positive correlation among S allele normalized frequency and annual mean, and minimum January temperatures. Our results lend weight to the idea that Chilean honeybee populations of colder areas have higher F allele frequencies, supporting previous claims that Mdh-1 allozymes of Apis mellifera are subject to temperature-mediated selection.     

Genetic changes in a barley population analyzed in terms of some life cycle components of selection

A bulk-hybrid population of barley (CC-XXI) was investigated for changes over a period of six generations, in (1) generation means and variances for three quantitative characters (2) phenotypic and genotypic frequencies at several marker loci and (3) the components of selection at several life cycle stages (germination and seedling survival, fertility, fecundity) for the dominant-recessive phenotypic classes at three of the above loci. The changes in mean and variance for the quantitative characters appeared to be due to both directional and stabilizing types of selection. The estimates of selective values from frequency data suggested heterozygote advantage along with unequal homozygote fitnesses at locuss and heterozygote disadvantage at locibl, r andv, from computations based on higher outcrossing rates in the presence of male-steriles.Of the components studied, fertility and fecundity appeared to account for a larger portion of the total selective differential than the pre-adult stages (germination and seedling establishment). Net selective values based on component analysis approximated the selective values obtained from the frequency data for locusr. For locis, andv, however, the two estimates were often quite different, although they generally agreed in direction. The discrepancies between the two sets of estimates were discussed in terms of the complexity of overall measures of fitness.     

The effect of selection on genetic balance when the population size is varying

It is known that a reformulation for variable population size of the classical Sewall Wright model for balance between two genotypes can lead, under some circumstances, to a situation of balanced polymorphism when there is no selection present. In this note it is shown that the presence of selection prohibits the possibility of balance and assures ultimate homozygosity with probability one.     

The effect of directed and stabilizing selection on the population size of Drosophila melanogaster

Population size as a measure of population fitness was assayed in laboratory lines of Drosophila melanogaster subjected to directional selection for positive photoactivity and stabilizing selection for a complex of metric wing characters. Three experimental designs were used: directional and stabilizing selection (DS); directional selection (D); unselected controls (C). Population size increased under all three experimental designs. The population size of DS and C lines as well as the rate of increase of the DS lines were higher as compared to the D lines. The results suggest that stabilizing selection maintains population fitness at the higher level.     

Analysis of complex allozyme polymorphisms in a barley population

Genotypes of 68,230 individuals taken from 10 generations (F₄–F₆, F₁₄–F₁₇, F₂₄–F₂₆) of an experimental population of barley were determined for four esterase loci. The results show that frequencies of gametic ditypes changed significantly over generations and that striking gametic phase disequilibrium developed within a few generations for each of the six pairwise combinations of loci which were monitored. The complex behavior of these four enzyme loci in the population is attributed to interactions between selection and restriction of recombination resulting from the effects of linkage and/or inbreeding.     

The inhibitory effect of NaCl on barley germination

Abstract The possibility that the nature of the inhibitory effect of NaCl is different during imbibition compared to germination was investigated. Germination in both NaCl and betaine (a non-toxic solute) improved with pre-imbibition in water. Seeds imbibed in inhibitory concentrations of either solute could be induced to germinate by brief exposure to water. Electron micrographs of tissue from seeds imbibed in 0.5 kmol m^?3 NaCl for 25 h showed cells identical to those in seeds imbibed in water for only 1 h, but seeds imbibed for 6 h in water exhibited many changes in ultrastructure. These results are consistent with the hypothesis that seed hydration must reach a critical value before germination can proceed, and that the inhibitory effect of NaCl is primarily osmotic in barley seeds that have not reached this hydration threshold. Although isotonic solutions of betaine and NaCl were equally inhibitory to germination, isotonic solutions of betaine and NaCl were not equally inhibitory to continued development in seeds which had been pre-imbibed in water. Calcium ions improved both germination and plumule emergence of pre-imbibed seeds in NaCl solutions, but calcium had little effect on pre-imbibed seeds placed in betaine. Very high concentrations of NaCl or betaine inhibited germination, but did not kill dry seeds. Both solutes, on the other hand, were lethal at high concentrations to germinating seeds. NaCl killed germinating seeds more rapidly than betaine, but calcium reduced the rate of killing to nearly that of betaine. We conclude that hydrated seeds are sensitive to both osmotic and toxic effects of NaCl and that calcium mitigates the toxic effect of NaCl, but not the osmotic effect.     

The effect of proline insertions on the thermostability of a barley alpha-glucosidase

The thermal stability of alpha-glucosidase is important because the conversion of starch to fermentable sugars during industrial production of ethanol (e.g. brewing, fuel ethanol production) typically takes place at temperatures of 65-73 degrees C. In this study we investigate the thermostability of alpha-glucosidases from four plant species, compare their deduced amino acid sequences, and test the effect of substituting a proline for the residue present in the wild-type enzyme on the thermostability of alpha-glucosidase. The alpha-glucosidase from barley (Hordeum vulgare) was significantly less thermostable than the other three alpha-glucosidases. A comparison of the published deduced amino acid sequences of these four alpha-glucosidases revealed conserved proline residues in the three most thermostable alpha-glucosidases that were not found in the barley enzyme. Site-directed mutagenesis was done on recombinant barley alpha-glucosidase to create proteins with prolines at these conserved positions. The thermostability (T(50)) of one of these mutant enzymes, T340P, was 10 degrees C higher than the non-mutated enzyme.     

Genetic variation in the acorn barnacle from allozymes to population genomics

Understanding the patterns of genetic variation within and among populations is a central problem in population and evolutionary genetics. We examine this question in the acorn barnacle, Semibalanus balanoides, in which the allozyme loci Mpi and Gpi have been implicated in balancing selection due to varying selective pressures at different spatial scales. We review the patterns of genetic variation at the Mpi locus, compare this to levels of population differentiation at mtDNA and microsatellites, and place these data in the context of genome-wide variation from high-throughput sequencing of population samples spanning the North Atlantic. Despite considerable geographic variation in the patterns of selection at the Mpi allozyme, this locus shows rather low levels of population differentiation at ecological and trans-oceanic scales (F(ST)?～?5%). Pooled population sequencing was performed on samples from Rhode Island (RI), Maine (ME), and Southwold, England (UK). Analysis of more than 650 million reads identified approximately 335,000 high-quality SNPs in 19 million base pairs of the S. balanoides genome. Much variation is shared across the Atlantic, but there are significant examples of strong population differentiation among samples from RI, ME, and UK. An F(ST) outlier screen of more than 22,000 contigs provided a genome-wide context for interpretation of earlier studies on allozymes, mtDNA, and microsatellites. F(ST) values for allozymes, mtDNA and microsatellites are close to the genome-wide average for random SNPs, with the exception of the trans-Atlantic F(ST) for mtDNA. The majority of F(ST) outliers were unique between individual pairs of populations, but some genes show shared patterns of excess differentiation. These data indicate that gene flow is high, that selection is strong on a subset of genes, and that a variety of genes are experiencing diversifying selection at large spatial scales. This survey of polymorphism in S. balanoides provides a number of genomic tools that promise to make this a powerful model for ecological genomics of the rocky intertidal.     

The effect of selection on genealogies

The coalescent process can describe the effects of selection at linked loci only if selection is so strong that genotype frequencies evolve deterministically. Here, we develop methods proposed by Kaplan, Darden, and Hudson to find the effects of weak selection. We show that the overall effect is given by an extension to Price's equation: the change in properties such as moments of coalescence times is equal to the covariance between those properties and the fitness of the sample of genes. The distribution of coalescence times differs substantially between allelic classes, even in the absence of selection. However, the average coalescence time between randomly chosen genes is insensitive to the current allele frequency and is affected significantly by purifying selection only if deleterious mutations are common and selection is strong (i.e., the product of population size and selection coefficient, Ns>3). Balancing selection increases mean coalescence times, but the effect becomes large only when mutation rates between allelic classes are low and when selection is extremely strong. Our analysis supports previous simulations that show that selection has surprisingly little effect on genealogies. Moreover, small fluctuations in allele frequency due to random drift can greatly reduce any such effects. This will make it difficult to detect the action of selection from neutral variation alone.     

Further analysis of complex allozyme polymorphisms in a barley population

New theory has recently been developed for two-locus models. In the light of this theory, an earlier analysis of esterase allozyme data from an experimental barley population has been modified to take proper account of initial gametic phase (linkage) disequilibria. The results show that the directions in which two-locus genotypic frequencies deviated from products of one-locus frequencies in this population followed those predicted by neutral descent theory. The observed departures were, however, much larger in size than predicted by the new descent measure theory, indicating that selection is operating in the population.     

The effect of artificial selection for coat color on fitness in a farm population of the sable (Martes zibellina)

The relationship between the response to artificial selection for darker coat color and fitness in a farm population of the sable (Martes zibellina L.) from the Pushkinskoe Fur Farm (Moscow oblast) was studied. The selection was performed during 41 years. By the moment of the study, a response to the selection for this character had been obtained: the coat color in the selected population had become darker, and the proportion of black animals in it increased. In addition, sables with black heads, which were absent in the original population, had appeared. Artificial selection was accompanied by a decrease in the fitness of the selected population, which was expressed in decreased female reproductive capacity parameters (the fertility, maturation rate, and duration of the reproductive period). A selection technique consisting in the use of only highly fertile animals in the selection originally made it possible to restore the fitness parameters to the initial level almost without a decrease in the dark shade of the fur. However, further selection led to a drastic decrease in fitness that could not be precluded by any selection method used. The possible ways to overcome this unfavorable effect of artificial selection are discussed.     

The population genetics of reinforcing selection

A common outcome of disruptive selection experiments between two differentiated populations which produce disadvantageous hybrids is an increase in homogamy. Experiments reported here result in another outcome when ‘classical’ selection experiments are redesigned. In these modified experiments, frequencies of genotypes in the mating population were not artificially maintained at parity but were instead determined from progeny proportions in the previous generation. In these selection lines another outcome, apart from an increase in homogamy, was demonstrated. Under a high selection coefficient against heterozygotes, elimination of a homozygote and the corresponding fixation of the other was observed. No selection line demonstrated the maintenance of two differentiated populations concurrently with the selection process of heterozygote disadvantage. A high number of generations of selection under this population genetical process is necessary to increase differences between two populations. However, the instability of gene frequencies which results in fixation or elimination of a homozygote is shown to be extremely rapid by comparison. Classical experiments were repeated and after 21 generations of selection there was no increase in divergence. For lower selection coefficients, high levels of introgression are apparent, and hence the genetical distinctness of the two populations decreases over time. This is in addition to the problem of an unstable equilibrium under selection against heterozygotes. Both aspects are important but not previously considered in experimental evidence for speciation models for which their implications are discussed.